MMAA (methylmalonic aciduria type A protein; the human cblA protein) is a mitochondrial GTPase of the SIMIBI-class G3E GTPase family, ArgK/MeaB subfamily (Pfam MeaB; InterPro GTPase_ArgK). It is imported into the mitochondrial matrix via a cleaved N-terminal transit peptide and functions as a G-protein chaperone that assists delivery and maintenance of the adenosylcobalamin (AdoCbl, vitamin B12) cofactor for the enzyme methylmalonyl-CoA mutase (MMUT/MUT). MMAA itself is not the mutase and does not isomerize methylmalonyl-CoA; instead, in a GTP-binding- and GTP-hydrolysis-dependent manner it gates the transfer of AdoCbl from the adenosyltransferase MMAB onto MMUT apoenzyme, protects the MMUT-bound cofactor from oxidative inactivation, and reactivates MMUT by promoting exchange of the damaged cofactor. Its low intrinsic GTPase activity is strongly stimulated by MMUT (which acts as a GAP), and MMAA forms a homodimer that assembles into nucleotide-sensitive interconverting oligomeric complexes with MMUT. Loss-of-function variants cause methylmalonic aciduria, cblA type (MACA), an autosomal recessive disorder of B12/AdoCbl metabolism that is frequently vitamin-B12(hydroxocobalamin)-responsive.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (PAN-GO) inference placing MMAA activity in the cytoplasm. MMAA is synthesized with a mitochondrial transit peptide and is imported into the mitochondrial matrix where it performs its chaperone/GTPase function on MMUT. Cytoplasmic/cytosolic detection is real (precursor pool / partial import), but the core functional compartment is the mitochondrial matrix, not the general cytoplasm. Reason: The gene product has a cleaved mitochondrial transit peptide and colocalizes with MMUT in mitochondria, so "cytoplasm" is not the core functional location; retain as a non-core localization signal rather than the primary site of action. Supporting Evidence: PMID:28943303 for the first time, the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0003924 GTPase activity | IBA GO_REF:0000033 | ACCEPT | Summary: GTPase activity is the central, phylogenetically conserved molecular function of MMAA and its ArgK/MeaB-subfamily orthologs. Directly demonstrated for the human protein and consistent with the conserved P-loop G-domain. Reason: Core molecular function; IBA is concordant with multiple experimental (IDA/IMP) annotations and with structural characterization of the GDP-bound protein. Supporting Evidence: PMID:20876572 We show that MMAA exhibits GTPase activity that is modulated by MUT |
| GO:0003924 GTPase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated (ARBA/InterPro/RHEA) assignment of GTPase activity mapped from the GTPase_ArgK domain and the GTP hydrolysis reaction (RHEA:19669). Correct. Reason: Electronic assignment agrees with the experimentally verified GTPase activity and the UniProt-annotated catalytic reaction GTP + H2O = GDP + phosphate + H(+). Supporting Evidence: PMID:28497574 This function of MMAA depends on its GTPase activity |
| GO:0005525 GTP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping of GTP binding from the GTPase_ArgK domain. MMAA binds GTP/GDP (crystallized in the GDP-bound form; defined GTP-binding loops), so this is correct. Reason: GTP binding is directly demonstrated experimentally and is an intrinsic property of the G-domain; electronic assignment is appropriate. Supporting Evidence: PMID:20876572 Formation of a stable MMAA-MUT complex is nucleotide-selective for MMAA |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic mapping from the UniProt Subcellular Location "Cytoplasm" keyword. MMAA is detected in the cytoplasm but its core functional site is the mitochondrial matrix. Reason: Consistent with the curated UniProt subcellular location; kept as a non-core localization since the protein functions after mitochondrial import. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic mapping from the UniProt Subcellular Location "Mitochondrion" keyword. This is the core functional compartment of MMAA. Reason: Agrees with experimental (IDA/HTP) localization and with the presence of a cleaved mitochondrial transit peptide. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005515 protein binding | IPI PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction with MMUT (UniProtKB:P22033). The biologically meaningful interaction is the GTP-dependent MMAA-MMUT complex through which AdoCbl is delivered; the bare "protein binding" term is uninformative about this function. Reason: Per curation guidance, generic "protein binding" is uninformative; the specific MMAA-MMUT interaction is better captured by the chaperone/complex-formation role and by the identical/homodimerization terms. Retained (not removed) as it records a real, experimentally supported interaction with MMUT. Supporting Evidence: PMID:20876572 the two proteins interact in vitro and in vivo |
