Glucose Induces Amphiphilic to Hydrophilic Conversion of a Subset of Glycosyl-Phosphatidylinositol-Anchored Ectoproteins in Yeast

Abstract

Previously, we have studied the lipolytic cleavage of a glycosyl-phosphatidylinositol (GPI)-anchored plasma membrane protein in yeast in response to a physiologically relevant external signal, i.e., transfer of spheroplasts from lactate to glucose medium (cf. Müller and Bandlow (1993)J. Cell. Biol.122, 325–336). In the present study the glucose-induced lipolytic processing ofmyo-[14C]inositol-labeled total GPI proteins of the plasma membrane and in particular of two such proteins, Gas1p and Gce1p, was examined in yeast spheroplasts. It was found that a small number of GPI proteins, among them Gce1p, are readily cleaved, whereas Gas1p and the majority of the GPI proteins are relatively little affected. Glucose-induced processing of Gce1p could be demonstrated also in intact cells. Increased GPI cleavage after exposure of cells or spheroplasts to glucose is not due to stimulation of cell surface expression of Gce1p, as the amount of total GPI-anchored Gce1p bound to plasma membranes is comparable in cells grown in glucose or lactate. In agreement with this, Brefeldin A added together with the label blocks transport of newly made Gce1p to the cell surface and, in the consequence, cleavage of labeled Gce1p in spheroplasted yeast cells. (The drug is ineffective in intact cells.) On the other hand, Brefeldin A does not significantly affect glucose-induced processing of inositol-labeled Gce1p at the plasma membrane when present during the period of nutritional upshift. We discuss that addition of glucose to the cells leads to the activation of a GPI-specific phospholipase which accepts only a subset of GPI proteins as substrates. This interpretation is additionally corroborated by the finding that purified [14C]inositol-labeled Gcep1p is lipolytically cleaved when incubated with Triton X-100-insoluble membrane complexes isolated from glucose-induced but not from uninduced spheroplasts. It is concluded that the phospholipase is present in these complexes and its state of activity is preserved during the preparation. GPI anchor cleavage in yeast appears to resemble strikingly the situation in insulin-responsive adipocytes subsequently to stimulation by insulin of glucose transport into these cells.