In the endoplasmic reticulum, the precursor form of urokinase plasminogen activator receptor is transamidated at residue 305, replacing the 30 carboxyterminal residues of the protein with an acylated glucosylphosphatidylinositol (acyl-GPI) moiety. The released carboxyterminal propeptide has no known function. The reaction is catalyzed by GPI transamidase, a complex of at least five proteins associated with the lumenal surface of the endoplasmic reticulum membrane (Yu et al. 1997; Ohishi et al. 2001; Hong et al. 2003).