| GO:0005515 protein binding | IPI PMID:28497574 Protein destabilization and loss of protein-protein interact... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction with MMUT (UniProtKB:P22033). As above, "protein binding" is uninformative; the functionally relevant interaction is the GTPase-stimulating, AdoCbl-gating MMAA-MMUT complex. Reason: Generic protein-binding term over-annotates a specific, functionally important MMAA-MMUT interaction; kept as a real interaction record rather than removed. Supporting Evidence: PMID:28497574 This function of MMAA depends on its GTPase activity, which is stimulated by an interaction with MUT |
| GO:0042802 identical protein binding | IPI PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | ACCEPT | Summary: Self-interaction (homodimer) supported by IntAct and by the crystal structure, in which MMAA is an alpha2 homodimer. Reason: The homodimeric state is structurally and biochemically verified and is relevant to the protein's oligomerization behavior with MMUT. Supporting Evidence: PMID:20876572 yet they show substantially different dimeric assembly and interaction, compared with their bacterial counterparts |
| GO:0009235 cobalamin metabolic process | TAS Reactome:R-HSA-9759218 | ACCEPT | Summary: Reactome traceable assertion placing MMAA in cobalamin (vitamin B12) metabolism. MMAA is required for intracellular handling of B12, specifically the mitochondrial delivery and maintenance of the AdoCbl cofactor on MMUT. Reason: Core biological process; consistent with experimental (IDA) annotations and with the cblA-type methylmalonic aciduria disease mechanism (reduced mitochondrial AdoCbl pool). Supporting Evidence: PMID:20876572 MMAA (methylmalonic aciduria type A), is implicated in the mitochondrial assembly of AdoCbl into MUT |
| GO:0005829 cytosol | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Immunofluorescence (Human Protein Atlas) detection in the cytosol. MMAA is synthesized as a cytosolic precursor and imported into mitochondria; the functionally relevant pool is mitochondrial. Reason: Real immunofluorescence localization but not the core site of function; the protein acts in the mitochondrial matrix after transit-peptide cleavage. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput mitochondrial proteomics detected MMAA in the mitochondrion, corroborating its curated mitochondrial localization. Reason: Consistent with the core mitochondrial localization established by targeted immunofluorescence and by the cleaved mitochondrial transit peptide. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0140104 molecular carrier activity | TAS Reactome:R-HSA-3159259 | MARK AS OVER ANNOTATED | Summary: Reactome TAS ("MMAA:MUT binds AdoCbl") assigning molecular carrier activity. MMAA does not itself carry cobalamin as cargo; it is the G-protein that gates the transfer of AdoCbl (produced by MMAB) onto MMUT in a GTP-dependent manner. "Molecular carrier activity" mislabels the gating/regulatory GTPase role as direct cofactor transport. Reason: The precise molecular function is GTPase activity; the AdoCbl delivery is a gated protein-protein handoff between MMAB and MUT that MMAA regulates rather than a carrier activity that MMAA itself enables. Retained (not removed) because MMAA is genuinely required for cofactor delivery, but flagged as over-annotated at the MF level. Supporting Evidence: PMID:20876572 our data point to a gatekeeping role for MMAA by favoring complex formation with MUT apoenzyme for AdoCbl assembly and releasing the AdoCbl-loaded holoenzyme from the complex, in a GTP-dependent manner PMID:28497574 all mutations interfered with gating the transfer of AdoCbl from MMAB to MUT |
