Abstract
Par-3 regulates animal cell polarity by targeting the Par complex proteins Par-6 and atypical protein kinase C (aPKC) to specific cortical sites. Although numerous physical interactions between Par-3 and the Par complex have been identified [1-6], we discovered a novel interaction between Par-3's second PDZ domain and a highly conserved aPKC PDZ-binding motif (PBM) that is required in the context of the full-length, purified Par-6-aPKC complex. We also found that Par-3 is phosphorylated by the full Par complex and phosphorylation induces dissociation of the Par-3 phosphorylation site from aPKC's kinase domain but does not disrupt the Par-3 PDZ2-aPKC PBM interaction. In asymmetrically dividing Drosophila neuroblasts, the aPKC PBM is required for cortical targeting, consistent with its role in mediating a persistent interaction with Par-3. Our results define a physical connection that targets the Par complex to polarized sites on the cell membrane.