Component of the GPI transamidase complex. May be involved in the recognition of either the GPI attachment signal or the lipid portion of GPI (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Forms a complex with PIGK/GPI8, PIGS, PIGT and GPAA1/GAA1. Belongs to the PIGU family.