Rafting across polarized cells

For raft proteins, direct delivery is not the travel route of choice, according to Roman Polishchuk, Jennifer Lippincott-Schwartz (NIH, Bethesda, MD), and colleagues. They find that, contrary to earlier evidence, glycosyl phosphatidylinositol (GPI)–anchored raft proteins make a detour to the basolateral membrane of polarized MDCK cells before crossing the cell to their final destination in the apical membrane.

Direct delivery had been plausible, as sorting of GPI-anchored proteins into rafts, and thus away from basolateral proteins, occurs in the trans-Golgi network (TGN) before either departs for plasma membranes. Few or no apical proteins were ever seen in the basolateral membrane. And finally, transport across the cell seemed unlikely: the main method of departure, clathrin-mediated endocytosis, did not transport rafts.

Lippincott-Schwartz recently found that rafts are continually endocytosed by a nonclathrin pathway, overcoming the...