elegans, PAR-3 appears to sit at or near the top of the hierarchy of polarity regulators, as it is required for the localization of proteins in all three polarity groups ( Bilder et al., 2003 Tanentzapf and Tepass, 2003 Harris and Peifer, 2004 Achilleos et al., 2010).Ĭell polarity is first evident in the C. Thus, polarity establishment occurs through a series of mutually antagonistic interactions between apical and basolateral polarity regulators. Likewise, the apical PAR and Crumbs groups prevent PAR-1 and Scribble group proteins from localizing to the apical domain. Together with the kinase PAR-1, the Scribble complex antagonizes proteins in the apical PAR and Crumbs groups to help restrict them to the apical surface. Finally, the Scribble complex, which includes the leucine-rich repeat protein Scribble, the membrane-associated guanylate kinase Discs large, and Lethal giant larvae, localizes to the basolateral surface. The Crumbs complex, which includes the transmembrane protein Crumbs and several interacting proteins, also localizes apically and is involved in maintaining this domain. The apical PAR group, which includes the PDZ domain scaffolding proteins PAR-3 and PAR-6, and the atypical PKC (aPKC) localize to the apical domain (PAR-6 and aPKC) and to junctions (PAR-3). 1 B St Johnston and Ahringer, 2010 Roignot et al., 2013 Rodriguez-Boulan and Macara, 2014). Polarity regulators fall into three main groups of proteins ( Fig. These cell surface cues translate polarity to the cortex by inducing the asymmetric localization of apical and basolateral polarity regulators. Polarity is established and oriented by cues that arise from cell–cell or cell–matrix interactions. Importantly, although many aspects of epithelial cell formation are deeply conserved, the mechanisms of epithelial cell formation can differ between species and even among the various epithelial cell types within a given species. The molecular events that lead to epithelial cell formation have been reviewed recently in detail ( St Johnston and Ahringer, 2010 Roignot et al., 2013 Rodriguez-Boulan and Macara, 2014) and are summarized briefly here for a prototypical cell. Much of our understanding of epithelial cell formation arises from in vitro studies of cultured mammalian cells and from in vivo genetic analyses in Drosophila and C. Epithelial cells are characterized by their distinct apical and basolateral surfaces as well as by defined junctional complexes that maintain polarity, promote adhesion, and limit diffusion across the cell layer ( Fig. The radial polarization of blastomeres is often, but not always, the first step in forming an epithelial cell layer on the surface of the embryo.
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