--- provider: openscientist model: openscientist-autonomous cached: false start_time: '2026-07-11T07:00:47.947744' end_time: '2026-07-11T07:14:01.668719' duration_seconds: 793.72 template_file: templates/gene_hypothesis_deep_research.md template_variables: organism: SCHPO gene: gaa1 gene_symbol: gaa1 uniprot_accession: Q9US48 taxon_id: NCBITaxon:284812 taxon_label: Schizosaccharomyces pombe 972h- focus_type: computational_prediction hypothesis_slug: prediction-cysteine-endopeptidase hypothesis_text: GO-GPT (via BioReason-Pro) predicts cysteine-type endopeptidase activity (GO:0004197) for S. pombe gaa1 (Q9US48), a subunit of the GPI transamidase (GPI-T) complex that attaches glycosylphosphatidylinositol anchors to the C-termini of GPI-anchored proteins. Independently assess whether gaa1 itself is the catalytic cysteine protease of the GPI-transamidase complex, or whether the catalytic transamidation/cysteine-protease activity resides in the Gpi8 (PIG-K) subunit while Gaa1 is a non-catalytic subunit involved in recognition/binding of the GPI-anchor attachment signal and stabilization of the complex -- which would make the cysteine-type endopeptidase prediction a subunit-level misassignment analogous to assigning a catalytic function to the wrong subunit of a complex. Determine whether the prediction is supported or refuted. term_context: '- Term: cysteine-type endopeptidase activity (GO:0004197)' reference_context: '- doi:10.64898/2026.03.19.712954' source_file: '' source_selector: '' source_context_yaml: "hypothesis: GO-GPT (via BioReason-Pro) predicts cysteine-type\ \ endopeptidase activity (GO:0004197) for\n S. pombe gaa1 (Q9US48), a subunit\ \ of the GPI transamidase (GPI-T) complex that attaches glycosylphosphatidylinositol\n\ \ anchors to the C-termini of GPI-anchored proteins. Independently assess whether\ \ gaa1 itself is the catalytic\n cysteine protease of the GPI-transamidase complex,\ \ or whether the catalytic transamidation/cysteine-protease\n activity resides\ \ in the Gpi8 (PIG-K) subunit while Gaa1 is a non-catalytic subunit involved in\ \ recognition/binding\n of the GPI-anchor attachment signal and stabilization\ \ of the complex -- which would make the cysteine-type\n endopeptidase prediction\ \ a subunit-level misassignment analogous to assigning a catalytic function to\n\ \ the wrong subunit of a complex. Determine whether the prediction is supported\ \ or refuted.\nfocus_type: computational_prediction\nterm_id: GO:0004197\nterm_label:\ \ cysteine-type endopeptidase activity\ncontext: []\nreference_id:\n- doi:10.64898/2026.03.19.712954" provider_config: timeout: 3600 max_retries: 3 parameters: allowed_domains: [] max_iterations: 3 use_hypotheses: false investigation_mode: autonomous poll_interval: 30 timeout: 7200 save_artifacts: true artifact_max_bytes: 5242880 citation_count: 7 artifact_count: 2 artifact_sources: openscientist_artifacts_zip: 2 artifacts: - filename: final_report.html path: openscientist_artifacts/final_report.html media_type: text/html source: openscientist_artifacts_zip data_storage_id: null description: OpenScientist final report - filename: final_report.pdf path: openscientist_artifacts/final_report.pdf media_type: application/pdf source: openscientist_artifacts_zip data_storage_id: null description: OpenScientist final report --- ## Question # AIGR Gene Hypothesis Deep Research You are evaluating one focused gene curation hypothesis for AI Gene Review. This is not a general gene overview. Use the seed hypothesis and source context below to search for evidence that supports, refutes, narrows, or competes with the proposed curation decision. ## Target Gene - **Organism code:** SCHPO - **Taxon:** Schizosaccharomyces pombe 972h- (NCBITaxon:284812) - **Gene directory:** gaa1 - **Gene symbol:** gaa1 - **UniProt accession:** Q9US48 ## Focus - **Focus type:** computational_prediction - **Hypothesis slug:** prediction-cysteine-endopeptidase - **Source file:** - **Source selector:** ## Seed Hypothesis GO-GPT (via BioReason-Pro) predicts cysteine-type endopeptidase activity (GO:0004197) for S. pombe gaa1 (Q9US48), a subunit of the GPI transamidase (GPI-T) complex that attaches glycosylphosphatidylinositol anchors to the C-termini of GPI-anchored proteins. Independently assess whether gaa1 itself is the catalytic