Membrane polarity in epithelial cells: protein sorting and establishment of polarized domains

Epithelial cells go to great trouble to organize the subdomains of their plasma membranes. The apical and basolateral surfaces of polarized epithelia are equipped with markedly distinct populations of channels, carriers, and pumps. This anisotropy is an absolute prerequisite for vectorial solute and fluid transport. The physiological properties of an individual epithelial cell type are determined not only by its census of transport proteins but also by the manner in which these proteins are segregated between the apical and basolateral portions of the plasmalemma (Curr. Top. Membr. 39: 37-86, 1991). To achieve this asymmetry, an epithelial cell must be able to establish distinct surface domains, to target newly synthesized transport proteins to their appropriate sites of functional residence, and to retain them there following their delivery. Studies of the cellular pathways involved in generating and maintaining the polarized state have begun to illuminate an elegant network of cell biological specializations that may be involved not only in establishing the distributions of transport proteins but in dynamically regulating their function as well.