Cell-free synthesis of glycosyl-phosphatidylinositol precursors for the glycolipid membrane anchor of Trypanosoma brucei variant surface glycoproteins. Structural characterization of putative biosynthetic intermediates.
Trypanosome variant surface glycoproteins exemplify a class of eukaryotic cell surface glycoproteins that rely on a carboxyl-terminal covalently-attached inositol-containing glycophospholipid for membrane attachment. The glycolipid anchor is acquired soon after translation of the polypeptide, apparently by replacement of a short carboxyl-terminal peptide sequence with a prefabricated glycolipid. A candidate glycolipid precursor (referred to as P2), and a related glycolipid (P3) have been identified recently in polar lipid extracts from trypanosomes. In this paper we describe the synthesis of P2 and P3 by trypanosome membranes. Analyses of organic solvent extracts from membranes incubated with radioactive sugar nucleotides (GDP-[3H]mannose or UDP-[3H]GlcNAc) showed a spectrum of labelled lipids, ranging from partially glycosylated species to the final products, P2 and P3. Structural analyses of these putative biosynthetic intermediates suggest that glycolipid assembly occurs via the sequential glycosylation of phosphatidylinositol.