Formation of a Bazooka–Stardust complex is essential for plasma membrane polarity in epithelia

Apical–basal polarity in Drosophila melanogaster epithelia depends on several evolutionarily conserved proteins that have been assigned to two distinct protein complexes: the Bazooka (Baz)–PAR-6 (partitioning defective 6)–atypical protein kinase C (aPKC) complex and the Crumbs (Crb)–Stardust (Sdt) complex. These proteins operate in a functional hierarchy, in which Baz is required for the proper subcellular localization of all other proteins. We investigated how these proteins interact and how this interaction is regulated. We show that Baz recruits Sdt to the plasma membrane by direct interaction between the Postsynaptic density 95/Discs large/Zonula occludens 1 (PDZ) domain of Sdt and a region of Baz that contains a phosphorylation site for aPKC. Phosphorylation of Baz causes the dissociation of the Baz–Sdt complex. Overexpression of a nonphosphorylatable version of Baz blocks the dissociation of Sdt from Baz, causing phenotypes very similar to those of crb and sdt mutations. Our findings provide a molecular mechanism for the phosphorylation-dependent interaction between the Baz–PAR-3 and Crb complexes during the establishment of epithelial polarity.