| GO:0009235 cobalamin metabolic process | IDA PMID:31056463 Allosteric Regulation of Oligomerization by a B(12) Traffick... | ACCEPT | Summary: Direct assay showing MMAA (CblA) allosterically regulates AdoCbl movement to/from MMUT; patient cblA mutations reduce the mitochondrial AdoCbl pool, placing MMAA upstream of or within cobalamin/AdoCbl metabolism. Reason: Experimentally supported role in the mitochondrial B12/AdoCbl trafficking pathway; core biological process for MMAA. Supporting Evidence: PMID:31056463 chaperone CblA is transduced via three "switch" elements that gate the movement |
| GO:0140104 molecular carrier activity | IDA PMID:31056463 Allosteric Regulation of Oligomerization by a B(12) Traffick... | MARK AS OVER ANNOTATED | Summary: IDA (MGI) assigning molecular carrier activity from the allosteric-oligomerization study. As with the Reactome annotation, MMAA gates B12 movement between MMAB and MMUT via its GTPase rather than acting as a direct molecular carrier of the cofactor. Reason: The paper demonstrates GTPase-gated cofactor movement and oligomer regulation, not a carrier/transport activity intrinsic to MMAA; the informative MF is GTPase activity. Kept as a record of the cofactor-movement role but flagged as over-annotated. Supporting Evidence: PMID:31056463 gate the movement of the B12 cofactor to and from MCM |
| GO:0009235 cobalamin metabolic process | IDA PMID:31056463 Allosteric Regulation of Oligomerization by a B(12) Traffick... | ACCEPT | Summary: Direct experimental involvement in cobalamin/AdoCbl metabolism: MMAA (CblA) gates B12 cofactor movement to/from MMUT, and cblA mutations lower the fibroblast AdoCbl pool. Reason: Core biological process supported by direct measurements of AdoCbl pool depletion in cblA patient cells and by the gating mechanism. Supporting Evidence: PMID:31056463 AdoCbl represents 15.1 |
| GO:0003924 GTPase activity | IDA PMID:21138732 Protection and reactivation of human methylmalonyl-CoA mutas... | ACCEPT | Summary: Direct assay of purified recombinant human MMAA showing GTP-hydrolysis-dependent reactivation of methylmalonyl-CoA mutase, demonstrating functional GTPase activity. Reason: Core molecular function, directly demonstrated; MMAA reactivates inactive mutase via GTP hydrolysis. Supporting Evidence: PMID:21138732 the addition of MMAA increases the enzymatic activity through GTP hydrolysis, indicating reactivation of MCM by exchange of the damaged cofactor |
| GO:0003924 GTPase activity | IDA PMID:28943303 Human MMAA induces the release of inactive cofactor and rest... | ACCEPT | Summary: Direct assay showing MMAA removes the damaged (oxidized) cofactor from inactive mutase through GTP hydrolysis, confirming functional GTPase activity. Reason: Core molecular function; directly demonstrated GTP-hydrolysis-coupled cofactor exchange. Supporting Evidence: PMID:28943303 hMMAA is able to remove the damaged cofactor through GTP hydrolysis |
| GO:0005515 protein binding | IPI PMID:21138732 Protection and reactivation of human methylmalonyl-CoA mutas... | MARK AS OVER ANNOTATED | Summary: Interaction with MMUT (UniProtKB:P22033) confirmed by yeast two-hybrid. The functionally relevant interaction is the MMAA-MMUT chaperone complex; "protein binding" is uninformative. Reason: Generic protein-binding term over-annotates the specific, experimentally verified MMAA-MMUT interaction; retained (not removed) as a real interaction. Supporting Evidence: PMID:21138732 Interaction between MCM and MMAA observed in vitro was confirmed in vivo by yeast two-hybrid system |
| GO:0005515 protein binding | IPI PMID:28943303 Human MMAA induces the release of inactive cofactor and rest... | MARK AS OVER ANNOTATED | Summary: Interaction with MMUT (UniProtKB:P22033). As above, the informative description is the MMAA-MMUT complex through which cofactor protection/exchange occurs, not bare "protein binding". Reason: Uninformative generic term; the specific MMAA-MMUT complex is better captured elsewhere. Kept as a real, experimentally supported interaction. Supporting Evidence: PMID:28943303 restores methylmalonyl-CoA mutase activity through their complex formation |
