ppgn-1 (Y38F2AR.7 / WBGene00021425 / UniProt G5EDB6) — research notes

Research journal for the AI GO-annotation review. Provenance is recorded inline as
[PMID:xxxx "verbatim quote"]. This is a dark gene: the deliverable is a precise,
defensible statement of what is and is not known.

1. Identity (verified from the UniProt record)

• UniProt G5EDB6 (TrEMBL, unreviewed), Caenorhabditis elegans, NCBITaxon:6239.
• Gene symbol ppgn-1; ORF Y38F2AR.7; WormBase WBGene00021425; RefSeq NP_500191.3.
• Protein name (UniProt SubName): Paraplegin. 747 aa, 82.8 kDa.
• The only primary reference on the UniProt entry is an unpublished nucleotide-sequence
  submission: Yamanaka, Yamada-Inagawa, Nishizono, Ogura (2006), title "Paraplegin
  homologue in C. elegans" (EMBL AB257343 / BAE96353.1). No functional paper.
• Domain architecture (UniProt/InterPro): N-terminal AAA+ ATPase module
  (AAA+ core IPR003959; AAA_lid_3 IPR041569; SMART AAA domain 317–462) and C-terminal
  peptidase M41 zinc-metallopeptidase module (IPR000642). Two predicted TM helices
  (Phobius: 111–133 and 225–245). N-terminal disordered/basic region 41–101 (matrix
  targeting/presequence-like). Cofactor Zn²⁺ (ARBA). SubCellular: Mitochondrion,
  multi-pass membrane protein.
• PANTHER classification: PTHR43655 "ATP-DEPENDENT PROTEASE", subfamily
  PTHR43655:SF8 PARAPLEGIN. MEROPS peptidase clan/family M41.A12.
• Reactome (electronic, projected): R-CEL-8949664 "Processing of SMDT1";
  R-CEL-9837999 "Mitochondrial protein degradation".

Bottom line on identity: despite the surrounding project framing, ppgn-1 is NOT an
epidermal/innate-immune/wound gene. It is the C. elegans paraplegin-subfamily
subunit of the mitochondrial m-AAA protease — an ATP-dependent, membrane-embedded
zinc metalloprotease of the inner mitochondrial membrane. All curation below is against
that identity.

2. What the m-AAA protease family does (KNOWN — orthologue/family biology)

The family is well characterised in yeast, mouse and human; ppgn-1's GO annotations are
IBA/IEA propagations from these orthologues.

• m-AAA proteases are a conserved, membrane-bound, ATP-dependent proteolytic machine of
  the mitochondrial inner membrane that performs protein quality control:
  PMID:16239145.
• Human paraplegin (SPG7) is a bona fide subunit of this complex, and the complex is
  ATP-dependent: PMID:16647881
  and PMID:16647881.
• The founding paper established paraplegin as a nuclear-encoded mitochondrial
  metalloprotease homologous to the yeast AAA ATPases with dual proteolytic + chaperone
  activity: PMID:9635427
  and localises to mitochondria: PMID:9635427.
• Besides bulk degradation, the m-AAA protease has a regulated processing role: it
  matures specific substrates. The conserved paradigm is maturation of the ribosomal
  protein MrpL32: PMID:16239145.
  This dual degrade-vs-process behaviour is explicit in the literature:
  PMID:21610694.
• Loss of function is pleiotropic across organisms (respiration, mito morphology, axon
  degeneration): PMID:16239145.
• In mammals the physiological complex is a hetero-/homo-hexamer of paraplegin (SPG7)
  and/or AFG3L2 (WITH/FROM of the ppgn-1 IBA annotations lists human SPG7 = UniProtKB
  Q9UQ90 and AFG3L2 = UniProtKB Q9Y4W6, plus mouse Afg3l1/Afg3l2 and yeast Yta10/Yta12).

3. Bioinformatic verification for THIS protein (KNOWN — from ppgn-1 sequence)

To guard against propagating enzyme annotations onto a possible pseudoenzyme, I scanned
the ppgn-1 sequence itself (see ppgn-1-bioinformatics/, reproducible
analyze_motifs.py). All three catalytic motifs are intact and correctly ordered:

• Walker A / P-loop GPPGCGKT at 325 (ATP binding).
• Walker B IIYIDE at 380 (Mg-ATP hydrolysis).
• HExxH zinc-binding catalytic motif HEAGH at 555, within the M41 domain,
  followed downstream by an acidic residue (…MLEHTD) — a complete FtsH-type Zn²⁺
  active site.

