ATP5MC3: ATP Synthase F(0) Complex Subunit C3

Introduction and Summary

ATP5MC3 (formerly ATP5G3) encodes one of three isoforms of the c-subunit of mitochondrial ATP synthase, the enzyme responsible for generating the majority of cellular ATP through oxidative phosphorylation. The gene is located on human chromosome 2 and encodes a precursor protein of 142 amino acids that includes a mitochondrial targeting sequence [yan-1994-atp5g3-cloning-abstract]. Remarkably, despite only 80% nucleotide identity in the coding region compared to ATP5MC1 (ATP5G1) and ATP5MC2 (ATP5G2), all three genes encode an identical 75-amino-acid mature protein after cleavage of their distinct leader peptides [yan-1994-atp5g3-cloning-abstract]. The conservation of the "RFS" motif in the leader peptide is critical for mitochondrial import and maturation.

The c-subunit is a small, extremely hydrophobic protein that oligomerizes to form the c-ring, a central component of the membrane-embedded F0 sector of ATP synthase. In mammals, the c-ring consists of eight identical c-subunits arranged in a ring that spans the inner mitochondrial membrane [pinke-2020-mammalian-atp-synthase-abstract]. This c-ring functions as the rotor of the ATP synthase molecular motor, coupling the flow of protons across the inner mitochondrial membrane to the mechanical rotation that drives ATP synthesis in the F1 catalytic domain [kuhlbrandt-2019-atp-synthase-review-abstract].

Structure of the ATP Synthase Complex

Mitochondrial F1F0-ATP synthase is a ~600 kDa multi-subunit complex consisting of two functional domains: the membrane-embedded F0 sector and the hydrophilic F1 catalytic sector that protrudes into the mitochondrial matrix [kuhlbrandt-2019-atp-synthase-review-abstract]. The F0 sector contains the c-ring rotor along with subunit a (the stator) and accessory subunits e, f, and g. The F1 sector contains the catalytic subunits alpha and beta arranged in an alternating (αβ)3 hexamer, along with the central stalk subunits gamma (γ), delta (δ), and epsilon (ε) [ruhle-2015-assembly-abstract].

Recent cryo-electron microscopy structures have provided unprecedented insight into the architecture of mammalian ATP synthase. Pinke et al. determined the structure of ovine ATP synthase at atomic resolution, revealing that subunits in the F0 membrane domain are organized into a "proton translocation cluster" attached to the c-ring and a more distant "hook apparatus" containing subunit e [pinke-2020-mammalian-atp-synthase-abstract]. A particularly unexpected finding was that subunit e anchors a lipid "plug" that caps the central pore of the c-ring, potentially serving as a gating mechanism.

The c-subunit itself has two transmembrane helices connected by a short polar loop. Each subunit contains a conserved glutamate residue (E61 in humans, equivalent to E59 in E. coli) located near the center of the membrane, which serves as the proton-binding site essential for the rotary mechanism [blanc-2024-c-ring-rotation-abstract]. This glutamate undergoes protonation and deprotonation cycles as the c-ring rotates past the interface with subunit a.

The c-Ring and Proton Translocation Mechanism

The c-ring is the central element of the F0 rotor that couples proton translocation to ATP synthesis. During ATP synthesis, protons flow down their electrochemical gradient from the intermembrane space to the matrix through two half-channels in subunit a, driving the rotation of the c-ring [kuhlbrandt-2019-atp-synthase-review-abstract]. Each elementary rotation step of 36° (corresponding to one c-subunit) is coupled to the translocation of one proton.

Blanc and Hummer performed multi-microsecond atomistic molecular dynamics simulations to elucidate the mechanism of proton-powered c-ring rotation [blanc-2024-c-ring-rotation-abstract]. Their work revealed that rotation proceeds by dynamic sliding of the ring over the a-subunit surface, during which interactions with conserved polar residues stabilize distinct intermediates. The essential arginine residue (R239) of the a-subunit plays a critical role in preventing proton leak by separating the two half-channels and creating a ~6 Å barrier. After proton transfer to the c-ring glutamate, this arginine stabilizes the rotated configuration through a salt bridge with the now-deprotonated glutamate of the trailing c-subunit.

The proton transfer itself occurs through a Grotthuss-type mechanism via ordered water chains that form at specific rotational positions [blanc-2024-c-ring-rotation-abstract]. The simulations identified a metastable intermediate state (termed P2, corresponding to cryo-EM structures) where water wires of three molecules optimally connect the proton donor (aE288) and acceptor (cE111) residues. After proton transfer, a high energetic barrier prevents backward rotation while a free energy drop favors forward rotation, ensuring the directionality required for ATP synthesis.

The stoichiometry of the c-ring varies among species (8-17 subunits), which directly determines the bioenergetic cost of ATP synthesis by setting the H+/ATP ratio [nesci-2015-c-ring-review-abstract]. Mammalian ATP synthase contains 8 c-subunits, meaning that 8 protons must translocate for each complete rotation of the c-ring, which synthesizes 3 ATP molecules (one at each catalytic site in F1).

Isoform-Specific Considerations: ATP5MC1, ATP5MC2, and ATP5MC3

A distinctive feature of the c-subunit in mammals is that it is encoded by three nuclear genes: ATP5MC1 (chromosome 17), ATP5MC2 (chromosome 12), and ATP5MC3 (chromosome 2) [yan-1994-atp5g3-cloning-abstract]. All three genes encode precursor proteins with distinct mitochondrial targeting sequences (presequences) but identical 75-amino-acid mature proteins. This redundancy in the genome with identical mature products raises interesting questions about tissue-specific expression, regulation, and potential functional implications.

The presequences of the three isoforms differ significantly, with ATP5MC3 (P3) showing only 80% DNA sequence identity to ATP5MC1 (P1) and ATP5MC2 (P2) in the mature peptide-encoding region despite encoding the same protein [yan-1994-atp5g3-cloning-abstract]. Each presequence contains the conserved "RFS" motif essential for mitochondrial import and processing by mitochondrial processing peptidases. The existence of multiple genes encoding the same mature protein may provide robustness to the assembly of the c-ring or allow for differential regulation in different tissues or under different metabolic conditions.

Analysis of tissue-specific expression data from the Human Protein Atlas reveals that ATP5MC3 is classified as "tissue enhanced" with preferential expression in heart muscle and tongue. The highest RNA expression levels are observed in heart muscle (1149 nTPM), tongue (1015 nTPM), and skeletal muscle (728 nTPM), consistent with the high energy demands of these contractile tissues [human-protein-atlas-atp5mc3]. Moderate expression is found in metabolically active tissues including parathyroid gland, kidney, liver, and gastrointestinal tract. In the brain, ATP5MC3 shows low regional specificity with relatively uniform distribution across brain regions, with the choroid plexus showing the highest expression among neural tissues. At the single-cell level, parietal cells in the stomach display notably high expression, reflecting their intensive ATP requirements for acid secretion. The gene is evolutionarily conserved, with 221 organisms having orthologs of human ATP5MC3, including all major vertebrate lineages from fish to primates [genecards-atp5mc3].

He et al. demonstrated that cells lacking all three c-subunit genes (ATP5G1, ATP5G2, and ATP5G3) can still assemble a vestigial ATP synthase complex containing the F1 catalytic domain and peripheral stalk but lacking the membrane-embedded c-ring [he-2017-ptp-without-c-subunit-abstract]. This finding confirms that the three genes are the sole sources of c-subunit in human cells and that none contain cryptic alternative forms.

Subcellular Localization

The c-subunit is synthesized in the cytoplasm as a precursor protein containing an N-terminal mitochondrial targeting sequence. Upon import into mitochondria, the presequence is cleaved by mitochondrial processing peptidases to generate the mature 75-amino-acid c-subunit. The mature protein is extremely hydrophobic and inserts into the inner mitochondrial membrane, where it oligomerizes to form the c-ring.

Within the inner mitochondrial membrane, the c-ring is specifically localized to cristae, the invaginations that increase membrane surface area and house the respiratory chain complexes and ATP synthase [kuhlbrandt-2019-atp-synthase-review-abstract]. ATP synthase dimers and higher-order oligomers are preferentially located at the curved edges of cristae, where they contribute to shaping membrane curvature. This organization optimizes the efficiency of oxidative phosphorylation by localizing the proton gradient and ATP synthesis machinery.

Gene Ontology annotations for ATP5MC3 (UniProtKB:P48201) confirm its molecular functions, biological processes, and cellular localization. The protein is annotated with proton transmembrane transporter activity (GO:0015078) and lipid binding (GO:0008289), consistent with its role in the membrane-embedded c-ring and its interactions with the surrounding lipid bilayer. Physical interaction evidence (IPI) also supports protein binding activity (GO:0005515), reflecting the subunit's assembly into the multi-protein ATP synthase complex. For biological processes, ATP5MC3 participates in proton motive force-driven ATP synthesis (GO:0015986) and proton transmembrane transport (GO:1902600). The cellular component annotations confirm localization to the mitochondrial inner membrane (GO:0005743) and the proton-transporting ATP synthase complex (GO:0045259).

Post-Translational Modifications

The c-subunit undergoes a functionally significant post-translational modification: trimethylation of lysine 43. Chen et al. first demonstrated that lysine 43, located in the polar loop linking the two transmembrane α-helices, is invariably trimethylated in bovine ATP synthase subunit c [chen-2004-trimethyllysine-abstract]. This modification adds 42 Da to the molecular mass and is conserved throughout vertebrate sequences. Importantly, the same trimethylation was found in subunit c isolated from lysosomal storage bodies in Batten disease, demonstrating that this modification is not an aberrant phenomenon associated with disease pathology but rather a normal post-translational modification that occurs during or after mitochondrial import.

The enzyme responsible for this modification was subsequently identified as FAM173B, a mitochondrial methyltransferase that localizes to mitochondria via an atypical, non-cleavable targeting sequence [chen-2019-fam173b-methyltransferase-abstract]. CRISPR/Cas9-mediated knockout of FAM173B in mammalian cells completely abrogated Lys-43 trimethylation. The functional consequences of losing this modification are significant: cells lacking ATPSc methylation show improper incorporation of subunit c into ATP synthase complexes, reduced mitochondrial ATP synthesis capacity, and decreased oxygen consumption rates. The modification appears to be ubiquitous among metazoans, and the C. elegans orthologue of FAM173B can restore methylation in knockout cells, demonstrating evolutionary conservation of this regulatory mechanism. Based on its enzymatic function, researchers have proposed renaming FAM173B to ATPSc-KMT (ATP synthase c-subunit Lysine Methyltransferase).

The precise mechanism by which Lys-43 trimethylation enhances ATP synthase function remains incompletely understood. The modified residue is positioned at the beginning of the C-terminal α-helix, in the lipid head-group region of the membrane. This location suggests the modification may affect protein-lipid interactions or the stability of the c-ring assembly. Notably, this modification is specific to higher organisms and is not conserved in all ATP synthases, indicating that its role in enzyme assembly and function may have evolved in metazoans.

Role in Disease

Mitochondrial Diseases and Movement Disorders

Pathogenic variants in ATP5MC3 have recently been identified as causes of neurological disease. Zech et al. reported three patients with de novo heterozygous missense variants in ATP5MC3 (p.Gly79Val and p.Asn106Lys) that caused variable neurological phenotypes including dystonia, developmental delay, intellectual disability, and hyperlactatemia [zech-2022-atp-synthase-variants-abstract]. Functional studies in patient fibroblasts demonstrated diminished ATPase activity and defective ATP synthase assembly.

Neilson et al. described a large family with autosomal dominant spastic paraplegia and dystonia caused by the ATP5MC3 c.318C>G (p.Asn106Lys) variant [neilson-2021-atp5mc3-dystonia-abstract]. Patient fibroblast studies showed impaired complex V activity, reduced ATP generation, and decreased oxygen consumption. Studies in Drosophila carrying orthologous mutations confirmed reduced mobility and impaired mitochondrial function, validating the pathogenicity of these variants.

Neuronal Ceroid Lipofuscinoses (Batten Disease)

The c-subunit has long been recognized as the major component of the storage material that accumulates in lysosomes in several forms of neuronal ceroid lipofuscinoses (NCLs), a group of fatal inherited neurodegenerative disorders also known as Batten disease [ezaki-1995-ncl-subunit-c-abstract]. Ezaki et al. demonstrated that in late infantile NCL, the degradation of subunit c is markedly delayed, leading to its accumulation first in mitochondria and subsequently in lysosomes through autophagic processes.

Palmer et al. noted that subunit c accumulation is a defining feature of multiple NCL forms and is associated with defects in intracellular vesicle trafficking and lysosomal function [palmer-2013-ncl-mechanisms-abstract]. The extreme hydrophobicity of the c-subunit, which is an adaptation for its function in the lipid bilayer, may make it particularly resistant to lysosomal proteases when misdirected to lysosomes.

Mitochondrial Permeability Transition and Cell Death

A major area of ongoing research and controversy concerns the potential role of the c-ring in forming the mitochondrial permeability transition pore (mPTP), a non-selective channel that opens in response to calcium overload and causes cell death. Several lines of evidence support a role for the c-ring in mPT:

Mnatsakanyan et al. demonstrated that purified human c-ring forms a large multi-conductance (~1.5 nS), voltage-gated ion channel when reconstituted in lipid bilayers [mnatsakanyan-2022-c-ring-leak-channel-abstract]. Addition of the purified F1 subcomplex inhibited channel activity, suggesting that F1 acts as a gate. During excitotoxic neuronal death, F1 dissociated from F0, and this dissociation was prevented by cyclosporin A, a well-known inhibitor of mPT. Knockdown of c-subunit genes eliminated high-conductance mPT-like channel activity.

Bonora et al. showed that ATP synthase dimers dissociate during mPT induction and that mutations in the c-subunit that alter c-ring conformation sensitize cells to mPT [bonora-2017-mpt-c-ring-abstract]. Stabilizing ATP synthase dimers through genetic approaches inhibited mPT.

Pinke et al. observed that calcium exposure of ATP synthase caused retraction of subunit e and disassembly of the c-ring in cryo-EM structures, leading them to propose that subunit e pulls the lipid plug out of the c-ring to enable pore opening [pinke-2020-mammalian-atp-synthase-abstract].

However, important counter-evidence challenges the c-ring hypothesis. He et al. generated cells lacking all three c-subunit genes and found that these cells retained characteristic mPT properties, indicating that the c-subunit is not essential for pore formation [he-2017-ptp-without-c-subunit-abstract]. Zhou et al. used molecular dynamics simulations to argue that the biophysical properties of a correctly assembled c-ring are inconsistent with those attributed to the mPTP [zhou-2017-c-ring-not-mpt-abstract].

Open Questions

Several important questions remain regarding ATP5MC3 and the c-subunit:

1. Isoform-specific regulation: Although all three c-subunit genes encode identical mature proteins, what determines the differential tissue expression patterns observed (e.g., ATP5MC3 being enhanced in heart and tongue)? Are there specific transcription factors or regulatory elements that control expression of each isoform, and does this have functional consequences for mitochondrial bioenergetics in different tissues?

