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 s urface proteins. Its structure and biosynthetic pathway are generally conse rved among eukaryotic organisms, with number of differences. In particular, mammalian and protozoan mannosyltransferases needed for addition of the fi rst mannose (GPI-MT-I) have different substrate specificities and are targe ts of species-specific inhibitors of GPI biosynthesis, GPI-MT-I, however, h as not been molecularly characterized. Characterization of GPI-MT-I would a lso help to clarify the topology of GPI biosynthesis, Here, we report a hum an cell line defective in GPI-MT-I and the gene responsible, PIG-M, PIG-M e ncodes a new type of mannosyltransferase of 423 amino acids, bearing multip le transmembrane domains, PIG-M has a functionally important DXD motif, a c haracteristic of many glycosyltransferases, within a domain facing the lume n of the endoplasmic reticulum (ER), indicating that transfer of the first mannose to GPI occurs on the lumenal side of the ER membrane.