Gating of the skeletal calcium release channel by ATP is inhibited by protein phosphatase 1 but not by Mg2+.

We have previously found that dephosphorylation/phosphorylation of the calcium release channel (CRC) of skeletal muscle confers channel sensitivity/insensitivity to the block by physiological [Mg2+] (approximately 1 mM). These studies have now been extended to modulation by ATP. Terminal cisternae vesicles of sarcoplasmic reticulum were incorporated into planar lipid bilayers. CRC gating by ATP (0.67 mM), in the absence of Ca2+ (< 1nM), was studied by treatment with protein kinase A (PKA) or phosphatase 1 (PPT1) and assayed in the presence and absence of free Mg2+ (1 mM). PPT1, PKA, and Mg2+ were directly applied to the bilayer using the microsyringe method, which controls the environment of the CRC in the bilayer for phosphorylation/ dephosphorylation cycles and for assays. PKA treated channels were activated by ATP to high open probabilities, while PPT1 treated channels were not activatable by ATP. Opening and closing of channels during cycles of PKA and PPT1 applications, respectively, provided evidence that the change of CRC activity is due to cyclic phosphorylation/dephosphorylation. Free Mg2+ (1 mM) did not block channels activated by ATP. The new finding is that channel gating by ATP can be controlled by the state of phosphorylation without inhibition by free Mg2+.