Structure of human glycosylphosphatidylinositol transamidase

Glycosylphosphatidylinositol (GPI) molecules are complex glycophospholipids and serve as membrane anchors for tethering many proteins to the cell surface. Attaching GPI to the protein in the endoplasmic reticulum (ER) is catalyzed by the transmembrane GPI transamidase (GPIT) complex, which is essential for maturation of the GPI-anchored proteins. The GPIT complex is known to be composed of five subunits: PIGK, PIGU, PIGT, PIGS and GPAA1. Here, we determined the structure of the human GPIT complex at a resolution of 3.1 angstrom using single-particle cryo-EM, elucidating its overall assembly. The PIGK subunit functions as the catalytic component, in which we identified a C206-H164-N58 triad that is critical for the transamination reaction. Transmembrane helices constitute a widely opened cleft, which is located underneath PIGK, serving as a GPI substrate-binding site. The ubiquitin E3 ligase RNF121 is visualized at the back of the complex and probably serves as a quality control factor for the GPIT complex. Cryo-EM structures of the human pentameric glycosylphosphatidylinositol transamidase (GPIT) complex reveal its subunit assembly and its catalytic and GPI-binding sites.

Automated summary

The GPI transamidase (GPIT) complex plays an important role in attaching GPI to proteins, facilitating the maturation of GPI-anchored proteins. The complex is composed of five subunits, with PIGK acting as the catalytic component and transmembrane helices serving as the GPI substrate-binding site. The ubiquitin E3 ligase RNF121 is also present, potentially functioning as a quality control factor for the GPIT complex.