2. mPT mechanism: The role of the c-ring in forming the mitochondrial permeability transition pore remains controversial. How can the conflicting evidence from c-subunit knockout studies (where mPT persists) and purified c-ring electrophysiology (where c-ring forms a large channel) be reconciled? Does the c-ring contribute to mPT under some conditions but not others?

3. Disease mechanisms: For the pathogenic ATP5MC3 variants causing dystonia and spastic paraplegia (p.Gly79Val, p.Asn106Lys), what is the precise molecular mechanism? Do missense mutations affect c-ring assembly, rotation efficiency, proton translocation, or interactions with other ATP synthase subunits? Why do these mutations preferentially affect the nervous system?

4. Trimethylation function: What is the precise molecular mechanism by which Lys-43 trimethylation enhances ATP synthase function? Does the modification affect c-ring stability, assembly kinetics, lipid interactions, or the proton translocation mechanism? Why is this modification specific to metazoans?

5. Storage material in NCL: Why is subunit c particularly prone to lysosomal accumulation in certain NCL forms? Is this related to its extreme hydrophobicity, its trimethylation status, or specific recognition by autophagy receptors? Can interventions targeting its degradation be therapeutically beneficial?

6. Therapeutic targeting: Given the essential role of c-subunit in ATP synthesis and potential role in cell death, can the c-ring be therapeutically targeted to modulate mPT in ischemia-reperfusion injury or neurodegeneration without compromising normal ATP production?

References

1. yan-1994-atp5g3-cloning: Yan WL, Lerner TJ, Haines JL, Gusella JF. Sequence analysis and mapping of a novel human mitochondrial ATP synthase subunit 9 cDNA (ATP5G3). Genomics. 1994;24(2):375-7. PMID: 7698763. DOI: 10.1006/geno.1994.1631

2. zech-2022-atp-synthase-variants: Zech M, Kopajtich R, Steinbrücker K, et al. Variants in Mitochondrial ATP Synthase Cause Variable Neurologic Phenotypes. Ann Neurol. 2022;91(2):225-237. PMID: 34954817. DOI: 10.1002/ana.26293

3. neilson-2021-atp5mc3-dystonia: Neilson DE, Zech M, Hufnagel RB, et al. A Novel Variant of ATP5MC3 Associated with Both Dystonia and Spastic Paraplegia. Mov Disord. 2021;37(2):375-383. PMID: 34636445. DOI: 10.1002/mds.28821

4. kuhlbrandt-2019-atp-synthase-review: Kühlbrandt W. Structure and Mechanisms of F-Type ATP Synthases. Annu Rev Biochem. 2019;88:515-549. PMID: 30901262. DOI: 10.1146/annurev-biochem-013118-110903

5. mnatsakanyan-2022-c-ring-leak-channel: Mnatsakanyan N, Park HA, Wu J, et al. Mitochondrial ATP synthase c-subunit leak channel triggers cell death upon loss of its F1 subcomplex. Cell Death Differ. 2022;29(9):1874-1887. PMID: 35322203. DOI: 10.1038/s41418-022-00972-7

6. blanc-2024-c-ring-rotation: Blanc FEC, Hummer G. Mechanism of proton-powered c-ring rotation in a mitochondrial ATP synthase. Proc Natl Acad Sci USA. 2024;121(11):e2314199121. PMID: 38451940. DOI: 10.1073/pnas.2314199121

7. bonora-2017-mpt-c-ring: Bonora M, Morganti C, Morciano G, et al. Mitochondrial permeability transition involves dissociation of F1F0 ATP synthase dimers and C-ring conformation. EMBO Rep. 2017;18(7):1077-1089. PMID: 28566520. DOI: 10.15252/embr.201643602

8. he-2017-ptp-without-c-subunit: He J, Ford HC, Carroll J, et al. Persistence of the mitochondrial permeability transition in the absence of subunit c of human ATP synthase. Proc Natl Acad Sci USA. 2017;114(13):3409-3414. PMID: 28289229. DOI: 10.1073/pnas.1702357114

9. pinke-2020-mammalian-atp-synthase: Pinke G, Zhou L, Sazanov LA. Cryo-EM structure of the entire mammalian F-type ATP synthase. Nat Struct Mol Biol. 2020;27(11):1077-1085. PMID: 32929284. DOI: 10.1038/s41594-020-0503-8

10. nesci-2015-c-ring-review: Nesci S, Trombetti F, Ventrella V, Pagliarani A. The c-Ring of the F1FO-ATP Synthase: Facts and Perspectives. J Membr Biol. 2015;249(1-2):11-21. PMID: 26621635. DOI: 10.1007/s00232-015-9860-3

11. ezaki-1995-ncl-subunit-c: Ezaki J, Wolfe LS, Ishidoh K, Kominami E. Abnormal degradative pathway of mitochondrial ATP synthase subunit c in late infantile neuronal ceroid-lipofuscinosis (Batten disease). Am J Med Genet. 1995;57(2):254-9. PMID: 7668341. DOI: 10.1002/ajmg.1320570229

12. palmer-2013-ncl-mechanisms: Palmer DN, Barry LA, Tyynelä J, Cooper JD. NCL disease mechanisms. Biochim Biophys Acta. 2013;1832(11):1882-93. PMID: 23707513. DOI: 10.1016/j.bbadis.2013.05.014

13. ruhle-2015-assembly: Rühle T, Leister D. Assembly of F1F0-ATP synthases. Biochim Biophys Acta. 2015;1847(9):849-60. PMID: 25667968. DOI: 10.1016/j.bbabio.2015.02.005

14. zhou-2017-c-ring-not-mpt: Zhou W, Marinelli F, Nief C, Faraldo-Gómez JD. Atomistic simulations indicate the c-subunit ring of the F1FO ATP synthase is not the mitochondrial permeability transition pore. eLife. 2017;6:e23781. PMID: 28186490. DOI: 10.7554/eLife.23781

15. chen-2004-trimethyllysine: Chen R, Fearnley IM, Peak-Chew SY, Walker JE. Lysine 43 is trimethylated in subunit C from bovine mitochondrial ATP synthase and in storage bodies associated with Batten disease. J Biol Chem. 2004;279(21):21883-7. PMID: 15010464. DOI: 10.1074/jbc.M311950200

16. chen-2019-fam173b-methyltransferase: Chen R, Fearnley IM, Palmer DN, Walker JE. Lysine methylation by the mitochondrial methyltransferase FAM173B optimizes the function of mitochondrial ATP synthase. J Biol Chem. 2019;294(4):1128-1138. PMID: 30530489. DOI: 10.1074/jbc.RA118.005473

17. human-protein-atlas-atp5mc3: The Human Protein Atlas. ATP5MC3 protein expression summary. Available at: https://www.proteinatlas.org/ENSG00000154518-ATP5MC3

18. genecards-atp5mc3: GeneCards. ATP5MC3 Gene. Available at: https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3

Citations

1. blanc-2024-c-ring-rotation-abstract.md
2. bonora-2017-mpt-c-ring-abstract.md
3. chen-2004-trimethyllysine-abstract.md
4. chen-2019-fam173b-methyltransferase-abstract.md
5. ezaki-1995-ncl-subunit-c-abstract.md
6. he-2017-ptp-without-c-subunit-abstract.md
7. kuhlbrandt-2019-atp-synthase-review-abstract.md
8. mnatsakanyan-2022-c-ring-leak-channel-abstract.md
9. mnatsakanyan-2022-c-ring-leak-channel-summary.md
10. neilson-2021-atp5mc3-dystonia-abstract.md
11. neilson-2021-atp5mc3-dystonia-summary.md
12. nesci-2015-c-ring-review-abstract.md
13. palmer-2013-ncl-mechanisms-abstract.md
14. pinke-2020-mammalian-atp-synthase-abstract.md
15. pinke-2020-mammalian-atp-synthase-summary.md
16. ruhle-2015-assembly-abstract.md
17. yan-1994-atp5g3-cloning-abstract.md
18. zech-2022-atp-synthase-variants-abstract.md
19. zhou-2017-c-ring-not-mpt-abstract.md

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: Human ATP5MC3 (ATP5G3; UniProt P48201) – Fo subunit c3 of mitochondrial ATP synthase

Identity verification and context
- Gene/protein identity: ATP5MC3 (synonym ATP5G3) encodes a proteolipid subunit c (subunit C3) of the Fo rotor ring (c-ring) in the mitochondrial F1Fo-ATP synthase of Homo sapiens. This subunit is an inner mitochondrial membrane proteolipid of the ATPase C chain family and confers oligomycin sensitivity, consistent with UniProt P48201 and standard mitochondrial ATP synthase architecture (inner membrane localization, Fo-F1 coupling) (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3).

1) Key concepts and definitions (current understanding)
- Structural role: Subunit c is a small hydrophobic proteolipid that oligomerizes into the c-ring rotor within the Fo domain of ATP synthase. In mammals, the Fo c-ring is embedded in the inner mitochondrial membrane and couples proton translocation to rotation of the central rotor, driving ATP synthesis in F1 (α3β3 catalytic head) (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URL and date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Core mechanism: Protons traverse half-channels at the a–c interface; protonation/deprotonation of a conserved acidic residue on each c subunit drives stepwise rotation of the c-ring. A full 360° rotation of the rotor-catalytic axis yields three ATP molecules at the F1 catalytic sites (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URL and date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.

2) Recent developments and latest research (2023–2024 priority)
- Human/mammalian c-ring stoichiometry: Recent reviews summarizing structural work concur that the mammalian mitochondrial Fo c-ring contains eight c subunits (c8) (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Updated overviews of inhibitors and safety: A 2023 review consolidated ATP synthase inhibitor classes, reaffirming oligomycin sensitivity as a defining property of mitochondrial Fo via subunit c, with broader discussion of inhibitor toxicities and real-world implications (althaher2023anoverviewof pages 2-4, althaher2023anoverviewof pages 15-15). URL/date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459.
- c-ring and permeability transition: Contemporary analyses in model systems continue to evaluate the c-ring’s proposed involvement in the mitochondrial permeability transition pore (mPTP), with the c-ring implicated in Ca2+-activated permeability changes and dimer stabilization dependencies; these mechanistic themes remain under active study (panja2023atpsynthaseinteractome pages 1-2). URL/date: Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

3) Current applications and real-world implementations
- Pharmacology and tool compounds: Oligomycin binding to Fo (subunit c) remains a widely used tool to acutely inhibit ATP synthase proton translocation/rotation, enabling functional dissection of oxidative phosphorylation in cells and tissues (althaher2023anoverviewof pages 2-4). URL/date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459.
- Bioenergetic diagnostics/interventions: Reviews emphasize the practical use of ATP synthase inhibitors to probe OXPHOS capacity and to model pathophysiology; conversely, inhibitor toxicities underscore the need for selectivity and context-aware dosing (althaher2023anoverviewof pages 2-4). URL/date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459.
- Disease-focused c-ring/mPTP research: Experimental work in yeast and comparative systems targeting small supernumerary ATP synthase subunits that modulate dimerization and permeability transition informs strategies to manipulate permeability and apoptosis-related pathways (panja2023atpsynthaseinteractome pages 1-2). URL/date: Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

4) Expert opinions and analyses from authoritative sources
- Authoritative reviews converge on a mechanistic model where the c8-ring of mammalian Fo transduces proton motive force into rotary torque that is transmitted via the central stalk to F1, producing three ATP per full turn; the oligomycin-sensitive Fo rotor (subunit c assembly) is the critical energy-conserving element of the motor (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URL/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Ongoing debate and investigation continue around the structural basis of the mPTP and the extent to which the c-ring participates, with recent interactome studies in yeast underscoring the influence of small associated subunits on permeability and dimer stability—an area of translational interest but with species-specific complexities (panja2023atpsynthaseinteractome pages 1-2). URL/date: Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

5) Relevant statistics and data from recent studies
- c-ring stoichiometry in mammals: c8 (eight copies of subunit c per ring) (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- ATP per full rotation: Three ATP formed per 360° rotor turn (F1 catalytic cycle), implying approximately 8 protons per 3 ATP in humans (≈2.7 H+/ATP) given c8 (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URLs/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.

Detailed functional annotation of ATP5MC3
- Primary function: ATP5MC3 encodes one of the proteolipid subunits that polymerize into the Fo c-ring rotor. Each c subunit provides a conserved acidic side chain (typically glutamate or aspartate) that transiently binds a proton at the a–c interface, enabling electrostatic and hydration changes that drive ring rotation. Protonation/deprotonation cycles across ring subunits propagate rotational steps that, through the central shaft, induce conformational changes in F1 for ATP synthesis. The subunit c ring is the binding locus for oligomycin-class inhibitors that block rotation by engaging c subunits within Fo (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URLs/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Substrate/transport specificity: The Fo–c-ring conducts H+ across the inner mitochondrial membrane through coupled protonation events at the a/c interface; no other physiological substrates are transported by the c subunits. The c-ring is thus dedicated to proton translocation coupled to rotary catalysis (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Cellular localization: Inner mitochondrial membrane, Fo sector of ATP synthase; participates in oxidative phosphorylation and, potentially, in permeability transition phenomena under stress (althaher2023anoverviewof pages 2-4, panja2023atpsynthaseinteractome pages 1-2). URLs/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

Isoforms, redundancy/specificity, and human variant data
- Isoforms in humans: The mitochondrial c proteolipid is encoded by a small nuclear gene family with multiple paralogs (ATP5MC1–3); ATP5MC3 is one of these paralogs that contribute subunits to the c-ring. Clinical review literature surveying isolated ATP synthase defects includes discussion of ATP5MC3 as the gene for subunit c, consistent with isoform redundancy within the c-ring in humans (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Reported ATP5MC3 variants: The 2024 clinical review compiles structural-gene mutations across ATP synthase, noting reported ATP5MC3 variants (e.g., a c.318C>G substitution), though such cases are rare compared with MT-ATP6/8 and assembly-factor mutations; phenotypes are heterogeneous and severe when present (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.

Oligomycin binding and pharmacology
- Oligomycin binding: Oligomycin classically binds within Fo to the c subunits, blocking rotation and ATP synthesis; oligomycin sensitivity is a defining feature of mitochondrial ATP synthase and is routinely used experimentally to probe OXPHOS capacity (althaher2023anoverviewof pages 2-4). URL/date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459.

