IDENTIFICATION OF TRIADIN AND OF HISTIDINE-RICH CA2-BINDING PROTEIN AS SUBSTRATES OF 60-KDA CALMODULIN-DEPENDENT PROTEIN-KINASE IN JUNCTIONAL TERMINAL CISTERNAE OF SARCOPLASMIC-RETICULUM OF RABBIT FAST MUSCLE()

The endogenous calmodulin-protein kinase system of sarcoplasmic reticu lum terminal cisternae of rabbit fast-twitch muscle was studied. Inves tigation of a single Ca2+-channel in terminal cisternae fused to plana r lipid bilayers demonstrated that the endogenous kinase inhibits the channel, although it remained unclear whether the phosphorylation site s are on the channel protein or on other junctional sarcoplasmic retic ulum specific proteins [Hain et al., (1994) Biophys. J. 67, 1823-1833] . Our results, which show that two junctional sarcoplasmic reticulum s pecific proteins,i.e., triadin and histidine-rich, Ca2+-binding protei n, but not the ryanodine receptor/Ca2+-channel protein, are phosphoryl ated by membrane-bound 60 kDa protein kinase, seem to be able to resol ve this ambiguity. Furthermore,such aprobably specific protein isoform of calmodulin-protein kinase, by its substrate specificity and exposu re to the cytoplasmic side of terminal cisternae at the junctional mem brane domain and based on protease sensitivity, also seems to possess some of the potential requirements for a regulatory role in the functi onal state of the Ca2+-channel. (C) 1995 Academic Press, Inc.