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.
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NAA35 (UniProt Q5VZE5) encodes the large auxiliary subunit of the heterotrimeric NatC N-terminal acetyltransferase complex in humans (aksnes2023natsata pages 1-2). This protein, also known as MAK10 homolog or EGAP, belongs to the evolutionarily conserved MAK10 family and functions as an essential structural component of the NatC complex alongside the catalytic subunit NAA30 and the small auxiliary subunit NAA38 (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2). NAA35's primary role is to serve as the central assembly scaffold that organizes the NatC complex architecture and mediates its interaction with ribosomes, thereby enabling co-translational N-terminal acetylation of specific substrate proteins (grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 9-10).
| Category | NAA35 / NatC summary | Key details specific to NAA35 | Evidence |
|---|---|---|---|
| Gene/protein identity | Human NAA35 (UniProt Q5VZE5; MAK10 family) encodes N-alpha-acetyltransferase 35, NatC auxiliary subunit, a core component of the heterotrimeric NatC N-terminal acetyltransferase complex. | NAA35 is the large auxiliary subunit of NatC; literature consistently pairs it with catalytic NAA30 and small auxiliary NAA38. | (grunwald2020divergentarchitectureof pages 1-2, aksnes2023natsata pages 1-2, varland2023nterminalacetylationshields pages 1-2) |
| Complex composition | NatC is a heterotrimer composed of NAA30 (catalytic GNAT-fold subunit), NAA35 (large auxiliary subunit), and NAA38 (small auxiliary subunit). All three are important for normal NatC activity. | NAA35 serves as the central assembly/scaffold subunit linking NAA30 and NAA38. | (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2) |
| Primary biochemical function of the complex | NatC catalyzes co-translational N-terminal acetylation: transfer of an acetyl group from acetyl-CoA to the free α-amino group of nascent polypeptides. | NAA35 is not the catalytic acetyltransferase; instead, it supports catalysis by organizing the NatC architecture and helping form the peptide-binding environment with NAA30. | (grunwald2020divergentarchitectureof pages 1-2, aksnes2023natsata pages 1-2, chang2023impactofprotein pages 4-5, grunwald2020divergentarchitectureof pages 9-10) |
| Subunit roles | NAA30 performs catalysis; NAA38 supports full activity/stability; NAA35 organizes the quaternary structure and contributes to ribosome association and substrate recognition. | NAA35 wraps around much of NAA30 and around NAA38, creating a highly intertwined complex and contributing residues/structure to the NAA30–NAA35 substrate-binding interface. | (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 4-5, grunwald2020divergentarchitectureof pages 9-10, grunwald2020divergentarchitectureof pages 8-9) |
| Substrate specificity of NatC | NatC preferentially acetylates initiator methionine-retaining N-termini where Met is followed by a hydrophobic or amphipathic residue. Canonical classes include ML, MF, MI, MW; expanded in vivo profiling also supports MY, MK, MM, MA, MV, MS in yeast, with some redundancy from other NATs for certain classes. | NAA35 helps create the deeper, more confined peptide-binding pocket with NAA30 that explains NatC specificity for Met-hydrophobic N-termini. | (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 1-2, grunwald2020divergentarchitectureof pages 6-7, damme2023expandedinvivo pages 1-2, grunwald2020divergentarchitectureof pages 9-10, nashed2023functionalmappingof pages 1-3) |
| Sequence determinants beyond position 2 | NatC mainly recognizes the first two residues, but positions 3 and 4 also contribute strongly to efficient binding/catalysis; structural and kinetic studies showed improved activity for peptides optimized at these positions. | The NAA30–NAA35 interface helps accommodate the first four residues of cognate substrates. | (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 6-7, grunwald2020divergentarchitectureof pages 9-10) |