cysteine protease of the GPI-transamidase complex, or whether the catalytic transamidation/cysteine-protease activity resides in the Gpi8 (PIG-K) subunit while Gaa1 is a non-catalytic subunit involved in recognition/binding of the GPI-anchor attachment signal and stabilization of the complex -- which would make the cysteine-type endopeptidase prediction a subunit-level misassignment analogous to assigning a catalytic function to the wrong subunit of a complex. Determine whether the prediction is supported or refuted. ## Term and Decision Context - Term: cysteine-type endopeptidase activity (GO:0004197) ## Reference Context - doi:10.64898/2026.03.19.712954 ## Source Context YAML ```yaml hypothesis: GO-GPT (via BioReason-Pro) predicts cysteine-type endopeptidase activity (GO:0004197) for S. pombe gaa1 (Q9US48), a subunit of the GPI transamidase (GPI-T) complex that attaches glycosylphosphatidylinositol anchors to the C-termini of GPI-anchored proteins. Independently assess whether gaa1 itself is the catalytic cysteine protease of the GPI-transamidase complex, or whether the catalytic transamidation/cysteine-protease activity resides in the Gpi8 (PIG-K) subunit while Gaa1 is a non-catalytic subunit involved in recognition/binding of the GPI-anchor attachment signal and stabilization of the complex -- which would make the cysteine-type endopeptidase prediction a subunit-level misassignment analogous to assigning a catalytic function to the wrong subunit of a complex. Determine whether the prediction is supported or refuted. focus_type: computational_prediction term_id: GO:0004197 term_label: cysteine-type endopeptidase activity context: [] reference_id: - doi:10.64898/2026.03.19.712954 ``` ## Research Objective Build a focused report that helps a curator decide whether this hypothesis should affect the gene review. Address the focus type directly: 1. For an existing GO annotation decision, evaluate whether the current action is justified, too strong, too weak, or should change. 2. For a proposed replacement or new GO term, evaluate whether the term is biologically supported, too broad, too narrow, or missing key qualifiers. 3. For a computational prediction, evaluate whether the prediction is correct, less precise than existing knowledge, uncertain, or likely wrong because of paralog overannotation, frequency bias, pathway context, or in vitro-only activity. 4. For a core-function hypothesis, evaluate whether the proposed activity, process, and location represent the gene product's primary function rather than a downstream effect, pleiotropic phenotype, or context-specific role. 5. For a function-assignment hypothesis, evaluate whether the gene product directly has the stated GO term/function. Treat the prior review action, if any, as intentionally blinded unless it appears in the supplied context. Use primary literature whenever possible. Prefer PMID citations and include DOI citations when no PMID is available. Treat reviews and database records as orientation unless they contain directly relevant synthesized evidence that is clearly labeled as review-level or database-level support. Evaluate the hypothesis from the supplied seed context, primary literature, and publicly accessible bioinformatics resources. Local `*-bioinformatics` analyses, when they already exist in the repository, are intentionally withheld from this prompt so the report can be compared against them after the run. Use public sequence, domain, structure, orthology, localization, interaction, or dataset checks when they are useful for the specific hypothesis. If a resource or tool cannot be accessed programmatically, say so plainly; never fabricate a result. Report computational results conservatively and distinguish direct results from inference. ## Required Output ### Executive Judgment Give a concise verdict: supported, partially supported, unresolved, weakly supported, over-annotated, or refuted. Explain the reasoning and the most important caveats. ### Evidence Matrix Create a table with one row per important evidence item: - Citation (PMID preferred) - Evidence type (direct assay, mutant phenotype, localization, interaction, structural/evolutionary, computational, review/database) - Supports / refutes / qualifies / competing - Claim tested - Key finding - Organism, tissue, cell