Biogenesis of endogenous plasma membrane proteins in epithelial cells.

Abstract

The plasma membrane (PM) is a complex and dynamic macromolecular assembly. Its complexity is particularly evident in polarized epithelial cells, where specialized domains exist to carry out diverse functions within one continuous and fluid lipid bilayer (57), The basolateral domain of epithelial cells faces the internal milieu and is functionally analogous to the PM of nonepithelial cells. It is also specialized for adhesion to the basal lamina and to other cells. The apical domain of epithelial cells faces the external medium and is considered the differentiated surface, since it displays the specialized structures and carries out the specialized functions that largely define each epithelium. Integral glycoproteins constitute a particularly interesting class of mem­ brane molecules in epithelial cells. They mediate a number of dynamic processes occurring at the PM and have been detected in both surface domains of epithelial cells (e.g. 36, 57), Most are transmembrane (e,g. 36), which means that unique sequences arc available at both the external and cytoplas­ mic faces of the bilayer for interaction with each environment. Furthermore, the presence of N-linked oligosaccharides on the external segments of these proteins places constraints on the intracellular pathways that they must