Affinage

PIGK

GPI-anchor transamidase · UniProt Q92643

Length
395 aa
Mass
45.3 kDa
Annotated
2026-06-10
21 papers in source corpus 14 papers cited in narrative 14 extracted findings
Cross-family judge vs UniProt: tie faithfulness: 7/7 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

PIGK (GPI8/PIG-K) is the catalytic cysteine-protease subunit of the endoplasmic reticulum GPI transamidase complex, which cleaves the C-terminal GPI attachment signal of precursor proteins and transfers a preassembled GPI anchor, thereby generating cell-surface GPI-anchored proteins (PMID:11483512, PMID:32220290). Within this five-subunit machine PIGK assembles with PIG-T, GAA1, PIG-S and PIG-U; PIG-T both stabilizes the complex and forms an intermolecular disulfide bond with PIGK that is required for full transamidase activity, while the soluble domains of PIGK and GAA1 interact directly to nucleate the catalytic core (PMID:11483512, PMID:12582175, PMID:28893510). Catalysis proceeds through a carbonyl/thioester intermediate formed with substrate, depends on a cysteine (Cys/His) mechanism inferred from chemical inhibition, and the assembled but enzymatically inactive complex is sufficient to hold the proform of substrate proteins (PMID:11483512, PMID:12582175, PMID:11042127, PMID:32081427). PIGK that fails to incorporate into the complex, or that lacks its partner PIGT, is unstable and cleared by Hrd1 (SYVN1)-mediated ERAD (PMID:34193731). Loss of PIGK function abolishes surface GPI-anchored protein expression and is the basis of a bi-allelic recessive neurodevelopmental disorder, with patient-derived variants failing to rescue surface GPI-AP levels in knockout cells (PMID:32220290, PMID:38902431). In vivo, PIGK loss impairs axonal voltage-gated sodium channel localization and neuronal excitability, causes neuronal apoptosis driven in part by excessive unfolded protein response activation, and is required for embryonic development (PMID:26133798, PMID:38902431, PMID:39521780, PMID:33392778). A non-canonical role promoting lipophagy through an ABHD5–PPARα axis in colorectal cancer cells has also been reported (PMID:40975507).

Mechanistic history

Synthesis pass · year-by-year structured walk · 13 steps
  1. 2000 High

    Establishing that GPI8 is itself the catalytic component answered whether the transamidase activity resides in this subunit rather than an associated factor.

    Evidence Cell-free reconstitution of GPI anchoring in lumenal-protein-depleted trypanosome membranes with recombinant Leishmania GPI8, abolished by iodoacetamide

    PMID:11042127

    Open questions at the time
    • Does not define the catalytic residues at atomic resolution
    • Performed in a protozoan system rather than the mammalian complex
  2. 2001 High

    Defining the multi-subunit composition showed that GPI8/PIGK acts within a stable complex and clarified how the complex is held together.

    Evidence Mouse F9 cell knockout, reciprocal co-immunoprecipitation, and in vitro transamidase assay detecting the carbonyl intermediate

    PMID:11483512

    Open questions at the time
    • Stoichiometry of the full complex not resolved
    • Roles of individual subunits in catalysis vs. assembly not fully separated
  3. 2001 Medium

    Cross-species complementation confirmed the catalytic function of GPI8 is evolutionarily conserved.

    Evidence Heterologous complementation of an S. cerevisiae GPI8 anchoring mutant by S. pombe GPI8

    PMID:11124699

    Open questions at the time
    • Single genetic readout
    • Does not address mammalian-specific regulation
  4. 2003 High

    Identifying the PIGK–PIGT disulfide bond explained a structural requirement for full enzymatic activity and showed the assembled complex captures substrate independently of catalysis.

    Evidence Site-directed cysteine-to-serine mutagenesis, in vitro transamidase assay, and co-purification of substrate proform with an inactive five-subunit complex

    PMID:12582175

    Open questions at the time
    • Disulfide is required for full but not all activity, leaving its precise mechanistic role open
    • Substrate-binding subunit interface not mapped
  5. 2010 Medium

    A low-resolution solution structure provided the first architectural view of the PIGK soluble domain.

