Epithelial cells typically display an apical-basal (A-B) polarity, which informs the localisation of cellular components and ensures epithelial tissue function. Although the establishment of A-B polarity has been widely studied, the mechanism underlying the co-ordination of polarity determinants remains unclear, as an increasing number of proteins involved and their post-translational modifications are identified. In this work, Shigeo Ohno and colleagues (Yamashita et al., 2020) tap into the complexity of this mechanism by elucidating novel interactions and modifications of PAR3, an apical polarity protein. The authors first show that the phosphatase PP1 interacts with PAR3, and that this interaction is mediated by the PAR3-binding protein ASPP2. Interestingly, the subsequent dephosphorylation event does not take place at the previously described phosphorylation sites in PAR3, Ser144 or Ser889, but at a novel site, Ser852, which is conserved among vertebrates and chordates. By using an S852A mutant of PAR3, the authors confirm that dephosphorylation of Ser852 is required for its appropriate localisation at cell–cell junctions. Furthermore, mutants of PAR3 in which Ser852 and/or Ser889 cannot be phosphorylated, form clusters in the cytoplasm and mislocalise to lateral membrane domains, which in turn leads to ectopic tight junction formation. Taken together, this study thus provides new insights into the organisation of PAR3 clusters and the establishment of A-B polarity.
