Mitoregulin (MTLN; also MOXI, MPM, LEMP, SMIM37) is a 56-amino-acid, vertebrate-conserved microprotein translated from a short ORF in a transcript long annotated as the non-coding RNA LINC00116. It is most abundant in heart, skeletal muscle and adipose tissue and is induced during myogenic and adipogenic differentiation. The peptide has a single transmembrane helix followed by a basic C-terminal tail and resides in mitochondrial membranes, most studies placing it in the inner membrane (one study places it mainly in the outer membrane). MTLN binds cardiolipin, self-associates into a ~66 kDa, probably hexameric, complex, and co-migrates with respiratory supercomplexes, complex I assembly intermediates and ATP synthase. It associates with the mitochondrial trifunctional protein (HADHA/HADHB), CPT1B and cytochrome b5 reductase CYB5R3. Loss of MTLN reduces respiratory supercomplex levels and complex I-linked respiration, lowers fatty acid beta-oxidation, destabilises cristae and cardiolipin, and leads to triglyceride accumulation, reduced exercise capacity and impaired muscle differentiation, whereas overexpression raises respiration, membrane potential, mitochondrial Ca2+ retention and fatty acid oxidation. It has no known catalytic activity and is thought to act as a membrane-organising factor that supports respiratory supercomplex assembly or stability and enhances fatty acid oxidation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:32243843 Mitoregulin Controls Ξ²-Oxidation in Human and Mouse Adipocyt... | REMOVE | Summary: IPI from co-immunoprecipitation of endogenous ATP synthase beta subunit (ATP5F1B) with MTLN-FLAG in human hPSC-derived adipocytes. Reason: Generic protein binding carries no functional information. The MTLN-ATP5F1B co-IP is plausible (both are mitochondrial membrane-associated and MTLN co-migrates with complex V on BN-PAGE), but the authors themselves describe it as possibly adipocyte-specific, no effect of MTLN on ATP synthase activity or assembly was shown, and no more specific molecular function can be derived from the interaction. Removal does not imply the interaction is false. Supporting Evidence: PMID:32243843 We verified the co-immunoprecipitation of exogenously expressed mitochondrial trifunctional enzyme, subunit beta (HADHB) and the endogenous ATP synthase beta subunit (ATP5B) using immunoblotting (Figures 4B, 4C, and S4BβS4D). PMID:32243843 The mitochondrial trifunctional enzyme interaction had been described earlier, but the found interaction with ATP5B seems to be a novel or adipocyte-specific process (Makarewich et al., 2018, Stein et al., 2018). |
| GO:0005515 protein binding | IPI PMID:32243843 Mitoregulin Controls Ξ²-Oxidation in Human and Mouse Adipocyt... | REMOVE | Summary: IPI from co-immunoprecipitation of HADHB (beta subunit of the mitochondrial trifunctional protein, MTP) with MTLN-FLAG in human adipocytes; the MTP interaction was also shown for mouse MOXI. Reason: The MTLN-MTP association is reproducible across labs (Friesen 2020 in human adipocytes; Makarewich 2018 in mouse muscle and HEK293 cells) and is the best mechanistic link between MTLN and fatty acid beta-oxidation. However, protein binding is uninformative, and the interaction does not define a molecular activity: MTLN is not required for MTP formation or stability, and whether it modulates MTP catalysis directly is untested. The biological consequence is captured by positive regulation of fatty acid beta-oxidation (GO:0032000). Removal does not imply the interaction is false. Supporting Evidence: PMID:32243843 We verified the co-immunoprecipitation of exogenously expressed mitochondrial trifunctional enzyme, subunit beta (HADHB) and the endogenous ATP synthase beta subunit (ATP5B) using immunoblotting (Figures 4B, 4C, and S4BβS4D). PMID:29949755 MOXI is a nuclear encoded peptide that localizes to the inner mitochondrial membrane (IMM), where it specifically interacts with the mitochondrial trifunctional protein (MTP), an IMM localized enzymatic complex that catalyzes the last three steps of long-chain fatty acid (LCFA) oxidation. PMID:29949755 indicating that MOXI is not required for MTP complex formation or stability PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. |
| GO:0005739 mitochondrion | IDA PMID:32243843 Mitoregulin Controls Ξ²-Oxidation in Human and Mouse Adipocyt... | ACCEPT | Summary: Mitochondrial localization of MTLN in human adipocytes by immunofluorescence co-localization with ATP5B/HK1 and by subcellular fractionation. Reason: Mitochondrial localization of MTLN is consistently reported by every group that has studied it, in human and mouse cells. The more specific sub-mitochondrial compartment (inner vs outer membrane) is debated, so the general term is a safe, correct annotation. Supporting Evidence: PMID:32243843 HK1, ATP5B, and MTLN-FLAG all localized together in the mitochondrial fraction, whereas only histone H3 was present in the nuclear fraction (Figure 3C). PMID:29949756 Mtln localizes to the inner mitochondrial membrane, where it binds cardiolipin and influences protein complex assembly. |
