THE FATTY-ACIDS IN UNREMODELLED TRYPANOSOME GLYCOSYL-PHOSPHATIDYLINOSITOLS

Glycolipid A, the precursor of the glycosyl-phosphatidylinositol (GPI) anchor of the trypanosome variant surface glycoprotein, is constructe d in two phases. First, the glycan is assembled on phosphatidylinosito l (PI), yielding a glycolipid termed A'. Second, glycolipid A' undergo es fatty acid remodelling, by deacylation and reacylation, to become t he dimyristoyl species glycolipid A. In this paper, we examine the fat ty acid content of glycolipid A' and its cellular progenitors. A' cont ains exclusively stearate at the sn-1 position and a complex mixture o f fatty acids (including 18:0, 18:1, 18:2, 20:4 and 22:6) at sn-2. Pre sumably these fatty acids derive from stearate-containing PI species w hich initially enter the biosynthetic pathway. We compared the diacylg lycerol species from glycolipid A' with those from phosphatidylinosito l to determine whether a subset of stearate-containing PIs is utilized for GPI biosynthesis. We found that the spectrum of stearate-containi ng diacylglycerols in PI is similar to that in A', although the propor tions of each compound differ. Total PI in general was highly enriched in stearate-containing species. Differences in composition between gl ycosylated PI and total cellular PI may be due to the substrate specif icity of the sugar transferase which initiates the GPI biosynthetic pa thway. Alternatively, the species of PI present at the endoplasmic ret iculum site of GPI biosynthesis may differ from those in total PI.