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.
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.
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research.md
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
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
gaa1 encodes a **GPI transamidase component** (Gaa1/GPAA1 family) required for efficient **attachment of GPI anchors** to secretory proteins in the ER.
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
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.
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.
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
This reaction occurs in the **endoplasmic reticulum (ER)** and is described as the final “commitment” step that produces a mature GPI-anchored protein.
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.
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.
NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s...
ACCEPT
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).
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
Across systems where it has been experimentally studied, Gaa1/GPAA1 is a **multi-pass ER membrane glycoprotein**.
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
the most defensible localization statement is **ER membrane**, inferred from conserved complex function and topology across eukaryotes.
NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s...
ACCEPT
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).
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
gaa1 encodes a **GPI transamidase component** (Gaa1/GPAA1 family) required for efficient **attachment of GPI anchors** to secretory proteins in the ER.
NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s...
MARK AS OVER ANNOTATED
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.
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research.md
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
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.
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
In yeast and fungal systems, GPI anchoring is a major contributor to **cell wall protein display** and surface proteome composition.
NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s...
ACCEPT
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).
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
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.
TAS PMID:26563290 Biosynthesis of GPI-anchored proteins: special emphasis on G...
ACCEPT
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.
Supporting Evidence:
file:SCHPO/gaa1/gaa1-deep-research.md
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
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.
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
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).
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
proproteins can bind Gaa1 even when the catalytic subunit Gpi8/PIGK is absent, consistent with a **substrate-recruitment** role for Gaa1.
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
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.
HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ...
KEEP AS NON CORE
Summary: Correct but less specific than GO:0005789 (endoplasmic reticulum membrane). Since gaa1 is a transmembrane protein, the membrane term is more accurate.
ORFeome cloning and global analysis of protein localization in the fission yeast Schizosaccharomyces pombe.
Core Functions
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.
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
file:SCHPO/gaa1/gaa1-deep-research-falcon.md
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.
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Review rationale: Gaa1 is a noncatalytic GPI-transamidase subunit. The cysteine-protease/transamidase active site resides in Gpi8/PIG-K, while Gaa1 contributes substrate/lipid recognition and complex positioning. GO:0004197 would therefore be a new subunit-level misassignment.