PIGS

PIGS (PIG-S) is an essential non-catalytic subunit of the endoplasmic reticulum GPI transamidase complex that transfers pre-assembled glycosylphosphatidylinositol (GPI) anchors onto substrate proteins (PMID:11483512). PIGS assembles into a stable complex with GAA1, GPI8, and PIG-T, and its loss abolishes GPI transfer by specifically blocking formation of the carbonyl intermediates that are the biochemical hallmark of the transamidation reaction (PMID:11483512). Structural characterization of the yeast ortholog (Gpi17p) indicates an elongated two-domain architecture, consistent with PIGS serving as a structural component spanning the transamidase complex (PMID:21134462). Beyond its role as a complex subunit and the carbonyl-intermediate step, no further catalytic or regulatory mechanism for PIGS has been characterized in the available corpus.

1. 2001 High

   Established that PIGS is an essential, dedicated subunit of the GPI transamidase machinery and pinpointed which step of the transamidation reaction it enables.

   Evidence Homologous recombination knockout in mouse F9 cells with a GPI anchoring/carbonyl-intermediate assay, plus co-immunoprecipitation of complex members and identification of the yeast ortholog Gpi17p

   PMID:11483512

   Open questions at the time

   • Whether PIGS contributes any catalytic activity or is purely structural/scaffolding within the complex was not resolved
   • The molecular role of PIGS in forming or stabilizing the carbonyl intermediate (e.g. substrate or GPI positioning) was not defined
   • No structural basis for how PIGS contacts GAA1, GPI8, or PIG-T was determined
2. 2010 Medium

   Provided the first structural view of the PIGS subunit, showing it is an elongated two-domain protein rather than a compact globular subunit.

   Evidence Small-angle X-ray scattering and circular dichroism on recombinant yeast ortholog (residues 38-467) produced in E. coli

   PMID:21134462

   Open questions at the time

   • Low-resolution solution envelope only; no atomic structure, no mutagenesis, and no functional validation
   • Performed on the yeast ortholog in isolation, not within the assembled transamidase complex or with the human protein
   • Does not assign which domain mediates complex assembly versus substrate handling

• The precise mechanistic contribution of PIGS to the transamidation chemistry and its contacts with the other transamidase subunits remain undefined.
• No high-resolution structure of human PIGS within the intact transamidase complex
• No definition of substrate or GPI-binding determinants on PIGS
• No link to disease established within this corpus