SELECTIVE DEPHOSPHORYLATION OF THE SUBUNITS OF SKELETAL-MUSCLE CALCIUM CHANNELS BY PURIFIED PHOSPHOPROTEIN PHOSPHATASES

Multiple sites on the alpha1 and beta subunits of purified skeletal muscle calcium channels are phosphorylated by cyclic AMP-dependent protein kinase, resulting in three different tryptic phosphopeptides derived from each subunit. Phosphoprotein phosphatases dephosphorylated these sites selectively. Phosphoprotein phosphatase 1 (PP1) and phosphoprotein phosphatase 2A (PP2A) dephosphorylated both alpha1 and beta subunits at similar rates, whereas calcineurin dephosphorylated beta subunits preferentially. PP1 dephosphorylated phosphopeptides 1, and 2 of the alpha1 subunit more rapidly than phosphopeptide 3. In contrast, PP2A dephosphorylated phosphopeptide 3 of the alpha1 subunit preferentially. All three phosphoprotein phosphatases preferentially dephosphorylated phosphopeptide 1 of the beta subunit and dephosphorylated phosphopeptides 2 and 3 more slowly. Mn2+ increased the rate and extent of dephosphorylation of all sites by calcineurin so that > 80% dephosphorylation of both alpha1 and beta subunits was obtained. The results demonstrate selective dephosphorylation of different phosphorylation sites on the alpha1 and beta subunits of skeletal muscle calcium channels by the three principal serine/threonine phosphoprotein phosphatases.