| GO:0005739 mitochondrion | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara orthology transfer of mitochondrion from mouse Mtln (Q8BT35). Reason: Correct and consistent with direct human data (Friesen 2020) and multiple mouse studies. Supporting Evidence: PMID:32243843 HK1, ATP5B, and MTLN-FLAG all localized together in the mitochondrial fraction, whereas only histone H3 was present in the nuclear fraction (Figure 3C). |
| GO:0005739 mitochondrion | ISS GO_REF:0000024 | ACCEPT | Summary: Curator ISS transfer of mitochondrion from mouse Mtln (Q8BT35). Reason: Correct; mitochondrial localization is established in both species. Supporting Evidence: PMID:32243843 HK1, ATP5B, and MTLN-FLAG all localized together in the mitochondrial fraction, whereas only histone H3 was present in the nuclear fraction (Figure 3C). PMID:29949756 Mtln localizes to the inner mitochondrial membrane, where it binds cardiolipin and influences protein complex assembly. |
| GO:0005743 mitochondrial inner membrane | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT/IBA inner mitochondrial membrane for the vertebrate mitoregulin family (PTN002201359), grounded in mouse Mtln experimental data. Reason: Three independent groups (Stein 2018, Makarewich 2018, Chugunova 2019) placed Mtln in the inner mitochondrial membrane by protease protection/fractionation, it binds the IMM-specific lipid cardiolipin, and it co-migrates with IMM respiratory complexes. Zhang et al. 2023 reported, using split-GFP topology reporters in human cells, that MTLN is primarily in the outer membrane; this remains unresolved and is raised in suggested_questions, but it is not sufficient to remove the IMM annotations, which are supported by several orthogonal datasets and by later work from the Boudreau lab on cristae/cardiolipin. Supporting Evidence: PMID:29949755 MOXI exhibited a PK proteolysis resistance pattern similar to that of the IMM marker HADHA, and distinct from the OMM protein TOMM20 (Figure 2B), thus substantiating the sub-mitochondrial localization of MOXI to the IMM. PMID:29949756 Mtln localizes to the inner mitochondrial membrane, where it binds cardiolipin and influences protein complex assembly. PMID:30796188 Our results perfectly match the conclusions of two independent groups, which showed that Mtln localizes in the inner membrane of mitochondria (25, 26). PMID:37664623 Here, by applying multiple orthogonal methods, we concluded that MTLN is primarily localized to the mitochondrial outer membrane and has a cytosolic tail. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated annotation (UniProt subcellular location SL-0168 and orthology) to mitochondrial inner membrane. Reason: Three independent groups (Stein 2018, Makarewich 2018, Chugunova 2019) placed Mtln in the inner mitochondrial membrane by protease protection/fractionation, it binds the IMM-specific lipid cardiolipin, and it co-migrates with IMM respiratory complexes. Zhang et al. 2023 reported, using split-GFP topology reporters in human cells, that MTLN is primarily in the outer membrane; this remains unresolved and is raised in suggested_questions, but it is not sufficient to remove the IMM annotations, which are supported by several orthogonal datasets and by later work from the Boudreau lab on cristae/cardiolipin. Supporting Evidence: PMID:29949755 MOXI exhibited a PK proteolysis resistance pattern similar to that of the IMM marker HADHA, and distinct from the OMM protein TOMM20 (Figure 2B), thus substantiating the sub-mitochondrial localization of MOXI to the IMM. PMID:29949756 Mtln localizes to the inner mitochondrial membrane, where it binds cardiolipin and influences protein complex assembly. |
