Initial enzyme for glycosylphosphatidylinositol biosynthesis requires PIG-P and is regulated by DPM2

Glycosylphosphatidylinositols (GPIs) are attached to the C-termini of many proteins, thereby acting as membrane anchors. Biosynthesis of GPI is initia ted by GPI-N-acetylglucosaminyltransferase (GPI-GnT), which transfers N-ace tylglucosamine from UDP-N-acetylglucosamine to phosphatidylinositol. GPI-Gn T is a uniquely complex glycosyltransferase, consisting of at least four pr oteins, PIG-A, PIG-H, PIG-C and GPI1. Here, we report that GPI-GnT requires another component, termed PIG-P, and that DPM2, which regulates dolichol-p hosphate-mannose synthase, also regulates GPI-GnT, PIG-P, a 134-amino acid protein having two hydrophobic domains, associates with PIG-A and GPI1. PIG -P is essential for GPI-GnT since a cell lacking PIG-P is GPI-anchor negati ve. DPM2, but not two other components of dolichol-phosphate-mannose syntha se, associates with GPI-GnT through interactions with PIG-A, PIG-C and GPI1 , Lec15 cell, a null mutant of DPM2, synthesizes early GPI intermediates, i ndicating that DPM2 is not essential for GPI-GnT; however, the enzyme activ ity is enhanced 3-fold in the presence of DPM2, These results reveal new es sential and regulatory components of GPI-GnT and imply co-regulation of GPI -GnT and the dolichol-phosphate-mannose synthase that generates a mannosyl donor for GPI.