type, or assay context - Confidence and limitations ### GO Curation Implications State the likely curation action as a lead requiring curator verification. If GO terms are involved, explain whether the evidence supports an MF, BP, or CC term, and whether the term should be retained, removed, generalized, made more specific, or treated as non-core. Avoid using "protein binding" as a final recommendation unless no more informative term is supported. ### Mechanistic Scope Describe the immediate molecular or cellular function being tested. Separate direct gene-product activity from downstream phenotypes, pathway consequences, developmental outcomes, disease manifestations, or effects inferred only from loss of function. ### Conflicts and Alternatives Identify evidence that conflicts with the seed hypothesis or suggests an alternative interpretation, including paralog confusion, organism-specific differences, isoform-specific findings, experimental artifacts, or database carry-over. ### Knowledge Gaps List explicit uncertainties that matter for curation. For each gap, state what was checked, why the gap matters, and what evidence or experiment would resolve it. ### Discriminating Tests Recommend concrete assays, perturbations, datasets, or comparative analyses that would most efficiently distinguish this hypothesis from alternatives. ### Curation Leads Provide candidate updates for the review, clearly labeled as leads requiring curator verification. Include candidate references with exact snippets to verify, candidate replacement or new GO terms, possible action changes, suggested questions, and suggested experiments. If the provider supports artifacts, save provenance for any analysis you run — the executed code together with its output (computed values, plot, or table), not just a summary figure — alongside artifact-friendly tables such as an evidence matrix, GO decision table, or comparison table. Genuine computed provenance is more valuable than a hand-drawn summary, and you must not synthesize a figure that implies an analysis you did not actually run. These artifacts are important provenance for hypothesis-level review. ## Output # AIGR Gene Hypothesis Deep Research — Final Report ## Target: *S. pombe* gaa1 (UniProt Q9US48) · GO:0004197 (cysteine-type endopeptidase activity) **Focus type:** computational_prediction · **Hypothesis slug:** prediction-cysteine-endopeptidase **Prediction source:** GO-GPT (via BioReason-Pro) · **Reference context:** doi:10.64898/2026.03.19.712954 --- ## Summary The GO-GPT/BioReason-Pro pipeline predicts **cysteine-type endopeptidase activity (GO:0004197)** for *Schizosaccharomyces pombe* **gaa1** (Q9US48), a subunit of the GPI transamidase (GPI-T) complex that attaches glycosylphosphatidylinositol (GPI) anchors to the C-termini of substrate proteins. This report evaluates, independently, whether gaa1 is itself the catalytic cysteine protease of the complex, or whether the catalytic transamidation/cysteine-protease chemistry resides in a different subunit — **Gpi8 (mammalian PIG-K)** — with gaa1 acting as a non-catalytic recognition/scaffolding component. **The prediction is REFUTED.** Every tier of evidence — classical yeast genetics and biochemistry, active-site mutagenesis, human and fungal cryo-EM structures, and direct protein-domain/annotation queries — places the catalytic cysteine-protease/transamidase activity in **Gpi8/PIG-K**, a caspase/legumain-related Peptidase C13 enzyme with a defined Cys–His(–Asn) catalytic triad. gaa1 carries **no** peptidase domain, **no** annotated catalytic residues, and **no** EC number; its role is to recognize/bind the GPI anchor and stabilize the pentameric complex. The prediction is therefore a textbook **subunit-level misassignment**: it attributes the catalytic chemistry of one subunit (Gpi8) to a different, non-catalytic subunit (gaa1). Two practical points sharpen the curation guidance. First, gaa1 does **not** currently carry GO:0004197, so the prediction would be a **new, incorrect molecular-function addition** rather than a correction of an existing annotation — the correct action is simply to reject it. Second, even the most generous alternative reading (that Gaa1/GPAA1 assists the amide-bond-forming second half-reaction) does not rescue the prediction: the only