| GO:0005743 mitochondrial inner membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Curator ISS transfer of mitochondrial inner membrane from mouse Mtln. Reason: Three independent groups (Stein 2018, Makarewich 2018, Chugunova 2019) placed Mtln in the inner mitochondrial membrane by protease protection/fractionation, it binds the IMM-specific lipid cardiolipin, and it co-migrates with IMM respiratory complexes. Zhang et al. 2023 reported, using split-GFP topology reporters in human cells, that MTLN is primarily in the outer membrane; this remains unresolved and is raised in suggested_questions, but it is not sufficient to remove the IMM annotations, which are supported by several orthogonal datasets and by later work from the Boudreau lab on cristae/cardiolipin. Supporting Evidence: PMID:29949755 MOXI exhibited a PK proteolysis resistance pattern similar to that of the IMM marker HADHA, and distinct from the OMM protein TOMM20 (Figure 2B), thus substantiating the sub-mitochondrial localization of MOXI to the IMM. PMID:30796188 Our results perfectly match the conclusions of two independent groups, which showed that Mtln localizes in the inner membrane of mitochondria (25, 26). |
| GO:0006629 lipid metabolic process | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara orthology transfer of lipid metabolic process from mouse Mtln. Reason: The source is the mouse IMP in Chugunova 2019 (Mtln knockout alters lipid composition via its partner CYB5R3). MTLN has no catalytic activity and does not itself carry out a lipid metabolic step; its documented effects (enhanced fatty acid beta-oxidation, maintained membrane lipid composition, cardiolipin protection) are regulatory/supportive. The broad parent term is not wrong as a grouping but is uninformative; the specific, better-supported claim is positive regulation of fatty acid beta-oxidation (GO:0032000), which the gene already carries. Supporting Evidence: PMID:30796188 Interaction of Mtln with NADH-dependent cytochrome b5 reductase stimulates complex I functioning most likely by providing a favorable lipid composition of the membrane. PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. |
| GO:0006629 lipid metabolic process | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator ISS transfer of lipid metabolic process from mouse Mtln (source IMP, PMID:30796188). Reason: The source is the mouse IMP in Chugunova 2019 (Mtln knockout alters lipid composition via its partner CYB5R3). MTLN has no catalytic activity and does not itself carry out a lipid metabolic step; its documented effects (enhanced fatty acid beta-oxidation, maintained membrane lipid composition, cardiolipin protection) are regulatory/supportive. The broad parent term is not wrong as a grouping but is uninformative; the specific, better-supported claim is positive regulation of fatty acid beta-oxidation (GO:0032000), which the gene already carries. Supporting Evidence: PMID:30796188 Interaction of Mtln with NADH-dependent cytochrome b5 reductase stimulates complex I functioning most likely by providing a favorable lipid composition of the membrane. PMID:37664623 We confirmed that CPT1B, but not unrelated protein CS, specifically interacts with endogenous MTLN by IP and western analysis (Figures 5C and S5A). |
| GO:0006635 fatty acid beta-oxidation | IMP PMID:32243843 Mitoregulin Controls Ξ²-Oxidation in Human and Mouse Adipocyt... | MODIFY | Summary: IMP from MTLN knockout/overexpression in human hPSC-derived adipocytes and mouse models: loss of MTLN reduces, and overexpression increases, fatty acid oxidation. Reason: The phenotype is solid, but MTLN does not perform any step of beta-oxidation: it is not an enzyme, it is not required for MTP formation or stability, and all groups describe it as augmenting the rate of FAO (through association with MTP, CPT1B, or indirectly via respiratory chain/membrane effects). By the participation test in the project guidelines, the direct process term overstates its role; the regulatory term positive regulation of fatty acid beta-oxidation (GO:0032000) captures what the IMP data show. Proposed replacements: positive regulation of fatty acid beta-oxidation Supporting Evidence: PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. PMID:29949755 indicating that MOXI is not required for MTP complex formation or stability PMID:32243843 The suppressive effect of MTLN on glycolysis and oxidative respiration and the increase in FAO suggests that MTLN causes adipocytes to favor fatty acid Ξ²-oxidation for energy production. PMID:29949756 Despite our observation that Mtln loss leads to deficiency in FAO, we believe that this occurs downstream of defects in respiratory chain activity |