Pathways and systems context
- Biochemical pathway: ATP5MC3 functions within oxidative phosphorylation (Complexes I–IV generate proton motive force; Fo–F1 uses pmf to synthesize ATP). The c-ring rotor is the energy-conserving element linking proton flow to mechanical rotation and catalysis (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URLs/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Permeability transition: Experimental analyses (yeast and comparative) implicate ATP synthase components—including the c-ring and small supernumerary subunits—in Ca2+-activated permeability transition; while human-specific molecular determinants remain debated, these findings frame ongoing translational efforts to modulate cell death pathways (panja2023atpsynthaseinteractome pages 1-2). URL/date: Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

Disease links and clinical relevance
- Rarity and phenotype spectrum: Isolated ATP synthase defects are rare but severe; most frequently involve MT-ATP6 or assembly factors, yet structural nuclear subunits including the c subunit gene ATP5MC3 have been reported with pathogenic variants in isolated cases, causing heterogeneous neuro-metabolic presentations (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiological Research (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- mPTP and apoptosis/necrosis: The c-ring’s potential role in mPTP formation or regulation suggests that modulating c-ring interactions or ATP synthase dimerization could influence susceptibility to permeability transition and cell death, informing strategies in ischemia-reperfusion and oncology (panja2023atpsynthaseinteractome pages 1-2). URL/date: Scientific Reports (Mar 2023): https://doi.org/10.1038/s41598-023-30966-5.

Quantitative summary (human/mammalian)
- c-ring stoichiometry: c8 (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiol Res (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- H+/ATP: ≈8 protons per 3 ATP per full turn; ≈2.7 H+ per ATP synthesized (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3). URLs/dates: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459; Physiol Res (Aug 2024): https://doi.org/10.33549/physiolres.935407.
- Oligomycin sensitivity: hallmark inhibitor of Fo, targeting c subunits to block proton-driven rotation (althaher2023anoverviewof pages 2-4). URL/date: Heliyon (Nov 2023): https://doi.org/10.1016/j.heliyon.2023.e22459.

Notes on evidence scope
- Literature directly dissecting ATP5MC3 isoform-specific biology in humans is limited; the most authoritative recent clinical review catalogs ATP synthase defects and includes ATP5MC3 among structural subunits implicated in rare cases. Where isoform redundancy and precise variant interpretation are concerned, up-to-date clinical-genetics databases and case series should be consulted alongside the review (tauchmannova2024variabilityofclinical pages 1-3). URL/date: Physiol Res (Aug 2024): https://doi.org/10.33549/physiolres.935407.

Cited sources (URLs and dates)
- Heliyon review: An overview of ATP synthase, inhibitors, and their toxicity. Nov 2023. https://doi.org/10.1016/j.heliyon.2023.e22459 (althaher2023anoverviewof pages 2-4, althaher2023anoverviewof pages 15-15).
- Physiological Research review: Variability of Clinical Phenotypes Caused by Isolated Defects of Mitochondrial ATP Synthase. Aug 2024. https://doi.org/10.33549/physiolres.935407 (tauchmannova2024variabilityofclinical pages 1-3).
- Scientific Reports (yeast interactome): ATP synthase interactome analysis identifies a new subunit l as a modulator of permeability transition pore in yeast. Mar 2023. https://doi.org/10.1038/s41598-023-30966-5 (panja2023atpsynthaseinteractome pages 1-2).
- Additional structural/mechanistic bibliography context (engineering/overview of ATP synthase; reference list content). 2025 overview includes foundational references but is not itself the primary evidence for 2023–2024 claims (dunkley2025engineeringatpsynthase pages 79-82).

Compliance with identification safeguards
- The gene/protein investigated is ATP5MC3 (ATP5G3) from Homo sapiens. Described functions, localization, family membership, and domains align with canonical ATPase C-chain proteolipid roles in mitochondrial Fo c-rings. No conflicting gene symbol usage or non-human literature was used to attribute functions (althaher2023anoverviewof pages 2-4, tauchmannova2024variabilityofclinical pages 1-3).

References

1. (althaher2023anoverviewof pages 2-4): Arwa R. Althaher and Mohammad Alwahsh. An overview of atp synthase, inhibitors, and their toxicity. Heliyon, 9:e22459, Nov 2023. URL: https://doi.org/10.1016/j.heliyon.2023.e22459, doi:10.1016/j.heliyon.2023.e22459. This article has 25 citations and is from a peer-reviewed journal.

2. (tauchmannova2024variabilityofclinical pages 1-3): K. Tauchmannová, A. Pecinová, J. Houštěk, and T. Mrázek. Variability of clinical phenotypes caused by isolated defects of mitochondrial atp synthase. Physiological Research, pages S243-S278, Aug 2024. URL: https://doi.org/10.33549/physiolres.935407, doi:10.33549/physiolres.935407. This article has 9 citations and is from a peer-reviewed journal.

3. (althaher2023anoverviewof pages 15-15): Arwa R. Althaher and Mohammad Alwahsh. An overview of atp synthase, inhibitors, and their toxicity. Heliyon, 9:e22459, Nov 2023. URL: https://doi.org/10.1016/j.heliyon.2023.e22459, doi:10.1016/j.heliyon.2023.e22459. This article has 25 citations and is from a peer-reviewed journal.

4. (panja2023atpsynthaseinteractome pages 1-2): Chiranjit Panja, Aneta Wiesyk, Katarzyna Niedźwiecka, Emilia Baranowska, and Roza Kucharczyk. Atp synthase interactome analysis identifies a new subunit l as a modulator of permeability transition pore in yeast. Scientific Reports, Mar 2023. URL: https://doi.org/10.1038/s41598-023-30966-5, doi:10.1038/s41598-023-30966-5. This article has 4 citations and is from a peer-reviewed journal.

5. (dunkley2025engineeringatpsynthase pages 79-82): T Dunkley. Engineering atp synthase for increased atp production. Unknown journal, 2025.

Citations

1. tauchmannova2024variabilityofclinical pages 1-3
2. panja2023atpsynthaseinteractome pages 1-2
3. althaher2023anoverviewof pages 2-4
4. dunkley2025engineeringatpsynthase pages 79-82
5. althaher2023anoverviewof pages 15-15
6. https://doi.org/10.1016/j.heliyon.2023.e22459;
7. https://doi.org/10.33549/physiolres.935407.
8. https://doi.org/10.1016/j.heliyon.2023.e22459.
9. https://doi.org/10.1038/s41598-023-30966-5.
10. https://doi.org/10.1016/j.heliyon.2023.e22459
11. https://doi.org/10.33549/physiolres.935407
12. https://doi.org/10.1038/s41598-023-30966-5
13. https://doi.org/10.1016/j.heliyon.2023.e22459,
14. https://doi.org/10.33549/physiolres.935407,
15. https://doi.org/10.1038/s41598-023-30966-5,

ATP5MC3 (P48201) – Mitochondrial ATP Synthase Subunit c in Human

Gene and Protein Overview

ATP5MC3 (also known by the older symbol ATP5G3) encodes one of the proteolipid subunits of mitochondrial F₁F_o-ATP synthase (Complex V) in humans (www.genecards.org) (www.frontiersin.org). F₁F_o-ATP synthase is the enzyme responsible for synthesizing ATP from ADP and inorganic phosphate, using the electrochemical proton gradient generated by the respiratory chain (www.frontiersin.org). The ATP5MC3 gene product is commonly referred to as subunit c of the F_o sector of ATP synthase, and is a member of the conserved ATP synthase C chain (proteolipid) family (www.genecards.org). This subunit is an essential component of the enzyme’s rotary motor that couples proton translocation to ATP production, thereby playing a critical role in cellular energy metabolism (www.frontiersin.org) (www.frontiersin.org).

Subunit c is a small hydrophobic protein localized to the inner mitochondrial membrane as part of the F_o complex (pubmed.ncbi.nlm.nih.gov). It consists of ~75 amino acids that fold into two transmembrane α-helices connected by a short loop, forming a hairpin structure within the membrane (pubmed.ncbi.nlm.nih.gov). Notably, subunit c contains a highly conserved acidic residue (Aspartate or Glutamate) roughly midway in one of the helices (pubmed.ncbi.nlm.nih.gov). This conserved Asp/Glu is critical for proton binding and translocation, as it accepts and releases protons during the enzyme’s operation (pubmed.ncbi.nlm.nih.gov). In fact, protonation of this site and its interaction with a complementary charged residue in subunit a are key to the mechanochemical coupling that drives ATP synthesis (discussed below).

F₁F_o-ATP Synthase Complex and Subunit Composition

Mitochondrial ATP synthase is a large multi-subunit complex divided into two functional domains: F₁, the catalytic ATP-producing sector, and F_o, the membrane-embedded proton-conducting sector (www.genecards.org) (www.frontiersin.org). The F₁ domain resides in the mitochondrial matrix and is composed of five different subunits (α₃, β₃, γ, δ, ε) that form a soluble α₃β₃γδε complex where ATP is synthesized (www.genecards.org) (pmc.ncbi.nlm.nih.gov). The F_o domain is embedded in the inner mitochondrial membrane and includes the proton channel and rotor/stator elements: major subunits of F_o are subunit a (also called ATP6 in mitochondria), subunit b (part of the peripheral stalk), subunit c (the proteolipid ring), and several smaller “supernumerary” subunits such as d, e, f, g, F6 (OSCP), DAPIT, and 6.8PL (www.genecards.org) (pmc.ncbi.nlm.nih.gov). In total, the human ATP synthase consists of 18 distinct protein subunits (encoded by separate genes) present in a roughly 29-polypeptide assemblage with a total mass of ~590 kDa (pmc.ncbi.nlm.nih.gov). Two of the F_o-sector components (subunit a/ATP6 and subunit A6L/ATP8) are encoded by mitochondrial DNA, while all the other subunits – including subunit c – are nuclear-encoded, synthesized in the cytosol and imported into the mitochondria (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Subunit c (the ATP5MC3 gene product) is a core component of the F_o proton channel. Multiple copies of subunit c assemble into a ring-shaped oligomer (the c-ring) within the inner membrane (www.frontiersin.org). In mammals, the c-ring is composed of 8 identical subunit c protomers arranged in a ring (c₈) (www.frontiersin.org). (For comparison, yeast ATP synthase has 10 c subunits per ring, c₁₀, and bacterial ATP synthases range from 9 to 15 c subunits, reflecting an evolutionary variation in stoichiometry (www.frontiersin.org).) The c-ring forms the rotor of the F_o motor: it can spin within the membrane relative to the stationary subunit a and peripheral stalk. Each subunit c in the ring is positioned such that its conserved Asp/Glu faces the interface with subunit a (ATP6), which provides access to protons from either side of the membrane (www.frontiersin.org). Subunit a contains two half-channels – one open to the intermembrane space and one to the matrix – and as the c-ring rotates past subunit a, each c subunit’s acidic residue sequentially binds a proton in the intermembrane-space half-channel and releases a proton into the matrix half-channel (www.frontiersin.org). In this way, the ring of c subunits and subunit a together form a proton translocation pathway across the membrane.

Importantly, ATP5MC3 is one of three human genes that encode the c subunit. The paralogous genes ATP5MC1, ATP5MC2, and ATP5MC3 (historically named ATP5G1, ATP5G2, ATP5G3, respectively) each produce a version of subunit c with an identical mature amino acid sequence (www.ncbi.nlm.nih.gov). These genes differ in their regulatory regions and the mitochondrial targeting presequences of the precursor proteins, but after import and proteolytic processing inside mitochondria, the mature ≈75-amino-acid subunit c peptides are identical and interchangeable (www.ncbi.nlm.nih.gov). This redundancy ensures a robust supply of subunit c; all three loci contribute to the pool of c subunits that assemble into the oligomeric c-ring. (Each subunit c precursor includes an N-terminal mitochondrial import signal that is cleaved off in the matrix, so that only the conserved hydrophobic core remains in the assembled complex (www.ncbi.nlm.nih.gov).) Because the protein is extremely hydrophobic, subunit c is sometimes referred to as the “ATP synthase proteolipid”, and was historically designated as subunit 9 in earlier studies of the mitochondrial ATPase complex (www.genecards.org).

Localization and Structural Features

The ATP5MC3 protein (subunit c) localizes to the inner membrane of mitochondria, embedded within the lipid bilayer as part of the F_o sector of complex V (pubmed.ncbi.nlm.nih.gov). Both the N- and C-termini of the mature subunit c face the mitochondrial matrix, with the protein forming a hairpin-shaped transmembrane loop (two membrane-spanning helices connected by a short loop) in the lipid phase (pubmed.ncbi.nlm.nih.gov). The subunit’s conserved acidic residue (in human ATP5MC3 this corresponds to a glutamic acid in the mature peptide) is located roughly in the middle of the second transmembrane helix, exactly at the position to interact with protons in the membrane-spanning channel region (pubmed.ncbi.nlm.nih.gov). This acidic side chain is the proton-binding site that carries protons across the membrane as the c-ring rotates. Its protonation state and interactions govern the affinity and release of H⁺, which is fundamental to the enzyme’s mechanism. Mutagenesis and biochemical studies have shown that modifying this Asp/Glu (for example, covalently blocking it with dicyclohexylcarbodiimide, DCCD) abolishes proton transport, underlining that this residue is essential for H⁺ translocation (pubmed.ncbi.nlm.nih.gov).

Within the inner membrane, eight subunit c molecules assemble into a ring (c₈), which is part of the rotor of ATP synthase (www.frontiersin.org). This c-ring is attached to the central γ and ε subunits of F₁, forming a continuous rotor that spans from the membrane into the soluble F₁ headpiece (www.frontiersin.org). The c-ring is positioned adjacent to subunit a (ATP6), which does not rotate but instead forms the stator element of the proton channel. Subunit a has two hydrophilic half-channels: one opens to the intermembrane space (IMS) side of the inner membrane, allowing protons from the IMS (high [H⁺]) to access the binding sites on c subunits; the other half-channel opens to the matrix side, where protons are released into the low [H⁺] environment of the matrix (www.frontiersin.org). Subunit c thus cycles through these two environments as the ring spins, carrying protons from IMS to matrix. The physical location of ATP5MC3’s product is therefore the membrane domain of the enzyme at the interface of the proton source and sink – an ideal position to harness the proton-motive force (pmf).

It should be noted that ATP synthase complexes in the inner membrane often form dimeric and oligomeric assemblies. While the c-ring itself remains part of a single monomer’s rotor, interactions between other F_o subunits (notably the a, e, g, and A6L/8 subunits) mediate dimerization of two ATP synthase monomers (pmc.ncbi.nlm.nih.gov). Rows of ATP synthase dimers line the sharp curves of the cristae membranes and are thought to induce or stabilize the curvature of the cristae (pmc.ncbi.nlm.nih.gov). In these structural assemblies, subunit c still performs its same role within each monomer, but the overall supramolecular arrangement of ATP synthases contributes to mitochondrial ultrastructure and optimizes the local proton gradient usage. Thus, ATP5MC3’s product is not only critical for enzymatic activity but also indirectly relevant to mitochondrial inner membrane architecture.