peptidase-like fold anywhere in the Gaa1 family is a **Zn/M28 metallopeptidase** superfamily hit (seen in human GPAA1, absent even from *S. pombe* gaa1), which is mechanistically **incompatible** with cysteine-type catalysis. --- ## Key Findings ### Finding 1 — gaa1 is a non-catalytic GPI-T subunit; the catalytic cysteine protease is Gpi8/PIG-K The GPI transamidase reaction proceeds through a covalent **thioester (carbonyl) intermediate**: an active-site cysteine attacks the carbonyl of the substrate's ω-site residue after cleavage of the C-terminal GPI-attachment signal peptide, then the GPI anchor's ethanolamine amino group displaces the enzyme to form the amide-linked anchor. The seed hypothesis asks which subunit supplies that catalytic cysteine. Classical genetics/biochemistry answers this directly. Ohishi et al. (2000) showed that *GAA1*-knockout cells fail to accumulate the carbonyl intermediate, but the essential cysteine and histidine residues that generate the intermediate belong to **Gpi8p**, not Gaa1p: "*cysteine and histidine residues of Gpi8p, which are conserved in members of a cysteine protease family, are essential for generation of a carbonyl intermediate... Gpi8p is a catalytic component that cleaves the GPI attachment signal peptide*" ([PMID:10793132](https://pubmed.ncbi.nlm.nih.gov/10793132/)). The GAA1 requirement is therefore **indirect** — it reflects the need for an intact, functional complex, not a catalytic activity intrinsic to Gaa1. Meyer et al. (2000) independently mapped Gpi8p's active site and identified it as a **caspase-related cysteine protease** whose catalytic Cys199 and His157 (S. cerevisiae numbering) are essential: "*gpi8 alleles mutated at Cys199 or His157 are nonfunctional*" ([PMID:10727241](https://pubmed.ncbi.nlm.nih.gov/10727241/)). This directly localizes the catalytic cysteine/histidine to Gpi8p. Structural biology confirms this across kingdoms. The human GPI-T cryo-EM structure places catalysis in PIGK (the Gpi8 ortholog): "*The PIGK subunit functions as the catalytic component, in which we identified a C206-H164-N58 triad that is critical for the transamination reaction*" ([PMID:35165458](https://pubmed.ncbi.nlm.nih.gov/35165458/)); GPAA1 — the direct ortholog of *S. pombe* gaa1 — is non-catalytic. The fungal GPI-T cryo-EM structure reinforces this, describing "*the dynamic accommodation of catalytic subunit Gpi8*" and casting Gaa1/Gab1 as GPI-anchor-binding/scaffold components ([PMID:41085069](https://pubmed.ncbi.nlm.nih.gov/41085069/)). Finally, direct UniProt provenance shows the domain asymmetry cleanly. *S. pombe* gaa1 (Q9US48) carries **only** Pfam PF04114 "Gaa1" (IPR007246) — no Peptidase_C13 domain, no annotated active site, no EC number, no cysteine-endopeptidase MF term — whereas *S. pombe* gpi8 (Q9USP5) carries Pfam PF01650 "Peptidase_C13" (IPR001096), annotated ACT_SITE His145/Cys187, EC 3.-.-.-, and GO:0003923. Genetics, mutagenesis, structure, and annotation all converge: **the cysteine-protease catalysis is Gpi8's, not gaa1's.** ### Finding 2 — Domain analysis: gaa1 has zero cysteine-protease signatures; the only peptidase-like fold in the Gaa1 family is a Zn/M28 metallopeptidase, not cysteine-type To exclude a "hidden" or divergent protease activity in gaa1, direct InterPro API queries were run against the target and its orthologs. *S. pombe* gaa1 (Q9US48) returns **only** Gaa1-component family signatures across all member databases — InterPro IPR007246, Pfam PF04114, PIRSF036762, PANTHER PTHR13304 — and **no** peptidase/protease domain and **no** catalytic-triad signature of any kind. By contrast, *S. pombe* gpi8 (Q9USP5) returns the full catalytic signature set: InterPro IPR001096 "Peptidase C13, legumain," Pfam PF01650 "Peptidase C13 family," PIRSF019663 "Transamidase/peptidase, legumain type," PIRSF500138 "catalytic/GPI8 component," and CATH G3DSA:3.40.50.1460 (the caspase/legumain-like fold). The human ortholog GPAA1 (O43292) carries the same Gaa1-family entries **plus** a CATH-Gene3D structural superfamily hit G3DSA:3.40.630.10, "Zn peptidases" — an **M28 metallopeptidase-like fold**. This matters for two reasons. First, a metallo/Zn-dependent peptidase fold is **mechanistically incompatible** with a cysteine-type endopeptidase (GO:0004197): one uses a metal ion and an activated-water