PP2A Antagonizes Phosphorylation of Bazooka by PAR-1 to Control Apical-Basal Polarity in Dividing Embryonic Neuroblasts

Bazooka/Par-3 (Baz) is a key regulator of cell polarity in epithelial cells and neuroblasts (NBs). Phosphorylation of Baz by PAR-1 and aPKC is required for its function in epithelia, but little is known about the dephosphorylation mechanisms that antagonize the activities of these kinases or about the relevance of Baz phosphorylation for NB polarity. We found that protein phosphatase 2A (PP2A) binds to Baz via its structural A subunit. By using phospho-specific antibodies, we show that PP2A dephosphorylates; Baz at the conserved serine residue 1085 and thereby antagonizes the kinase activity of PAR-1. Loss of PP2A function leads to complete reversal of polarity in NBs, giving rise to an "upside-down" polarity phenotype. Overexpression of PAR-1 or Baz, or mutation of 14-3-3 proteins that bind phosphorylated Baz, causes essentially the same phenotype, indicating that the balance of PAR-1 and PP2A effects on Baz phosphorylation determines NIB polarity.