The first step of glycosylphosphatidylinositol biosynthesis is mediated by a complex of pig-a, pig-h, pig-c and gpi1

Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidylinositol (PI). This chemically simple step is genetically complex because three genes are required in both mammals and yeast. Mammalian PIG-A and PIG-C are homologous to yeast GPI3 and GPI2, respectively; however, mammalian PIG-H is not homologous to yeast GPI1. Here, we report cloning of a human homolog of GPI1 (hGPI1) and demonstrate that four mammalian gene products form a protein complex in the endoplasmic reticulum membrane. PIG-L, which is involved in the second step in GPI synthesis, GlcNAc-PI de-N-acetylation, did not associate with the isolated complex. The protein complex had GPI–GlcNAc transferase (GPI–GnT) activity in vitro, but did notmediate the second reaction. Bovine PI was utilized ~100-fold more efficiently than soybean PI as a substrate, and lyso PI was a very inefficient substrate. These results suggest that GPI– GnT recognizes the fatty acyl chains of PI. The unusu-ally complex organization of GPI–GnTmay be relevant to selective usage of PI and/or regulation