A Possible Role of Protein Phosphorylation in the Inactivation of a Ca²⁺-Induced Ca²⁺ Release Channel from Skeletal Muscle Sarcoplasmic Reticulum

Abstract

The Ca²⁺-induced Ca²⁺ release channel in the heavy fraction of the sarcoplasmic reticulum (SR) from rabbit skeletal muscle is inactivated during ATP-dependent Ca²⁺ uptake (Morii, H., Takisawa, H., & Yamamoto, T. (1985) J. Biol. Chem. 260, 11536-11541). AMP, one of the adenine nucleotides which activate the Ca²⁺ release, delayed the onset of the channel inactivation when added early during the course of the Ca²⁺ uptake. However, AMP could no longer activate the channel but accel-erated the inactivation when added during the later phase of the Ca2+ uptake. In SR passively loaded with Ca²⁺, the Ca²⁺ channel which had been activated by AMP and Ca²⁺ was not spontaneously inactivated. Similarly, during GTP-dependent Ca²⁺ uptake, the channel activated by AMP was not inactivated. In addition acid phosphatase markedly delayed the onset of the inactivation during ATP-dependent Ca₂₊ uptake, without affecting Ca²⁺ ATPase activity or GTP-dependent Ca₂₊ uptake by heavy SR. The effect of the phosphatase was completely blocked by ruthenium red, a potent inhibitor of the channel. These results suggest that the channel is inactivated through an ATP-dependent process, presumably phosphorylation of proteins in the SR membrane. This was supported by the findings that the reactiva-tion of the inactivated channel by added Ca²⁺ was markedly accelerated by the addi-tion of acid phosphatase and that several proteins of heavy SR were phosphorylated during ATP-dependent Ca²⁺ uptake.