ABSTRACT
INTRODUCTION Epithelia form boundaries between different compartments in the body and allow vectorial flow of molecules and ions from one to the other side (Simons and Fuller, 1985). To serve the transport functions and to protect the cells from external environment, proteins and lipids have to be distributed polarized over the apical and basolateral plasma membrane domains. Generation and maintenance of surface polarity requires sorting and polarized transport of newly synthesized surface molecules (Rodriguez-Boulan and Powell, 1992; Simons, 1995). In recent studies determinants for intracellular sorting into the apical and basolateral pathways have been identified. The two routes appear to use fundamentally different mechanisms. Apical and basolateral determinants are not restricted to proteins in epithelial cells, but appear to function in all cell types to generate polarity. Therefore, epithelial cells provide a good tool to study general mechanisms of membrane trafficking which cannot be easily analysed in cells lacking obvious surface polarity. The aim of this review is to analyse the sorting mechanisms responsible for the polarized distribution of plasma membrane proteins in epithelia.