nucleophile, the other uses a catalytic cysteine thiolate. Second, *S. pombe* gaa1 did **not** even return this metallopeptidase CATH hit, so there is no protease-like structural signal in the target at all. The domain analysis thereby closes the loophole: if any enzymatic role for Gaa1 exists (some models propose Gaa1/GPAA1 contributes to the amide-bond-forming second half-reaction), it would be **metallo/synthetase** in character, not cysteine-type. Either way, GO:0004197 is contradicted at the domain level. ### Finding 3 — Current annotation status: gaa1 has no GO:0004197; its sole MF term is GO:0003923 (TAS) — the prediction would be a novel, incorrect addition A QuickGO annotation query for Q9US48 confirms that **GO:0004197 is not present** in gaa1's current record. Its only molecular-function annotation is **GO:0003923 (GPI-anchor transamidase activity)**, evidence code TAS, reference PMID:26563290. Its other annotations are biological-process terms (GO:0016255 attachment of GPI anchor to protein, IBA/NAS; GO:0006506 GPI anchor biosynthetic process, IEA; GO:0031505 fungal-type cell wall organization, NAS) and cellular-component terms (GO:0042765 GPI-anchor transamidase complex; GO:0005789 ER membrane; GO:0005783 ER; GO:0098553 lumenal side of ER membrane, IC). No cysteine-protease/endopeptidase term exists anywhere in the record. The curation implication is concrete: the prediction is **not** a duplicate of an existing annotation — it is a proposal to **add a new** molecular-function term. Because that term is refuted by all available evidence, the correct action is to **reject the addition**. Separately (and not as justification for the prediction), the existing GO:0003923 TAS annotation on gaa1 attributes whole-complex catalytic activity to a non-catalytic subunit; by the same subunit-assignment logic it is a candidate for curator review, though annotating whole-complex activity to each subunit is an accepted GO convention. The bottom line for this hypothesis is unchanged: **cysteine-type endopeptidase activity is not supported for gaa1.** --- ## Mechanistic Model / Interpretation The GPI transamidase (GPI-T) is a pentameric, ER-membrane–embedded enzyme that catalyzes the second major post-translational step in GPI anchoring: it cleaves the C-terminal GPI-attachment signal peptide of a nascent substrate at the ω-site and, in the same catalytic cycle, replaces it with a preassembled GPI glycolipid via an amide (transamidation) bond. The five subunits and their roles are summarized below. ``` GPI Transamidase (GPI-T) complex — subunit roles ------------------------------------------------------------------ Subunit (S. pombe / human) Role ------------------------------------------------------------------ Gpi8 / PIG-K CATALYTIC cysteine protease (Peptidase C13 / legumain-caspase-like). Active-site Cys-His(-Asn) triad forms the thioester carbonyl intermediate. <-- GO:0004197 HERE Gaa1 / GPAA1 (TARGET) Non-catalytic. Binds/recognizes the GPI anchor; stabilizes the complex; proposed role in the amide-bond (second) half- reaction. NO cysteine-protease domain. Gpi16 / PIG-T Scaffold; disulfide-links to Gpi8. Gpi17 / PIG-S Scaffold / substrate handling. Cdc91 / PIG-U Accessory. ------------------------------------------------------------------ Reaction (catalysis at Gpi8/PIG-K, NOT Gaa1): Substrate-omega-C(=O)-signal-peptide | Gpi8 active-site Cys thiolate attacks v Substrate-omega-C(=O)-S-Gpi8 (thioester "carbonyl intermediate") | GPI-ethanolamine-NH2 nucleophile (Gaa1 assists binding) v Substrate-omega-C(=O)-NH-GPI + Gpi8-SH (regenerated) ``` The prediction under review takes a property that is real for the *complex* and for *one specific subunit (Gpi8/PIG-K)* and mis-attributes it to a *different subunit (Gaa1)*. All four evidence tiers place the catalytic cysteine in Gpi8: (1) *GAA1*-null cells lose intermediate formation only because the complex is non-functional, while the actual catalytic Cys/His are Gpi8's (PMID:10793132); (2) direct mutagenesis pinpoints Gpi8's Cys199/His157 (PMID:10727241); (3) human and fungal cryo-EM place the Cys–His–Asn triad in PIGK/Gpi8 and label Gaa1/GPAA1 non-catalytic (PMID:35165458, PMID:41085069); and (4) domain databases show gaa1 has no protease signature while gpi8 carries the full Peptidase C13/legumain