| Example substrates / substrate classes | Known or cited NatC-dependent targets include UBE2M/UBC12, UBE2F, ARFRP1 (Arl3 in yeast), ARL8B, viral Gag, and mitochondrial precursor proteins with NatC-compatible N-termini. | Through supporting NatC-mediated acetylation of these proteins, NAA35 indirectly affects their stability, targeting, and pathway function. | (varland2023nterminalacetylationshields pages 1-2, varland2023nterminalacetylationshields pages 3-4, grunwald2020divergentarchitectureof pages 9-10, nashed2023functionalmappingof pages 1-3, aksnes2019cotranslationalposttranslationaland pages 4-5) |
| Cellular localization | NatC is a ribosome-associated NAT that acts co-translationally at or near the ribosomal exit tunnel on nascent chains; major NATs including NatC are associated with mono- and polyribosomes. | NAA35 contains the principal ribosome-binding surface of NatC. | (varland2023nterminalacetylationshields pages 1-2, grunwald2020divergentarchitectureof pages 8-9, grunwald2020divergentarchitectureof pages 10-11, aksnes2019cotranslationalposttranslationaland pages 4-5) |
| NAA35 tip region | Structural work identified an elongated ~30 Å protruding “NAA35 tip” region formed mainly by helices near the C-terminus. | The tip region is electropositive and required for efficient ribosome association; alanine substitutions in this region reduced NatC co-sedimentation with ribosomes without major catalytic defects in vitro. | (grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 8-9, grunwald2020divergentarchitectureof pages 10-11) |
| Structural fold/features of NAA35 | NAA35 is mostly α-helical with a unique fold lacking close structural homologs in the PDB; it also contains short β-strands in the N- and C-terminal regions. | Its uniqueness supports a specialized NatC-specific scaffolding role rather than a generic NAT auxiliary architecture. | (grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 9-10) |
| Assembly architecture | NAA35 forms the central assembly hub of NatC. It wraps around almost the entire circumference of NAA38 and around three quarters of NAA30, helping generate a central tunnel and peptide-binding groove. | NAA35 connects NAA38 to NAA30 and is essential for proper NatC integrity; the N-terminus of NAA35 is important for NAA38 interaction. | (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 4-5, grunwald2020divergentarchitectureof pages 9-10) |
| Contribution to catalysis | The catalytic residues reside in NAA30, but NatC catalysis depends on proper subunit organization. Mutations in selected NAA35 residues had only modest direct kinetic effects, whereas loss of NAA38 reduced kcat strongly; overall, all three subunits are required for full in vivo function. | NAA35’s main mechanistic role is architectural and positional, enabling efficient catalysis rather than donating the catalytic residues itself. | (grunwald2020divergentarchitectureof pages 6-7, damme2023expandedinvivo pages 1-2, grunwald2020divergentarchitectureof pages 9-10) |
| Protein quality control pathway | A major recent function of NatC is to shield Met-hydrophobic N-termini from degradation. In human cells, NatC loss exposes proteins to recognition by the UBR4–KCMF1 arm of the Arg/N-degron pathway. | NAA35 knockout showed strong positive genetic interactions with UBR4, KCMF1, UBR2, UBE2A and other quality-control genes, supporting NAA35’s role in this pathway as part of NatC. | (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2, varland2023nterminalacetylationshields pages 3-4) |
| Vesicle/Golgi trafficking pathway | NatC supports subcellular targeting of specific small GTPases and trafficking proteins, including Golgi and lysosomal targeting pathways. Loss of NatC perturbs Golgi vesicle transport and related trafficking modules. | NAA35 knockout screens were enriched for negative genetic interactions with genes involved in Golgi vesicle transport, endosomal transport, virion assembly, vesicle organization, and included ARL1, SYS1, RAB2A, ARFRP1. | (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2, varland2023nterminalacetylationshields pages 3-4, aksnes2019cotranslationalposttranslationaland pages 4-5) |
