title: Phosphorylation of skeletal-muscle troponin I and troponin T by phospholipid-sensitive Ca2+-dependent protein kinase and its inhibition by troponin C and tropomyosin.
abstract: Skeletal-muscle troponin I and troponin T were found to be rapidly phosphorylated by cardiac phospholipid-sensitive Ca2+-dependent protein kinase, with Km values of 6.66 and 0.13 microM respectively. Stoichiometric phosphorylation of skeletal troponin I (endogenous phosphate content 0.7 mol/mol) indicated that the Ca2+-dependent enzyme and cyclic AMP-dependent protein kinase incorporated 0.9 and 0.8 mol/mol respectively. The same experiments with skeletal troponin T (endogenous phosphate content 1.9 mol/mol) revealed a maximal phosphorylation of 2 mol/mol by the Ca2+-dependent enzyme, whereas the cyclic AMP-dependent enzyme was unable to phosphorylate troponin T. The Ca2+-dependent enzyme phosphorylated both serine and threonine residues in skeletal and cardiac troponin I or troponin T; the cyclic AMP-dependent enzyme, in comparison, phosphorylated only serine in skeletal and cardiac troponin I. Although an equimolar amount of skeletal or cardiac troponin C markedly inhibited (80-90%) phosphorylation of skeletal and cardiac troponin I by the Ca2+-dependent enzyme, these troponin C preparations inhibited only phosphorylation of skeletal troponin I, but not that of cardiac troponin I, by the cyclic AMP-dependent enzyme. Calmodulin and Ca2+-binding protein S-100a could mimic the inhibitory effect of troponin C. A tissue specificity appeared to exist for the skeletal troponin T-skeletal troponin C interaction. Inhibition of troponin T phosphorylation by an equimolar amount of troponin C was lower than that of troponin I phosphorylation; these findings might explain in part why troponin T was the major substrate for the Ca2+-dependent enzyme in the troponin complex. The present studies indicate that skeletal and cardiac troponin I and troponin T were effective substrates for phospholipid-sensitive Ca2+-dependent protein kinase, suggesting a potential involvement of this Ca2+-effector enzyme in the regulation of myofibrillar activity.
mesh_terms: D000818:Animals; D002135:Calcium-Binding Proteins / Q000494:pharmacology; D002417:Cattle; D004591:Electrophoresis, Polyacrylamide Gel; D009132:Muscles / Q000378:metabolism*; D009206:Myocardium / Q000201:enzymology; D010743:Phospholipids / Q000494:pharmacology; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism*; D011817:Rabbits; D014335:Tropomyosin / Q000494:pharmacology; D014336:Troponin / Q000378:metabolism* / Q000494:pharmacology; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T
chemical_list: D002135:Calcium-Binding Proteins; D010743:Phospholipids; D047428:Protein Kinase Inhibitors; D014335:Tropomyosin; D014336:Troponin; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T; D011494:Protein Kinases