| GO:0032000 positive regulation of fatty acid beta-oxidation | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara orthology transfer of positive regulation of fatty acid beta-oxidation from mouse Mtln (source IMP, PMID:29949755). Reason: Supported in human adipocytes (Friesen 2020) and in mouse heart and muscle (Makarewich 2018): loss of MTLN lowers and overexpression raises fatty acid oxidation, and MTLN associates with the mitochondrial trifunctional protein and CPT1B. This regulatory term is the correct way to capture the FAO phenotype. The precise mechanism (direct modulation of MTP/CPT1B vs secondary to respiratory chain and membrane effects) is unresolved, but all mechanisms are consistent with positive regulation. Supporting Evidence: PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. PMID:29949755 MOXI is a nuclear encoded peptide that localizes to the inner mitochondrial membrane (IMM), where it specifically interacts with the mitochondrial trifunctional protein (MTP), an IMM localized enzymatic complex that catalyzes the last three steps of long-chain fatty acid (LCFA) oxidation. |
| GO:0032000 positive regulation of fatty acid beta-oxidation | ISS GO_REF:0000024 | ACCEPT | Summary: Curator ISS transfer of positive regulation of fatty acid beta-oxidation from mouse Mtln. Reason: Supported in human adipocytes (Friesen 2020) and in mouse heart and muscle (Makarewich 2018): loss of MTLN lowers and overexpression raises fatty acid oxidation, and MTLN associates with the mitochondrial trifunctional protein and CPT1B. This regulatory term is the correct way to capture the FAO phenotype. The precise mechanism (direct modulation of MTP/CPT1B vs secondary to respiratory chain and membrane effects) is unresolved, but all mechanisms are consistent with positive regulation. Supporting Evidence: PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. PMID:32243843 The suppressive effect of MTLN on glycolysis and oxidative respiration and the increase in FAO suggests that MTLN causes adipocytes to favor fatty acid Ξ²-oxidation for energy production. |
| GO:0045333 cellular respiration | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara orthology transfer of cellular respiration from mouse Mtln. Reason: The source is the mouse IMP in Lin et al. 2019 (MPM), where Mtln gain/loss changed oxygen consumption and ATP production; human HeLa overexpression (Stein 2018) and mouse KO data (complex I activity, supercomplexes) agree. MTLN is not a respiratory chain component and does not change the intrinsic activity of purified complex I; it supports respiration indirectly, via supercomplex/membrane organisation. The broad term is acceptable as a non-core grouping; the more informative participation claim is mitochondrial respirasome assembly (GO:0097250). Supporting Evidence: PMID:31296841 Mechanistical investigations based on both gain- and loss-of function studies revealed that MPM increased oxygen consumption and ATP production of mitochondria. PMID:30796188 Unlike the integral components and assembly factors of NADH:ubiquinone oxidoreductase, Mtln does not alter its enzymatic activity directly. |
| GO:0045333 cellular respiration | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator ISS transfer of cellular respiration from mouse Mtln (source IMP, PMID:31296841). Reason: The source is the mouse IMP in Lin et al. 2019 (MPM), where Mtln gain/loss changed oxygen consumption and ATP production; human HeLa overexpression (Stein 2018) and mouse KO data (complex I activity, supercomplexes) agree. MTLN is not a respiratory chain component and does not change the intrinsic activity of purified complex I; it supports respiration indirectly, via supercomplex/membrane organisation. The broad term is acceptable as a non-core grouping; the more informative participation claim is mitochondrial respirasome assembly (GO:0097250). Supporting Evidence: PMID:31296841 Mechanistical investigations based on both gain- and loss-of function studies revealed that MPM increased oxygen consumption and ATP production of mitochondria. PMID:30796188 Unlike the integral components and assembly factors of NADH:ubiquinone oxidoreductase, Mtln does not alter its enzymatic activity directly. |
| GO:0051146 striated muscle cell differentiation | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara orthology transfer of striated muscle cell differentiation from mouse Mtln. Reason: Source is the mouse IMP in Lin et al. 2019 (MPM silencing impairs C2C12 differentiation; KO mice have smaller fibres and impaired regeneration), corroborated by Wang et al. 2020 (LEMP, mouse and zebrafish). The authors attribute the effect to enhanced mitochondrial respiration, and PGC-1alpha overexpression rescues it, so MTLN acts on differentiation indirectly through its mitochondrial role. Retained as a non-core, downstream developmental process. Supporting Evidence: PMID:31296841 These results suggest that MPM may promote myogenic differentiation by enhancing mitochondrial respiration. PMID:32393776 In this study, we identified a putative trans-membrane micropeptide, LEMP, which functions in myogenesis in both zebrafish and mice. |