catalytic signature. A subtle but useful refinement: even the *possibility* that Gaa1 contributes catalytically to transamidation is constrained to be **non-cysteine** in nature. The only peptidase-like structural fold detected anywhere in the Gaa1 family is a **Zn/M28 metallopeptidase** superfamily hit (in human GPAA1, and absent even from *S. pombe* gaa1). A metallopeptidase-like fold cannot support GO:0004197, which is defined by cysteine-thiolate catalysis. So both the "Gaa1 is purely structural/recognition" model and the "Gaa1 assists the amide-bond second half-reaction" model are consistent with **refuting** cysteine-type endopeptidase activity. --- ## Evidence Base / Evidence Matrix | Citation | Evidence type | Direction | Claim tested | Key finding | Context | Confidence & limitations | |---|---|---|---|---|---|---| | [PMID:10793132](https://pubmed.ncbi.nlm.nih.gov/10793132/) (Ohishi 2000) | Mutant phenotype + biochemistry | **Refutes** | Does gaa1 supply the catalytic Cys/His? | *GAA1*-null cells fail to form the carbonyl intermediate, but the essential Cys/His belong to Gpi8p ("Gpi8p is a catalytic component that cleaves the GPI attachment signal peptide"). | Mouse F9 cells / yeast; in vitro GPI-anchoring | High. GAA1 requirement is indirect (complex integrity), not catalytic. | | [PMID:10727241](https://pubmed.ncbi.nlm.nih.gov/10727241/) (Meyer 2000) | Direct assay / mutagenesis | **Refutes** | Where is the GPI-T active site? | Gpi8p is caspase-related; Cys199/His157 mutants are nonfunctional. | *S. cerevisiae* | High. Directly identifies catalytic residues in Gpi8, not gaa1. | | [PMID:35165458](https://pubmed.ncbi.nlm.nih.gov/35165458/) (human GPI-T cryo-EM) | Structural | **Refutes** | Which subunit holds the catalytic triad? | PIGK holds the Cys206–His164–Asn58 triad; GPAA1 (gaa1 ortholog) non-catalytic. | *Homo sapiens*, cryo-EM ~3.1 Å | High. Direct structural localization of catalysis to PIGK/Gpi8. | | [PMID:41085069](https://pubmed.ncbi.nlm.nih.gov/41085069/) (fungal GPI-T cryo-EM) | Structural | **Refutes** | Subunit roles in fungal GPI-T | Gpi8 explicitly the "catalytic subunit"; Gaa1/Gab1 GPI-binding/scaffold. | Fungal, cryo-EM | High. Kingdom-matched to *S. pombe*. | | UniProt Q9US48 (gaa1) — direct fetch | Sequence/domain (database) | **Refutes** | Does gaa1 carry a protease domain/active site? | Only PF04114 "Gaa1" (IPR007246); no Peptidase_C13, no ACT_SITE, no EC. | *S. pombe* | High. Direct provenance; database-level. | | UniProt Q9USP5 (gpi8) — direct fetch | Sequence/domain (database) | **Contrast** | Where do catalytic annotations map? | PF01650 Peptidase_C13, ACT_SITE His145/Cys187, EC 3.-.-.-, GO:0003923. | *S. pombe* | High. Shows catalytic annotation belongs to gpi8. | | InterPro API (executed) | Structural/evolutionary (database) | **Refutes** | Any cysteine-protease signature in gaa1? | gaa1: zero peptidase signatures. gpi8: IPR001096 legumain, CATH caspase/legumain fold (3.40.50.1460). | *S. pombe* | High. Direct, reproducible. | | InterPro API (executed) | Structural (database) | **Qualifies** | Is the "Gaa1 is peptidase-like" idea cysteine-type? | Human GPAA1 (O43292) shows CATH "Zn peptidases" (M28-like) fold — metallo, not cysteine; *S. pombe* gaa1 lacks even this hit. | Human vs *S. pombe* | Moderate; fold-level only, argues against GO:0004197. | | QuickGO (executed) | Review/database | **Qualifies** | Does gaa1 already carry GO:0004197? | No. Sole MF = GO:0003923 (transamidase activity, TAS, PMID:26563290). | *S. pombe* | High. Prediction would be a new, incorrect MF addition. | | [PMID:27208238](https://pubmed.ncbi.nlm.nih.gov/27208238/) (Arabidopsis AtGPI8) | Mutant phenotype | **Refutes** (context) | Which subunit is the Cys protease across kingdoms? | AtGPI8/PIG-K described as "a Cys protease that transfers an assembled GPI anchor to proteins." | *Arabidopsis thaliana* | Medium-high. Confirms Cys-protease identity is GPI8's in plants too. | | [PMID:12958211](https://pubmed.ncbi.nlm.nih.gov/12958211/) (Nagamune 2003) | Interaction/composition | **Qualifies** | Is GAA1 a conserved subunit distinct from GPI8? | GAA1 and GPI8 are distinct, separately conserved core subunits across eukaryotes. | *Trypanosoma brucei* | Medium. Reinforces GAA1 ≠ GPI8. | --- ## GO Curation Implications **Current live annotation set for gaa1 (Q9US48):** the only MF term is **GO:0003923 (GPI-anchor transamidase activity), evidence TAS, ref PMID:26563290**. **GO:0004197 is NOT currently annotated.