Role of calmodulin in skeletal muscle sarcoplasmic reticulum.
Three proteins having Mr of 20 000, 35 000, and 57 000 were phosphorylated by a calmodulin-dependent system in fast skeletal muscle sarcoplasmic reticulum (SR). The 20 000-dalton phosphoprotein was an acidic proteolipid distinct from phospholamban, which was present in cardiac SR preparations. The 57 000-dalton phosphoprotein became phosphorylated very rapidly (t1/2 = 5-10 s at 0 degrees C) and was distinct from calsequestrin and the 53 000-dalton glycoprotein, as judged from the electrophoretic mobility on neutral Laemmli gels and from its detergent-extraction characteristics. None of the three phosphoproteins interacted directly with calmodulin, implying that they were not the regulatory subunit of the calmodulin-dependent kinase. The calmodulin-dependent kinase(s) responsible for the phosphorylation of the three protein substrates was (were) membrane bound. Its Km(ATP) was about 200 microM, and at the probable physiological calmodulin concentration of 1-2 microM, its Km (Ca) was 0.7 microM. The 57 000-dalton phosphoprotein was dephosphorylated by an endogenous phosphatase activity, which was also activated by the Ca-calmodulin complex.
D000818:Animals; D001921:Brain; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D002147:Calmodulin / Q000494:pharmacology*; D002417:Cattle; D006593:Hexokinase / Q000378:metabolism; D008970:Molecular Weight; D009132:Muscles / Q000378:metabolism*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011510:Proteolipids / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D011510:Proteolipids; D011494:Protein Kinases; D006593:Hexokinase; D010749:Phosphoprotein Phosphatases