Binding of the catalytic subunit of protein phosphatase-1 to the ryanodine-sensitive calcium release channel protein

A number of studies have reported that the activity of the ryanodine-sensitive calcium release channel (ryanodine receptor) in the junctional sarcoplasmic reticulum of skeletal and cardiac muscle can be modulated by protein phosphorylation-dephosphorylation through activation of endogenous protein kinases and/or by addition of exogenous protein kinases and protein phosphatases. In this study, we have investigated the possibility that protein phosphatase-1 (PP1) is targeted to the junctional sarcoplasmic reticulum by the direct isolation of PP1-binding proteins on PP1-Sepharose affinity columns. The results show that the ryanodine receptor of both skeletal and cardiac muscle bind to this affinity support, and are released at supraphysiological salt concentrations in a relatively pure state. Reciprocal experiments demonstrated that PP1 binds to the immobilized muscle ryanodine receptor. The direct binding of PP1 to the ryanodine receptor was supported by the finding that tryptic fragments of the receptor were retained on PP1-Sepharose. The ability of PP1 to dephosphorylate the ryanodine receptor that was phosphorylated by protein kinase A was also demonstrated. These studies show that PP1 is targeted to the junctional sarcoplasmic reticulum by binding to the ryanodine receptor, and provide a biochemical basis for the possibility that PP1 may play a role in the regulation of calcium flux via protein phosphorylation-dephosphorylation mechanisms.