Fig. 2. The core mechanisms of apical-basal cell polarity. Cell-polarity complexes that contain the Par3-Par6-aPKC protein complex (and the Crumbs-Pals1-Patj complex; not shown) localize to the apical membrane domain and promote apical membrane identity. The function of Par3-Par6-aPKC complex is counteracted by the Dlg, Scrib, Lgl and Par1 proteins that localize to the basal membrane domain, and promote basolateral-membrane-domain identity. aPKC counteracts the activity of basal polarity proteins via direct phosphorylation and `inactivation' of Lgl. Although the mechanisms that are responsible for negative regulation of Par3-Par6-aPKC activity by the Dlg, Scrib and Lgl proteins are not well understood, basal Par1 can negatively regulate the apical polarity complex by phosphorylation and inactivation of apical Par3.