    Evidence Circular dichroism and SAXS ab initio modeling of the yeast PIGK soluble domain (residues 24–337)

    PMID:21134462

    Open questions at the time
    • No high-resolution or catalytic-site structure
    • No functional validation of the modeled architecture
  6. 2017 Medium

    Reconstituting the PIGK–GAA1 soluble-domain interaction showed which subunit pair forms the catalytic core and its stoichiometry.

    Evidence Recombinant GST pulldown, native gel, and size-exclusion chromatography of yeast Gpi8 and Gaa1 soluble domains showing α2β2 assembly

    PMID:28893510

    Open questions at the time
    • Soluble domains only, not the full membrane complex
    • How this core engages PIGT/PIGS/PIGU not addressed
  7. 2020 Medium

    Direct enzymatic characterization confirmed the cysteine-protease/metal-dependent endopeptidase nature of the catalytic subunit and linked it to GPI precursor accumulation.

    Evidence Cell-free endopeptidase assay with CaGPI8 heterozygous mutants and Cys/His-modifying chemical inhibitors

    PMID:32081427

    Open questions at the time
    • Catalytic residues not individually mutated in this assay
    • Metal dependence vs. classic cysteine-protease chemistry not fully reconciled
  8. 2020 High

    Bi-allelic PIGK variants were established as a loss-of-function cause of decreased surface GPI-anchored proteins in humans.

    Evidence Flow cytometry of patient blood/fibroblasts and overexpression rescue in patient cells and PIGK-knockout lines across multiple families

    PMID:32220290

    Open questions at the time
    • Mechanism linking GPI-AP loss to the neurodevelopmental phenotype not resolved here
    • Genotype–phenotype correlations across variants incomplete
  9. 2021 Medium

    Identifying Hrd1-mediated ERAD as the quality-control route for unassembled PIGK explained how stoichiometry of the complex is enforced.

    Evidence Proteasome inhibition and Hrd1 (SYVN1) knockdown/knockout with PIGK stability immunoblotting, showing PIGK is unstable without PIGT

    PMID:34193731

    Open questions at the time
    • Ubiquitination sites on PIGK not mapped
    • Single lab, not reconstituted
  10. 2021 Medium

    In vivo models established PIGK as essential for development and neuronal survival.

    Evidence Morpholino knockdown in zebrafish causing motor neuron dysplasia and apoptosis, and patient-variant knock-in mice showing embryonic lethality

    PMID:33392778

    Open questions at the time
    • Embryonic lethality limits adult phenotype analysis
    • Cell-type-specific contributions not dissected
  11. 2024 Medium

    Functional and in vivo work connected loss of GPI-anchored proteins to sodium channel mislocalization and impaired neuronal excitability.

    Evidence CHO rescue assay quantifying transamidase activity of p.Met161Val plus zebrafish neuronal sodium-channel immunofluorescence and electrophysiology

    PMID:26133798 PMID:38902431

    Open questions at the time
    • Identity of the relevant GPI-anchored protein(s) mediating channel localization unknown
    • Mechanistic link between GPI-AP loss and channel trafficking not defined
  12. 2024 Medium

    iPSC and conditional-knockout models linked PIGK deficiency to UPR-driven apoptosis as a disease mechanism.

    Evidence Pcp2-cre conditional knockout mice with Purkinje cell loss and patient-mutant iPSC NPCs showing UPR activation rescued by UPR inhibitor

    PMID:39521780

    Open questions at the time
    • Whether UPR is the primary vs. secondary driver of neurodegeneration unclear
    • Link from GPI-AP loss to UPR induction not mechanistically traced
  13. 2025 Low

    A proposed non-canonical PIGK function in cancer suggested a lipophagy-promoting role distinct from its transamidase activity.