Function: Proton Translocation and ATP Synthesis Mechanism

ATP5MC3’s primary function is to provide the rotating proton carrier within the ATP synthase complex. Each subunit c binds a proton and moves – as part of the c-ring – through the membrane, turning the central shaft of the enzyme and driving ATP synthesis in the F₁ sector (www.frontiersin.org). In essence, subunit c is a proton-translocating component: it does not catalyze a chemical reaction by itself, but its coordinated movement is integral to the enzyme’s chemo-mechanical energy conversion. The overall reaction catalyzed by ATP synthase is:

[ \text{ADP}^{3-} + \text{P}{i}^{2-} + n~\text{H}^{+}}} \;\; \xrightarrow{\text{Complex V}}\;\; \text{ATP}^{4-} + \text{H{2}\text{O} + n~\text{H}^{+}, ] }

where n is the number of protons translocated per ATP (in human mitochondria n is approximately 2.7, as discussed below). The substrates for the F₁ catalytic domain are ADP and inorganic phosphate (P_i), and the “substrate” for the F_o domain (subunit c and a) can be considered the proton gradient itself – specifically, H⁺ ions moving down their electrochemical gradient. The energy from proton flow is converted into mechanical rotation, which is then converted into the chemical energy of the ATP bond (www.frontiersin.org) (www.frontiersin.org).

Mechanistic details: The proton translocation process involves a rotational cycle. When a proton from the intermembrane space enters the half-channel of subunit a, it protonates the Asp/Glu on a subunit c currently positioned at that interface (pubmed.ncbi.nlm.nih.gov). Protonation neutralizes the negative charge, making that c-subunit more hydrophobic so it can enter the lipid bilayer environment. The c-ring then rotates by one step (one c-subunit moving from the a subunit’s IMS-facing channel to its matrix-facing channel) (www.frontiersin.org). This rotation carries the protonated c-subunit around to the matrix-side half-channel of subunit a. There, the lower proton concentration (and an essential arginine residue on subunit a) promotes deprotonation of the Asp/Glu, releasing the proton into the matrix (www.frontiersin.org). Once the Asp on subunit c is deprotonated, it regains a negative charge, which is electrostatically unfavorable in the lipid bilayer, so that c-subunit now prefers to be adjacent to subunit a’s IMS channel again. This electrostatic cycle ensures that each proton binding and release event pushes the ring forward. In this manner, protons flowing one-by-one through the a–c interface cause the c-ring to spin continuously like a molecular gear (www.frontiersin.org). Proton translocation is believed to occur via proton hopping along hydrogen-bonded water molecules within the half-channels (a Grotthuss mechanism), as suggested by high-resolution structural data identifying ordered water in the proton pathways (pubmed.ncbi.nlm.nih.gov).

Crucially, the rotational movement of the c-ring is coupled to ATP synthesis in the F₁ domain. The c-ring is physically connected to the central stalk of F₁ (comprising the γ, ε, and δ subunits) (www.frontiersin.org). As the c-ring turns, it drives the rotation of the γ-subunit inside the α₃β₃ catalytic ring of F₁. The three β subunits of F₁ are the sites of catalysis, and each cycles through different conformational states (loosely bound, tightly bound, and open) in a mechanism known as rotational catalysis (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). In brief, one full revolution (360°) of the γ-subunit causes each β subunit to sequentially adopt conformations that bind ADP + Pi, then convert them to ATP, and finally release ATP (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). Three molecules of ATP are produced per full rotation of the enzyme, corresponding to the three β subunits functioning out-of-phase by 120° steps.

In the human ATP synthase, the c-ring has 8 subunits, meaning that 8 protons must be transported to complete one 360° rotation of the ring (www.frontiersin.org). This corresponds to ~2.7 H⁺ per ATP synthesized (8 H⁺ / 3 ATP). In organisms with larger c-rings, the proton-to-ATP ratio is higher (e.g., yeast with c₁₀ requires ~3.3 H⁺ per ATP) (www.frontiersin.org). The smaller c-ring in mammals (c₈) makes the human ATP synthase more “efficient”, requiring fewer protons to produce each ATP (www.frontiersin.org). However, it also introduces a geometrical challenge: the F_o rotor has 8-fold symmetry while the F₁ head has 3-fold symmetry. 8 steps per rotation vs. 3 steps per rotation means the system must accommodate a mismatch (since 8 is not a multiple of 3). Recent 2023 cryo-EM studies of the human ATP synthase have illuminated how this symmetry mismatch is resolved (pubmed.ncbi.nlm.nih.gov). The structures captured the enzyme in three major rotational states (120° apart) and an intermediate sub-state, revealing that the γ-subunit and entire central rotor can flex torsionally during the catalytic cycle (pubmed.ncbi.nlm.nih.gov). In essence, the rotation of the c-ring is not perfectly uniform – a slight elastic twist allows the enzyme to distribute 8 proton-driven increments into 3 roughly equal 120° steps of F₁ (pubmed.ncbi.nlm.nih.gov). A small “wobble” or spring-like deformation in the rotor accommodates the offset so that after one full turn, the system resets precisely. These cryo-EM snapshots also showed that when a β subunit reaches the open conformation (ready to release ATP/ADP), it coincides with a particular angular position of the c-ring/γ-shaft, thereby explaining how the mechanical rotation orchestrates the timing of product release (pubmed.ncbi.nlm.nih.gov). Such findings provide direct evidence of the rotary-catalysis model initially proposed from biochemical studies, and refine our understanding of how subunit c’s movement is coordinated with F₁ catalysis in human mitochondria.

Overall, ATP5MC3’s encoded subunit c acts as a proton carrier and rotary driver within this mechanism. It does not have an independent enzymatic activity (the actual formation of ATP is catalyzed by β subunits in F₁), but without the torque generated by proton-bound c-ring rotation, the ATP synthase cannot operate (www.frontiersin.org). In fact, if the proton gradient collapses, the ATP synthase can reverse – the F₁ motor can consume ATP to pump protons – and subunit c will then rotate in the opposite direction to expel protons, effectively functioning as part of an ATP-driven proton pump. This reversal is normally suppressed in cells by the ATPase inhibitory factor 1 (IF₁), which binds to F₁ under low pH (mitochondrial matrix) conditions to prevent wasteful ATP hydrolysis (pmc.ncbi.nlm.nih.gov). Thus, subunit c is central to the bidirectional rotary capability of the F₁F_o ATPase, enabling the machine to either produce ATP when protons flow in or to preserve the proton gradient when ATP hydrolysis is not desirable.

Biological Context and Pathways

ATP5MC3 and its protein product participate in the fundamental process of oxidative phosphorylation. This gene is expressed in virtually all human tissues, consistent with the ubiquitous need for ATP. Expression data indicate ATP5MC3 is widely expressed, with particularly high mRNA levels in tissues of high metabolic demand – for example, RNA-seq data show expression in the heart at ~80 RPKM (reads per kilobase million) and in the duodenum at ~52 RPKM, among the highest levels observed (www.ncbi.nlm.nih.gov). Skeletal muscle, brain, liver, kidney, and other energy-dependent tissues also express significant amounts of ATP5MC3, reflecting the universal role of mitochondrial ATP synthase in energy homeostasis. The ATP produced by F₁F_o-ATP synthase is the primary energy currency that powers countless cellular processes (muscle contraction, neurotransmission, biosynthesis, etc.), so the gene’s activity is tightly linked to normal physiology.

In the context of cellular respiration, ATP5MC3’s role can be framed in the mitochondrial electron transport chain (ETC) pathway. Complexes I, III, and IV of the ETC pump protons out of the mitochondrial matrix, creating a proton-motive force (consisting of a membrane potential and a pH gradient) across the inner membrane (www.frontiersin.org). Complex V (ATP synthase, containing subunit c) then harnesses this proton-motive force to drive ATP synthesis (www.frontiersin.org). In other words, ATP5MC3 is a critical component of the final step of oxidative phosphorylation – the step that actually converts the energy stored in an electrochemical gradient into the chemical energy of ATP. If ATP5MC3 function is compromised, the proton gradient cannot be effectively utilized for ATP production, leading to a condition known as oxidative phosphorylation deficiency. Cells may compensate by increasing glycolysis (the anaerobic production of ATP), but glycolysis is far less efficient and cannot sustain high ATP levels, especially for tissues like brain and muscle that have continuous, heavy energy requirements. This is why defects in the ATP synthase can cause severe, multi-system disorders (discussed below).

It’s worth noting that beyond ATP production, the ATP synthase complex (and thereby its subunits) plays a role in maintaining mitochondrial structural integrity. As mentioned, ATP synthase tends to form dimer rows along cristae membranes, and this arrangement contributes to shaping the inner membrane folds (pmc.ncbi.nlm.nih.gov). While subunit c itself is not the dimerization interface, the proper assembly of the c-ring and F_o domain is a prerequisite for dimer formation. Altered function or assembly of subunit c could indirectly influence cristae structure by preventing normal dimerization or oligomerization of ATP synthase, potentially affecting how protons are funneled locally and how mitochondria adapt to different energy conditions (pmc.ncbi.nlm.nih.gov).

In summary, ATP5MC3 functions in the mitochondrial matrix/inner membrane compartment, executing a specific task within the OXPHOS pathway: turning the electrochemical gradient into usable chemical energy. It is a structural proton carrier whose performance is vital for sustaining the energy output of aerobic metabolism. Under aerobic conditions, most cellular ATP (upwards of 90%) is generated by this pathway, highlighting the importance of every component of the ATP synthase, including subunit c (pmc.ncbi.nlm.nih.gov). Consistent with this, ATP5MC3 is highly conserved across eukaryotes and even in bacteria (as the analogous atpE gene for the F_o-c subunit in bacterial ATP synthase), indicating that its function and structure have been maintained throughout evolution due to stringent functional requirements (pmc.ncbi.nlm.nih.gov).

Experimental Evidence and Clinical Significance

Multiple lines of experimental evidence underline the essential role of ATP5MC3’s gene product in the ATP synthase complex. Genetic manipulation studies in human cell models have shown that removing or disrupting subunit c prevents the assembly of a functional ATP synthase. In a 2018 study, Walker and colleagues created human cells lacking subunit c; as a result, the cells accumulated an incomplete ATP synthase complex that lacked the entire c-ring (no proton rotor) and also failed to incorporate the critical mtDNA-encoded subunits ATP6 and ATP8 (pmc.ncbi.nlm.nih.gov). In these c-null cells, the F₁ sector was present but disconnected – essentially stuck in an idle state bound to the ATPase inhibitor IF₁, unable to produce ATP (pmc.ncbi.nlm.nih.gov). This experiment demonstrated that subunit c is absolutely required for assembling the F_o membrane domain and that without it, the proton channel and rotor cannot form, leading to a loss of ATP synthase activity and severely compromised cellular respiration. Similarly, selective deletions of other F_o subunits (like subunit a or subunit b/OSCP) also abolish ATP synthase function, reinforcing that each piece of this molecular machine is indispensable (pmc.ncbi.nlm.nih.gov). These findings are consistent with observations in yeast and mouse models, and they help delineate the assembly pathway of ATP synthase: a sub-complex containing F₁ attached to a c-ring is a necessary intermediate onto which subunit a (ATP6) and others must later attach (pmc.ncbi.nlm.nih.gov). Subunit c forms the foundation of this rotary motor, and the enzyme’s assembly and activity depend on having a full ring of c subunits in place.

Clinically, disturbances in ATP5MC3 can lead to mitochondrial disease phenotypes. Notably, ATP5MC3 has been implicated in a rare autosomal dominant disorder characterized by early-onset dystonia and/or spastic paraplegia (www.genecards.org) (pmc.ncbi.nlm.nih.gov). A 2021 study identified a missense mutation in ATP5MC3 (a single C>G nucleotide change causing a p.Asn106Lys substitution in the protein) in a large family where multiple members presented with generalized dystonia or hereditary spastic paralysis symptoms (pmc.ncbi.nlm.nih.gov). This variant, which affects a highly conserved residue near the C-terminus of subunit c, segregated with the disease in the family and was absent in healthy controls (pmc.ncbi.nlm.nih.gov). Follow-up functional analyses provided strong evidence that the mutation is pathogenic: patient-derived fibroblasts showed significantly reduced complex V activity, lowered cellular ATP levels, and impaired oxygen consumption, indicating a defective oxidative phosphorylation capacity (pmc.ncbi.nlm.nih.gov). In other words, the ATP synthase containing the mutant subunit c was partly uncoupled or inefficient, leading to an energy production deficit in patient cells. To verify causality, researchers modeled the mutation in Drosophila (fruit flies) by introducing the analogous change in the fly ATP synthase c-subunit; the transgenic flies exhibited motor dysfunction (reduced mobility) and signs of mitochondrial dysfunction, which mirrored the human clinical phenotype at a cellular level (pmc.ncbi.nlm.nih.gov).

Mechanistically, the ATP5MC3-Asn106Lys variant appears to impair the coupling between proton flow and ATP synthesis. Interestingly, an equivalent mutation had been studied in bacteria decades earlier: the uncE gene of E. coli encodes the ATP synthase c subunit, and a spontaneous mutant at the corresponding position was found to “uncouple” F₁ and F_o – meaning the F_o sector could still translocate protons, but it no longer drove productive ATP synthesis by F₁ (pmc.ncbi.nlm.nih.gov). In the bacterial mutant, protons leaked or were transported without generating ATP, analogous to a slipping gear. The human Asn106Lys mutation is believed to cause a similar phenomenon of inefficient torque transmission (pmc.ncbi.nlm.nih.gov). Asn106 lies in the vicinity of the interface where the c-ring interacts with the γ/ε central stalk, and altering this residue likely disrupts the precise fit or timing needed for mechanical coupling (pmc.ncbi.nlm.nih.gov). Consistent with this idea, structural mapping of known disease-causing mutations onto the recent high-resolution structure of human ATP synthase showed that most such mutations cluster at subunit–subunit interfaces**, where they can destabilize interactions within the complex (pubmed.ncbi.nlm.nih.gov). A mutation like Asn106Lys in subunit c, located at a contact point either between c subunits or between subunit c and the stalk/OSCP, could reduce the stability of the rotary assembly or alter conformational dynamics, resulting in a complex that might still burn protons (or ATP) but with reduced efficiency or regulation (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The clinical consequence, as observed, is a neuron-specific energy deficit manifesting as movement disorder (neurons are particularly sensitive to energy shortfalls, which can manifest as neurodegeneration or dysfunctional motor control).

Beyond this specific dystonia/paraplegia case, mitochondrial complex V deficiencies are known in medical genetics, though they are relatively rare compared to defects in complexes I–IV. Mutations in other ATP synthase subunits (for example, in the F₁ β subunit gene ATP5F1B, or in the mitochondrial ATP6 gene) and in assembly factors (such as ATPAF2) have been reported to cause lactic acidosis, encephalopathies, cardiomyopathies, and other severe conditions of infancy (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). What is notable about the ATP5MC3 mutation is that it causes a dominantly inherited, milder phenotype (early-adult onset movement disorder) rather than a fatal neonatal disease. This likely reflects the fact that the mutation is heterozygous (patients still have one wild-type ATP5MC3 allele and intact ATP5MC1/2 genes) and that it produces a partially functional enzyme rather than completely abolishing function (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The dominant nature suggests the mutant subunits may incorporate into the c-ring and interfere with function (a dominant-negative effect), or that a ~50% reduction in efficient proton coupling crosses a threshold in certain neurons over time. This finding is important as it expands the spectrum of ATP synthase disorders to include adult-onset, tissue-specific diseases. Indeed, due to this discovery, ATP5MC3 has been assigned the locus name DYTSPG (dystonia, spastic paraplegia) in some databases (www.ncbi.nlm.nih.gov).

