Compartmentation and topology of glucosylceramide synthesis.

Evidence is presented supporting a model for glucosylceramide formation on the apoplastic side of the plasma membrane in plants. Glucosylceramide synthase and sterol glucosyltransferase were both localized to the plasma membrane. Whereas sterol glucosylation was sensitive to proteolytic enzymes, ceramide glucosylation was not. These results are consistent with our model in which steryl glucoside is synthesized on the cytosolic side of the membrane and then translocated across the membrane where it donates glucose to ceramide.