| Neddylation pathway | NatC-mediated acetylation of UBE2M and UBE2F increases their affinity for cognate E3 ligases, promoting cullin neddylation. | NAA35 participates indirectly by enabling NatC to acetylate these NEDD8-conjugating enzymes. | (varland2023nterminalacetylationshields pages 1-2, aksnes2019cotranslationalposttranslationaland pages 4-5) |
| Mitochondrial biology | NatC deficiency has been linked to reduced expression of mitochondrial proteins, loss of membrane potential, and mitochondrial fragmentation in human cells; yeast and comparative analyses also support a role in mitochondrial precursor handling/import. | NAA35, via NatC, likely contributes to acetylation of mitochondrial precursor proteins with NatC-compatible N-termini and thereby to mitochondrial proteome integrity. | (grunwald2020divergentarchitectureof pages 2-3, nashed2023functionalmappingof pages 1-3) |
| Development, aging, and organismal phenotypes | NatC perturbation affects cell growth, apoptosis, development, motility, and longevity across systems. In flies, loss of NatC caused reduced longevity and age-dependent motility defects; in zebrafish, NAA30 or NAA35 loss impaired development. | NAA35 is therefore functionally important beyond biochemistry, even though its effects are mediated through the NatC complex. | (grunwald2020divergentarchitectureof pages 2-3, varland2023nterminalacetylationshields pages 1-2) |
| Human disease relevance | Reviews and primary studies connect NAT dysfunction broadly to human disease; NatC components have emerging disease links. A de novo NAA35 variant has been reported in a patient cohort with cerebral palsy, though causal certainty remains limited. | Direct disease evidence for NAA35 is still sparse compared with NAA30/NAA10/NAA15, so current interpretation is suggestive rather than definitive. | (grunwald2020divergentarchitectureof pages 1-2, aksnes2023natsata pages 1-2) |
Table: This table summarizes the identity, composition, function, localization, substrate specificity, structural biology, and pathway roles of human NAA35 as the large auxiliary subunit of the NatC complex. It is useful for quickly distinguishing NAA35’s scaffolding and ribosome-binding roles from the catalytic activity carried out by NAA30.
The NatC complex catalyzes N-terminal acetylation (Nt-acetylation), which is one of the most abundant protein modifications in eukaryotes, affecting approximately 80-90% of the human proteome (aksnes2023natsata pages 1-2, varland2023nterminalacetylationshields pages 1-2). This irreversible modification involves the transfer of an acetyl group from acetyl-coenzyme A to the free α-amino group at the N-terminus of nascent polypeptide chains (grunwald2020divergentarchitectureof pages 1-2, aksnes2023natsata pages 1-2). NatC is one of the three major ribosome-associated N-terminal acetyltransferases (along with NatA and NatB) that collectively ensure the majority of cellular proteins receive this modification (aksnes2023natsata pages 1-2).
It is crucial to emphasize that NAA35 itself does not possess catalytic acetyltransferase activity. The catalytic function resides exclusively in the NAA30 subunit, which contains the conserved GNAT (GCN5-related N-acetyltransferase) fold (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, chang2023impactofprotein pages 4-5). Instead, NAA35 functions as the central assembly hub that holds the NatC complex together and creates the appropriate environment for catalysis (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 9-10).
The heterotrimeric NatC complex exhibits a unique architecture that is strikingly different from other NAT complexes, primarily due to the specialized structure and interactions of NAA35 (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3). NAA35 forms extensive interactions with both other subunits: it wraps around almost the entire circumference of the small NAA38 subunit (burying approximately 1,970 Ų) and encloses about three-quarters of the NAA30 catalytic subunit in a more condensed, ring-like structure (burying approximately 1,890 Ų) (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 2-3). This highly intertwined quaternary assembly generates a central tunnel and peptide-binding groove that are essential for substrate recognition (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 2-3).
