Impact of Baz/PAR-3 phosphorylation by aPKC on cell polarity

The apical-basal polarization and the formation of cell-cell junctions are important key steps in the development of epithelia to ensure barrier formation, directed transport and mechanical integrity of the tissue. In this context the PAR complex, which consists of the multi modular scaffold proteins Bazooka (Baz, PAR-3 in mammals), PAR-6 and the atypical protein kinase C (aPKC), is of essential significance. The proteins in this complex operate in a specific hierarchy, in which Baz plays a key role in the proper subcellular localization of other apical markers. Later in epithelial development, the scaffolding protein Baz and the transmembrane protein Crumbs (Crb) function to some extent redundantly to determine the apical domain of epithelial cells and further direct the formation of apical junctions by recruiting aPKC. On the other hand, the formation of the active PAR complex is counterbalanced by the basolateral kinase PAR-1. However, it is still not entirely understood, how aPKC functions in positioning Baz at the apical junctions. In this study the interaction between aPKC and Baz/PAR-3 was characterized in more detail. aPKC can interact with the PDZ2-3 domain and the aPKC-binding motif of Baz. Furthermore, five serines/threonines within the BazPDZ2-3 region and seven serines/threonines of the mammalian homolog PAR-3PDZ2-3, respectively, could be identified to be phosphorylated by aPKC. Impaired Baz PDZ-phosphorylation disrupts apical junctional accumulation of Baz/aPKC and AJ formation during late cellularization/early gastrulation in the Drosophila epidermis. These defects can be rescued by ectopic expression of Crb or activated aPKC as well as by inhibition of PAR-1. Thus PDZ-phosphorylation of Baz by aPKC is essential to recruit and activate aPKC in the absence of Crb in order to restrict basolateral PAR-1 activity to ensure correct apical-basal polarization and AJ formation of epithelial cells. As PAR-6 has a supposed PDZ-binding motif (PBM), we questioned if this PBM is important for PAR-6 localization and function. Experiments made in this study give a first hint to a certain role of the PDZ-binding motif in the correct function of PAR-6