PIG-M transfers the first mannose to glycosylphosphatidylinositol on the lumenal side of the ER

Glycosylphosphatidylinositol (GPI) acts as a membrane anchor of many cell surface proteins. Its structure and biosynthetic pathway are generally conserved among eukaryotic organisms, with number of differences. In particular, mammalian and protozoan mannosyltransferases needed for addition of the first mannose (GPI-MT-I) have different substrate specificities and are targets of species-specific inhibitors of GPI biosynthesis, GPI-MT-I, however, has not been molecularly characterized. Characterization of GPI-MT-I would also help to clarify the topology of GPI biosynthesis, Here, we report a human cell line defective in GPI-MT-I and the gene responsible, PIG-M, PIG-M encodes a new type of mannosyltransferase of 423 amino acids, bearing multiple transmembrane domains, PIG-M has a functionally important DXD motif, a characteristic of many glycosyltransferases, within a domain facing the lumen of the endoplasmic reticulum (ER), indicating that transfer of the first mannose to GPI occurs on the lumenal side of the ER membrane.