The cAMP-dependent protein kinase (PKA) controls a large number of cellular functions. One critical PKA substrate in the brain and heart is the L-type Ca2+ channel Cav1.2, the activity of which is upregulated by PKA. The main PKA phosphorylation site is serine 1928 in the central pore forming α11.2 subunit of Cav1.2. PKA is bound to Cav1.2 within a macromolecular signaling complex consisting of the β2 adrenergic receptor, trimeric G8 protein, and adenylyl cyclase for fast, localized, and hence specific signaling [Davare, M. A., Avdonin, V., Hall, D. D., Peden, E. M., Buret, A., Weinberg, R. J., Horne, M. C., Hoshi, T., and Hell, J. W. (2001) Science 293, 98-101]. Protein phosphatase 2A (PP2A) serves to effectively balance serine 1928 phosphorylation by PKA through its association with the Cav1.2 complex [Davare, M. A., Horne, M. C., and Hell, J. W. (2000) J. Biol. Chem. 275, 39710-39717]. We now show that native PP2A holoenzymes, as well as the catalytic subunit itself, bind to α11.2 immediately downstream of serine 1928. Of those holoenzymes, only heterotrimeric PP2A containing B′ and B″ subunits copurify with α11.2. Preventing the binding of PP2A by truncating α11.2 28 residues downstream of serine 1928 hampers its dephosphorylation in intact cells. Our results demonstrate for the first time that a stable interaction of PP2A with Cav1.2 is required for effective reversal of PKA-mediated channel phosphorylation. Accordingly, PKA as well as PP2A are constitutively associated with Cav1.2 for its proper regulation by phosphorylation and dephosphorylation of serine 1928.
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