id: Q9US48 gene_symbol: gaa1 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:284812 label: Schizosaccharomyces pombe 972h- description: |- Essential subunit of the GPI-anchor transamidase complex (GPIT) in the endoplasmic reticulum. The complex catalyzes the terminal step of GPI anchor attachment, transferring preassembled GPI glycolipids to specific proteins at their C-terminal GPI-anchor signal sequence. Gaa1 contains a luminal domain with an M28 family metallopeptidase-like fold; while earlier work proposed this domain as the catalytic metallopeptide synthetase, the 2022 cryo-EM structure of the human complex assigns the catalytic cysteine protease to PIGK (Gpi8 ortholog) and supports a structural/substrate-positioning role for Gaa1, including substrate recruitment and GPI lipid recognition. The multi-pass transmembrane protein anchors the complex in the ER membrane and positions the catalytic machinery for GPI attachment. Direct S. pombe biochemical data are limited; functional assignments are strongly supported by orthology to the conserved Gaa1/GPAA1 family, in which a S. pombe ortholog is explicitly recognized. existing_annotations: - term: id: GO:0016255 label: attachment of GPI anchor to protein evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: This is the core biological process for gaa1. The protein is directly responsible for catalyzing the terminal step of GPI anchor attachment to proteins in the ER. supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: Gaa1 is a core component of the GPI:protein transamidase (GPIT) enzyme complex that catalyzes the final step of GPI anchor attachment to proteins in the endoplasmic reticulum - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- gaa1 encodes a **GPI transamidase component** (Gaa1/GPAA1 family) required for efficient **attachment of GPI anchors** to secretory proteins in the ER. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- Cross-species sequence analysis explicitly includes **a *Schizosaccharomyces pombe* Gaa1 ortholog** and shows conservation of the function-critical proline motif in the last TM segment, supporting that *S. pombe* gaa1/Q9US48 is a true family member with conserved mechanistic features. action: ACCEPT - term: id: GO:0042765 label: GPI-anchor transamidase complex evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: part_of review: summary: Core cellular component annotation. Gaa1 is an essential structural and substrate-positioning subunit of this five-protein complex. supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: Gaa1 resides in the ER as an integral membrane protein and is a stable structural subunit of the GPIT enzyme complex (GO:0042765) - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- Likely a core subunit of the **five-subunit GPI transamidase (GPIT/GPI-T)** with orthologs of PIGK/Gpi8, PIGT/Gpi16, PIGS/Gpi17, and PIGU/Gab1/Cdc91. action: ACCEPT - term: id: GO:0005789 label: endoplasmic reticulum membrane evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in review: summary: Correct localization. Gaa1 is a multi-pass transmembrane protein anchored in the ER membrane where GPI attachment occurs. supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: Gaa1 is an integral membrane glycoprotein of the ER. It is a multi-pass membrane protein embedded in the ER membrane as part of the GPIT complex - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- Across systems where it has been experimentally studied, Gaa1/GPAA1 is a **multi-pass ER membrane glycoprotein**. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- the most defensible localization statement is **ER membrane**, inferred from conserved complex function and topology across eukaryotes. action: ACCEPT - term: id: GO:0006506 label: GPI anchor biosynthetic process evidence_type: IEA original_reference_id: GO_REF:0000120 qualifier: involved_in review: summary: This term is correct but less specific than GO:0016255 (attachment of GPI anchor to protein). Gaa1 functions in the terminal attachment step, which is part of the broader GPI anchor biosynthetic process. Falcon notes that this final transamidation step is the commitment step producing a mature GPI-anchored protein. action: KEEP_AS_NON_CORE supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- This reaction occurs in the **endoplasmic reticulum (ER)** and is described as the final “commitment” step that produces a mature GPI-anchored protein. - term: id: GO:0016020 label: membrane evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: located_in review: summary: True but uninformative. Gaa1 is a multi-pass membrane protein, so the claim is correct, but it is entailed by the accepted, more specific GO:0005789 (endoplasmic reticulum membrane) and GO:0042765 (GPI-anchor transamidase complex) annotations and adds nothing beyond them. action: TOO_GENERAL - term: id: GO:0042765 label: GPI-anchor transamidase complex evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: part_of review: summary: Correct; the InterPro-based assignment agrees with the phylogenetic (IBA) annotation to the same term, and the two lines of evidence reinforce each other. Gaa1 is a core subunit of the GPI-anchor transamidase complex. action: ACCEPT - term: id: GO:0005789 label: endoplasmic reticulum membrane evidence_type: NAS original_reference_id: PMID:15003443 qualifier: located_in review: summary: Correct localization, consistent with the IEA annotation to the same term. The cited paper is a genome-wide GPI-site prediction study and does not itself address Gaa1 localization; the claim rests on the conserved topology of Gaa1/GPAA1 as a multi-pass ER membrane subunit of the GPI transamidase (quoted below). action: ACCEPT supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- Across systems where it has been experimentally studied, Gaa1/GPAA1 is a **multi-pass ER membrane glycoprotein**. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- the most defensible localization statement is **ER membrane**, inferred from conserved complex function and topology across eukaryotes. - term: id: GO:0016255 label: attachment of GPI anchor to protein evidence_type: NAS original_reference_id: PMID:15003443 qualifier: involved_in review: summary: Correct; consistent with the IBA annotation to the same term. The cited paper is a genome-wide GPI-site prediction study and does not itself assay Gaa1; the claim rests on Gaa1 being a GPI transamidase component required for GPI anchor attachment (quoted below). action: ACCEPT supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- gaa1 encodes a **GPI transamidase component** (Gaa1/GPAA1 family) required for efficient **attachment of GPI anchors** to secretory proteins in the ER. - term: id: GO:0031505 label: fungal-type cell wall organization evidence_type: NAS original_reference_id: PMID:15003443 qualifier: involved_in review: summary: While GPI-anchored proteins contribute to cell wall organization in fungi, this is an indirect downstream effect. The direct function of gaa1 is GPI anchor attachment, not cell wall organization itself; this annotation overshoots the gene's direct role and is marked as over-annotated. supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: In S. pombe (and other fungi), numerous cell wall enzymes and adhesins are GPI-anchored; therefore, Gaa1 is indirectly critical for cell wall assembly and integrity - reference_id: PMID:15003443 supporting_text: A sensitive predictor for potential GPI lipid modification sites in fungal protein sequences and its application to genome-wide studies for Aspergillus nidulans, Candida albicans, Neurospora crassa, Saccharomyces cerevisiae and Schizosaccharomyces pombe. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- In yeast and fungal systems, GPI anchoring is a major contributor to **cell wall protein display** and surface proteome composition. action: MARK_AS_OVER_ANNOTATED - term: id: GO:0042765 label: GPI-anchor transamidase complex evidence_type: NAS original_reference_id: PMID:15003443 qualifier: part_of review: summary: Correct; consistent with the IBA and IEA annotations to the same term. The cited paper does not itself address complex membership; the claim rests on Gaa1/GPAA1 being a core subunit of the five-subunit GPI transamidase (quoted below). action: ACCEPT supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- Likely a core subunit of the **five-subunit GPI transamidase (GPIT/GPI-T)** with orthologs of PIGK/Gpi8, PIGT/Gpi16, PIGS/Gpi17, and PIGU/Gab1/Cdc91. - term: id: GO:0003923 label: GPI-anchor transamidase activity evidence_type: TAS original_reference_id: PMID:26563290 qualifier: contributes_to review: summary: |- Core molecular function. GO:0003923 is the complex-level catalytic activity of the GPI transamidase complex that gaa1 is part of, and the annotation is retained as core for gaa1 as a subunit. Note on mechanism: earlier modeling (Eisenhaber 2014; Su 2020) proposed that the M28-family-like luminal domain of Gaa1/GPAA1 is itself the peptide-bond-forming metallopeptide synthetase. Falcon deep research flags that this has been revised by the 2022 near-atomic cryo-EM structure of human GPI-T, which assigns the catalytic cysteine protease to PIGK (the Gpi8 ortholog) and indicates Gaa1/GPAA1 is more likely a structural/substrate-positioning subunit contributing to substrate recruitment and GPI lipid recognition rather than the principal catalyst. The complex-level GO:0003923 annotation remains appropriate for gaa1 as an essential subunit, but it should not be read as asserting that Gaa1 alone performs the peptide-bond chemistry. supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: structural and bioinformatic studies revealed that Gaa1's luminal domain is homologous to M28 family metallo-peptidases, suggesting that Gaa1 itself is the enzyme that catalyzes the second step of the transamidation - reference_id: PMID:26563290 supporting_text: 'Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling.' - reference_id: PMID:35551457 supporting_text: |- Structure-based mutagenesis suggests a legumain-like mechanism for the recognition and cleavage of proprotein substrates, and an endogenous GPI in the structure defines a composite cavity for the lipid substrate. