| Claim/Topic | Organism/System | Key finding | Quantitative details | Evidence type | Citation (include DOI URL and publication date) |
|---|---|---|---|---|---|
| **S. pombe gaa1/Q9US48 identity** | *Schizosaccharomyces pombe* (in multispecies alignment) | A Gaa1 ortholog from *S. pombe* is explicitly included in cross-species alignment of the last TM segment; the family-defining conserved proline linked to GPI recognition is present, supporting that Q9US48/gaa1 belongs to the Gaa1/GPAA1 GPI-transamidase family. | Conserved proline in a GXXP/GXP-like motif in the last TM segment. | Comparative sequence conservation; family inference | Vainauskas & Menon, **2004-02**, JBC, DOI: https://doi.org/10.1074/jbc.M312191200 (pqac-00000010) |
| **S. pombe gaa1/Q9US48 function (inferred)** | *S. pombe* gaa1 / UniProt Q9US48 | Best-supported annotation is **GPI transamidase component Gaa1**, involved in attachment of a preassembled GPI anchor to precursor proteins after C-terminal signal processing. Direct *S. pombe*-specific biochemical evidence was not retrieved, so this is inferred from strong orthology/family conservation. | No direct *S. pombe* kinetic data retrieved. | Orthology-based functional inference from conserved GPIT subunit family | Conserved-family evidence summarized from Gaa1/GPAA1 studies (pqac-00000003, pqac-00000010, pqac-00000017) |
| **S. pombe gaa1/Q9US48 localization (inferred)** | *S. pombe* gaa1 / eukaryotic Gaa1 family | Likely an **ER membrane** protein with a large luminal domain, because Gaa1/GPAA1 is ER-localized across experimentally studied systems and functions in the ER-resident GPI transamidase complex. | Human GPAA1/Gaa1 studied as multi-pass membrane glycoprotein; 7 TM spans in 2002 work, 8 TMHs in 2022 cryo-EM model. | Inference from conserved topology and complex localization | Vainauskas et al., **2002-08**, JBC, DOI: https://doi.org/10.1074/jbc.M205402200; Xu et al., **2022-05**, Nat Commun, DOI: https://doi.org/10.1038/s41467-022-30250-6 (pqac-00000003, pqac-00000005, pqac-00000008) |
| **S. pombe gaa1/Q9US48 complex membership (inferred)** | *S. pombe* gaa1 / eukaryotic GPIT | 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. | Human structure resolved a 1:1:1:1:1 heteropentamer. | Orthology/family inference supported by conserved complex architecture | Ohishi et al., **2000-05**, Mol Biol Cell, DOI: https://doi.org/10.1091/mbc.11.5.1523; Xu et al., **2022-05**, Nat Commun, DOI: https://doi.org/10.1038/s41467-022-30250-6 (pqac-00000005, pqac-00000017) |
| **2002 structural role of Gaa1** | Human Gaa1 in GPIT | Gaa1 is an **ER-localized membrane glycoprotein**; its **large luminal domain** mediates interaction with other GPIT subunits, while C-terminal TM segments are required for a **functional** complex. | Detergent-extracted Gaa1-containing complexes sedimented at ~**17 S**. | Primary experimental cell biology and structure-function analysis | Vainauskas et al., **2002-08**, JBC, DOI: https://doi.org/10.1074/jbc.M205402200 (pqac-00000003, pqac-00000014) |
| **2002 substrate-recognition role** | Human Gaa1/GPIT | Pro-protein substrates can bind **Gaa1 in the absence of Gpi8**, implying a key **substrate-recognition/recruitment** role for Gaa1 within GPIT. | No catalytic rate reported. | Primary experimental interaction analysis | Vainauskas et al., **2002-08**, JBC, DOI: https://doi.org/10.1074/jbc.M205402200 (pqac-00000014) |
| **2004 GPI recognition by Gaa1 TM segment** | Human Gaa1/GPIT with cross-species comparison | A conserved **proline in the last TM segment** is required for **GPI recognition** by GPIT; mutant complexes can assemble and bind proprotein yet fail to co-precipitate GPI efficiently. | Example: **P609L** lost H8/GPI co-precipitation, whereas **W611L** retained it. | Primary mutational/biochemical evidence | Vainauskas & Menon, **2004-02**, JBC, DOI: https://doi.org/10.1074/jbc.M312191200 (pqac-00000000, pqac-00000002) |
| **2022 GPIT architecture** | Human GPI transamidase cryo-EM | Near-atomic structure showed an **equimolar heteropentameric** complex with a luminal catalytic assembly and transmembrane core; GPAA1 forms a major membrane-embedded scaffold with a portico-like architecture. | **2.53 Å** resolution; **2,393 residues** modeled (**94.4%** complete); **24 TMHs** total; GPAA1 contributes **8 TMHs**. | Primary structural biology (cryo-EM) | Xu et al., **2022-05**, Nat Commun, DOI: https://doi.org/10.1038/s41467-022-30250-6 (pqac-00000005, pqac-00000006, pqac-00000008) |
