Lipid raft association of carboxypeptidase E is necessary for its function as a regulated secretory pathway sorting receptor

Membrane carboxypeptidase E (CPE) is a sorting receptor for targeting prohormones, such as pro-opiomelanocortin, to the regulated secretory pathway in endocrine cells. Its membrane association is necessary for it to bind a prohormone sorting signal at the trans-Golgi network (TGN) to facilitate targeting. In this study, we examined the lipid interaction of CPE in bovine pituitary secretory granule membranes, which are derived from the TGN. We show that CPE is associated with detergent-resistant lipid domains, or rafts, within secretory granule membranes. Lipid analysis revealed that these rafts are enriched in glycosphingolipids and cholesterol. Pulse-chase and subcellular fractionation experiments in AtT-20 cells show that the association of CPE with membrane rafts occurred only after it reached the Golgi. Cholesterol depletion resulted in dissociation of CPE from secretory granule membranes and decreased the binding of prohormones to membranes, lit vivo cholesterol depletion using lovastatin resulted in the lack of sorting of CPE and its cargo to the regulated secretory pathway. We propose that the sorting receptor function of CPE necessitates its interaction with glycosphingolipid-cholesterol rafts at the TGN, thereby anchoring it in position to bind to its prohormone cargo.