| GO:0005737 cytoplasm | IDA PMID:28943303 Human MMAA induces the release of inactive cofactor and rest... | KEEP AS NON CORE | Summary: Direct localization detected MMAA in the cytoplasm in addition to mitochondria. The protein's core functional pool is mitochondrial (matrix), consistent with its transit peptide and MMUT colocalization. Reason: Real localization but not the core functional compartment; MMAA acts on MMUT after mitochondrial import. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005739 mitochondrion | IDA PMID:28943303 Human MMAA induces the release of inactive cofactor and rest... | ACCEPT | Summary: Direct in vivo demonstration that MMAA localizes to and colocalizes with MMUT in human fibroblast mitochondria. This is the core functional compartment. Reason: Core cellular localization, directly demonstrated and consistent with the cleaved mitochondrial transit peptide. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0003924 GTPase activity | IDA PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | ACCEPT | Summary: Structural and biochemical study directly demonstrating that human MMAA is a GTPase whose activity is modulated by MMUT; crystallized in the GDP-bound form. Reason: Core molecular function, directly demonstrated with structural support (PDB 2WWW). Supporting Evidence: PMID:20876572 We show that MMAA exhibits GTPase activity that is modulated by MUT |
| GO:0003924 GTPase activity | IMP PMID:28497574 Protein destabilization and loss of protein-protein interact... | ACCEPT | Summary: Mutational analysis of 15 purified MACA mutant proteins showed wild-type-like intrinsic GTPase activity for all but the GTP-binding-domain variant, confirming GTPase activity as an intrinsic MMAA function and dissecting the MUT-stimulated (GAP) component. Reason: Core molecular function; IMP evidence corroborates the intrinsic GTPase activity and its MUT-dependent stimulation. Supporting Evidence: PMID:28497574 All 15 purified mutant proteins demonstrated wild-type like intrinsic GTPase activity |
| GO:0005525 GTP binding | IDA PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | ACCEPT | Summary: Direct demonstration of GTP/GDP binding by human MMAA, crystallized in the GDP-bound form with defined GTP-binding loops; complex formation with MMUT is nucleotide-selective. Reason: GTP binding is directly demonstrated and structurally characterized; intrinsic property of the G-domain. Supporting Evidence: PMID:20876572 Formation of a stable MMAA-MUT complex is nucleotide-selective for MMAA |
| GO:0005525 GTP binding | IDA PMID:28497574 Protein destabilization and loss of protein-protein interact... | ACCEPT | Summary: Direct assays of GTP/GDP binding across MACA mutants; only the GTP-binding-domain variant (p.Asp292Val) showed decreased GTP binding, confirming GTP binding as an intrinsic MMAA property. Reason: GTP binding directly demonstrated; consistent with the conserved G-domain. Supporting Evidence: PMID:28497574 only one (p.Asp292Val), where the mutation is in the GTP binding domain, revealed decreased GTP binding |
| GO:0009235 cobalamin metabolic process | TAS PMID:28497574 Protein destabilization and loss of protein-protein interact... | ACCEPT | Summary: MMAA regulates incorporation of the AdoCbl cofactor into MMUT, placing it in cobalamin/AdoCbl metabolism; cblA mutations impair AdoCbl transfer and cause disease. Reason: Core biological process; strongly supported by the mechanistic and disease-variant data. Supporting Evidence: PMID:28497574 MMAA regulates the incorporation of the cofactor adenosylcobalamin (AdoCbl), generated from the MMAB adenosyltransferase, into the destination enzyme methylmalonyl-CoA mutase (MUT) |
| GO:0042802 identical protein binding | IDA PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | ACCEPT | Summary: Direct structural demonstration that MMAA self-associates as a homodimer (alpha2), distinct from its bacterial counterparts. Reason: Homodimerization is structurally verified and relevant to MMAA oligomerization behavior. Supporting Evidence: PMID:20876572 yet they show substantially different dimeric assembly and interaction, compared with their bacterial counterparts |