| **2022 catalytic assignment revises GPAA1 role** | Human GPIT | Structure and mutagenesis support **PIGK** as the catalytic cysteine protease; GPAA1’s soluble domain resembles a Zn-protease fold but tested acidic/histidine residues were **not required** in the cell assay, arguing GPAA1 is more likely **structural/substrate-positioning** rather than the principal catalyst. | GPAA1 D/E/H substitutions did **not** reduce CD59 staining; PIGK **H164A** or **C206S** abolished activity; **R60E** left **9.8%** of WT activity. | Primary structural biology plus mutagenesis | Xu et al., **2022-05**, Nat Commun, DOI: https://doi.org/10.1038/s41467-022-30250-6 (pqac-00000004, pqac-00000005) |
| **2022 substrate selectivity model** | Human GPIT | The active site forms an elongated cavity spanning from the membrane toward the catalytic dyad, with the **distance to the membrane** proposed as a **molecular ruler** for selecting valid GPI-attachment signals. | Cavity extends ~**22 Å** from membrane toward catalytic dyad; **12/22** mapped pathogenic mutations clustered near catalytic/GPI-binding regions. | Primary structural/mechanistic inference | Xu et al., **2022-05**, Nat Commun, DOI: https://doi.org/10.1038/s41467-022-30250-6 (pqac-00000004, pqac-00000006) |
| **GAA1/GPAA1 catalytic hypothesis from modeling** | Human GPAA1 lumenal domain | Modeling work proposed GPAA1 as an **M28-family metallo-peptide synthetase** with likely **single-Zn** chemistry and dynamic flaps around the active site, offering a mechanistic explanation for peptide-bond formation to phosphoethanolamine. | Predicted one Zn favored over two; two flaps show anti-correlated “breathing” dynamics. | Computational structural inference | Su et al., **2020-09**, Biology Direct, DOI: https://doi.org/10.1186/s13062-020-00266-3 (pqac-00000001, pqac-00000015) |
| **ω-site specificity concept** | Eukaryotic GAA1/GPAA1 literature | Classical GPAA1-centered model proposes transfer to proteins bearing a GPI-attachment ω-site with limited residue tolerance. | Permissive ω-site residues summarized as **Ala, Asn, Asp, Cys, Gly, Ser**. | Review/synthesis of prior biochemical literature | Eisenhaber et al., **2014-04**, Cell Cycle, DOI: https://doi.org/10.4161/cc.28761 (pqac-00000007, pqac-00000012) |
| **2024 ERAD regulation of GPI-T biogenesis** | Human HEK293T cells and mouse brown adipose tissue | SEL1L–HRD1 ERAD regulates GPI-anchored protein biogenesis by targeting **PIGK** for degradation, thereby indirectly controlling the function of the whole GPIT complex containing **GPAA1/GAA1**. | Screen identified **>100** high-confidence endogenous ERAD substrates, with ~**88%** cell-type specificity. | Primary proteomics and cell biology | Wei et al., **2024-01**, Nat Commun, DOI: https://doi.org/10.1038/s41467-024-44948-2 (pqac-00000011) |
| **2024 quantitative screen characteristics** | Human ERAD interactome | In the SEL1L-centered interactome, many candidate substrates had features common to secretory-pathway proteins, consistent with surveillance of GPI-T/GPI-AP biogenesis. | **55** SEL1L interactors; **61%** membrane proteins, **69%** glycosylated, **31%** with disulfide bonds. | Primary proteomics dataset | Wei et al., **2024-01**, Nat Commun, DOI: https://doi.org/10.1038/s41467-024-44948-2 (pqac-00000013) |
| **2024 relevance to disease and GPI-AP output** | Human GPI-T / ERAD | Several disease-associated **PIGK** variants are ERAD substrates; because GPIT has five core subunits including GPAA1, this work highlights post-translational quality control as an important regulator of the GPI-anchoring pathway. | Context includes **>150** human GPI-anchored proteins. | Primary mechanistic study with disease-variant analysis | Wei et al., **2024-01**, Nat Commun, DOI: https://doi.org/10.1038/s41467-024-44948-2 (pqac-00000011, pqac-00000013) |


*Table: This table summarizes what is directly known versus inferred for *S. pombe* gaa1/Q9US48, then places it in the broader mechanistic context of GAA1/GPAA1 research from landmark 2002, 2004, 2022, and 2024 studies. It is useful for separating species-specific evidence from orthology-based annotation and recent pathway-level advances.*