** BP: GO:0016255, GO:0006506, GO:0031505. CC: GO:0042765, GO:0005789, GO:0005783, GO:0098553. | GO term | Aspect | Action for gaa1 | Rationale | |---|---|---|---| | GO:0004197 cysteine-type endopeptidase activity | MF | **Do not add / reject** | Catalysis is Gpi8's; gaa1 has no protease domain/active site | | GO:0003923 GPI-anchor transamidase activity | MF | Retain (flag as whole-complex convention) | Currently annotated (TAS); belongs most directly to Gpi8 | | GO:0016255 / GO:0006506 (BP) | BP | Retain | Well-supported role in GPI attachment | | GO:0042765 GPI-anchor transamidase complex (CC) | CC | Retain | gaa1 is a bona fide complex subunit | **Leads (curator verification required):** - **Reject** the addition of **GO:0004197** to gaa1 (Q9US48). This would be a new, incorrect MF term — a subunit-level misassignment; the catalytic cysteine-protease/transamidase activity belongs to Gpi8/PIG-K. - **Review (lower priority)** the existing GO:0003923 (TAS) on gaa1, which attributes whole-complex catalytic activity to a non-catalytic subunit. This is a judgment call (whole-complex activity is sometimes annotated to each subunit by convention), not a clear error. - The catalytic MF term (GO:0003923; mechanistically GO:0004197) correctly belongs to **gpi8 (Q9USP5)**, where GO:0003923 is already annotated. - **MF for gaa1:** No catalytic MF is supported by direct evidence. If a molecular-function statement is desired, the defensible level is complex-membership/recognition, not enzymatic activity. Avoid "protein binding" as the terminal recommendation; the informative characterization is *non-catalytic structural/recognition subunit of the GPI-T complex*. --- ## Mechanistic Scope The immediate molecular function under test is **cysteine-type endopeptidase (peptide-bond hydrolysis via a catalytic cysteine)**. The relevant chemistry — nucleophilic attack by an active-site Cys thiolate on the substrate ω-site carbonyl to form a covalent thioester intermediate, followed by aminolysis by the GPI ethanolamine to form the amide-linked anchor — is a **direct gene-product activity of Gpi8/PIG-K**. For **gaa1**, the tested activity is **absent**: gaa1's direct contributions are (i) recognition/binding of the GPI-attachment signal and/or the GPI glycolipid, and (ii) structural stabilization of the pentameric complex. The observed loss of intermediate formation in *GAA1*-null cells (PMID:10793132) is a **downstream, loss-of-function consequence** of complex dysfunction — not evidence of a catalytic activity intrinsic to gaa1. Distinguishing these is the crux: mutant phenotypes of a non-catalytic subunit can mimic loss of catalysis without the subunit being catalytic. --- ## Conflicts and Alternatives - **Apparent conflict (resolved):** GAA1 knockout abolishes carbonyl-intermediate formation (PMID:10793132), which could naively suggest Gaa1 is catalytic. The same paper resolves this: the essential catalytic Cys/His are in Gpi8p; Gaa1 is a required partner, not the enzyme. - **Paralog/subunit confusion (likely source of the prediction):** gaa1 is a *subunit of a cysteine-protease-containing complex*; association/frequency bias can propagate the complex's catalytic term onto any member. This is precisely the misassignment the seed hypothesis anticipates. - **Alternative activity for GAA1 (M28-like/second half-reaction):** Some literature proposes GPAA1 assists amide-bond formation and resembles M28 metallopeptidases; this is corroborated here at the fold level (human GPAA1 carries a CATH "Zn peptidases"/M28-like hit). Even if catalytically relevant, this is a metallo/amide-synthetase character, **mechanistically incompatible with GO:0004197**. *S. pombe* gaa1 returns no protease fold signature at all, so the prediction is unsupported even under the most generous reading. - **Organism-specific differences:** None that rescue the hypothesis. The Cys-protease identity of GPI8/PIG-K is conserved from yeast to human to plant (PMID:10727241, 35165458, 27208238), and the distinctness of GAA1 vs GPI8 is conserved even in trypanosomes (PMID:12958211). - **Database carry-over:** No existing GO:0004197 annotation on gaa1 to carry over; the current record's MF is GO:0003923 only. --- ## Limitations and Knowledge Gaps 1. **Does *S. pombe* gaa1 have any weak intrinsic proteolytic/amide-forming activity?