| GO:0051146 striated muscle cell differentiation | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator ISS transfer of striated muscle cell differentiation from mouse Mtln (source IMP, PMID:31296841). Reason: Source is the mouse IMP in Lin et al. 2019 (MPM silencing impairs C2C12 differentiation; KO mice have smaller fibres and impaired regeneration), corroborated by Wang et al. 2020 (LEMP, mouse and zebrafish). The authors attribute the effect to enhanced mitochondrial respiration, and PGC-1alpha overexpression rescues it, so MTLN acts on differentiation indirectly through its mitochondrial role. Retained as a non-core, downstream developmental process. Supporting Evidence: PMID:31296841 These results suggest that MPM may promote myogenic differentiation by enhancing mitochondrial respiration. PMID:32393776 In this study, we identified a putative trans-membrane micropeptide, LEMP, which functions in myogenesis in both zebrafish and mice. |
| GO:0070328 triglyceride homeostasis | IMP PMID:32243843 Mitoregulin Controls Ξ²-Oxidation in Human and Mouse Adipocyt... | KEEP AS NON CORE | Summary: IMP from MTLN knockout in human hPSC-adipocytes (triglyceride accumulation) and mouse (altered serum lipids). Reason: The phenotype is reproducible (Friesen 2020 human adipocytes; Averina 2023 KO mice obese on high-fat diet with elevated serum TG), but it is an indirect consequence of reduced fatty acid oxidation/mitochondrial oxidative capacity rather than something MTLN does to triglycerides. MTLN does not act on TG synthesis, storage or lipolysis machinery directly. Kept as a non-core downstream process; the core claim is positive regulation of fatty acid beta-oxidation. Supporting Evidence: PMID:32243843 Interaction with HADHB and accumulation of TGs in MTLN KO hPSC-adipocytes implicates MTLN in long-chain fatty acid catabolism. PMID:32243843 Our data suggest MTLN augments the rate of Ξ²-oxidation of long-chain fatty acids, possibly through direct interaction with HADHB. |
| GO:0097250 mitochondrial respirasome assembly | IDA PMID:29949756 Mitoregulin: A lncRNA-Encoded Microprotein that Supports Mit... | ACCEPT | Summary: Mitochondrial respirasome assembly, from Stein et al. 2018 (supercomplex loss in Mtln-KO tissues, co-migration of Mtln with supercomplexes). Reason: Mtln knockout mice show reduced respiratory supercomplexes and reduced complex I activity within I+III2 and respirasome (I+III2+IV) bands, while individual complex levels are unchanged; Mtln co-migrates with supercomplexes and the complex I assembly module on BN-PAGE and binds cardiolipin, a lipid required for supercomplex stability. This is consistent with a structural, membrane-organising contribution to supercomplex assembly/stability rather than a downstream effect, and it is the most specific process term for MTLN. Caveats: evidence is from loss-of-function and co-migration rather than reconstitution; human overexpression did not change supercomplex levels; and Chugunova 2019 found complex I+III activity independent of Mtln. The UniProt function line (assembly "and/or stability") reflects this uncertainty. Supporting Evidence: PMID:29949756 decreases in respiratory supercomplexes were evident in Mtln-KO hearts (Figure 5C) and skeletal muscles (Figure S7B) PMID:29949756 Mtln-KO skeletal muscle samples also showed reduced CI activities within both I+III2 supercomplexes and respirasomes (I+III2+IV) (Figure S7C). PMID:29949756 Subsequent BN-PAGE western blot revealed that Mtln co-migrates with several notable complexes, including the CI assembly module (CIA; Alston et al., 2016; Heide et al., 2012), complex V, and various supercomplexes PMID:29949756 Mtln localizes to the inner mitochondrial membrane, where it binds cardiolipin and influences protein complex assembly. |
| GO:0097250 mitochondrial respirasome assembly | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara orthology transfer of mitochondrial respirasome assembly from mouse Mtln (source IMP, PMID:29949756). Reason: Mtln knockout mice show reduced respiratory supercomplexes and reduced complex I activity within I+III2 and respirasome (I+III2+IV) bands, while individual complex levels are unchanged; Mtln co-migrates with supercomplexes and the complex I assembly module on BN-PAGE and binds cardiolipin, a lipid required for supercomplex stability. This is consistent with a structural, membrane-organising contribution to supercomplex assembly/stability rather than a downstream effect, and it is the most specific process term for MTLN. Caveats: evidence is from loss-of-function and co-migration rather than reconstitution; human overexpression did not change supercomplex levels; and Chugunova 2019 found complex I+III activity independent of Mtln. The UniProt function line (assembly "and/or stability") reflects this uncertainty. Supporting Evidence: PMID:29949756 decreases in respiratory supercomplexes were evident in Mtln-KO hearts (Figure 5C) and skeletal muscles (Figure S7B) PMID:29949756 Mtln-KO skeletal muscle samples also showed reduced CI activities within both I+III2 supercomplexes and respirasomes (I+III2+IV) (Figure S7C). |
