PGAP1

PGAP1 is an endoplasmic reticulum-resident GPI inositol-deacylase that catalyzes a key post-attachment remodeling step of glycosylphosphatidylinositol-anchored proteins (GPI-APs), removing the acyl chain linked to the inositol of the GPI anchor immediately after GPI is transferred to a nascent protein (PMID:14734546). It is a serine hydrolase whose activity depends on a conserved catalytic serine, mutation of which abolishes deacylation (PMID:14734546). Cryo-EM structures resolve a 10-transmembrane architecture in which GPI-AP acyl chains are accommodated in a guitar-shaped hydrophobic cavity, while luminal glycan-mediated interactions confer substrate fidelity and prevent hydrolysis of bulk membrane lipids, with substrate entry and product release modeled as a 'drawing compass' motion (PMID:38167496). This deacylation is required for efficient ER-to-Golgi maturation and trafficking of GPI-APs (PMID:14734546). The enzymatic function is conserved across kingdoms: the Arabidopsis ortholog HLD1/AtPGAP1 supports GPI-AP membrane release and is essential for self-incompatibility (PMID:35316654). Loss of PGAP1 activity causes developmental and reproductive defects in mice, including otocephaly, perinatal lethality, and male infertility from failed sperm–zona pellucida adhesion (PMID:17711852), and in humans biallelic loss-of-function mutations abolish deacylase activity and cause intellectual disability and encephalopathy (PMID:24784135, PMID:25804403).

1. 2004 High

   Established the molecular identity and catalytic mechanism of PGAP1, answering what enzyme performs GPI inositol deacylation and how it acts.

   Evidence Catalytic serine mutagenesis with functional rescue in deacylase-deficient CHO cells and PI-PLC resistance readout

   PMID:14734546

   Open questions at the time

   • No structural basis for catalysis or substrate selectivity at this stage
   • Physiological consequence of deacylation not yet defined
2. 2004 High

   Linked the deacylation reaction to a cell-biological function, showing it is required for efficient ER-to-Golgi trafficking of GPI-APs.

   Evidence Pulse-chase and maturation/trafficking assays in isogenic PGAP1-deficient CHO cells

   PMID:14734546

   Open questions at the time

   • Mechanism by which retained acyl chain delays export unresolved
   • Whether all GPI-APs depend equally on deacylation unknown
3. 2007 High

   Demonstrated the in vivo physiological requirement for PGAP1, connecting deacylation to organismal development and sperm function.

   Evidence Constitutive PGAP1 knockout mouse with fertility, IVF, and zona pellucida binding assays

   PMID:17711852

   Open questions at the time

   • Which specific GPI-AP substrates mediate sperm-zona adhesion not identified
   • Cause of otocephaly at the GPI-AP level not defined
4. 2014 High

   Confirmed PGAP1 as the sole deacylase for this remodeling step in human cells and tied its loss to human disease.

   Evidence PI-PLC sensitivity assay on patient B lymphoblastoid cells with wild-type cDNA rescue

   PMID:24784135

   Open questions at the time

   • Genotype-phenotype relationship across mutation types not established
   • Downstream GPI-AP defects causing neurological phenotype unmapped
5. 2015 High

   Extended the human disease link by functionally validating additional loss-of-function variants and dose-dependence in carriers.

   Evidence Mutant vs wild-type construct rescue in deacylase-deficient CHO cells and PI-PLC sensitivity in patient and heterozygous carrier LCLs

   PMID:25804403

   Open questions at the time

   • Clinical spectrum across PGAP1 variants not delineated
   • No structural rationale for variant severity
6. 2022 Medium

   Showed deep evolutionary conservation of the deacylase function, establishing its role in GPI-AP membrane release in plants.

   Evidence Self-incompatibility suppressor screen in Arabidopsis with GFP-SKU5 GPI-AP release reporter and in vivo deacylase assay

   PMID:35316654

   Open questions at the time

   • Performed in non-mammalian system; direct relevance to mammalian PGAP1 inferred not shown
   • Link between deacylation and GPI-AP cleavage/release mechanistically incomplete
7. 2024 High

   Resolved the structural basis of catalysis and substrate fidelity, explaining how PGAP1 selects GPI-AP acyl chains over bulk membrane lipids.

   Evidence Cryo-EM structures at 2.66–2.84 Å with biochemical activity assays and substrate-cavity analysis

   PMID:38167496

   Open questions at the time

   • No structure of an enzyme-substrate or product complex confirming the 'drawing compass' model
   • Regulation of enzyme activity in the ER not addressed

• The specific GPI-AP substrates whose mistrafficking underlies the neurological and reproductive phenotypes remain unidentified.
• No substrate-resolved map linking deacylation failure to tissue-specific disease
• Regulatory inputs controlling PGAP1 activity unknown