    Evidence Colorectal cancer cell proliferation/autophagy assays with PIGK overexpression/knockdown, ABHD5 analysis, and xenografts

    PMID:40975507

    Open questions at the time
    • Correlative single-lab evidence not independently confirmed
    • Mechanistic disconnect from the established ER transamidase role unresolved

Open questions

Synthesis pass · forward-looking unresolved questions
  • How loss of specific GPI-anchored proteins leads to the cell-type-specific neuronal and developmental phenotypes, and whether the cancer-associated lipophagy role is genuine, remain open.
  • No high-resolution structure of the human catalytic site
  • Causal GPI-AP substrates for neuronal phenotypes unidentified
  • Non-canonical functions not independently validated

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 4 GO:0016787 hydrolase activity 2
Localization
GO:0005783 endoplasmic reticulum 2
Pathway
R-HSA-392499 Metabolism of proteins 2
Partners
GAA1PIGSPIGTSYVN1
Complex memberships
GPI transamidase complex

Evidence

Reading pass · 14 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2001 PIGK (GPI8) is a core catalytic subunit of the GPI transamidase complex, which also contains GAA1, PIG-S, and PIG-T. PIG-S and PIG-T form a protein complex with GAA1 and GPI8; PIG-T stabilizes the complex by maintaining expression of GAA1 and GPI8. GPI transamidase forms a carbonyl intermediate with substrate proteins during the transamidation reaction. Gene knockout by homologous recombination in mouse F9 cells, co-immunoprecipitation, in vitro transamidase assay The EMBO journal High 11483512
2003 PIGK (GPI8) and PIG-T form a functionally important intermolecular disulfide bond between conserved cysteine residues. Mutation of these cysteines to serine reduced but did not abolish GPI transamidase activity in vitro, indicating the disulfide bond is required for full activity but not essential. An inactive GPI transamidase complex (containing non-functional GPI8 and four other subunits) co-purifies with the proform of substrate proteins, demonstrating all five subunits are sufficient to hold substrate proteins. Site-directed mutagenesis of conserved cysteines, in vitro transamidase assay, co-purification/immunoprecipitation with antibodies against GPI8 and PIG-T The Journal of biological chemistry High 12582175
2000 Trypanosome GPI8 is a soluble lumenal ER protein that constitutes the catalytic component of the GPI transamidase. Recombinant Leishmania GPI8 (expressed in E. coli) reconstitutes GPI anchoring activity in high-pH-washed trypanosome membranes depleted of lumenal ER proteins, and this activity is abolished by iodoacetamide treatment (indicating cysteine dependence). Cell-free reconstitution assay, high-pH washing of membranes, recombinant protein complementation, iodoacetamide inhibition, immunoblotting The Biochemical journal High 11042127
2017 The soluble domains of Gpi8 and Gaa1 (yeast GPI transamidase subunits) interact directly and specifically without requiring other subunits, forming a complex with apparent α2β2 stoichiometry (two Gpi8 homodimers associated with two Gaa1 subunits). Recombinant protein co-purification (GST pulldown), native gel analysis, size exclusion chromatography Archives of biochemistry and biophysics Medium 28893510
2010 The soluble domain of yeast PIG-K (residues 24–337) adopts a secondary structure comprising ~52% α-helix and ~12% β-sheet, and forms an elongated particle in solution with an egg-like portion and a small globular segment linked by a stalk (~1.9 nm), as determined by SAXS-based low-resolution solution structure. Circular dichroism spectroscopy, small-angle X-ray scattering (SAXS) with ab initio modeling Journal of structural biology Medium 21134462
2020 Candida albicans Gpi8 (the GPI transamidase catalytic subunit) functions as a metal-dependent endopeptidase. The heterozygous CaGPI8 strain accumulates complete precursor GPI glycolipids, has reduced cell surface GPI-APs, and shows low endopeptidase activity in a cell-free assay. Activity is sensitive to agents that modify Cys/His residues, consistent with a cysteine protease mechanism. Cell-free endopeptidase assay, CaGPI8 heterozygous mutant and revertant analysis, chemical inhibition with Cys/His modifying agents Biochemical and biophysical research communications Medium 32081427
