Apicobasal polarity complexes.

Asymmetric distribution of proteins and other molecules within cells leads to cell polarization. One of the most studied examples occurs in epithelial cells that polarize to form apical and basolateral surfaces (Nelson, 2003). Although we know much about directed protein trafficking after cells are polarized, less is known about the initial mechanisms that lead to polarization. Genetic and biochemical studies in mammalian systems and lower organisms have begun to reveal the pathways that control this process. The current model is that apical and basolateral protein complexes are mutually antagonistic, which leads to the distribution of proteins in a polarized fashion (Bilder et al., 2003; Tanentzapf and Tepass, 2003). In this model, distinct protein kinases become localized in a polarized fashion and, through phosphorylation, control the localization of other proteins (see below).