Sarcoplasmic reticular Ca2+ release channel is phosphorylated at serine 2843 in intact rabbit skeletal muscle

The Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum (SR) can be phosphorylated on serine 2843 in isolated heavy SR (HSR) fraction by associated protein kinase(s). Laser desorption mass spectrometry of the major [32P]phosphopeptide demonstrates that in HSR the channel is already phosphorylated before the in vitro phosphorylation starts. However, occasionally ca. 60% of the receptor cannot be phosphorylated, the degree of in vitro phosphorylation is generally low. We suggest an interaction of the channel with a not yet identified effector molecule which might determine the phosphorylation degree in addition to the enzymatic activities of associated kinase(s) and phosphatase(s).