Role of B Regulatory Subunits of Protein Phosphatase Type 2A in Myosin II Assembly Control in Dictyostelium discoideum

In Dictyostelium discoideum, myosin II resides predominantly in a soluble pool as the result of phosphorylation of the myosin heavy chain (MHC), and dephosphorylation of the MHC is required for myosin II filament assembly, recruitment to the cytoskeleton, and force production. Protein phosphatase type 2A (PP2A) was identified in earlier studies in Dictyostelium as a key biochemical activity that can drive MHC dephosphorylation. We report here gene targeting and cell biological studies addressing the roles of candidate PP2A B regulatory subunits (phr2aB alpha and phr2aB beta) in myosin II assembly control in vivo. Dictyostelium phr2aB alpha- and phr2aB beta-null cells show delayed development, reduction in the assembly of myosin II in cytoskeletal ghost assays, and defects in cytokinesis when grown in suspension compared to parental cell lines. These results demonstrate that the PP2A B subunits phr2aB alpha and phr2aB beta contribute to myosin II assembly control in vivo, with phr2aB alpha having the predominant role facilitating MHC dephosphorylation to facilitate filament assembly.