| GO:0042803 protein homodimerization activity | IDA PMID:20876572 Structures of the human GTPase MMAA and vitamin B12-dependen... | ACCEPT | Summary: MMAA forms a homodimer, directly demonstrated in the crystal structure. This self-association underlies its nucleotide-sensitive oligomerization with MMUT. Reason: Homodimerization activity is directly and structurally supported; a genuine, specific molecular property. Supporting Evidence: PMID:20876572 yet they show substantially different dimeric assembly and interaction, compared with their bacterial counterparts |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-3322135 | ACCEPT | Summary: Reactome TAS placing MMAA in the mitochondrial matrix (the "Defective MMAA does not protect MUT" reaction). This is the specific compartment where MMAA acts on MMUT. Reason: Consistent with the cleaved mitochondrial transit peptide, matrix-localized MMUT, and the demonstrated mitochondrial colocalization; the most precise functional location. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-3159259 | ACCEPT | Summary: Reactome TAS placing MMAA in the mitochondrial matrix (the "MMAA:MUT binds AdoCbl" reaction). Correct core functional location. Reason: Matrix localization is consistent with mitochondrial import and MMUT colocalization. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-3322971 | ACCEPT | Summary: Reactome TAS placing MMAA in the mitochondrial matrix. Consistent with its functional compartment. Reason: Matrix localization is well supported; core functional location. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-71010 | ACCEPT | Summary: Reactome TAS placing MMAA in the mitochondrial matrix (associated with the MUT isomerization reaction module). Consistent with the functional compartment. Reason: Matrix localization is well supported by import signal and MMUT colocalization. Supporting Evidence: PMID:28943303 the in vivo localization of hMMAA and its colocalization with hMCM in human fibroblasts mitochondria were demonstrated |
| GO:0043085 positive regulation of catalytic activity | IDA PMID:21138732 Protection and reactivation of human methylmalonyl-CoA mutas... | NEW | Summary: MMAA reactivates inactive methylmalonyl-CoA mutase and protects it from inactivation, thereby positively regulating the catalytic activity of MMUT. This chaperone/reactivase role is well documented but is not captured by the existing GOA terms. Reason: Proposed new annotation capturing MMAA's protectase/reactivase function toward MMUT, demonstrated in vitro with purified proteins. Supports the corresponding core function. Supporting Evidence: PMID:21138732 the addition of MMAA increases the enzymatic activity through GTP hydrolysis, indicating reactivation of MCM by exchange of the damaged cofactor PMID:28943303 restores methylmalonyl-CoA mutase activity through their complex formation |
| GO:0050821 protein stabilization | IDA PMID:28943303 Human MMAA induces the release of inactive cofactor and rest... | NEW | Summary: By forming a complex with MMUT, MMAA slows formation of the oxidized inactive cofactor (OH2Cbl) and thereby protects/maintains functional MMUT, i.e. stabilizes the active holoenzyme. Documented mechanistically but not present in the existing GOA set. Reason: Proposed new annotation capturing MMAA's protective (protectase) role toward MMUT; supports the corresponding core function. Supporting Evidence: PMID:28943303 the complex formation of hMCM/hMMAA decreases the rate of oxidized cofactor formation, protecting the hMCM enzyme PMID:21138732 it prevents inactivation by guarding MCM |
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Download this section (compressed HTML)Q: Is MMAA best described at the molecular-function level by a dedicated cofactor-loading / GTPase-dependent chaperone term rather than the generic "molecular carrier activity", given that it gates a protein-protein cofactor handoff rather than transporting cobalamin itself?
Q: What is the physiological significance of the cytoplasmic/cytosolic MMAA pool detected by immunofluorescence and proteomics, versus the functionally characterized mitochondrial matrix pool?
Experiment: Reconstitute the full human mitochondrial B12 pathway (MMAB/ATR, MMAA/CblA, MMUT) in vitro and use single-turnover/stopped-flow spectroscopy to quantify how GTP binding vs hydrolysis by MMAA controls the directionality of AdoCbl loading and cob(II)alamin offloading.
Experiment: Use cryo-EM of the nucleotide-sensitive MMAA-MMUT oligomers to define the loading-ready state and map how cblA switch-region patient variants shift the oligomeric equilibrium.
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