SPECIFIC REQUIREMENTS FOR THE ER TO GOLGI TRANSPORT OF GPI-ANCHORED PROTEINS IN YEAST

GPI-anchored proteins are attached to the membrane via a glycosylphosp hatidylinositol-(GPI) anchor whose carbohydrate core is conserved in a ll eukaryotes. Apart from membrane attachment, the precise role of the GPI-anchor is not known, but it has been proposed to play a role in p rotein sorting, We have investigated the transport of the yeast GPI-an chored protein Gas1p, We identified two mutant strains in volved in ve ry different cellular processes that are blocked selectively in the tr ansport of GPI-anchored proteins before arrival to the Golgi. The end8 -1/lcb1-100 mutant is defective in ceramide synthesis. In vitro data s uggest a requirement for ceramides after the exit from the ER. We ther efore propose that ceramides might function in the fusion of a GPI-con taining vesicle with the Golgi, but we cannot exclude a role in the ER , The second mutant that blocks the transport of GPI-anchored proteins to the Golgi is ret1-1, a mutant in the cx-subunit of coatomer. In bo th mutants, GPI-anchor attachment is normal and in ret1-1 cells, the G PI-anchors are remodeled with ceramide to the same extent as in wild-t ype cells.