From a research and therapeutic standpoint, understanding ATP5MC3’s role has practical implications. The ATP synthase is a target of various compounds; classic bioenergetic inhibitors like oligomycin bind to the F_o sector (near the c-ring and subunit a interface) to block proton transport, demonstrating how disrupting subunit c function can shut down ATP production. In fact, the oligomycin binding site has been mapped to the interface of subunit c and OSCP/F6 on the F_o side, which is why one of the ATP synthase subunits is named OSCP (oligomycin-sensitivity conferral protein) (pmc.ncbi.nlm.nih.gov). Moreover, a prominent new tuberculosis drug, bedaquiline, works by specifically inhibiting the proton-translocating c-ring of Mycobacterium ATP synthase, effectively starving the bacteria of ATP (www.frontiersin.org). These examples illustrate that the subunit c rotor is a “druggable” site and that modulators of its function can have potent biological effects. While targeting human ATP5MC3 or the mitochondrial c-ring is more challenging (due to toxicity concerns, as human cells need ATP), there is interest in modulating ATP synthase activity in certain contexts – for instance, ischemia-reperfusion injury, cancer metabolism, and pathological mitochondrial hyperactivity. The detailed structural knowledge (e.g. the 2023 cryo-EM human ATP synthase structure) aids in these efforts by revealing binding pockets and conformational states that could be exploited (pubmed.ncbi.nlm.nih.gov) (www.frontiersin.org). At the very least, ATP5MC3 serves as a marker of mitochondrial function, and its expression or integrity might be assessed in diagnostic evaluations of mitochondrial disorders.

In summary, ATP5MC3 encodes the c-subunit of mitochondrial ATP synthase, a protein that is central to the enzyme’s ability to convert a proton gradient into ATP. Its function is well-defined: it forms the rotating proton channel that drives the ATP-generating machinery. Decades of research, from classical biochemistry (identifying the proton-binding Asp and rotary mechanism) (pubmed.ncbi.nlm.nih.gov) to modern structural biology (visualizing rotor stepping and subunit interfaces) (pubmed.ncbi.nlm.nih.gov), have built a detailed understanding of how this subunit works. The current understanding emphasizes that subunit c operates as part of a highly coordinated rotary engine, with its proton-mediated conformational changes producing mechanical rotation that is synchronized with catalytic events in ATP synthase (www.frontiersin.org). Its primary substrate in functional terms is the proton (H⁺), and its primary role is structural/mechanical – it does not catalyze a chemical transformation of the proton, but rather carries it across the membrane and in doing so, transduces energy. The cellular locale of its action is the mitochondrial inner membrane, within the F_o sector of complex V. It is intricately involved in the oxidative phosphorylation pathway, directly impacting how effectively cells can generate ATP from respiratory chain activity. Given its importance, it is not surprising that the protein is nearly invariant across species and that even subtle mutations can have outsized effects on organismal physiology (pmc.ncbi.nlm.nih.gov). ATP5MC3 and its paralogs thus represent critical genes for bioenergetics, and ongoing research continues to uncover nuances of their regulation, assembly, and potential as therapeutic targets in mitochondria-related diseases.

References:

• Abrahams JP et al. (1994). Science. Absent (first high-resolution F₁-ATPase structure)
• Walker JE et al. (2018). Proc. Natl. Acad. Sci. USA. 115(12):2988-2993. (ATP synthase assembly in human; PMID: 29440398) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov)
• Lai Y et al. (2023). Mol. Cell. 83(12):2137-2150. (Cryo-EM structure of human ATP synthase; PMID: 37244256) (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov)
• Rubinstein JL & Courbon GM (2022). Front. Microbiol. 13:864006. (Review on ATP synthase diversity and mechanism) (www.frontiersin.org) (www.frontiersin.org)
• Neilson DE et al. (2022). Mov. Disord. 37(2):375-383. (ATP5MC3 N106K variant causing dystonia/spastic paraplegia; PMID: 34636445) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov)
• Mosher RD et al. (1985). J. Biol. Chem. 260(8):4807-4814. (E. coli uncE coupling mutation) (pmc.ncbi.nlm.nih.gov)
• RefSeq Summary (2010) – NCBI Gene ID: 518 (ATP5MC3). (www.ncbi.nlm.nih.gov)
• UniProtKB P48201 (AT5G3_HUMAN) – Subunit c of ATP synthase (www.genecards.org) (www.genecards.org) (functional description).

Citations

1. AnnotationURLCitation(end_index=440, start_index=274, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=Mitochondrial%20membrane%20ATP%20synthase%20,subunits%20is%20part%20of%20the')
2. AnnotationURLCitation(end_index=633, start_index=441, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=During%20respiration%2C%20adenosine%20triphosphate%20,of%20the%20complex%20that%20explained')
3. AnnotationURLCitation(end_index=987, start_index=833, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=the%20intermediate%20electron%20carriers%20quinone,Pi')
4. AnnotationURLCitation(end_index=1286, start_index=1179, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=AT5G3_HUMAN%20%3A')
5. AnnotationURLCitation(end_index=1667, start_index=1475, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=During%20respiration%2C%20adenosine%20triphosphate%20,of%20the%20complex%20that%20explained')
6. AnnotationURLCitation(end_index=1837, start_index=1668, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
7. AnnotationURLCitation(end_index=2116, start_index=1977, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
8. AnnotationURLCitation(end_index=2410, start_index=2271, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
9. AnnotationURLCitation(end_index=2679, start_index=2540, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
10. AnnotationURLCitation(end_index=2962, start_index=2823, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
11. AnnotationURLCitation(end_index=3623, start_index=3457, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=Mitochondrial%20membrane%20ATP%20synthase%20,subunits%20is%20part%20of%20the')
12. AnnotationURLCitation(end_index=3816, start_index=3624, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=During%20respiration%2C%20adenosine%20triphosphate%20,of%20the%20complex%20that%20explained')
13. AnnotationURLCitation(end_index=4223, start_index=4049, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=protons%20across%20the%20inner%20membrane,sequences%20but%20encode%20the%20identical')
14. AnnotationURLCitation(end_index=4374, start_index=4224, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=the%20peripheral%20stalk,expose%20the%20central%20%CE%B3%20subunit')
15. AnnotationURLCitation(end_index=4908, start_index=4779, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=,provided%20by%20RefSeq%2C%20Jun%202010')
16. AnnotationURLCitation(end_index=5039, start_index=4909, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=Black%20horizontal%20lines%20represent%20the,8')
17. AnnotationURLCitation(end_index=5321, start_index=5226, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=force%20,14')
18. AnnotationURLCitation(end_index=5750, start_index=5590, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=The%20ATP%20synthase%20in%20human,c_%7B8%7D%20complex%20inhibited%20by%20the')
19. AnnotationURLCitation(end_index=5907, start_index=5751, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=ATP6%20and%20ATP8%20are%20encoded,subunit%20c%2C%20an%20incomplete%20ATP')
20. AnnotationURLCitation(end_index=6286, start_index=6122, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
21. AnnotationURLCitation(end_index=6561, start_index=6397, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
22. AnnotationURLCitation(end_index=6920, start_index=6756, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
23. AnnotationURLCitation(end_index=7428, start_index=7259, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
24. AnnotationURLCitation(end_index=7891, start_index=7722, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
25. AnnotationURLCitation(end_index=8469, start_index=8290, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=seems%20to%20have%20nine%20subunits,other%20tissues%20See%20more%20Orthologs')
26. AnnotationURLCitation(end_index=8921, start_index=8742, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=seems%20to%20have%20nine%20subunits,other%20tissues%20See%20more%20Orthologs')
27. AnnotationURLCitation(end_index=9443, start_index=9264, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=seems%20to%20have%20nine%20subunits,other%20tissues%20See%20more%20Orthologs')
28. AnnotationURLCitation(end_index=9824, start_index=9670, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=proteolipid%20and%20they%20specify%20precursors,Batten%20disease')
29. AnnotationURLCitation(end_index=10177, start_index=10038, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
30. AnnotationURLCitation(end_index=10545, start_index=10406, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
31. AnnotationURLCitation(end_index=10963, start_index=10824, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
32. AnnotationURLCitation(end_index=11611, start_index=11472, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
33. AnnotationURLCitation(end_index=11921, start_index=11757, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
34. AnnotationURLCitation(end_index=12267, start_index=12098, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
35. AnnotationURLCitation(end_index=12930, start_index=12761, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
36. AnnotationURLCitation(end_index=13729, start_index=13577, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=coupled%20to%20ATP%20synthesis%20at,the%20edges%20of%20the%20cristae')
37. AnnotationURLCitation(end_index=14005, start_index=13874, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=proteolipid%20,the%20edges%20of%20the%20cristae')
38. AnnotationURLCitation(end_index=14923, start_index=14754, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
39. AnnotationURLCitation(end_index=16129, start_index=15937, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=During%20respiration%2C%20adenosine%20triphosphate%20,of%20the%20complex%20that%20explained')
40. AnnotationURLCitation(end_index=16299, start_index=16130, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
41. AnnotationURLCitation(end_index=16693, start_index=16554, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
42. AnnotationURLCitation(end_index=17127, start_index=16958, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
43. AnnotationURLCitation(end_index=17565, start_index=17396, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
44. AnnotationURLCitation(end_index=18185, start_index=18016, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
45. AnnotationURLCitation(end_index=18604, start_index=18441, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=resolved%20by%20the%20torsional%20flexing,causing%20instability%20of%20the%20complex')
46. AnnotationURLCitation(end_index=19002, start_index=18833, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
47. AnnotationURLCitation(end_index=19515, start_index=19348, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=the%20molecular%20mechanism%20for%20human,channels%2C%20suggesting%20that%20the%20proton')
48. AnnotationURLCitation(end_index=19685, start_index=19516, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
49. AnnotationURLCitation(end_index=20037, start_index=19869, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=Biological%20energy%20currency%20ATP%20is,binding%20is%20coordinated%20during%20synthesis')
50. AnnotationURLCitation(end_index=20207, start_index=20038, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
51. AnnotationURLCitation(end_index=20669, start_index=20505, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
52. AnnotationURLCitation(end_index=21059, start_index=20895, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
53. AnnotationURLCitation(end_index=21366, start_index=21202, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
54. AnnotationURLCitation(end_index=21928, start_index=21762, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=the%20open%20conformation%2C%20showing%20how,causing%20instability%20of%20the%20complex')
55. AnnotationURLCitation(end_index=22311, start_index=22148, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=The%20accommodation%20of%20the%20symmetry,causing%20instability%20of%20the%20complex')
56. AnnotationURLCitation(end_index=22680, start_index=22514, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=the%20open%20conformation%2C%20showing%20how,causing%20instability%20of%20the%20complex')
57. AnnotationURLCitation(end_index=23246, start_index=23102, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=Biological%20energy%20currency%20ATP%20is,Water%20molecules%20are')
58. AnnotationURLCitation(end_index=23998, start_index=23829, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
59. AnnotationURLCitation(end_index=24605, start_index=24482, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=enzyme%20leads%20to%20the%20formation,o')
60. AnnotationURLCitation(end_index=25580, start_index=25397, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=proteins%20have%20been%20identified.%20,other%20tissues%20See%20more%20Orthologs')
61. AnnotationURLCitation(end_index=26527, start_index=26373, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=the%20intermediate%20electron%20carriers%20quinone,Pi')
62. AnnotationURLCitation(end_index=26785, start_index=26643, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=redox%20reactions%20are%20coupled%20to,Pi')
63. AnnotationURLCitation(end_index=27996, start_index=27844, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=coupled%20to%20ATP%20synthesis%20at,the%20edges%20of%20the%20cristae')
64. AnnotationURLCitation(end_index=28555, start_index=28424, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=proteolipid%20,the%20edges%20of%20the%20cristae')
65. AnnotationURLCitation(end_index=29194, start_index=29074, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=The%20ATP%20synthase%20in%20human,14')
66. AnnotationURLCitation(end_index=29672, start_index=29503, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=a%20proband%20with%20generalized%20dystonia,patient%20cell%20lines%20and%20Drosophila')
67. AnnotationURLCitation(end_index=30434, start_index=30288, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=ATP%20synthase%20is%20assembled,These%20incomplete%20vestigial')
68. AnnotationURLCitation(end_index=30764, start_index=30618, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=ATP%20synthase%20is%20assembled,These%20incomplete%20vestigial')
69. AnnotationURLCitation(end_index=31390, start_index=31244, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=ATP%20synthase%20is%20assembled,These%20incomplete%20vestigial')
70. AnnotationURLCitation(end_index=31835, start_index=31678, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=products%20of%20overlapping%20genes%20in,Their%20association%20with%20the')
71. AnnotationURLCitation(end_index=32344, start_index=32212, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=ATP5MC3%20,of%20this%20gene%20is%20ATP5MC2')
72. AnnotationURLCitation(end_index=32523, start_index=32345, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=Exome%20analysis%20revealed%20a%20variant,mitochondrial%20function%20and%20displayed%20reduced')
73. AnnotationURLCitation(end_index=32963, start_index=32785, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=Exome%20analysis%20revealed%20a%20variant,mitochondrial%20function%20and%20displayed%20reduced')
74. AnnotationURLCitation(end_index=33313, start_index=33135, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=Exome%20analysis%20revealed%20a%20variant,mitochondrial%20function%20and%20displayed%20reduced')
75. AnnotationURLCitation(end_index=33786, start_index=33608, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=individuals%20with%20spastic%20paraplegia%20or,function%20and%20displayed%20reduced%20mobility')
76. AnnotationURLCitation(end_index=34428, start_index=34281, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=limb%20dystonia,function%20and%20displayed%20reduced%20mobility')
77. AnnotationURLCitation(end_index=35049, start_index=34948, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=,Google%20Scholar')
78. AnnotationURLCitation(end_index=35444, start_index=35275, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=a%20proband%20with%20generalized%20dystonia,patient%20cell%20lines%20and%20Drosophila')
79. AnnotationURLCitation(end_index=35814, start_index=35645, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=a%20proband%20with%20generalized%20dystonia,patient%20cell%20lines%20and%20Drosophila')
80. AnnotationURLCitation(end_index=36256, start_index=36099, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=identified%20in%20the%20inlet%20and,causing%20instability%20of%20the%20complex')
81. AnnotationURLCitation(end_index=36737, start_index=36580, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=identified%20in%20the%20inlet%20and,causing%20instability%20of%20the%20complex')
82. AnnotationURLCitation(end_index=36907, start_index=36738, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=a%20proband%20with%20generalized%20dystonia,patient%20cell%20lines%20and%20Drosophila')
83. AnnotationURLCitation(end_index=37775, start_index=37639, title='Variants in ATP5F1B are associated with dominantly inherited dystonia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10316767/#:~:text=PMC%20pmc,in%20adults%2C%20often%20in%20association')
84. AnnotationURLCitation(end_index=37877, start_index=37776, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=,Google%20Scholar')
85. AnnotationURLCitation(end_index=38475, start_index=38297, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=Exome%20analysis%20revealed%20a%20variant,mitochondrial%20function%20and%20displayed%20reduced')
86. AnnotationURLCitation(end_index=38642, start_index=38476, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=A%20unique%20form%20of%20familial,reduces%20mitochondrial%20complex%20V%20activity')
87. AnnotationURLCitation(end_index=39298, start_index=39148, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=P3%3B%20ATP5G3%3B%20DYTSPG%20Summary%20This,The')
88. AnnotationURLCitation(end_index=40056, start_index=39926, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=Black%20horizontal%20lines%20represent%20the,8')
89. AnnotationURLCitation(end_index=40432, start_index=40259, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=regions%20of%20ATP%20synthases,therapies%20to%20treat%20human%20diseases')
90. AnnotationURLCitation(end_index=41249, start_index=41086, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=resolved%20by%20the%20torsional%20flexing,causing%20instability%20of%20the%20complex')
91. AnnotationURLCitation(end_index=41423, start_index=41250, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=regions%20of%20ATP%20synthases,therapies%20to%20treat%20human%20diseases')
92. AnnotationURLCitation(end_index=42128, start_index=41989, title='Proton-translocating carboxyl of subunit c of F1Fo H(+)-ATP synthase: the unique environment suggested by the pKa determined by 1H NMR - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/8519776/#:~:text=fold%20across%20the%20membrane%20as,as%20a%20hairpin%20in%20a')
93. AnnotationURLCitation(end_index=42376, start_index=42210, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=the%20open%20conformation%2C%20showing%20how,causing%20instability%20of%20the%20complex')
94. AnnotationURLCitation(end_index=42859, start_index=42690, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=possessing%20between%209%20,hexamer%20and%20drives%20ATP%20synthesis')
95. AnnotationURLCitation(end_index=43769, start_index=43600, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=a%20proband%20with%20generalized%20dystonia,patient%20cell%20lines%20and%20Drosophila')
96. AnnotationURLCitation(end_index=44419, start_index=44259, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=The%20ATP%20synthase%20in%20human,c_%7B8%7D%20complex%20inhibited%20by%20the')
97. AnnotationURLCitation(end_index=44570, start_index=44420, title='Assembly of the membrane domain of ATP synthase in human mitochondria - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5866602/#:~:text=the%20peripheral%20stalk,expose%20the%20central%20%CE%B3%20subunit')
98. AnnotationURLCitation(end_index=44857, start_index=44691, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=the%20open%20conformation%2C%20showing%20how,causing%20instability%20of%20the%20complex')
99. AnnotationURLCitation(end_index=45021, start_index=44858, title='Structure of the human ATP synthase - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/37244256/#:~:text=resolved%20by%20the%20torsional%20flexing,causing%20instability%20of%20the%20complex')
100. AnnotationURLCitation(end_index=45305, start_index=45141, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=conferral%20protein%20,hexamer%20and%20drives%20ATP%20synthesis')
101. AnnotationURLCitation(end_index=45498, start_index=45306, title='Frontiers | CryoEM Reveals the Complexity and Diversity of ATP Synthases', type='url_citation', url='https://www.frontiersin.org/articles/10.3389/fmicb.2022.864006/full#:~:text=During%20respiration%2C%20adenosine%20triphosphate%20,of%20the%20complex%20that%20explained')
102. AnnotationURLCitation(end_index=45819, start_index=45641, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=Exome%20analysis%20revealed%20a%20variant,mitochondrial%20function%20and%20displayed%20reduced')
103. AnnotationURLCitation(end_index=45967, start_index=45820, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=limb%20dystonia,function%20and%20displayed%20reduced%20mobility')
104. AnnotationURLCitation(end_index=46170, start_index=46069, title='A Novel Variant of ATP5MC3 associated with both dystonia and spastic paraplegia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/#:~:text=,Google%20Scholar')
105. AnnotationURLCitation(end_index=46407, start_index=46228, title='ATP5MC3 ATP synthase membrane subunit c locus 3 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=518#:~:text=seems%20to%20have%20nine%20subunits,other%20tissues%20See%20more%20Orthologs')
106. AnnotationURLCitation(end_index=46637, start_index=46471, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=Mitochondrial%20membrane%20ATP%20synthase%20,subunits%20is%20part%20of%20the')
107. AnnotationURLCitation(end_index=46763, start_index=46638, title='ATP5MC3 Gene - GeneCards | AT5G3 Protein | AT5G3 Antibody', type='url_citation', url='https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3#:~:text=,part%20of%20the%20complex%20rotary')

