Functional analysis of the first mannosyltransferase (PIG-M) involved in glycosylphosphatidylinositol synthesis in Plasmodium falciparum.

The mammalian glycosylphosphatidylinositol (GPI) anchor consists of three mannoses attached to acylated GlcN-(acyl)PI to form Man(3)-GlcN-(acyl)PI. The first of the three mannose groups is attached to an intermediate to generate Man-GlcN-(acyl)PI by the first mannosyltransferase (GPI-MT-I). Mammalian and protozoan GPI-MT-I have different substrate specificities. PIG-M encodes the mammalial GPI-MT-I which has 423 amino acids and multiple transmembrane domains. In this work we cloned PIG-M homologues from humans, Plasmodium falciparum (PfPIG-M), and Saccharomyces cerevisiae (GPI14), to test whether they could complement GPI-MT-I-deficient mammalian cells, since this biosynthetic step is likely to be a good target for selective screening of inhibitors against many pathogenic organisms. PfPIG-M partially restored cell surface expression of the GPI-anchored protein CD59 in PIG-M deficient mammalian cells, and first mannose transfer activity in vitro; however, this was not the case for GPI14.