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- In the literature, the **precise catalytic assignment** for GPAA1/Gaa1 has evolved. A 2022 near-atomic cryo-EM structure of human GPI-T supports **PIGK** as the catalytic cysteine protease (with an essential catalytic dyad) and provides evidence that GPAA1’s luminal domain—despite a protease-like fold—may be more **structural/substrate-positioning** rather than the main catalytic center (based on mutagenesis and a cell-surface CD59 reporter assay). - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- proproteins can bind Gaa1 even when the catalytic subunit Gpi8/PIGK is absent, consistent with a **substrate-recruitment** role for Gaa1. - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- The last transmembrane segment of Gaa1 is implicated in **GPI recognition**: truncations and point mutations (including a conserved proline) disrupt GPI co-precipitation while preserving assembly and proprotein binding, indicating a role in binding/presenting the lipid substrate. action: ACCEPT - term: id: GO:0005783 label: endoplasmic reticulum evidence_type: HDA original_reference_id: PMID:16823372 qualifier: is_active_in review: summary: Correct but less specific than GO:0005789 (endoplasmic reticulum membrane). Since gaa1 is a transmembrane protein, the membrane term is more accurate. action: KEEP_AS_NON_CORE supported_by: - reference_id: PMID:16823372 supporting_text: ORFeome cloning and global analysis of protein localization in the fission yeast Schizosaccharomyces pombe. references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms. findings: [] - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: GO_REF:0000044 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt. findings: [] - id: GO_REF:0000120 title: Combined Automated Annotation using Multiple IEA Methods. findings: [] - id: PMID:15003443 title: A sensitive predictor for potential GPI lipid modification sites in fungal protein sequences and its application to genome-wide studies for Aspergillus nidulans, Candida albicans, Neurospora crassa, Saccharomyces cerevisiae and Schizosaccharomyces pombe. findings: [] - id: PMID:16823372 title: ORFeome cloning and global analysis of protein localization in the fission yeast Schizosaccharomyces pombe. findings: [] - id: PMID:26563290 title: 'Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling.' findings: [] - id: PMID:35551457 title: Molecular insights into biogenesis of glycosylphosphatidylinositol anchor proteins. findings: - statement: | The 2.53-Å cryo-EM structure of human GPI transamidase reveals an equimolar heteropentameric complex. Structure-based mutagenesis supports a legumain-like (cysteine protease) mechanism for recognition and cleavage of proprotein substrates by the catalytic subunit (PIGK/Gpi8 ortholog), with Gaa1/GPAA1 contributing a structural and substrate/lipid-positioning role rather than the principal catalysis. supporting_text: |- Structure-based mutagenesis suggests a legumain-like mechanism for the recognition and cleavage of proprotein substrates, and an endogenous GPI in the structure defines a composite cavity for the lipid substrate. reference_section_type: ABSTRACT - id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md title: Falcon deep research report on S. pombe gaa1 (Q9US48) findings: - statement: | gaa1 encodes a GPI transamidase component (Gaa1/GPAA1 family) required for efficient attachment of preassembled GPI anchors to secretory proteins in the ER. Direct S. pombe biochemical data were not retrieved; the assignment is inferred from strong orthology and family conservation. supporting_text: |- gaa1 encodes a **GPI transamidase component** (Gaa1/GPAA1 family) required for efficient **attachment of GPI anchors** to secretory proteins in the ER. reference_section_type: OTHER - statement: | A cross-species alignment of the last transmembrane segment of Gaa1 orthologs explicitly includes a S. pombe Gaa1 ortholog and shows conservation of the function-critical proline motif, confirming Q9US48 is a bona fide member of the Gaa1/GPAA1 GPI-transamidase family. supporting_text: |- Cross-species sequence analysis explicitly includes **a *Schizosaccharomyces pombe* Gaa1 ortholog** and shows conservation of the function-critical proline motif in the last TM segment, supporting that *S. pombe* gaa1/Q9US48 is a true family member with conserved mechanistic features. reference_section_type: OTHER - statement: | The precise catalytic assignment for Gaa1/GPAA1 has been revised. A 2022 near-atomic cryo-EM structure of human GPI-T supports PIGK (Gpi8 