2021 Unassembled PIGK (not incorporated into the GPI transamidase complex) is degraded via the proteasome-dependent ERAD pathway, with Hrd1 (SYVN1) identified as the responsible ubiquitin E3 ligase. PIGK is unstable without its partner PIGT. Proteasome inhibitor treatment, Hrd1 knockdown/knockout, immunoblotting of PIGK stability Cell structure and function Medium 34193731
2020 Bi-allelic PIGK variants cause decreased cell surface presence of GPI-anchored proteins, as shown by flow cytometry of blood and fibroblasts from affected individuals. Overexpression of wild-type PIGK in patient fibroblasts or PIGK-knockout cells rescued GPI-AP surface levels, while two tested patient-derived mutant variants failed to rescue, demonstrating these variants are loss-of-function. Flow cytometry for GPI-AP surface levels, PIGK overexpression rescue in patient fibroblasts and knockout cell lines American journal of human genetics High 32220290
2015 Pigk mutation in zebrafish (macho mutant) causes touch insensitivity by reducing voltage-gated sodium current (INa) amplitude and abolishing action potential firing in Rohon-Beard sensory neurons. Ubiquitous expression of wild-type pigk rescues touch response; expression restricted to sensory neurons rescues INa and action potential firing but not the full behavioral touch response, indicating PIGK is required in cells beyond sensory neurons for circuit-level behavior. ENU mutagenesis screen, whole-cell patch clamp electrophysiology, morpholino knockdown phenocopy, transgenic wild-type pigk rescue (ubiquitous and sensory-neuron-specific) Journal of neurophysiology High 26133798
2024 The PIGK p.Met161Val missense variant reduces GPI transamidase activity in PIGK-deficient CHO rescue experiments. In pigk-deficient zebrafish, axonal localization of voltage-gated sodium channels and action potential generation are impaired in neurons, linking loss of GPI-anchored proteins to neuronal sodium channel mislocalization. CHO cell rescue assay for GPI transamidase activity, pigk mutant zebrafish neuronal immunofluorescence for sodium channel localization, electrophysiology Journal of human genetics Medium 38902431
2024 PIGK deficiency in Purkinje cells (Pcp2-cko mice) leads to cerebellar atrophy, ataxia, and progressive Purkinje cell loss accompanied by increased apoptosis and neuroinflammation. In iPSCs carrying a PIGK c.87dupT mutation, increased apoptosis and accelerated neural rosette/NPC differentiation are associated with excessive unfolded protein response (UPR) activation; UPR inhibitor treatment rescues these phenotypes. Conditional knockout mouse (Pcp2-cre), iPSC disease model with patient-derived PIGK mutant, RNA-seq of NPCs, UPR pathway inhibitor rescue experiments Cell death & disease Medium 39521780
2021 PIGK knockdown in zebrafish causes severe developmental defects including primary motor neuronal dysplasia and extensive neural cell apoptosis. Homozygous knock-in mouse models bearing patient-derived PIGK variants (affecting Asp204 or producing truncation) result in complete embryonic lethality, demonstrating PIGK is essential for embryonic development. Morpholino knockdown in zebrafish, knock-in mouse models, immunofluorescence for apoptosis markers Human genetics Medium 33392778
2001 Schizosaccharomyces pombe GPI8 (SpGPI8) encodes a 411 amino acid protein with 53.5% identity to S. cerevisiae GPI8 and functionally complements a S. cerevisiae GPI8 anchoring mutant, demonstrating conservation of catalytic function. Heterologous complementation of S. cerevisiae GPI8 mutant Yeast (Chichester, England) Medium 11124699
2025 PIGK upregulates ABHD5 expression, activating a PIGK-ABHD5-PPARα signaling axis that promotes lipophagy and suppresses colorectal cancer cell proliferation. In vitro PIGK overexpression promotes autophagy; this effect is mediated through ABHD5. In vitro CRC cell proliferation and autophagy assays, PIGK overexpression and knockdown, ABHD5 expression analysis, xenograft mouse tumorigenesis experiments Cellular signalling Low 40975507