NatC exhibits specificity for proteins that retain their initiator methionine followed by a hydrophobic or amphipathic amino acid (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 1-2, damme2023expandedinvivo pages 1-2). The canonical substrate classes include N-termini starting with ML (methionine-leucine), MF (methionine-phenylalanine), MI (methionine-isoleucine), and MW (methionine-tryptophan) (grunwald2020divergentarchitectureof pages 1-2, damme2023expandedinvivo pages 1-2). Expanded proteome-wide profiling in yeast has revealed additional substrate types including MY, MK, MM, MA, MV, and MS, though some of these may show redundancy with other NATs (damme2023expandedinvivo pages 1-2).
Importantly, the peptide-binding site of NatC is formed at the interface between NAA30 and NAA35, with both subunits contributing to substrate recognition (grunwald2020divergentarchitectureof pages 3-4, grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 6-7). Structural studies revealed that the first four amino acids of cognate substrates are recognized at this NAA30-NAA35 interface, with positions 3 and 4 also contributing significantly to binding affinity and catalytic efficiency (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 6-7, grunwald2020divergentarchitectureof pages 9-10). This deep, confined peptide-binding pocket created by the NAA30-NAA35 interface explains NatC's preference for hydrophobic N-termini and distinguishes it from the more open binding sites of other NATs (grunwald2020divergentarchitectureof pages 9-10).
All three NatC subunits—NAA30, NAA35, and NAA38—are essential for full enzymatic activity in vivo (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2). Deletion of any subunit in yeast results in complete loss of NatC-mediated acetylation for model substrates (grunwald2020divergentarchitectureof pages 9-10). While NAA30 contains the catalytic residues and NAA35 provides the structural framework, NAA38 also plays a critical stabilizing role; deletion of NAA38 reduced the catalytic turnover number (kcat) to only 6% of wild-type activity (grunwald2020divergentarchitectureof pages 7-8). NAA38 appears to stabilize the N-terminus of NAA35, which forms part of the extended peptide-binding pocket (grunwald2020divergentarchitectureof pages 9-10).
NAA35, as part of the NatC complex, localizes to ribosomes where it functions co-translationally (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 1-2, grunwald2020divergentarchitectureof pages 8-9, aksnes2019cotranslationalposttranslationaland pages 4-5). The major NATs, including NatC, are associated with mono- and polyribosomes and act on nascent polypeptide chains as they emerge from the ribosomal exit tunnel (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 8-9). This co-translational mode of action distinguishes the ribosome-associated NATs from post-translational acetyltransferases like NatF and NatH (aksnes2023natsata pages 1-2, aksnes2019cotranslationalposttranslationaland pages 4-5).
A distinctive structural feature of NAA35 is an elongated extension termed the "NAA35 tip region," which protrudes approximately 30 Å from the main body of the NatC complex (grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 8-9). This tip is formed primarily by helices α20 and α21 near the C-terminus of NAA35 (grunwald2020divergentarchitectureof pages 2-3). Structural modeling and biochemical co-sedimentation assays demonstrated that this electropositive tip region directly contacts ribosomal RNA and is essential for the stable association of NatC with ribosomes (grunwald2020divergentarchitectureof pages 8-9, grunwald2020divergentarchitectureof pages 10-11).
Mutagenesis studies identified several distinct electropositive regions (EPRs) on the NatC surface, with EPR2 located on the NAA35 tip containing 11 positively charged residues (grunwald2020divergentarchitectureof pages 8-9). Alanine substitution of four lysine residues (K500, K501, K503, K504) in the Tip1 mutant significantly reduced ribosome binding without substantially affecting intrinsic catalytic activity, confirming the tip's dedicated role in ribosome recruitment (grunwald2020divergentarchitectureof pages 8-9). Importantly, the NAA38 subunit is not required for ribosome association, as the NAA35-NAA30 heterodimer could still associate with ribosomes (grunwald2020divergentarchitectureof pages 8-9).