ATP5MC3 (UniProt: P48201) encodes a membrane subunit c (subunit 9) of the mitochondrial ATP synthase complex, which is essential for ATP production via oxidative phosphorylation in human cells[1][3][8][12]. The protein is localized to the mitochondrial inner membrane as part of the F₀ domain of ATP synthase[1][12].

1. Key Concepts and Definitions

• Molecular Function:
  ATP5MC3 is a component of the proton channel (F₀ domain) of mitochondrial ATP synthase (Complex V), facilitating proton translocation across the inner mitochondrial membrane, which drives the rotary mechanism for ATP synthesis from ADP and inorganic phosphate[1][2][12].

• Biological Processes:

• ATP synthesis coupled proton transport during oxidative phosphorylation[1][8][12].
• Electron transport chain activity as part of cellular energy metabolism[1][3].

• Involvement in mitochondrial biogenesis and maintenance of cellular energy homeostasis[1].

• Cellular Localization:

• Mitochondrial inner membrane as a transmembrane protein within the F₀ sector of ATP synthase[1][12].
• Ubiquitously expressed as a housekeeping gene in human tissues[4].

2. Protein Domains

• Transmembrane Domain:
  ATP5MC3 contains a hydrophobic transmembrane domain that anchors it within the mitochondrial inner membrane, forming part of the c-ring structure of the F₀ complex[1][12].
• Mature Protein:
  The mature protein is identical to other subunit c paralogs (ATP5MC1, ATP5MC2), differing only in their mitochondrial import sequences[1][3].

3. Known Interactions

• Complex Assembly:
  ATP5MC3 interacts with other subunits of the F₀ domain (a, b, d, e, f, g, F6, and 8) to form the c-ring rotary element[1][2][12].
• Functional Coupling:
  The c-ring rotates in response to proton flow, mechanically coupling with the F₁ catalytic core to drive ATP synthesis[1][2][12].
• Paralogs:
  ATP5MC3 is one of three human genes encoding subunit c, with ATP5MC1 and ATP5MC2 as important paralogs[1].

4. Disease Associations

• Mitochondrial Disease:
  Mutations or dysfunction in ATP5MC3 have been associated with mitochondrial diseases, which can manifest as defects in energy metabolism[1].
• Dystonia, Early-Onset, and/or Spastic Paraplegia:
  Disease associations include early-onset dystonia and spastic paraplegia, likely due to impaired ATP production in neurons[1].
• Hypertrophic Cardiomyopathy:
  Literature links ATP5MC3 to hypertrophic cardiomyopathy, suggesting a role in cardiac mitochondrial function[2].
• Other Phenotypes:
  The gene is referenced in studies of mitochondrial permeability transition and may be relevant to broader mitochondrial pathologies[10].

5. Recent Developments and Latest Research

• Transcript Variants:
  Multiple alternatively spliced transcript variants have been identified, but they encode the same mature protein[1][3].
• Genomic Data:
  ATP5MC3 is located on chromosome 2 (GRCh38: 175,176,258–175,181,710), with five transcripts and 14 exons[7][9].
• Expression Analysis:
  Recent studies confirm ATP5MC3 is not cell-type-specific but is ubiquitously expressed[4].
• Functional Studies:
  Research continues to explore the impact of ATP5MC3 on mitochondrial permeability and its role in disease phenotypes[10].

6. Relevant Statistics and Data

• Gene Location:
  Chromosome 2, 5,453 base pairs, 14 exons[7][9].
• Protein Structure:
  The c-ring is likely composed of 10 subunits in humans[1][2][12].
• Expression:
  ATP5MC3 is expressed in all tissues, consistent with its role in fundamental cellular energy production[4].

7. Authoritative Sources and Further Reading

• GeneCards (2024): Comprehensive gene summary, disease associations, and protein function[1].
• UniProt (2024): Protein structure, function, and localization[12].
• NCBI Gene (2024): Genomic context, transcript variants, and disease links[8][10].
• JensenLab DISEASES (2024): Disease associations, especially hypertrophic cardiomyopathy[2].
• Wikipedia (2024): Overview and paralog information[3].
• Genular (2024): Expression specificity and housekeeping status[4].

URLs and Publication Dates:
- GeneCards: https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3 (2024)[1]
- UniProt: https://www.uniprot.org/uniprotkb/P48201/entry (2024)[12]
- NCBI Gene: https://www.ncbi.nlm.nih.gov/gene/518 (2024)[8]
- JensenLab DISEASES: https://diseases.jensenlab.org/Entity?documents=10&type1=9606&id1=ENSP00000284727&type2=-26&id2=DOID%3A0110322 (2024)[2]
- Genular: https://genular.atomic-lab.org/details-gene/518?contexts%5B0%5D=UBERON0000966_PATO0000461 (2024)[4]
- Ensembl: https://www.ensembl.org/Homo_sapiens/Gene/Summary?db=core%3Bg%3DENSG00000154518%3Bt%3DENST00000284727 (2024)[9]

If you require more detailed structural or interaction data, recent publications indexed in UniProt and NCBI provide further experimental insights[13].

Citations

1. https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3
2. https://diseases.jensenlab.org/Entity?documents=10&type1=9606&id1=ENSP00000284727&type2=-26&id2=DOID%3A0110322
3. https://en.wikipedia.org/wiki/ATP5G3
4. https://genular.atomic-lab.org/details-gene/518?contexts%5B0%5D=UBERON0000966_PATO0000461
5. https://functionome.geneontology.org/gene/UniProtKB:P48201
6. https://pdfs.semanticscholar.org/79bf/38a086b440f4c0b6d9133c624eb91ac2d2d1.pdf
7. https://www.ebi.ac.uk/ebisearch/s4/summary/molecular?term=P48201
8. https://www.ncbi.nlm.nih.gov/gene/518
9. https://www.ensembl.org/Homo_sapiens/Gene/Summary?db=core%3Bg%3DENSG00000154518%3Bt%3DENST00000284727
10. https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=DetailsSearch&Term=518
11. http://biogps.org/gene/518/
12. https://www.uniprot.org/uniprotkb/P48201/entry
13. https://www.uniprot.org/uniprotkb/P48201/publications

The user is asking for a comprehensive research report on the ATP5MC3 gene in humans. Let me break down what I need to cover:

1. The gene ATP5MC3 (encoding ATP synthase membrane subunit C locus 3, also known as P48201)
2. Its primary function - especially the biochemical reaction it catalyzes/substrate specificity
3. Its localization - where in the cell it functions
4. Biological processes and pathways it's involved in
5. Evidence from experimental, structural, evolutionary, or bioinformatic analysis

Let me plan my approach:

Planning:

1. Introduction/Overview: What is ATP5MC3? Basic identification, localization, disease associations
2. Structural Properties: The protein structure, hairpin formation, transmembrane helices
3. Molecular Function: The specific role in ATP synthase, proton translocation mechanism
4. Biochemical Mechanism: How the c-subunit participates in ATP synthesis, proton-coupled rotation
5. Complex Assembly and Organization: How ATP5MC3 assembles into the c-ring, stoichiometry
6. Post-Translational Modifications: Lysine methylation by FAM173B
7. Clinical/Pathological Significance: Disease associations, particularly dystonia and spastic paraplegia
8. Evolutionary Context: Conservation across organisms
9. Broader Cellular Context: Role in mitochondrial bioenergetics, oxidative phosphorylation
10. Conclusion: Summary and implications

Let me go through the search results and organize them:

Key Information from Sources:

From [1][8][16][27][32][53][56]: ATP5MC3 encodes a subunit of mitochondrial ATP synthase, specifically subunit c of the Fo complex. It catalyzes ATP synthesis using an electrochemical gradient of protons. One of three genes encoding this subunit with distinct import sequences but identical mature protein.

ATP5MC3 is located on chromosome 2 with NCBI Gene ID 518. The c subunit functions as the primary transmembrane component across V-type, A-type, and F-type ATP synthases, where it oligomerizes into a rotor ring containing a critical aspartate residue (in F-ATPases) or glutamate (in V-ATPases) that mediates proton transport. The c subunit exhibits amyloidogenic properties, capable of forming β-sheet structures and oligomers in calcium-dependent conditions.

The protein features a characteristic hairpin architecture composed of two transmembrane α-helices, with Asp61 in the second helix playing a crucial role in proton translocation. Post-translational methylation occurs at lysine-43. The c-ring rotates at speeds up to 150 Hz through interaction with the a-subunit, a mechanism recently elucidated via cryo-EM and molecular dynamics simulations that detail both the rotational dynamics and proton transfer pathways. Specific residues like Asp61 are essential for F0 sector function and proton movement across the membrane.

A pathogenic variant (c.318C>G, p.Asn106Lys) in ATP5MC3 has been identified in patients with dystonia and spastic paraplegia, resulting in diminished complex V activity. The c-subunits function cooperatively during rotation, enabling efficient energy coupling. FAM173B methyltransferase catalyzes trimethylation at Lys-43, while the c-ring itself can form a voltage-gated ion channel involved in mitochondrial permeability transition. The oligomeric c-ring structure, along with the broader F1Fo ATP synthase architecture, represents a sophisticated molecular motor system critical for oxidative phosphorylation and mitochondrial biogenesis.

Recent structural insights from cryo-EM have revealed rotary states and dimer configurations, showing how the c-oligomer undergoes hairpin folding with specific packing arrangements. Subunit coupling mechanisms drive γ-subunit rotation, while the OSCP hinge provides flexible coupling through rotary substates. ATP synthase dimers form ribbon-like structures that shape mitochondrial membranes and facilitate cristae formation, with c-ring stoichiometry adapting to meet cellular physiological demands. The F1-ATPase catalytic cycle proceeds through six distinct steps, operating via a 3×120° rotary mechanism that reflects the evolutionary refinement of ATP synthase across diverse organisms.

