Protein targeting pathways and sorting signals in epithelial cells

A large body of knowledge has been accumulated in the last two decades on the nature of the mechanisms responsible for the polarization of epithelial cells (Rodriguez- Boulan and Nelson, 1989; Rodriguez-Boulan and Powell, 1992). Three fundamental types of mechanisms have emerged that contribute to epithelial polarity (Figure 7.1): (1) intracellular sorting and polarized delivery of proteins and lipids to the cell surface (Figure 7.1, center); (2) “trapping” interactions with domain-restricted cytoskeleton elements (Figure 7.1, right); and (3) diffusive restriction by the tight junctions (Figure 7.1, left). This chapter deals primarily with the first type.