** Checked: no domain, active site, or assay supports this. Matters because a positive result could justify a *different* (metallo/synthetase) MF, but never cysteine-endopeptidase. Resolution: reconstituted GPI-T activity assays with catalytically dead gpi8 vs gaa1 point mutants. 2. **Structural triad absence for *S. pombe* gaa1.** Orthologs (human PIGK, yeast) were assessed structurally; pombe gaa1 has not been individually solved. Resolution: map Q9US48 onto the human/yeast GPI-T cryo-EM structures (or AlphaFold model) to confirm no Cys–His–Asn triad in the Gaa1 fold. 3. **Provenance of the GO-GPT prediction.** The reference DOI (2026.03.19.712954) was not programmatically accessible in this run; its internal reasoning could not be inspected. This does not change the verdict. 4. **Positive MF for gaa1 remains under-specified.** The evidence robustly refutes GO:0004197 and supports a non-catalytic recognition/stabilization role, but a precise, primary-evidence-backed "GPI-attachment-signal/GPI-anchor binding" MF term was not pinned to a specific GO ID here. --- ## Discriminating Tests 1. **Active-site swap/mutagenesis:** Reconstitute GPI-T with catalytically dead gpi8 (Cys→Ala/Ser) vs wild-type gaa1, and vice versa; only gpi8 active-site mutation should abolish transamidation — already demonstrated in orthologs (PMID:10727241). 2. **Thioester-intermediate trapping / activity-based probes:** Cysteine-protease-directed covalent probes on isolated GPI-T, with subunit identification by MS. Prediction: label maps to Gpi8/PIG-K, not gaa1. 3. **Structural triad mapping:** Superpose an AlphaFold model of Q9US48/gaa1 onto the human PIGK catalytic pocket; confirm the absence of a nucleophilic cysteine positioned for catalysis. 4. **Domain/HMM scan (done at annotation level):** Confirm gaa1 lacks Peptidase_C13 (PF01650) and any MEROPS clan CD cysteine-protease signature — only PF04114 is present. --- ## Proposed Follow-up Actions / Curation Leads (require curator verification) - **Action:** Reject the GO:0004197 prediction for gaa1 (Q9US48). Do not add cysteine-type endopeptidase activity. - **Candidate references to attach:** PMID:10793132, PMID:10727241, PMID:35165458, PMID:41085069. - Verify snippet (PMID:10793132): *"cysteine and histidine residues of Gpi8p, which are conserved in members of a cysteine protease family, are essential for generation of a carbonyl intermediate ... Gpi8p is a catalytic component that cleaves the GPI attachment signal peptide."* - Verify snippet (PMID:10727241): *"gpi8 alleles mutated at Cys199 or His157 are nonfunctional."* - Verify snippet (PMID:35165458): *"The PIGK subunit functions as the catalytic component, in which we identified a C206-H164-N58 triad that is critical for the transamination reaction."* - Verify snippet (PMID:41085069): *"the dynamic accommodation of catalytic subunit Gpi8."* - **Retain existing gaa1 annotations:** GO:0042765 (CC), GO:0016255 (BP), GO:0006506 (BP), GO:0005789 (CC). - **Route the catalytic term correctly:** the cysteine-protease/transamidase MF belongs to gpi8 (Q9USP5), where GO:0003923 is already annotated. - **Suggested curator question:** Should gaa1 carry any positive MF term beyond complex membership? Current evidence supports only a non-catalytic recognition/scaffolding role. --- ### Provenance / Artifacts - Direct UniProt fetches (executed): Q9US48 (gaa1) and Q9USP5 (gpi8) domain/active-site/GO lines. - InterPro API queries (executed): gaa1 (Q9US48), gpi8 (Q9USP5), human GPAA1 (O43292). - QuickGO annotation snapshot (executed): Q9US48 MF/BP/CC terms. - Literature: PMIDs 10793132, 10727241, 35165458, 41085069, 27208238, 12958211. **Bottom line:** The cysteine-type endopeptidase (GO:0004197) prediction for gaa1 is **REFUTED** — a subunit-level misassignment of Gpi8/PIG-K's catalytic activity. gaa1 is a non-catalytic GPI-transamidase subunit that binds the GPI anchor and stabilizes the complex; it should not receive GO:0004197. ## Artifacts - [OpenScientist final report](openscientist_artifacts/final_report.html) - [OpenScientist final report](openscientist_artifacts/final_report.pdf)