Phosphorylation and dephosphorylation of Ser852 and Ser889 control clustering, localization, and function of PAR3.

Cell polarity is essential for various asymmetric cellular events, where the partitioning defective (PAR) protein, PAR3, plays a unique role as a cellular landmark to establish polarity. In epithelial cells, PAR3 localizes at the subapical border such as the tight junction in vertebrates and functions as an apical determinant. Although there is much information about the regulators of PAR3 localization, the mechanism involved in PAR3 concentration and localization to the specific membrane domain remains an important question to be clarified. In this study, we demonstrate that ASPP2, which controls PAR3 localization, links PAR3 and protein phosphatase 1 (PP1). The ASPP2-PP1 complex dephosphorylates a novel phosphorylation site, Ser852, of PAR3. Furthermore, Ser852- or Ser889-unphosphorylatable PAR3 mutants form protein clusters and ectopically localize to the lateral membrane. Concomitance of clustering and ectopic localization suggests that PAR3 localization is a consequence of local clustering. We also demonst rate that unphosphorylatable forms of PAR3 exhibited a low molecular turnover and failed to coordinate rapid reconstruction of the tight junction, supporting that both phosphorylated and dephosphorylated states are essential for the functional integrity of PAR3.