Establishing and maintaining epithelial cell polarity. Roles of protein sorting, delivery and retention.

Polarized epithelial cells are distinguished from many other cell types by the presence of distinct plasma membrane domains, termed apical and basal-lateral, which face different biological compartments separated by the epithelium (Rodriguez-Boulan and Nelson, 1989). An important problem is to understand the mechanisms involved in the development of the structural and functional organization of these membrane domains. It has been generally thought that membrane domains are generated and maintained by sorting membrane proteins in the trans-Go\gi network (TGN) into different vesicle populations, which are then delivered directly to the appropriate membrane domain (Simons and Wandinger-Ness, 1990). Here, we summarize recent studies on potential signals involved in the sorting of proteins in the TGN, and discuss them in the broader context of how cells generate cell surface polarity during development. We will consider three sequential steps that are part of the protein sorting machinery in these cells, and which together appear to be involved in generating and maintaining polarized distributions of proteins on the epithelial cell surface: (1) sorting of apical and basallateral membrane proteins in the TGN into different populations of transport vesicles; (2) transport to, and capture and docking of the transport vesicle at the appropriate cell surface domain; (3) retention of the protein in the membrane domain.