ortholog) as the catalytic cysteine protease, while Gaa1/GPAA1's luminal domain, despite a protease-like fold, is more likely structural/substrate- positioning rather than the principal catalytic center. supporting_text: |- In the literature, the **precise catalytic assignment** for GPAA1/Gaa1 has evolved. A 2022 near-atomic cryo-EM structure of human GPI-T supports **PIGK** as the catalytic cysteine protease (with an essential catalytic dyad) and provides evidence that GPAA1’s luminal domain—despite a protease-like fold—may be more **structural/substrate-positioning** rather than the main catalytic center (based on mutagenesis and a cell-surface CD59 reporter assay). reference_section_type: OTHER - statement: | The most conservative functional annotation for Gaa1 is a structural/recognition subunit of the ER GPI transamidase complex required for GPI-anchor attachment, likely contributing to substrate recruitment and/or GPI lipid engagement rather than being the protease that cleaves the signal peptide. supporting_text: |- the most conservative functional annotation is: **structural/recognition subunit of the ER GPI transamidase complex required for GPI-anchor attachment**, likely contributing to **substrate recruitment and/or GPI lipid engagement** rather than being the protease that cleaves the signal peptide. reference_section_type: OTHER - statement: | Proproteins can bind Gaa1 even when the catalytic subunit Gpi8/PIGK is absent, consistent with a substrate-recruitment role for Gaa1 within the transamidase complex. supporting_text: |- proproteins can bind Gaa1 even when the catalytic subunit Gpi8/PIGK is absent, consistent with a **substrate-recruitment** role for Gaa1. reference_section_type: OTHER - statement: | The last transmembrane segment of Gaa1, including a conserved proline, is implicated in GPI (lipid) recognition; mutations disrupt GPI co-precipitation while preserving complex assembly and proprotein binding. supporting_text: |- The last transmembrane segment of Gaa1 is implicated in **GPI recognition**: truncations and point mutations (including a conserved proline) disrupt GPI co-precipitation while preserving assembly and proprotein binding, indicating a role in binding/presenting the lipid substrate. reference_section_type: OTHER - statement: | Gaa1/GPAA1 is a multi-pass ER membrane glycoprotein across all systems where it has been experimentally studied; ER membrane is the most defensible localization for S. pombe gaa1, inferred from conserved complex function and topology across eukaryotes. supporting_text: |- Across systems where it has been experimentally studied, Gaa1/GPAA1 is a **multi-pass ER membrane glycoprotein**. reference_section_type: OTHER - statement: | Gaa1 is likely a core subunit of the five-subunit GPI transamidase complex, alongside orthologs of PIGK/Gpi8, PIGT/Gpi16, PIGS/Gpi17, and PIGU/Gab1/Cdc91. supporting_text: |- Likely a core subunit of the **five-subunit GPI transamidase (GPIT/GPI-T)** with orthologs of PIGK/Gpi8, PIGT/Gpi16, PIGS/Gpi17, and PIGU/Gab1/Cdc91. reference_section_type: OTHER core_functions: - description: |- Gaa1 is an essential structural and recognition subunit of the GPI-anchor transamidase complex in the endoplasmic reticulum. The complex catalyzes the terminal transamidation step that transfers preassembled GPI glycolipid anchors to specific proteins at their C-terminal GPI signal sequences. The luminal domain of Gaa1/GPAA1 has an M28-family metallopeptidase-like fold; while earlier models proposed it as the catalytic metallopeptide synthetase, the 2022 cryo-EM structure of human GPI-T assigns the catalytic cysteine protease to PIGK (Gpi8 ortholog) and supports a structural/substrate-positioning role for Gaa1, including substrate (proprotein) recruitment and GPI lipid recognition via the last transmembrane segment. Multiple transmembrane segments anchor and position the catalytic machinery in the ER membrane. gaa1 contributes the complex- level GPI-anchor transamidase activity (GO:0003923) as an essential subunit. molecular_function: id: GO:0003923 label: GPI-anchor transamidase activity directly_involved_in: - id: GO:0016255 label: attachment of GPI anchor to protein locations: - id: GO:0005789 label: endoplasmic reticulum membrane in_complex: id: GO:0042765 label: GPI-anchor transamidase complex supported_by: - reference_id: file:SCHPO/gaa1/gaa1-deep-research.md supporting_text: Gaa1 is a core component of the GPI:protein transamidase (GPIT) enzyme complex that catalyzes the final step of GPI anchor attachment to proteins in the endoplasmic reticulum - reference_id: file:SCHPO/gaa1/gaa1-deep-research-falcon.md supporting_text: |- the most conservative functional annotation is: **structural/recognition subunit of the ER GPI transamidase complex required for GPI-anchor attachment**, likely contributing to **substrate recruitment and/or GPI lipid engagement** rather than being the protease that cleaves the signal peptide.