PAR-3 and the aPKC–PAR-6 complex – which are crucial proteins for establishing apicobasal polarity in epithelial cells – are thought to regulate the establishment of the apical domain, as well as the maturation of epithelial junctional structures such as tight junctions (TJs). The molecular mechanisms that underpin their roles have been unclear, although it has recently been shown that PAR-3 can regulate TJ development without interacting with aPKC–PAR-6. By contrast, Shigeo Ohno and colleagues (p. 1595) now show that apical-domain formation requires an interaction between PAR-3 and aPKC–PAR-6. The authors show that, in MDCK cells, knocking down PAR-3 inhibits exocytic delivery of apical proteins to the plasma membrane and eventually results in apical-domain mislocalisation; moreover, the cells fail to form normal cysts in 3D culture, and instead develop cysts that have multiple lumens. These defects can be rescued by wild-type PAR-3, but not by a PAR-3 mutant that cannot interact with aPKC. They next observe that, in depolarised MDCK cells, aPKC and PAR-6 (but not PAR-3) accumulate on apical vacuoles, and that these are targeted to PAR-3-containing primordial cell-cell contact sites as repolarisation progresses. Thus, the authors conclude, the formation of a PAR-3–aPKC–PAR-6 complex is necessary for apical-domain establishment.
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