ATP5MC3: Structural and Functional Characterization of a Critical Component of Mitochondrial ATP Synthase

The ATP5MC3 gene encodes a subunit of the mitochondrial ATP synthase complex, specifically the membrane-embedded subunit c of the F₀ portion of the enzyme, also known as the oligomycin sensitivity-conferring protein (OSCP)-interacting proteolipid subunit. This 75-residue peptide plays a central mechanistic role in coupling the electrochemical proton gradient across the inner mitochondrial membrane to the synthesis of adenosine triphosphate (ATP), the universal energy currency of the cell. Recent investigations using cryo-electron microscopy, molecular dynamics simulations, and functional genomics have revealed that ATP5MC3-encoded subunit c participates in a sophisticated rotary mechanism that converts proton translocation into mechanical rotation, ultimately driving ATP synthesis with remarkable efficiency. This report synthesizes current understanding of ATP5MC3's molecular structure, biochemical function, cellular localization, assembly into higher-order complexes, post-translational modification, disease associations, and evolutionary conservation, drawing on both classical biochemical studies and contemporary structural biology approaches.

Molecular Architecture and Structural Features of Subunit c

ATP5MC3 encodes a small, highly hydrophobic protein that functions as an integral membrane component of the F₀ complex of ATP synthase[1][8][16]. The mature protein, which is 75 residues in length, adopts a characteristic hairpin structure composed of two transmembrane α-helices connected by a small cytoplasmic loop[31]. This distinctive fold is highly conserved from bacteria through eukaryotes, indicating its ancient evolutionary origin and critical functional importance[31]. The structural organization places the first transmembrane helix toward the inner surface of the oligomeric assembly, while the second transmembrane helix orients toward the periphery, creating a specific geometric arrangement that is essential for function[34][54].

The critical functional residue in subunit c is aspartate at position 61 in the second transmembrane helix, which serves as the principal proton-binding site during the catalytic cycle[7][10]. This aspartate residue is positioned at the interface between two interacting c-subunits within the c-ring assembly, occupying a hydrophobic environment shielded from the lipid bilayer by the packing arrangement of the transmembrane helices[34][54][55]. The positioning of Asp61 is particularly significant, as it lies within the center of four transmembrane helices from two neighboring subunits, creating a microenvironment that is uniquely suited to facilitating proton transfer[34]. Structural studies using nuclear magnetic resonance spectroscopy on isolated subunit c have confirmed the hairpin conformation and validated predictions from genetic studies regarding the positioning of functionally important residues[31]. The conserved glycine motif with a GxGxGxG pattern in the transmembrane helices facilitates the tight packing of helices that is essential for maintaining the structural integrity of the c-ring and for allowing the proper presentation of the aspartate residue during catalysis[51].

Beyond the transmembrane helices, subunit c contains a small loop region connecting the two helices on the cytoplasmic side of the membrane. Notably, position 106 in this loop contains a highly conserved residue that serves as a point of contact between the F₀ and F₁ portions of ATP synthase[13][21][50]. This contact region is critical for transmitting the rotary motion generated by proton translocation in F₀ to the catalytic machinery of F₁. A naturally occurring mutation at this position—a change from lysine to asparagine (p.Asn106Lys)—has been identified as causing neurological disease, highlighting the functional importance of this structural feature[13][21].

The Oligomeric c-Ring: Assembly, Stoichiometry, and Quaternary Structure

In the native ATP synthase complex, ATP5MC3-encoded subunit c proteins do not function as monomers but rather self-assemble into highly ordered oligomeric rings, termed c-rings[25][34][54][55]. The assembly of individual c-subunits into these rings occurs through specific interactions between the hydrophobic surfaces of adjacent monomers, with the "front face" of one subunit packing against the "back face" of the neighboring subunit[34][54]. This arrangement creates a compact hollow cylinder with an outer diameter of approximately 55-60 Ångströms and an inner cavity with a minimal diameter of 11-12 Ångströms[34][54]. Phospholipids are believed to pack within this inner space in the native membrane environment, suggesting that lipid molecules are integral to the structural integrity and potentially the functional properties of the c-ring[34].

The stoichiometry of the c-ring—that is, the number of c-subunit monomers comprising each ring—is species-dependent and varies between 8 and 17 subunits across different organisms[28][44][51]. In mammalian mitochondria, human ATP synthase c-rings typically contain approximately 10 c-subunits, though some evidence suggests variation between tissues and physiological states[25][51]. The stoichiometry of the c-ring is a critical determinant of ATP synthase efficiency, as it directly impacts the ion-to-ATP ratio, a fundamental parameter of cellular bioenergetics[51]. The number of c-subunits in the ring also determines the angular increment by which the rotor advances with each proton translocation event; for a c₁₀ ring, each proton translocation event advances the rotor by approximately 36 degrees, corresponding to a complete 360-degree rotation after ten sequential protonation-deprotonation cycles[38][51].

Recent molecular dynamics and crystallographic studies have revealed that the c-ring stoichiometry is not absolutely fixed but rather can adapt to cellular needs and evolutionary pressures[51]. In the alkaliphilic bacterium Bacillus pseudofirmus, whose ATP synthase must operate efficiently at extraordinarily high pH values (above 10), the c-ring contains thirteen subunits, which may confer advantages for functioning under conditions of very low proton-motive force[51]. The precise stoichiometry appears to be determined by subtle structural features in the GxGxGxG motif and surrounding residues that control the tightness of helix packing and the geometry of subunit-subunit interactions[51]. This adaptation of c-ring stoichiometry represents an elegant example of how evolutionary pressures shape the bioenergetic capabilities of organisms inhabiting extreme environments.

Biochemical Function: The Proton-Translocation Mechanism

The primary biochemical function of ATP5MC3-encoded subunit c is to facilitate directional proton translocation across the inner mitochondrial membrane, coupled to the rotation of the c-ring relative to the stationary a-subunit[1][8][10]. This process is the direct result of the electrochemical proton gradient established by the electron transport chain, where complexes I, III, and IV pump protons from the mitochondrial matrix into the intermembrane space, generating both a concentration gradient and an electrical potential across the membrane[1][39].

The mechanism of proton translocation involves a precisely orchestrated cycle of proton binding and release involving Asp61 of the c-subunit and specific residues of the a-subunit, particularly the critical arginine 210[10][38][41]. Protons from the intermembrane space enter the F₀ complex through an entry half-channel formed at the interface between subunits a and c[38][41]. This entry pathway is composed of hydrophilic residues from both the a-subunit and two neighboring c-subunits, creating an aqueous environment that allows protons to reach the buried aspartate residue[38][41]. Once a proton reaches Asp61 on one of the c-subunits positioned at the a-subunit interface, it protonates the aspartate carboxyl group, converting it from a negatively charged deprotonated state to a neutral protonated form[7][10].

This protonation event triggers a profound conformational change in the c-subunit, particularly affecting the C-terminal transmembrane helix, which undergoes a substantial rotation relative to the N-terminal helix[7][38]. The structural change associated with Asp61 protonation has been quantified through nuclear magnetic resonance and cryo-electron microscopy studies, revealing a twist of approximately 140 degrees between the two helices[7][38]. This conformational change alters the electrochemical interactions between the c-subunit and the a-subunit, reducing the favorable electrostatic interactions and allowing the c-ring to rotate forward[9][38].

The rotational movement is driven by the favorable change in the electrostatic environment; as the protonated Asp61 moves away from the hydrophilic environment of the a-subunit into the hydrophobic lipid bilayer, the c-ring must rotate to bring a new, unprotonated aspartate residue from a neighboring c-subunit into the proton-binding site at the a-subunit interface[9][38]. This rotational movement has been termed the "power stroke" and represents the force that ultimately drives ATP synthesis[9]. As the c-ring rotates and the newly incoming aspartate prepares to be protonated, the previously protonated Asp61 moves to a region near the exit half-channel, also formed at the a/c interface[38][41]. Here, the hydrophilic environment facilitates deprotonation of the aspartate, releasing the proton into the matrix side of the membrane[38][41]. The exit half-channel is also composed of hydrophilic residues from the a-subunit and c-subunits, ensuring that the released proton can proceed toward the matrix[38][41].

Recent molecular dynamics simulations of complete rotary cycles have provided unprecedented atomic-level detail regarding this mechanism[9]. These simulations, performing multi-microsecond atomistic molecular dynamics calculations, have demonstrated that rotation proceeds through dynamic sliding of the ring over the a-subunit surface, with specific intermediates being stabilized by interactions with conserved polar residues[9]. Crucially, these simulations have identified ordered water chains that organize for Grotthuss-type proton transfer—a mechanism in which proton transfer occurs not through free diffusion but through sequential protonation and deprotonation of amino acid residues and water molecules, allowing rapid proton transport without bulk water movement[9].

The simulations have also revealed asymmetry in the free energy landscape that strongly favors forward rotation over backward rotation[9]. After a proton binds to an incoming c-subunit aspartate, a high free energy barrier is established that prevents backward rotation, while the overall free energy gradient strongly favors continued forward rotation[9]. This ratchet-like mechanism, sometimes termed a Brownian ratchet combined with a power stroke, ensures the thermodynamic irreversibility of the process and the directional rotation necessary for ATP synthesis to occur[9].

Cellular Localization and Mitochondrial Targeting

ATP5MC3 encodes a protein that is synthesized on ribosomes in the cytoplasm and subsequently imported into mitochondria through a well-characterized targeting mechanism[1][6][26]. The mature, functional form of subunit c lacks an N-terminal targeting sequence; instead, it begins directly with the first transmembrane helix[6][26]. However, the nascent protein synthesized from the ATP5MC3 mRNA contains a mitochondrial targeting sequence (MTS) at its N-terminus that is recognized by the translocase of the outer mitochondrial membrane (TOM) complex and the translocase of the inner mitochondrial membrane (TIM) complex[6][26].

Notably, the ATP5MC3 gene is one of three genes—ATP5MC1, ATP5MC2, and ATP5MC3—that encode the identical mature c-subunit protein, each with a distinct mitochondrial import sequence[1][6][8][16][26][27]. After successful translocation across both mitochondrial membranes, the mitochondrial protease MPP (mitochondrial processing peptidase) cleaves the targeting sequence, releasing the mature 75-residue peptide[6][26]. This redundancy of genes encoding the same protein but with different targeting sequences represents an interesting evolutionary feature, possibly allowing fine-tuning of c-subunit import rates or spatial distribution within the mitochondrial compartments[6][26].

Once imported and processed, the mature ATP5MC3-encoded subunit c immediately associates with other c-subunit molecules—products of ATP5MC1 and ATP5MC2—to form the c-ring assembly in the inner mitochondrial membrane[1][6][25]. The c-ring then integrates into the complete F₀ complex through interaction with other subunits, particularly the a-subunit, which is encoded by the mtDNA gene ATP6 and is therefore translated directly on ribosomes localized to the inner mitochondrial membrane[25]. The physical location of c-ring assembly occurs directly at the inner mitochondrial membrane, where the nascent c-subunits can immediately participate in rotor formation.

Integration into the ATP Synthase Complex and Supramolecular Organization

The ATP5MC3-encoded c-subunit, as a component of the c-ring, represents a crucial structural element of the larger F₀-F₁ ATP synthase complex[1][8][25][33]. This molecular machine consists of two functionally distinct domains: the F₁ catalytic head, which extends into the mitochondrial matrix and contains the catalytic sites for ATP synthesis, and the F₀ domain, which is embedded in the inner mitochondrial membrane and functions as a proton-translocating turbine[1][8][25][33]. The c-ring, of which ATP5MC3-encoded subunits are constituents, forms the core of the rotary element of the F₀ domain, interacting directly with the stationary a-subunit through which protons are channeled[25][33].

The central stalk of ATP synthase, which is composed of the γ, δ, and ε subunits of the F₁ domain, is firmly attached to the c-ring, such that rotation of the c-ring driven by proton translocation directly drives rotation of the γ-subunit within the F₁ catalytic core[8][25][33][44]. This mechanical coupling between F₀ and F₁ is absolute and remarkably efficient—the rotational motion generated by proton translocation is directly converted into conformational changes at the catalytic sites of the β-subunits, where ATP synthesis occurs[33][36][44].

Recent cryo-electron microscopy structures have revealed that the coupling between the F₀ and F₁ domains is not rigid but rather involves a degree of flexibility, particularly at the oligomycin sensitivity-conferring protein (OSCP) subunit, which contains an interdomain hinge[44][57]. This flexible coupling mechanism allows the F₁ head to rotate through approximately 30 degrees at the beginning of each 120-degree primary rotary step, facilitating the coordination between the stoichiometrically mismatched F₀ domain (with 10 c-subunits) and the F₁ domain (with threefold symmetry)[44][57].

In mammalian mitochondria, ATP synthases do not exist as isolated monomers but rather assemble into highly organized supramolecular structures. ATP synthase monomers dimerize through interactions mediated by subunits e, k, i/j, and others, forming stable dimers[22][43][46]. These dimers then organize into long, linear ribbons that are localized specifically at the apex of the cristae—the infoldings of the inner mitochondrial membrane that dramatically increase the surface area available for oxidative phosphorylation[43][46]. The organization of ATP synthases into these dimer ribbons is not an accidental feature but rather serves important functional purposes; the ribbons bend the lipid bilayer locally, with a radius of curvature of approximately 17 nanometers[43][46]. This local membrane curvature creates a "proton trap" effect, concentrating protons in the restricted volume of the cristae lumen and thereby significantly increasing the local pH gradient experienced by the ATP synthase[43][46]. This geometry-dependent enhancement of the local pH gradient is thought to be particularly important for ATP synthesis under conditions of limited proton availability or high energy demand.

Post-Translational Modification and Regulatory Mechanisms

An important regulatory feature of ATP5MC3-encoded subunit c involves post-translational modification by lysine methylation. In cells, the mature c-subunit undergoes trimethylation at lysine 43, a modification catalyzed by the mitochondrial methyltransferase FAM173B[20][26]. Recent investigations have demonstrated that this lysine methylation is essential for optimal ATP synthase function and that disruption of the methyltransferase FAM173B results in failure of the c-subunit to incorporate properly into the ATP synthase complex[20].

In cells lacking functional FAM173B, c-subunits can still associate with other ATP synthase subunits and can form complex assemblies, but these complexes exhibit markedly reduced ATP-generating capacity[20]. Furthermore, cellular respiration is substantially impaired in the absence of proper c-subunit methylation, indicating that this post-translational modification has profound effects on mitochondrial bioenergetics[20]. The specific role of the Lys43 methylation appears to be promoting the proper folding of the c-subunit into its native α-helical hairpin conformation, as c-subunits lacking this modification are more prone to aggregation and misfolding[20][26].

Interestingly, under stress conditions characterized by elevated calcium and oxidative stress, unmethylated or unfolded c-subunits have been found to adopt alternative conformations, particularly β-sheet structures[6][26][29]. The c-subunit in β-sheet conformation is itself an amyloidogenic peptide capable of self-assembly into oligomers and fibrils, a process that is notably enhanced by elevated calcium concentrations[6][26][29]. These misfolded oligomers can form ion channels in artificial lipid bilayers, suggesting a potential role in stress-induced permeabilization of the inner mitochondrial membrane[6][28][29]. This apparent duality of function—normal ATP synthase participation versus stress-induced ion channel formation—represents an intriguing example of how the same polypeptide can adopt functionally distinct conformations depending on cellular conditions.

