Regulation of the sarcoplasmic reticular Ca2+ transport ATPase by phosphorylation and dephosphorylation

At 0.1 mg/ml protein and 0.45 micrometers free Ca2+ l mol trichloroacetic acid precipitable phosphate is incorporated into 100,000 g SR protein as hydroxylamine sensitive acylphosphate. At nearly physiological protein concentration (ca. 7 mg/ml) a total of ca. 0.8 mol phosphate/100,000 g protein is incorporated, from which a fraction of 0.3 mol/100,000 g protein is insensitive to the hydroxylamine treatment, i.e. it is alkylphosphate. Phosphorylase kinase accelerates the alkylphosphate incorporation ca. 3-fold and enhances its final level to 0.7 mol/100,000 g protein. At 1.6 nm free Ca2+ alkylphosphate incorporation occurs at high SR concentration to a maximal extent of 0.5 mol/ 100,000 g protein. The incorporated alkylphosphate is present in comparable amounts in the 100,000 Mr Ca2+-transport ATP-ase and a polypeptide of Mr 9,000.