Interpretation: ppgn-1 is a catalytically competent m-AAA protease at the sequence
level, not a degenerate pseudoenzyme. This supports the enzymatic GO terms for this
specific protein, not merely by family membership. See
file:worm/ppgn-1/ppgn-1-bioinformatics/RESULTS.md.

4. C. elegans context — and the paralog trap (NOT-known for ppgn-1)

• C. elegans has (at least) two m-AAA protease subunit genes: the well-studied
  spg-7 (WBGene00004978; the AFG3L2-type subunit, and the gene whose RNAi is the
  canonical trigger of the mitochondrial unfolded-protein response, UPR^mt, and of
  detoxification/immune gene induction — WormBase/literature summaries) and the
  understudied ppgn-1 (paraplegin subfamily, this gene).
• Crucial curation caveat: the large C. elegans mitochondrial-protease functional
  literature (UPR^mt, drug resistance, detoxification, pathogen resistance) is about
  spg-7, NOT ppgn-1. None of it can be attributed to ppgn-1. The GOA annotations for
  ppgn-1 are correctly all IBA/IEA (no experimental evidence code), reflecting this.
• WITH/FROM of the ppgn-1 IBA annotations includes WB:WBGene00004978 (spg-7) as a
  co-orthologue in the PANTHER tree, i.e. the two worm genes are placed together in the
  m-AAA clade.

5. Explicit KNOWN vs NOT-known ledger for ppgn-1

KNOWN (high confidence, from orthology + domain/motif evidence):
- Encodes a paraplegin-subfamily m-AAA protease subunit (AAA+ ATPase + M41 Zn-peptidase).
- Retains intact ATP-binding (Walker A), ATP-hydrolysis (Walker B) and Zn-peptidase
(HExxH) catalytic motifs → catalytically competent in principle.
- Predicted mitochondrial, integral inner-membrane (two TM helices), matrix-facing
catalytic domains — the canonical m-AAA topology.
- Family function: ATP-dependent proteolytic quality control + regulated processing of
inner-membrane / matrix substrates.

NOT KNOWN (the real gaps for ppgn-1 specifically):
- No C. elegans experimental characterisation of ppgn-1: no mutant/RNAi phenotype,
no localisation, no biochemistry has been reported for this gene (all GO evidence is
IBA/IEA; the only UniProt reference is an unpublished sequence submission).
- Substrates unknown: which inner-membrane proteins ppgn-1 degrades or matures in the
worm is undetermined (no worm equivalent of the MrpL32 result attributed to ppgn-1).
- Complex composition unknown: whether ppgn-1 assembles with the AFG3L2-type subunit
spg-7 into a hetero-hexamer, forms a homo-oligomer, or is catalytically redundant with
spg-7 is untested.
- Physiological role unknown: whether ppgn-1 (as opposed to spg-7) contributes to
UPR^mt, respiration, mitochondrial morphology, fertility, longevity, stress/pathogen
responses, or is functionally dispensable, is unknown.
- Regulation unknown: expression, tissue distribution (Bgee: "Expressed in germ line
and 4 other cell types" — coarse, electronic), and conditions under which it acts are
uncharacterised.

6. Annotation-review plan (summary)

All 10 GOA annotations are IBA/IEA and are consistent with a catalytically competent
paraplegin-subfamily m-AAA protease; none is contradicted, so none is REMOVEd.
- Core, well-supported family propagations → ACCEPT: mitochondrial protein processing
(IBA), m-AAA complex (IBA), metalloendopeptidase activity (IBA + IEA), ATP-dependent
peptidase activity (IEA), ATP binding (IEA), ATP hydrolysis activity (IEA).
- Correct but non-specific / non-core → KEEP_AS_NON_CORE with a note: proteolysis
(GO:0006508, generic parent of the specific processing/QC role), mitochondrion
(GO:0005739, generic parent of inner membrane), membrane (GO:0016020, generic parent
of inner membrane).
- The persistent uncertainty (no worm data) is captured in knowledge_gaps, not by
down-grading correct family propagations.

7. Deep research status

just deep-research-falcon worm ppgn-1 --fallback perplexity-lite was run FOREGROUND and
retried. Falcon timed out at the 600 s server limit on both attempts; the perplexity-lite
fallback returned HTTP 401 (account quota exhausted). No -deep-research-*.md file was
produced. Per repo policy I did not fabricate one. This review therefore rests on the
UniProt/InterPro record, the GOA evidence codes, the primary m-AAA/paraplegin literature
cited above (PubMed-verified, cached in publications/), and the reproducible sequence
analysis in ppgn-1-bioinformatics/.