A Possible Role of Protein Phosphorylation in the Inactivation of a Ca2+-Induced Ca2+ Release Channel from Skeletal Muscle Sarcoplasmic Reticulum1

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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 accelerated 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 reactivation of the inactivated channel by added Ca2+ was markedly accelerated by the addition of acid phosphatase and that several proteins of heavy SR were phosphorylated during ATP-dependent Ca2+ uptake.