Proton-Coupling Mechanism: Sequential Catalysis and Cooperative Function

The function of ATP5MC3-encoded subunit c within the larger ATP synthase must be understood not as the action of an isolated component but rather as cooperative participation in a coordinated rotary catalytic cycle. Early biochemical and genetic evidence suggested a model in which the essential Asp61 residue of subunit c works in concert with specific residues of the a-subunit, particularly the critical Arg210, to create a precisely positioned ion-binding site[7][10][17]. Suppressor mutation analysis provided early insights into this interaction; third-site suppressor mutations that restored function to otherwise defective ATP synthases were found to map to specific helices of both subunit c and subunit a, providing genetic evidence for direct physical interaction between these subunits[17].

More recent evidence suggests that the proton-coupling mechanism may be even more sophisticated than previously appreciated. Rather than a simple sequential model in which only one c-subunit participates in proton transfer at any given moment, contemporary molecular dynamics simulations and biochemical studies have revealed that multiple c-subunits contribute to the functional mechanism[9][14][49][52]. Specifically, multiple c-subunits positioned at different angles around the a-subunit interface appear to participate in a cooperative process, with the waiting times for proton uptake being effectively shared among several subunits[14][49][52].

This cooperative mechanism has been demonstrated through elegant experiments in which two different point mutations were introduced at different positions around the c-ring of reconstituted ATP synthase, and the activity of the enzyme was measured as a function of the distance between the two mutations[14][49][52]. The experiments revealed that ATP synthesis activity is highest when the two mutated c-subunits are positioned close to each other in the ring and decreases as the distance between them increases[14][49][52]. This distance-dependent effect provides unambiguous evidence that at least three neighboring c-subunits on the a/c interface cooperate during the c-ring rotation process[14][49][52].

Genetic Defects and Human Neurological Disease

ATP5MC3 mutations have emerged as a significant cause of human genetic disease, particularly severe neurological conditions characterized by movement disorders. A pivotal discovery in this area was the identification of a novel missense variant (c.318C>G, resulting in p.Asn106Lys) in families with autosomal dominant dystonia and spastic paraplegia[13][21][37]. The variant was initially discovered in a large multigenerational family in which affected members showed an unusual clinical combination of either generalized dystonia with childhood onset or spastic paraplegia with adult onset[13][21][37].

The proband of the primary family presented with unilateral upper limb onset dystonia at age 2 years, which progressively generalized to involve the trunk and all limbs, resulting in loss of ambulation by adolescence[13][21][37]. The same proband subsequently showed benefit from bilateral pallidal deep brain stimulation, a surgical intervention typically effective for primary generalized dystonia[13][21][37]. Other family members, including the proband's father and grandfather, presented with progressive spastic paraplegia characterized by lower extremity weakness and spasticity, with onset after age 20 years[13][21][37]. The incomplete penetrance and variable expression of the phenotype within the same family, together with the unusual combination of dystonia and spastic paraplegia, represented a novel clinical entity[13][21][37].

Functional studies of patient-derived fibroblasts from affected family members demonstrated significantly reduced complex V activity compared to control cells, along with impaired ATP generation and decreased oxygen consumption, indicating compromised mitochondrial oxidative phosphorylation capacity[13][21][37]. Transgenic Drosophila models carrying the orthologous mutation also exhibited impaired mitochondrial function and significantly reduced mobility in standard geotaxis assays, providing independent animal model evidence for the pathogenicity of the variant[13][21][37].

The mechanistic basis for the p.Asn106Lys variant has been elucidated through structural and comparative analysis. The substitution occurs in a conserved loop region of subunit c that serves as the critical point of contact between the F₀ and F₁ domains of ATP synthase[13][21][37]. This loop region shows complete amino acid homology from humans through Drosophila, underscoring its functional importance[13][21][37]. Notably, a naturally occurring mutation at the equivalent position in the Escherichia coli ATP synthase ortholog (atpE gene)—the p.N102E substitution—has been previously shown to uncouple ATP hydrolysis from proton translocation while preserving proton translocation capacity[13][21][37]. The presence of the mutant allele is hypothesized to interfere with the normal F₁-F₀ interaction through a dominant-negative mechanism, rather than simple haploinsufficiency[13][21][37]. Supporting this model, overexpression of the mutant form in Drosophila resulted in statistically significant impairment of mobility, whereas overexpression of wildtype protein did not[13][21][37].

The pathological findings in these patients suggest that the reduced ATP synthase activity leads to insufficient ATP production and accumulation of reactive oxygen species, which in turn damages the nervous system, particularly neurons in the basal ganglia and spinal cord that have extremely high energy demands[13][21][37]. The differential presentation of dystonia versus spastic paraplegia within the same family may reflect variable penetrance, epigenetic factors, or sex-specific effects, though these mechanisms remain to be fully elucidated.

Role in Oxidative Phosphorylation and Mitochondrial Biogenesis

The broader biological significance of ATP5MC3 lies in its essential role in oxidative phosphorylation, the aerobic process by which cells generate ATP from substrates in the presence of oxygen[1][15][18][39]. Approximately 90-95 percent of the ATP generated by eukaryotic cells under aerobic conditions is produced through oxidative phosphorylation in mitochondria, making this process absolutely critical for cellular function and survival[1][15][18][39]. The ATP5MC3-encoded subunit c is not merely an incidental component of this system but rather represents a critical catalytic element without which proton-coupled ATP synthesis cannot occur.

The efficiency of ATP synthesis is determined in part by the stoichiometry of protons to ATP, a relationship that is directly influenced by the number of c-subunits in the c-ring[51]. Each complete rotation of the c-ring, driven by proton translocation across the membrane, results in a defined number of ATP molecules being synthesized at the F₁ catalytic sites, determined by the matching of c-ring stoichiometry to the geometry of the F₁ catalytic sites[51]. This optimization represents an elegant solution to the bioenergetic requirements of different cell types and organisms; tissues with high ATP demand, such as cardiac muscle and brain, contain mitochondria packed with numerous cristae, dramatically increasing the surface area available for ATP synthesis[43][46].

The expression of ATP5MC3 is coordinately regulated with other genes encoding OXPHOS complex components through transcriptional programs that are activated in response to cellular energy demands[15][18]. The nuclear respiratory factors 1 and 2 (NRF1 and NRF2), which are transcriptional regulators that act on nuclear genes coding for OXPHOS subunits, play central roles in this regulation[18]. Additionally, mitochondrial transcription factor A (Tfam), which regulates transcription of genes encoded on the mitochondrial genome (including ATP6, encoding the critical a-subunit), is also transcriptionally regulated in response to cellular energy demands[18]. This integrated regulation ensures that the stoichiometric balance of nuclear-encoded and mitochondrially-encoded OXPHOS subunits is maintained, facilitating efficient assembly of functional ATP synthase complexes.

Evolutionary Conservation and Origin of ATP Synthases

The ATP synthase complex represents one of the most ancient and conserved molecular machines known to biology, likely predating the divergence of Archaea and Bacteria and possibly even the Last Universal Common Ancestor (LUCA) of all life[45]. The subunit c proteins, encoded in humans by ATP5MC3 and its paralogs, represent remarkably conserved molecules with clear orthologs identifiable in virtually all organisms from bacteria through archaea, plants, and animals[45]. Comparative sequence analysis reveals that the core structural features—including the hairpin fold with two transmembrane helices and the critical Asp61 proton-binding residue—are conserved across these vastly different organisms.

Phylogenetic analyses of ATP synthase genes, combined with phylogenetic analysis of the subunit c and other ATP synthase components, suggest that the divergence of ATP synthase into the F-type lineage (found in bacteria, plant chloroplasts, and animal mitochondria) and the A/V-type lineage (found in archaea and vacuoles) occurred very early in cellular evolution, dating to more than four billion years ago[45]. The retention of nearly identical catalytic mechanisms across all organisms suggests intense purifying selection, indicating that the precise details of the proton-translocation mechanism have been optimized by evolution and cannot be substantially altered without loss of function[45].

In eukaryotes, the mitochondrial ATP synthase represents an endosymbiotic inheritance from the alpha-proteobacterial ancestor that was engulfed to form mitochondria[45]. The ATP5MC3 gene in humans represents a nuclear copy of what was originally a bacterial ATP synthase gene, a transfer event that occurred at the time of endosymbiosis and presumably involved intron insertion and acquisition of a mitochondrial targeting sequence[45]. The existence of three separate genes (ATP5MC1, ATP5MC2, and ATP5MC3) encoding the identical mature c-subunit protein represents a more recent duplication event within eukaryotic evolution, possibly reflecting the increased demand for ATP synthase in multicellular organisms or providing redundancy that buffers against deleterious mutations.

Therapeutic Implications and Future Directions

The identification of ATP5MC3 mutations as causative of human neurological disease opens new avenues for therapeutic intervention and diagnosis of mitochondrial disorders. Patients with unexplained dystonia or spastic paraplegia of unknown etiology might benefit from genetic screening for ATP5MC3 variants, particularly when accompanied by evidence of mitochondrial dysfunction such as elevated lactate or reduced ATP production. Furthermore, understanding the molecular basis of disease-associated ATP5MC3 variants could inform therapeutic strategies aimed at enhancing ATP synthase function or compensating for impaired energy production in affected neurons.

The observation that ATP5MC3-encoded subunit c can undergo conformational transitions to form amyloidogenic β-sheet structures under stress conditions[6][26][29] also raises intriguing questions about potential links to neurodegenerative diseases characterized by protein aggregation. While α-synuclein and amyloid-β are well-established contributors to Parkinson's disease and Alzheimer's disease respectively, the possibility of c-subunit aggregation contributing to neurotoxicity in specific contexts warrants further investigation[6][26][29].

The development of cryo-electron microscopy techniques has enabled the determination of high-resolution structures of ATP synthase in multiple rotatory states, providing unprecedented detail regarding the dynamic conformational changes that occur during catalysis[19][22][44][57]. Continued structural investigations, potentially including time-resolved crystallography or cryo-EM of ATP synthase trapped in specific catalytic intermediates, may further illuminate the mechanism and suggest new targets for pharmaceutical intervention or bioengineering approaches aimed at enhancing ATP production in cells with compromised mitochondrial function.

Conclusion

The ATP5MC3 gene encodes a small but functionally critical component of the mitochondrial ATP synthase complex, the molecular machine responsible for generating the vast majority of cellular ATP under aerobic conditions. The product of ATP5MC3, the membrane-spanning subunit c of the F₀ domain, represents a remarkable example of evolutionary conservation, with its hairpin structure and precise catalytic mechanism preserved across billions of years of evolution. Through a sophisticated proton-coupling mechanism involving the sequential protonation and deprotonation of Asp61 and cooperative interactions among multiple c-subunits in the c-ring, the ATP5MC3 gene product converts the electrochemical proton gradient into mechanical rotation that ultimately drives ATP synthesis.

The quaternary assembly of ATP5MC3-encoded subunits into the c-ring, integration into the larger ATP synthase complex, and oligomerization into higher-order supramolecular structures represent additional levels of biological organization that contribute to the efficiency and regulation of cellular energy production. Post-translational modification through lysine methylation by FAM173B represents an important regulatory mechanism ensuring proper folding and functional incorporation of the c-subunit. The identification of disease-associated mutations in ATP5MC3 has revealed the severe neurological consequences of compromised ATP synthase function, with implications for understanding dystonia, spastic paraplegia, and potentially other neurodegenerative conditions.

Future investigations should continue to employ contemporary structural biology approaches, functional genomics, and animal models to elucidate the remaining mechanistic questions regarding ATP synthase function and regulation. The therapeutic potential of enhancing ATP synthase activity or correcting the effects of pathogenic variants presents an important frontier in mitochondrial medicine, with potential benefits extending far beyond the rare genetic disorders directly caused by ATP5MC3 mutations to potentially including more common mitochondrial dysfunction-associated conditions.

Citations

1. https://www.genecards.org/cgi-bin/carddisp.pl?gene=ATP5MC3
2. https://www.ncbi.nlm.nih.gov/gene/518
3. https://en.wikipedia.org/wiki/ATP_synthase_subunit_C
4. https://www.uniprot.org/uniprotkb/P48201/entry
5. https://www.uniprot.org/uniprotkb/Q3ZC75/entry
6. https://www.nature.com/articles/s41598-021-88157-z
7. https://pubmed.ncbi.nlm.nih.gov/10580496/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC10945847/
9. https://pubmed.ncbi.nlm.nih.gov/1331039/
10. https://www.embopress.org/doi/10.1038/sj.embor.7400646
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC8840961/
12. https://elifesciences.org/articles/69096
13. https://pmc.ncbi.nlm.nih.gov/articles/PMC6523352/
14. https://pubmed.ncbi.nlm.nih.gov/8300584/
15. https://pmc.ncbi.nlm.nih.gov/articles/PMC3211033/
16. https://www.annualreviews.org/content/journals/10.1146/annurev-biophys-060724-110838
17. https://pubmed.ncbi.nlm.nih.gov/30530489/
18. https://pmc.ncbi.nlm.nih.gov/articles/PMC9244403/
19. https://pubmed.ncbi.nlm.nih.gov/31213526/
20. https://movementdisorders.onlinelibrary.wiley.com/doi/abs/10.1002/mds.28821
21. https://pmc.ncbi.nlm.nih.gov/articles/PMC3278611/
22. https://www.nature.com/articles/s41418-022-00972-7
23. https://pubmed.ncbi.nlm.nih.gov/33888826/
24. https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=DetailsSearch&Term=518
25. https://pubmed.ncbi.nlm.nih.gov/8218294/
26. https://pmc.ncbi.nlm.nih.gov/articles/PMC9447477/
27. https://www.pnas.org/doi/10.1073/pnas.96.14.7785
28. https://www.pnas.org/doi/10.1073/pnas.0708746105
29. https://earth.callutheran.edu/Academic_Programs/Departments/BioDev/omm/jsmolnew/atp_synthase/atp_synthase.html
30. https://jackwestin.com/resources/mcat-content/oxidative-phosphorylation/atp-synthase-chemiosmotic-coupling
31. https://pmc.ncbi.nlm.nih.gov/articles/PMC6056720/
32. https://pmc.ncbi.nlm.nih.gov/articles/PMC2442323/
33. https://pubmed.ncbi.nlm.nih.gov/18323778/
34. https://www.science.org/doi/10.1126/science.aaw9128
35. https://pmc.ncbi.nlm.nih.gov/articles/PMC10656485/
36. https://www.pnas.org/doi/10.1073/pnas.1816556116
37. https://pmc.ncbi.nlm.nih.gov/articles/PMC2222616/
38. https://pmc.ncbi.nlm.nih.gov/articles/PMC3651500/
39. https://pmc.ncbi.nlm.nih.gov/articles/PMC8809890/
40. https://pmc.ncbi.nlm.nih.gov/articles/PMC22139/
41. https://www.nature.com/articles/s41467-021-25029-0
42. https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793(03)01101-3
43. https://pmc.ncbi.nlm.nih.gov/articles/PMC7703542/