ppgn-1 (Y38F2AR.7) encodes the Caenorhabditis elegans paraplegin-subfamily subunit of the mitochondrial m-AAA protease, an ATP-dependent proteolytic machine embedded in the inner mitochondrial membrane. The protein has the diagnostic two-module architecture of this family: an N-terminal AAA+ ATPase module that uses ATP binding and hydrolysis to unfold and translocate substrates, and a C-terminal M41 (FtsH-type) zinc-metallopeptidase module that cleaves peptide bonds; the catalytic residues of both modules (Walker A P-loop GPPGCGKT, Walker B IIYIDE, and the HExxH zinc-binding motif HEAGH) are intact in the ppgn-1 sequence. It is predicted to be an integral, multi-pass inner-membrane protein with matrix-facing catalytic domains and an N-terminal mitochondrial targeting region. Within the conserved m-AAA complex, family members carry out two coupled activities: quality-control degradation of misfolded or unassembled inner-membrane proteins, and regulated proteolytic maturation of specific substrates (the paradigm being processing of the ribosomal protein MrpL32 to control mitochondrial ribosome assembly and translation). The orthologous human subunit is paraplegin (SPG7), whose loss causes autosomal-recessive hereditary spastic paraplegia. The specific in vivo role, substrates, and complex partners of ppgn-1 in C. elegans have not been experimentally determined; C. elegans additionally carries the well-studied AFG3L2-type m-AAA subunit spg-7.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0034982 mitochondrial protein processing | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) propagation of the conserved m-AAA processing role from paraplegin/AFG3L2 orthologs. Consistent with ppgn-1's intact M41 protease domain and the family paradigm of regulated substrate maturation (e.g. MrpL32). Retained as a core function, with the caveat that no C. elegans substrate has been shown. Reason: ppgn-1 is a clear paraplegin-subfamily m-AAA subunit (PANTHER PTHR43655:SF8) with a complete M41 zinc-peptidase module (HExxH motif HEAGH intact at residue 555), so the conserved processing activity is appropriately propagated. The direct substrate in the worm is unknown (see knowledge_gaps). Supporting Evidence: PMID:16239145 The mitochondrial ribosomal protein MrpL32 is processed by the m-AAA protease, allowing its association with preassembled ribosomal particles and completion of ribosome assembly in close proximity to the inner membrane. |
| GO:0005745 m-AAA complex | IBA GO_REF:0000033 | ACCEPT | Summary: IBA propagation that ppgn-1 is a subunit of the inner-membrane m-AAA protease complex. Well supported by orthology to paraplegin (a conserved m-AAA subunit) and by the shared two-module (AAA+ + M41) architecture. Core. Reason: Paraplegin is a conserved subunit of the m-AAA complex; ppgn-1 is its C. elegans ortholog. The specific composition of the worm complex (homo-oligomer vs hetero-oligomer with the AFG3L2-type subunit spg-7) is not experimentally established. Supporting Evidence: PMID:16647881 paraplegin, which is a conserved subunit of the ubiquitous and ATP-dependent m-AAA protease in mitochondria. |
| GO:0004222 metalloendopeptidase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA propagation of zinc-dependent endopeptidase activity from the family. Directly corroborated for the ppgn-1 sequence by the intact M41 HExxH zinc-binding catalytic motif (HEAGH at residue 555, within the Peptidase_M41 domain). Core molecular function. Reason: The C-terminal M41 peptidase domain (InterPro IPR000642) and the intact HExxH motif support metalloendopeptidase chemistry for this specific protein, not merely by family membership. Supporting Evidence: PMID:9635427 Paraplegin is highly homologous to the yeast mitochondrial ATPases, AFG3, RCA1, and YME1, which have both proteolytic and chaperon-like activities at the inner mitochondrial membrane. |
| GO:0004176 ATP-dependent peptidase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (IEA) assignment of ATP-dependent peptidase activity. This is the most complete single molecular-function term for an m-AAA protease because it captures the coupling of the AAA+ ATPase to the M41 peptidase. Supported by the intact Walker A/B (GPPGCGKT, IIYIDE) and HExxH motifs in the ppgn-1 sequence. Core. Reason: ppgn-1 has both an intact AAA+ ATPase module (Walker A GPPGCGKT at 325, Walker B IIYIDE at 380) and an intact M41 peptidase module (HExxH at 555), the defining combination for ATP-dependent proteolysis. Supporting Evidence: PMID:16239145 AAA proteases comprise a conserved family of membrane bound ATP-dependent proteases that ensures the quality control of mitochondrial inner-membrane proteins. |
