| Aspect | Summary |
|---|---|
| Molecular function / reaction | **Bst1 is the Schizosaccharomyces pombe ortholog of the PGAP1/BST1 GPI inositol-deacylase family**, predicted to hydrolyze and remove the inositol-linked acyl chain from newly attached GPI anchors in the ER, initiating post-attachment GPI lipid remodeling; this chemistry is established directly for yeast/mammalian orthologs and is consistent with the S. pombe protein family assignment and phenotypes (pqac-00000024, pqac-00000004, pqac-00000006, pqac-00000008, pqac-00000013). |
| Substrate | The immediate substrate is a **nascent triacylated GPI-anchored protein (GPI-AP3)** carrying an acyl chain on the inositol 2-position; deacylation yields a diacylated GPI-anchored protein (GPI-AP2). More generally, Bst1/PGAP1 acts on **newly GPI-anchored proteins in the ER lumen**, not bulk membrane phospholipids (pqac-00000008, pqac-00000009, pqac-00000010, pqac-00000011). |
| Pathway step | Bst1 acts **after GPI attachment to protein and before efficient ER export**, as the first remodeling step in the post-attachment GPI lipid-remodeling pathway; upstream, Gwt1/PIG-W adds the inositol-linked acyl chain, and downstream remodeling includes Per1/Gup1/Cwh43-dependent lipid changes that promote secretion and membrane organization (pqac-00000009, pqac-00000011, pqac-00000013, pqac-00000004). |
| Localization | In **S. pombe**, functional Bst1 localizes to the **ER (cortical and nuclear-envelope-associated ER)** and **cytoplasmic puncta enriched for early/cis-Golgi association**; 66% of Bst1 puncta overlap with Anp1-GFP (cis-Golgi/ER-Golgi transport marker) versus 12% with Sec72-GFP (trans-Golgi) (pqac-00000022, pqac-00000021). |
| Key phenotypes | **bst1Δ and bst1-s27** mutants are **rounder/swollen, hyperseptated, defective in contractile-ring constriction, septation, and daughter-cell separation**, and show impaired secretion/targeting of septum-remodeling glucanases **Eng1** and **Agn1**; Bst1 loss also perturbs early secretory trafficking, with increased COPII puncta, altered Anp1 distribution, increased tubular ER, and reduced acid phosphatase secretion (pqac-00000023, pqac-00000022, pqac-00000021, pqac-00000020). |
| Quantitative data | In S. pombe, **~23% WT** cells had one septum versus **~52% bst1-s27/bst1Δ** cells with one septum and **~30%** with ≥2 septa; contractile-ring constriction increased from **34.2 ± 3.5 min (WT)** to **78.4 ± 31.5 min (bst1Δ)**; Bst1 puncta overlap was **66% with Anp1** and **12% with Sec72**; Sec13 puncta increased from **2.1 ± 1.6 to 8.5 ± 4.7** and Sec24 puncta from **2.8 ± 1.8 to 6.2 ± 3.6** per focal plane in bst1 mutants; ync13 lysis fell from **77%** to **15% (bst1-s27)** or **9% (bst1Δ)** in suppressor backgrounds (pqac-00000022, pqac-00000021, pqac-00000023). |
| Genetic interactions | **bst1-s27** was isolated as a **spontaneous suppressor of ync13**; bst1 loss suppresses ync13 colony-formation/lysis defects, likely by reducing secretion of glucanases that would otherwise promote premature cell separation on defective septa. In double mutants, Eng1 recruitment is further impaired, supporting a trafficking-based interaction rather than direct pathway identity with Ync13 (pqac-00000023, pqac-00000024, pqac-00000019, pqac-00000021). |
| Supporting evidence in other organisms | In **S. cerevisiae and mammals**, BST1/PGAP1 was shown experimentally to mediate **inositol deacylation**, and loss causes **delayed ER-to-Golgi transport** of GPI-anchored proteins such as Gas1/DAF and defects in GPI-AP quality control; a **2024 structure** resolved PGAP1 as a **10-TM enzyme** with a **guitar-shaped substrate cavity** and serine hydrolase-type catalysis, strongly supporting the inferred biochemical role of S. pombe Bst1 (pqac-00000004, pqac-00000006, pqac-00000007, pqac-00000008, pqac-00000016, pqac-00000017). |


*Table: This table summarizes the current functional annotation of Schizosaccharomyces pombe bst1 (SPAC824.02/Q9UT41), integrating direct fission-yeast evidence with conserved mechanistic evidence from BST1/PGAP1 family studies. It highlights the enzyme’s reaction, pathway role, localization, phenotypes, quantitative findings, and cross-species support.*