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 mu scle calcium channels are phosphorylated by cyclic AMP-dependent prote in kinase, resulting in three different tryptic phosphopeptides derive d from each subunit. Phosphoprotein phosphatases dephosphorylated thes e sites selectively. Phosphoprotein phosphatase 1 (PP1) and phosphopro tein phosphatase 2A (PP2A) dephosphorylated both alpha1 and beta subun its at similar rates, whereas calcineurin dephosphorylated beta subuni ts preferentially. PP1 dephosphorylated phosphopeptides 1, and 2 of th e alpha1 subunit more rapidly than phosphopeptide 3. In contrast, PP2A dephosphorylated phosphopeptide 3 of the alpha1 subunit preferentiall y. All three phosphoprotein phosphatases preferentially dephosphorylat ed phosphopeptide 1 of the beta subunit and dephosphorylated phosphope ptides 2 and 3 more slowly. Mn2+ increased the rate and extent of deph osphorylation of all sites by calcineurin so that > 80% dephosphorylat ion of both alpha1 and beta subunits was obtained. The results demonst rate selective dephosphorylation of different phosphorylation sites on the alpha1 and beta subunits of skeletal muscle calcium channels by t he three principal serine/threonine phosphoprotein phosphatases.