| GO:0004222 metalloendopeptidase activity | IEA GO_REF:0000002 | ACCEPT | Summary: Second annotation of metalloendopeptidase activity, here from the InterPro M41 signature (IEA). Same well-supported molecular function as the IBA annotation above; kept as a corroborating electronic assignment. Reason: Redundant with the IBA metalloendopeptidase annotation but independently supported by the InterPro Peptidase_M41 signature (IPR000642) and the intact HExxH motif. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (IEA) assignment of ATP binding, an activity of the AAA+ module. Corroborated by the intact Walker A P-loop (GPPGCGKT at residue 325) in the ppgn-1 sequence. A component activity of the ATP-dependent protease. Reason: The N-terminal AAA+ ATPase domain (SMART SM00382; InterPro IPR003959) with an intact Walker A motif provides a direct structural basis for ATP binding. |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Subcellular-location mapping (IEA) to mitochondrion. Correct but non-specific: the family resides in the inner mitochondrial membrane, captured more precisely in core_functions (GO:0005743). Retained but marked non-core because it is a generic parent of the informative location. Reason: Consistent with the orthologous localization (paraplegin localizes to mitochondria) and with two predicted TM helices; the more informative term is mitochondrial inner membrane, so this generic term is not the core localization statement. Supporting Evidence: PMID:9635427 Immunofluorescence analysis and import experiments showed that Paraplegin localizes to mitochondria. |
| GO:0006508 proteolysis | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro (IEA) generic proteolysis term. Correct but uninformative: the specific biological roles (protein quality-control degradation and mitochondrial protein processing) are captured in core_functions. Retained as a non-core parent term. Reason: GO:0006508 is a high-level parent of the specific m-AAA processes; it is not wrong, but the informative annotations are mitochondrial protein processing (GO:0034982) and protein quality control (GO:0006515). |
| GO:0016020 membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Subcellular-location mapping (IEA) to the generic term membrane. The protein is a multi-pass integral membrane protein, but the informative location is the mitochondrial inner membrane (GO:0005743), used in core_functions. Non-core parent. Reason: Correct (two predicted TM helices at 111-133 and 225-245; UniProt multi-pass membrane protein) but far too generic to be a core localization statement. |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (IEA) assignment of ATP hydrolysis activity, the motor activity of the AAA+ module that drives substrate unfolding/translocation. Corroborated by the intact Walker B motif (IIYIDE at residue 380) in the ppgn-1 sequence. A component activity of the ATP-dependent protease. Reason: The AAA+ ATPase module with intact Walker A and Walker B motifs supports ATP hydrolysis; this is the energy-providing half of the ATP-dependent protease. |
| GO:0005743 mitochondrial inner membrane | ISS | NEW | Summary: Proposed refinement of the generic mitochondrion/membrane localizations to the specific compartment where m-AAA proteases reside. Inferred from orthology to paraplegin and from the two predicted TM helices flanking matrix-facing catalytic domains (canonical m-AAA topology). Reason: The generic GOA terms (GO:0005739 mitochondrion, GO:0016020 membrane) understate the known compartment. The m-AAA protease is an integral protein of the mitochondrial inner membrane; this ISS annotation captures the informative location while the worm-specific topology remains to be experimentally mapped. Supporting Evidence: PMID:16647881 The m-AAA protease carries out protein quality control in the inner membrane of the mitochondria |
| GO:0006515 protein quality control for misfolded or incompletely synthesized proteins | ISS | NEW | Summary: Proposed refinement of the generic proteolysis annotation to the specific quality-control process performed by the m-AAA protease: ATP-dependent degradation of misfolded/unassembled inner-membrane proteins. Inferred from orthology and the intact AAA+ + M41 catalytic modules. Reason: GO:0006508 (proteolysis) is uninformative; the conserved biological role of the family is inner-membrane protein quality control. This ISS annotation records that specific process, complementing the mitochondrial protein processing (GO:0034982) role. The endogenous worm substrates are not yet identified. Supporting Evidence: PMID:16239145 AAA proteases comprise a conserved family of membrane bound ATP-dependent proteases that ensures the quality control of mitochondrial inner-membrane proteins. |
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Download this section (compressed HTML)Q: Does ppgn-1 form a functional m-AAA protease complex on its own, or does it require the AFG3L2-type subunit spg-7, and are the two subunits functionally redundant in C. elegans?
Suggested experts: C. elegans mitochondrial biologists, m-AAA protease / AAA+ protease structural biochemists
Q: What are the endogenous substrates of ppgn-1 in C. elegans, and does it mature a MrpL32 homolog to regulate mitochondrial ribosome assembly as in yeast and mouse?
Suggested experts: Mitochondrial proteostasis / ribosome assembly researchers
Experiment: Generate a ppgn-1 null allele (CRISPR) and, in parallel with spg-7 RNAi/mutant, assay respiration, mitochondrial morphology, UPRmt (hsp-6p::GFP), fertility and lifespan to determine ppgn-1's non-redundant contribution.
