GPI biosynthesis in mammalian cells: CoA-dependent acylation of GlcN-PI

We have used microsomes prepared from murine lymphoma cell lines to investigate the individual reactions by which glycosylphosphatidylinositol (GPI) is synthesized in mammalian cells. Previously, GTP was found to specifically stimulate the second reaction in the pathway, the deacetylation of GlcNAc-PI to GlcN-PI. An additional GPI precursor was detected in incubations with GTP and was found to be GlcN-PI(acyl), the glycolipid proposed to be the third intermediate in mammalian GPI biosynthesis. Investigation into the factors that affect the formation of GlcN-PI(acyl) revealed that, in the presence of GTP, the addition of either CoA or palmitoyl-CoA to the incubation greatly enhanced the amount of this product made. CoA stimulation of this reaction persisted even when ATP was depleted and no formation of acyl-CoA was possible, indicating that the free CoA rather than an acyl-CoA is the actual effector of GlcN-PI acylation. Therefore, we propose that the third reaction in mammalian GPI biosynthesis is catalyzed by a CoA-dependent transacylase rather than an acyl-CoA acyltransferase.