High molecular weight proteins in cardiac and skeletal muscle junctional sarcoplasmic reticulum vesicles bind calmodulin, are phosphorylated, and are degraded by Ca2+-activated protease

A unique set of high molecular weight proteins was identified in junctional sarcoplasmic reticulum (SR) vesicles isolated from both cardiac muscle and skeletal muscle. These high Mr proteins were not present in free SR vesicles isolated from either tissue, nor were they observed in purified sarcolemmal fractions. The junctional SR high Mr proteins migrated as doublets in sodium dodecyl sulfate-polyacrylamide gels and exhibited apparent Mr values between 290,000 and 350,000. The high Mr proteins bound calmodulin; they were the principal proteins labeled in the cardiac and skeletal muscle SR subfractions by azido-125I-calmodulin. The high Mr proteins were also substrates for an endogenous Ca2+-calmodulin-dependent protein kinase activity, as well as exogenously added catalytic subunit of cAMP-dependent protein kinase. In addition, the junctional SR high Mr proteins were the major SR proteins degraded by a Ca2+-activated protease purified from smooth muscle. Control experiments verified the separation of junctional SR vesicles and free SR vesicles from both muscle types. Junctional SR vesicles were enriched in calsequestrin, and they exhibited Ca2+ uptake which was stimulated up to 10-fold by either ryanodine or ruthenium red. Free SR vesicles were deficient in calsequestrin and were insensitive to these two agents. Localization of the cardiac and skeletal muscle high Mr proteins to the junctional SR, coupled with demonstration of their nearly identical biochemical properties, suggests that the proteins are homologous and are likely to have similar functions in both types of striated muscle.