The crystal structure of the Saccharomyces cerevisiae NatC complex was solved to 2.40-2.45 Å resolution, providing detailed insights into the molecular organization of the complex (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3). NAA35 adopts a predominantly α-helical structure with short β-strands in the N- and C-terminal regions (grunwald2020divergentarchitectureof pages 2-3). Notably, database searches revealed no close structural homologs for NAA35 in the Protein Data Bank, indicating that it has evolved a unique, specialized architecture for its role in the NatC complex (grunwald2020divergentarchitectureof pages 2-3, grunwald2020divergentarchitectureof pages 9-10).
The architecture of NatC differs markedly from the heterodimeric NatA and NatB complexes (grunwald2020divergentarchitectureof pages 4-5). While the auxiliary subunits of NatA (NAA15) and NatB (NAA25) primarily engulf the N-terminal portions of their catalytic subunits, NAA35 wraps around both the N-terminal and C-terminal halves of NAA30, creating a more extensive and intertwined structure (grunwald2020divergentarchitectureof pages 4-5). Furthermore, the β6-β7 loop of NAA30, which is critical for substrate binding, makes direct contact with NAA35 in the NatC complex—a feature not observed in NatA or NatB (grunwald2020divergentarchitectureof pages 4-5).
Structural studies with cofactor and ligand-bound NatC complexes revealed dynamic, sequence-specific conformational changes upon substrate binding (grunwald2020divergentarchitectureof pages 6-7, grunwald2020divergentarchitectureof pages 7-8). Binding of substrate peptides, particularly those with optimal sequences like the viral Gag peptide (MLRFV), induced large conformational rearrangements in the β6-β7 loop of NAA30 and movement of the α1-α2 loop, resulting in constriction of the central tunnel (grunwald2020divergentarchitectureof pages 6-7, grunwald2020divergentarchitectureof pages 7-8). These ligand-induced conformational changes are essential for efficient catalysis and demonstrate the structural plasticity of the NatC active site (grunwald2020divergentarchitectureof pages 6-7).
Although NAA35 does not directly participate in catalysis, it supports the catalytic mechanism by maintaining the proper positioning of active site residues in NAA30 (grunwald2020divergentarchitectureof pages 7-8, grunwald2020divergentarchitectureof pages 9-10). The proposed catalytic mechanism involves key residues in NAA30 including Glu118 (general base), Tyr80 (coordinating a catalytic water), and Tyr130 (general acid) (grunwald2020divergentarchitectureof pages 7-8). NAA35's role is primarily architectural, ensuring that the catalytic machinery is properly organized and that substrates can access the active site efficiently (grunwald2020divergentarchitectureof pages 7-8, grunwald2020divergentarchitectureof pages 9-10).
The NatC complex and its constituent subunits, including NAA35, are evolutionarily conserved from yeast to humans (grunwald2020divergentarchitectureof pages 1-2, damme2023expandedinvivo pages 1-2). Studies have demonstrated functional conservation, with human NAA30 able to rescue yeast deletion mutant phenotypes and partially restore the yeast NatC Nt-acetylome (damme2023expandedinvivo pages 1-2). This conservation underscores the fundamental biological importance of NatC-mediated acetylation across eukaryotes (damme2023expandedinvivo pages 1-2).
NAA35, through its role in the NatC complex, participates in multiple critical cellular pathways and processes. The functional impact of NatC extends far beyond simple protein modification, influencing protein stability, localization, complex formation, and quality control.
One of the most significant recent discoveries is that NatC-mediated N-terminal acetylation serves as a protective mechanism against protein degradation (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2). Genome-wide CRISPR knockout screens in human cells revealed strong negative genetic interactions between NatC subunits (including NAA35) and components of the Arg/N-degron pathway, particularly the E3 ubiquitin ligase complex UBR4-KCMF1 (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2, varland2023nterminalacetylationshields pages 3-4).
Both the UBR4-KCMF1 ubiquitin ligase complex and the NatC acetyltransferase recognize proteins bearing an unacetylated N-terminal methionine followed by a hydrophobic residue (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2). When NatC is absent, these proteins remain unacetylated and become targets for UBR4-KCMF1-mediated ubiquitination and proteasomal degradation (varland2023nterminalacetylationshields pages 1-2). Conversely, NatC-mediated acetylation shields these N-termini, preventing recognition by the Arg/N-degron pathway and thereby stabilizing the proteins (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2). This interplay between NatC and the UBR4-KCMF1 pathway represents a fundamental mechanism of protein quality control and proteostasis regulation (varland2023nterminalacetylationshields pages 1-2).
