EXPRESSION OF A VARIANT SURFACE GLYCOPROTEIN OF TRYPANOSOMA GAMBIENSEIN PROCYCLIC FORMS OF TRYPANOSOMA-BRUCEI SHOWS THAT THE CELL-TYPE DICTATES THE NATURE OF THE GLYCOSYLPHOSPHATIDYLINOSITOL MEMBRANE ANCHOR ATTACHED TO THE GLYCOPROTEIN

Procyclic forms of Trypanosoma brucei have been genetically modified t o express the major metacyclic variant surface glycoprotein (VSG varia nt AnTat 11.17) of Trypanosoma gambiense. The VSG is expressed in an i ntact membrane-bound form that can be detected over the entire plasma membrane, together with procyclin, and as a series of lower-molecular- mass fragments that are mostly soluble degradation products. The prese nce of degraded VSG in the cells and the culture medium suggests that VSG is not efficiently processed and/or efficiently folded when expres sed in procyclic cells. The level of procyclin expressed on the surfac e of these cells is slightly reduced, although there is no difference in procyclin mRNA levels. The intact membrane-bound form of the VSG is N-glycosylated with oligomannose structures and contains a glycosylph osphatidylinositol (GPI) membrane anchor that can be biosynthetically labelled with [H-3]ethanolamine. The anchor is sensitive to mammalian GPI-specific phospholipase D but, like the anchor of procyclin, it is resistant to the action of bacterial phosphatidylinositol-specific pho spholipase C. This pattern of phospholipase sensitivity suggests that the GPI anchor acquired by VSG when expressed in procyclics is acylate d on the inositol ring and therefore resembles a procyclic procyclin-t ype anchor rather than a trypomastigote VSG-type anchor with respect t o the lipid structure. The VSG expressed in procyclics was sensitive t o the action of a mixture of sialidase, beta-galactosidase and beta-he xosaminidase, suggesting that the VSG GPI anchor also contains a sialy lated polylactosamine side-chain modification similar to that describe d for procyclin. These results indicate that the nature of the protein expressed has little influence on the post-translational modification s performed in the secretory pathway of procyclic trypanosomes.