Source papers

Stage 0 corpus · 21 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2001 PIG-S and PIG-T, essential for GPI anchor attachment to proteins, form a complex with GAA1 and GPI8. The EMBO journal 143 11483512
2003 Essential roles for GPI-anchored proteins in African trypanosomes revealed using mutants deficient in GPI8. Molecular biology of the cell 92 12631733
2019 The Temperature-Dependent Retention of Introns in GPI8 Transcripts Contributes to a Drooping and Fragile Shoot Phenotype in Rice. International journal of molecular sciences 73 31906256
2003 Two subunits of glycosylphosphatidylinositol transamidase, GPI8 and PIG-T, form a functionally important intermolecular disulfide bridge. The Journal of biological chemistry 68 12582175
2020 Bi-allelic Variants in the GPI Transamidase Subunit PIGK Cause a Neurodevelopmental Syndrome with Hypotonia, Cerebellar Atrophy, and Epilepsy. American journal of human genetics 33 32220290
2006 Recent developments in the molecular, biochemical and functional characterization of GPI8 and the GPI-anchoring mechanism [review]. Molecular membrane biology 33 16785205
2000 Soluble GPI8 restores glycosylphosphatidylinositol anchoring in a trypanosome cell-free system depleted of lumenal endoplasmic reticulum proteins. The Biochemical journal 30 11042127
2016 The Glycosylphosphatidylinositol Anchor Biosynthesis Genes GPI12, GAA1, and GPI8 Are Essential for Cell-Wall Integrity and Pathogenicity of the Maize Anthracnose Fungus Colletotrichum graminicola. Molecular plant-microbe interactions : MPMI 25 27937175
2021 Loss of PIGK function causes severe infantile encephalopathy and extensive neuronal apoptosis. Human genetics 15 33392778
2010 Structural insight into the glycosylphosphatidylinositol transamidase subunits PIG-K and PIG-S from yeast. Journal of structural biology 12 21134462
2012 A single nucleotide polymorphism in the human PIGK gene associates with low PIGK expression in colorectal cancer patients. International journal of oncology 9 22824918
2017 The soluble domains of Gpi8 and Gaa1, two subunits of glycosylphosphatidylinositol transamidase (GPI-T), assemble into a complex. Archives of biochemistry and biophysics 8 28893510
2015 pigk Mutation underlies macho behavior and affects Rohon-Beard cell excitability. Journal of neurophysiology 7 26133798
2001 The Schizosaccharomyces pombe GPI8 gene complements a Saccharomyces cerevisiae GPI8 anchoring mutant. Yeast (Chichester, England) 7 11124699
2021 Hrd1-dependent Degradation of the Unassembled PIGK Subunit of the GPI Transamidase Complex. Cell structure and function 4 34193731
2020 The caspase-like Gpi8 subunit of Candida albicans GPI transamidase is a metal-dependent endopeptidase. Biochemical and biophysical research communications 4 32081427
2024 PIGK defects induce apoptosis in Purkinje cells and acceleration of neuroectodermal differentiation. Cell death & disease 3 39521780
2016 Microsatellite polymorphism located immediately upstream of the phosphatidylinositol glycan, class K gene (PIGK) affects its expression, which correlates with tyrosinase activity in human melanocytes. Journal of dermatological science 2 27919619
2025 PIGK regulates lipophagy in colorectal cancer through ABHD5. Cellular signalling 1 40975507
2024 A novel homozygous variant of the PIGK gene caused by paternal disomy in a patient with neurodevelopmental disorder, cerebellar atrophy, and seizures. Journal of human genetics 1 38902431
2026 A Novel Phenotypic Presentation of Absence Seizures in a 15-Month-Old with PIGK-Related GPI Biosynthesis Disorder: A Case Report. Case reports in neurology 0 41836309

Missed literature

Know a paper Affinage missed for PIGK? Flag it for the maintainers and the community.

No submissions yet.