Mediates GPI anchoring in the endoplasmic reticulum, by replacing a protein's C-terminal GPI attachment signal peptide with a pre-assembled GPI. During this transamidation reaction, the GPI transamidase forms a carbonyl intermediate with the substrate protein. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Forms a complex with CDC91, GPI16, GPI17 and GAA1. The disulfide bond between GPI8 and GPI16 is important for normal enzyme activity. Present with 1560 molecules/cell in log phase SD medium. Belongs to the peptidase C13 family.