NatC-mediated acetylation is crucial for the correct subcellular targeting of numerous proteins, particularly those involved in Golgi and vesicle trafficking (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 1-2, varland2023nterminalacetylationshields pages 3-4). Genetic interaction screens with NAA35 knockout cells showed enrichment for genes involved in Golgi vesicle transport, endosomal transport, and virion assembly (varland2023nterminalacetylationshields pages 2-3, varland2023nterminalacetylationshields pages 3-4).
Well-characterized examples include the small GTPases ARFRP1 (Arl3 in yeast) and ARL8B, whose acetylation by NatC is essential for their proper localization (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 1-2, aksnes2019cotranslationalposttranslationaland pages 4-5). ARFRP1/Arl3 requires NatC-mediated acetylation of its N-terminal methionine for targeting to the trans-Golgi network, a process mediated by interaction between the acetylated N-terminus and the membrane protein SYS1 (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 1-2). Similarly, ARL8B depends on NatC acetylation for correct targeting to lysosomes (varland2023nterminalacetylationshields pages 1-2). Loss of NatC leads to mislocalization of these proteins and disruption of organelle function (grunwald2020divergentarchitectureof pages 1-2, varland2023nterminalacetylationshields pages 2-3).
NatC plays an important role in the neddylation pathway by acetylating the NEDD8-conjugating enzymes UBE2M (UBC12 in yeast) and UBE2F (varland2023nterminalacetylationshields pages 1-2, aksnes2019cotranslationalposttranslationaland pages 4-5). The acetylated N-terminus of UBE2M interacts with a hydrophobic pocket in the E3 ligase adaptor DCN1 (DCUN1D1 in mammals), significantly enhancing the affinity of this interaction and promoting efficient cullin neddylation (aksnes2019cotranslationalposttranslationaland pages 4-5). Cullin neddylation is essential for the activation of cullin-RING E3 ubiquitin ligases, which regulate numerous cellular processes including cell cycle progression and protein degradation (varland2023nterminalacetylationshields pages 1-2). Thus, NAA35, through the NatC complex, indirectly influences the ubiquitin-proteasome system via this acetylation-dependent mechanism.
NatC has been implicated in mitochondrial protein biology through multiple lines of evidence (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2, nashed2023functionalmappingof pages 1-3). In yeast, deletion of NatC subunits results in reduced growth on non-fermentable carbon sources (such as glycerol), suggesting defects in mitochondrial function (grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2). In human cells, depletion of NAA30 leads to reduced expression of mitochondrial proteins, loss of mitochondrial membrane potential, and mitochondrial fragmentation (grunwald2020divergentarchitectureof pages 2-3, damme2023expandedinvivo pages 1-2).
Recent functional genomics work in yeast identified that mitochondrial targeting sequences (MTS) show a strong over-representation of hydrophobic residues at position 2, precisely matching the substrate specificity of NatC (nashed2023functionalmappingof pages 1-3). Co-translational purification of NatC-associated ribosomes confirmed that mitochondrial precursor proteins are indeed targeted by NatC during translation (nashed2023functionalmappingof pages 1-3). Systematic mutagenesis of position 2 in a model mitochondrial protein confirmed that this residue is critical for efficient mitochondrial import, providing a molecular explanation for the mitochondrial defects observed in NatC-depleted cells (nashed2023functionalmappingof pages 1-3).
