PHOSPHATIDYLETHANOLAMINE IS THE DONOR OF THE TERMINAL PHOSPHOETHANOLAMINE GROUP IN TRYPANOSOME GLYCOSYLPHOSPHATIDYLINOSITOLS

A variety of eukaryotic cell surface proteins, including the variant s urface glycoproteins of African trypanosomes, rely on a covalently att ached lipid, glycosylphosphatidylinositol (GPI), for membrane attachme nt. GPI anchors are synthesized in the endoplasmic reticulum by stepwi se glycosylation of phosphatidylinositol (via UDP-GlcNAc and dolichol- P-mannose) followed by the addition of phosphoethanolamine. The experi ments described in this paper are aimed at identifying the biosyntheti c origin of the terminal phosphoethanolamine group. We show that trypa nosome GPIs can be labelled via CDP-[H-3]ethanolamine or [beta-P-32]CD P-ethanolamine in a cell-free system, indicating that phosphoethanolam ine is acquired en bloc. In pulse-chase experiments with CDP-[H-3]etha nolamine we show that the GPI phosphoethanolamine is not derived direc tly from CDP-ethanolamine, but instead from a relatively stable metabo lite, such as phosphatidylethanolamine (PE), generated from CDP-ethano lamine in the cell-free system. To test the possibility that PE is the immediate donor of the GPI phosphoethanolamine moiety, we describe me tabolic labelling experiments with [H-3]serine and show that GPIs can be labelled in the absence of detectable radiolabelled CDP-ethanolamin e, presumably via [H-3]PE generated from [H-3]phosphatidylserine (PS). The data support the proposal that the terminal phosphoethanolamine g roup in trypanosome GPIs is derived from PE.