Intracellular routes of apical and basolateral plasma membrane proteins to the surface of epithelial cells.

The asymmetric distribution of pumps, channels, receptors and enzymes between the apical and basolateral domains of the plasma membrane is essential for the vectorial function of transporting and secretory epithelia. Enveloped viruses that bud in a polarized fashion from the surface of epithelial cells in culture provide an excellent system to study the biogenesis of this asymmetry. Using this system, evidence has been accumulated indicating that apical and basolateral plasma membrane proteins follow the same pathway until they reach the Golgi apparatus, where they codistribute. At the level of the Golgi, or immediately after leaving this organelle, apical and basolateral glycoproteins are incorporated into separate "carrier vesicles", which transport them to the respective cell surface. Future work should establish the structural and biochemical nature of these vesicles and the molecular basis of this vectorial delivery of plasma membrane proteins in epithelial cells.