Type: Reverse genetics / phenotyping
Experiment: Endogenously tag ppgn-1, purify from worm mitochondria, and identify complex partners (test for spg-7 hetero-oligomerization) and, using a catalytically dead HExxH mutant as a substrate trap, capture processed/degraded substrates.
Type: Affinity purification / mass spectrometry
Experiment: Determine ppgn-1 tissue expression and sub-mitochondrial topology with an endogenous fluorescent tag and protease-protection assays, to confirm the predicted inner-membrane, matrix-facing catalytic-domain topology.
Type: Localization
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: ppgn-1 has no experimental functional characterization in C. elegans: no loss-of-function (mutant or RNAi) phenotype, no localization, and no biochemical assay has been reported for this gene. Every GO annotation is phylogenetic (IBA) or electronic (IEA), and the only UniProt-linked reference is an unpublished nucleotide sequence submission.
OPEN BIOLOGY WHOLLY_DARK
What is known: The protein is confidently a paraplegin-subfamily m-AAA protease subunit (PANTHER PTHR43655:SF8; AAA+ + M41 architecture) with intact catalytic motifs, and the human ortholog paraplegin (SPG7) and yeast/mouse orthologs are well characterized. What is missing is any direct evidence for ppgn-1 itself in the worm.
Significance: Without any worm-level data, ppgn-1's function is entirely an orthology inference; its non-redundant contribution (if any) to C. elegans mitochondrial physiology is unknown.
What would resolve it: Generate and phenotype a ppgn-1 deletion/RNAi allele (respiration, mitochondrial morphology, UPRmt reporters, fertility, lifespan), and determine its expression and subcellular localization with an endogenous tag.
Provenance (the field's own admissions):
Gap: The endogenous substrate(s) of ppgn-1 in C. elegans are unidentified. It is unknown whether the worm protease matures a functional homolog of MrpL32 (regulated processing) and/or which misfolded inner-membrane proteins it degrades (quality control), and whether ppgn-1 and the AFG3L2-type subunit spg-7 act on the same or distinct substrate sets.
OPEN BIOLOGY MF_DARK
What is known: The conserved m-AAA protease has a defined dual activity (processing vs degradation), and MrpL32 maturation is the textbook processing substrate in yeast and mouse. ppgn-1 retains an intact HExxH metallopeptidase active site, so it is catalytically capable in principle.
Significance: Substrate identity is what distinguishes a housekeeping protease from a specific regulatory node; it is the rate-limiting unknown for placing ppgn-1 in a pathway.
What would resolve it: Substrate-trapping (catalytically inactive HExxH mutant) plus comparative proteomics of ppgn-1 vs spg-7 loss-of-function to identify processed/stabilized substrates.
Provenance (the field's own admissions):
Gap: The oligomeric composition of the C. elegans m-AAA complex containing ppgn-1 is undetermined: whether ppgn-1 forms a homo-oligomer, assembles into a hetero-oligomer with the AFG3L2-type subunit spg-7, and whether the two worm subunits are functionally redundant, is untested.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: In mammals the m-AAA protease is a hexamer of paraplegin (SPG7) and/or AFG3L2, and one catalytically dead subunit can be tolerated within an otherwise active complex. C. elegans has both a paraplegin-type (ppgn-1) and an AFG3L2-type (spg-7) subunit, but their assembly relationship is not reported.
Significance: Complex composition determines whether ppgn-1 loss can be buffered by spg-7 and how to interpret single-gene phenotypes.
What would resolve it: Co-immunoprecipitation / blue-native PAGE of endogenously tagged ppgn-1 and spg-7, and epistasis/double-mutant analysis to test redundancy.
Provenance (the field's own admissions):
Gap: Whether ppgn-1 (as opposed to its paralog spg-7) contributes to any organismal process in C. elegans - the mitochondrial unfolded-protein response, respiration, mitochondrial dynamics, fertility, longevity, or stress/pathogen resistance - is unknown. The large C. elegans mito-protease functional literature concerns spg-7 and cannot be attributed to ppgn-1.
OPEN BIOLOGY BP_DARK
What is known: Loss of m-AAA proteases across organisms causes respiratory deficiency, altered mitochondrial morphology and axonal degeneration; none of these readouts has been tested specifically for ppgn-1 in the worm.
Significance: Assigning ppgn-1 a biological process (vs treating it as a redundant paralog) requires worm phenotypes; this determines its relevance to mitochondrial surveillance and stress signaling.
What would resolve it: Test ppgn-1 loss-of-function against UPRmt (hsp-6p::GFP), respiration, mitochondrial morphology, and pathogen/stress-resistance assays, side by side with spg-7.
Provenance (the field's own admissions):
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