Identification of a glycolipid precursor of the Trypanosoma brucei variant surface glycoprotein

The variant surface glycoprotein (VSG) of Trypanosoma brucei has a glycolipid covalently attached to its C terminus. This glycolipid, which anchors the protein to the cell membrane, is attached to the VSG polypeptide within 1 min after translation (Bangs, J.D., Hereld, D., Krakow, J.L., Hart, G.W., and Englund, P.T. (1985) Proc. Natl. Sci. U.S.A. 82, 3207-3211). This rapid processing suggests that, prior to incorporation, the glycolipid may exist in the cell as a preformed precursor which is transferred to the VSG polypeptide en bloc. We have isolated a molecule which has properties consistent with it being a VSG glycolipid precursor. It is highly polar and can be labeled by [³H] myristate but not by [³H]palmitate. It reaches steady state during continuous labeling with [³H]myristate and shows rapid turnover in pulse-chase experiments, suggesting that it is a metabolic intermediate rather than an end product. When treated with HNO₂ it liberates phosphatidylinositol, as does VSG (Ferguson, M.A.J., Low, M.G., and Cross, G.A.M. (1985) J. Biol. Chem. 260, 14547-14555). Also, like VSG, it releases a compound which co-migrates on thin layer chromatography with dimyristylglycerol when treated with the purified endogenous phospholipase C from trypanosomes. After treatment with this lipase, the putative precursor can be immunoprecipitated by antibodies directed against the C-terminal cross-reactive antigenic determinant of the VSG. These data provide strong evidence that this glycolipid is a VSG precursor.