Galpha sub(12) Directly Interacts with PP2A: Evidence For Galpha sub(12)-Stimulated PP2A Phosphatase Activity And Dephosphorylation Of Microtubule-Associated Protein, Tau

The Galpha sub(12/13) family of heterotrimeric G proteins modulate multiple cellular processes including regulation of the actin cytoskeleton. Galpha sub(12/13) interact with several cytoskeletal/scaffolding proteins, and in a yeast two-hybrid screen with Galpha sub(12), we detected an interaction with the scaffolding subunit (Aalpha) of the Ser/Thr phosphatase, protein phosphatase 2A (PP2A). PP2A dephosphorylates multiple substrates including tau, a microtubule-associated protein that is hyperphosphorylated in neurofibrillary tangles. The interaction of Aalpha and Galpha sub(12) was confirmed by coimmunoprecipitation studies in transfected COS cells and by glutathione S-transferase (GST)-Galpha sub(12) pull-downs from cell lysates of primary neurons. The interaction was specific for Aalpha and Galpha sub(12) and was independent of Galpha sub(12) conformation. Endogenous Aalpha and Galpha sub(12) colocalized by immunofluorescent microscopy in Caco-2 cells and in neurons. In vitro reconstitution of GST-Galpha sub(12) or recombinant Galpha sub(12) with PP2A core enzyme resulted in -300% stimulation of PP2A activity that was not detected with other Galpha subunits and was similar with GTPgammaS-and GDP-liganded Galpha sub(12). When tau and active kinase (Cdk5 and p25) were cotransfected in to COS cells, there was robust tau phosphorylation. Co-expression of wild type or QLalpha sub(12) with tau and the active kinase resulted in 60 +/-15% reductions in tau phosphorylation. In primary cortical neurons stimulated with lysophosphatitic acid, a 50% decrease in tau phosphorylation was observed. The Galpha sub(12) effect on tau phosphorylation was inhibited by the PP2A inhibitor, okadaic acid (50 nM), in COS cells and neurons. Taken together, these findings reveal novel, direct regulation of PP2A activity by Galpha sub(12) and potential in vivo modulation of PP2A target proteins including tau.