Protein Phosphatase-1α Interacts with and Dephosphorylates Polycystin-1

Polycystin signaling is likely to be regulated by phosphorylation. While a number of potential protein kinases and their target phosphorylation sites on polycystin-1 have been identified, the corresponding phosphatases have not been extensively studied. We have now determined that polycystin-1 is a regulatory subunit for protein phosphatase-1 alpha (PP1 alpha). Sequence analysis has revealed the presence of a highly conserved PP1-interaction motif in the cytosolic, C-terminal tail of polycystin-1; and we have shown that transfected PP1 alpha specifically co-immunoprecipitates with a polycystin-1 C-tail construct. To determine whether PP1 alpha dephosphorylates polycystin-1, a PKA-phosphorylated GST-polycystin-1 fusion protein was shown to be dephosphorylated by PP1 alpha but not by PP2B (calcineurin). Mutations within the PP1-binding motif of polycystin-1, including an autosomal dominant polycystic kidney disease (ADPKD)-associated mutation, significantly reduced PP1 alpha-mediated dephosphorylation of polycystin-1. The results suggest that polycystin-1 forms a holoenzyme complex with PP1 alpha via a conserved PP1-binding motif within the polycystin-1 C-tail, and that PKA-phosphorylated polycystin-1 serves as a substrate for the holoenzyme.