Gα₁₂ directly interacts with PP2A: Evidence for Gα¹²-stimulated PP2A phosphatase activity and dephosphorylation of microtubule-associated protein, tau

The Gα_12/13 family of heterotrimeric G proteins modulate multiple cellular processes including regulation of the actin cytoskeleton. Gα_12/13 interact with several cytoskeletal/scaffolding proteins, and in a yeast two-hybrid screen with Gα₁₂, we detected an interaction with the scaffolding subunit (Aα) of the Ser/Thr phosphatase, protein phosphatase 2A (PP2A). PP2A dephosphorylates multiple substrates including tau, a microtubule-associated protein that is hyperphosphorylated in neurofibrilary tangles. The interaction of Aα and Gα₁₂ was confirmed by coimmunoprecipitation studies in transfected COS cells and by glutathione S-transferase (GST)- Gα₁₂ pull-downs from, cell lysates of primary neurons. The interaction was specific for Aα and Gα₁₂ and was independent of Gα₁₂ conformation. Endogenous Aα and Gα₁₂ colocalized by immunofluorescent microscopy in Caco-2 cells and in neurons. In vitro reconstitution of GST-Gα₁₂ or recombinant Gα₁₂ with PP2A core enzyme resulted in ∼300% stimulation of PP2A activity that was not detected with other Gα subunits and was similar with GTPγS- and GDP-liganded Gα₁₂. 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 QLα₁₂ 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 Gα₁₂ 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 Gα₁₂ and potential in vivo modulation of PP2A target proteins including tau.