Loss of NatC function has profound effects on organismal development, aging, and lifespan across multiple model systems (grunwald2020divergentarchitectureof pages 1-2, grunwald2020divergentarchitectureof pages 2-3, varland2023nterminalacetylationshields pages 1-2). In zebrafish, knockout of NAA30 or NAA35 led to decreased cell proliferation, increased apoptosis, poor blood vessel formation, and embryonic lethality (grunwald2020divergentarchitectureof pages 1-2).
In Drosophila melanogaster, loss of NatC was associated with male sterility, reduced longevity, and age-dependent loss of motility due to developmental muscle defects (varland2023nterminalacetylationshields pages 1-2). Remarkably, muscle-specific overexpression of UbcE2M (the fly ortholog of UBE2M), one of the key proteins targeted for degradation in NatC knockout conditions, was sufficient to suppress many of the defects caused by NatC deletion, including motility and longevity phenotypes (varland2023nterminalacetylationshields pages 1-2). This finding demonstrates that NatC-mediated protection of specific substrate proteins from degradation has direct physiological consequences for healthy aging and tissue function.
In yeast, NatC deletion affects stress responses, with all three subunit mutants showing osmotic sensitivity and reduced growth under high salt stress (damme2023expandedinvivo pages 1-2). Additionally, NatC has been implicated in starvation-induced nuclear-to-cytosolic relocalization of the proteasome, an age-dependent process (damme2023expandedinvivo pages 1-2).
While direct disease associations for NAA35 remain limited compared to other NAT subunits, emerging evidence suggests potential roles in human pathology (grunwald2020divergentarchitectureof pages 1-2, aksnes2023natsata pages 1-2). The catalytic subunit NAA30 has been shown to be upregulated in glioblastoma, and knockdown of NAA30 in glioblastoma-initiating cells reduced their viability, sphere-forming ability, and hypoxia tolerance (grunwald2020divergentarchitectureof pages 1-2). Mice transplanted with NAA30-depleted glioblastoma cells showed prolonged survival, indicating that NatC may serve as a therapeutic target in certain cancers (grunwald2020divergentarchitectureof pages 1-2).
A potentially pathogenic de novo variant in NAA35 has been identified in patients with cerebral palsy, a heterogeneous group of disorders affecting movement and posture, though the causal relationship requires further validation (grunwald2020divergentarchitectureof pages 1-2). Given the essential nature of NatC and its broad effects on protein homeostasis, it is plausible that NAA35 variants could contribute to developmental or neurological disorders, but more research is needed to establish definitive disease links.
NAA35 functions as the large auxiliary subunit and central scaffold of the evolutionarily conserved NatC N-terminal acetyltransferase complex. While NAA35 does not possess intrinsic catalytic activity, it is absolutely essential for NatC function. NAA35's primary roles include: (1) organizing the heterotrimeric architecture by wrapping around both NAA30 and NAA38, (2) contributing residues to the substrate-binding interface with NAA30 that determines specificity for methionine-hydrophobic N-termini, and (3) mediating ribosome association through a specialized electropositive tip region.
NAA35 enables NatC to function co-translationally at the ribosomal exit tunnel, acetylating nascent polypeptides with N-terminal sequences beginning with Met followed by hydrophobic residues. This acetylation modification has profound biological consequences, including protection of proteins from degradation by the Arg/N-degron pathway, facilitation of subcellular targeting to organelles such as the Golgi and mitochondria, and enhancement of neddylation pathway activity.
The structural biology of NAA35 reveals a unique, specialized architecture with no close homologs, reflecting its evolved function in coordinating NatC assembly and ribosome binding. Recent advances in understanding the NatC-UBR4-KCMF1 interplay have revealed N-terminal acetylation as a critical mechanism of protein quality control. The conservation of NAA35 and NatC from yeast to humans, combined with emerging disease associations and the complex's involvement in aging and development, underscores the fundamental importance of this auxiliary subunit in cellular physiology.
Future research directions include elucidating additional NatC substrates and their specific functional outcomes, understanding how NatC activity is regulated in different cellular contexts, and exploring the therapeutic potential of modulating NatC function in diseases such as cancer and developmental disorders.
References
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