The compound diisopropylfluorophosphate (DFP) selectively inhibits an inositol deacylase activity in living trypanosomes that, together with the previously described phenylmethylsulfonyl fluoride (PMSF)-sensitive inositol acyltransferase, maintains a dynamic equilibrium between the glycosylphosphatidylinositol (GPI) anchor precursor, glycolipid A [NH(CH)PO-6Manax1-2Mana1-6Mana1-4GlcNa1-6myo -inositol-1-PO-sn-1,2-dimyristoylglcerol], and its inositol acylated form, glycolipid C. Experiments using DFP in living trypanosomes and a trypanosome cell-free system suggest that earlier GPI intermediates are also in equilibrium between their inositol acylated and non-acylated forms. However, unlike mammalian and yeast cells, bloodstream form trypanosomes do not appear to produce an inositol acylated form of glucosaminyl-phosphatidylinositol (GlcN-PI). A specific function of inositol acylation in trypanosomes may be to enhance the efficiency of ethanolamine phosphate addition to the ManGlcN-(acyl)PI intermediate. Inositol deacylation appears to be a prerequisite for fatty acid remodelling of GPI intermediates that leads to the exclusive presence of myristic acid in glycolipid A and, ultimately, in the variant surface glycoprotein (VSG). In the presence of DFP, the de novo synthesis of GPI precursors cannot proceed beyond glycolipid C' (the un-remodelled version of glycolipid C) and lyso-glycolipid C'. Under these conditions glycolipid C'-type GPI anchors appear on newly synthesized VSG molecules. However, the efficiencies of both anchor addition to VSG and N-glycosylation of VSG were significantly reduced. A modified model of the GPI biosynthetic pathway in bloodstream form African trypanosomes incorporating these findings is presented.
|Number of pages||14|
|Publication status||Published - 1 Jan 1995|