| GO:0097250 mitochondrial respirasome assembly | ISS GO_REF:0000024 | ACCEPT | Summary: Curator ISS transfer of mitochondrial respirasome assembly from mouse Mtln. Reason: Mtln knockout mice show reduced respiratory supercomplexes and reduced complex I activity within I+III2 and respirasome (I+III2+IV) bands, while individual complex levels are unchanged; Mtln co-migrates with supercomplexes and the complex I assembly module on BN-PAGE and binds cardiolipin, a lipid required for supercomplex stability. This is consistent with a structural, membrane-organising contribution to supercomplex assembly/stability rather than a downstream effect, and it is the most specific process term for MTLN. Caveats: evidence is from loss-of-function and co-migration rather than reconstitution; human overexpression did not change supercomplex levels; and Chugunova 2019 found complex I+III activity independent of Mtln. The UniProt function line (assembly "and/or stability") reflects this uncertainty. Supporting Evidence: PMID:29949756 decreases in respiratory supercomplexes were evident in Mtln-KO hearts (Figure 5C) and skeletal muscles (Figure S7B) PMID:29949756 Subsequent BN-PAGE western blot revealed that Mtln co-migrates with several notable complexes, including the CI assembly module (CIA; Alston et al., 2016; Heide et al., 2012), complex V, and various supercomplexes |
| GO:1901612 cardiolipin binding | ISS GO_REF:0000024 | NEW | Summary: Proposed new annotation: mitoregulin binds cardiolipin, the signature phospholipid of the inner mitochondrial membrane. Reason: Cardiolipin binding is the most direct molecular activity reported for MTLN: cardiolipin-coated beads pull down Mtln at least as well as ATP synthase subunit c, a lipid-strip assay shows preference for cardiolipin over other membrane lipids, and synthetic human Mtln inserts into mitochondrial-like lipid monolayers and alters their packing. The binding assays used mouse tissue lysate, so ISS from mouse (identical membrane-proximal basic residues) is used for human. This is a molecular function, so the process-participation test does not apply; the binding is plausibly the basis for the supercomplex and membrane-stability phenotypes. Supporting Evidence: PMID:29949756 cardiolipin-coated beads pulled down Mtln from wild-type mouse cardiac tissue lysates to a similar or greater degree compared to subunit c, an ATP synthase subunit with high affinity for cardiolipin (Figure 2F) PMID:29949756 A lipid-strip binding assay further supported that Mtln preferentially engages cardiolipin relative to other common cell membrane lipids (Figure 2G). PMID:41555203 Mtln strongly binds CL,1 and our Langmuir analyses demonstrate that Mtln adsorbs onto (and/or into) mitochondrial-like lipid monolayers and alters monolayer properties, reducing lipid density and enhancing elasticity. PMID:41555203 Synthetic human Mtln was synthesized by Biomatik USA, LLC |
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Download this section (compressed HTML)Q: Is MTLN primarily an inner- or outer-mitochondrial-membrane protein, and does the answer differ by cell type or expression level? Split-GFP topology in human cells (Zhang 2023) conflicts with protease-protection data from three other groups; should GO:0005741 mitochondrial outer membrane be added alongside or instead of GO:0005743?
Q: Is the MTLN effect on fatty acid beta-oxidation a direct modulation of MTP or CPT1B activity, or a secondary consequence of improved respiratory chain and membrane organisation, as the Stein 2018 authors argue?
Q: Does the ~66 kDa MTLN homo-oligomer have an activity of its own (e.g. a pore or channel), which would give a more informative molecular function than cardiolipin binding?
Experiment: Reconstitute purified MTLN with cardiolipin-containing liposomes and isolated complexes I, III and IV (or MTP) to test whether MTLN directly promotes supercomplex formation or MTP activity, including cardiolipin-binding-deficient mutants of the basic C-terminal tail.
Experiment: Resolve submitochondrial localisation of endogenous MTLN in human muscle cells and adipocytes by APEX2 proximity labelling targeted to matrix, IMS and cytosol, combined with cryo-ET immunogold, without overexpression.
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