title: Effects of phosphorylated and unphosphorylated C-protein on cardiac actomyosin ATPase.
abstract: C-protein, a component of the thick filaments of striated muscles, is reversibly phosphorylated and dephosphorylated in heart. It has been hypothesized that C-protein may be involved in regulating contraction, because the extent of C-protein phosphorylation correlates with the rate of cardiac relaxation. To test this hypothesis, the effects of phosphorylated and unphosphorylated C-protein on the actin-activated ATPase activity of myosin filaments prepared from DEAE-Sephadex-purified myosin were examined. Unphosphorylated C-protein (0.1 microM to 1.5 microM) stimulated actin-activated myosin ATPase activity in a dose-dependent manner. With a myosin: C-protein molar ratio of approximately 1, actin-activated myosin ATPase activity was elevated up to 3.2 times that of the control. Phosphorylated C-protein (2.5 mol PO4/mol C-protein) stimulated the activity somewhat less (2.5 times that of control). The stimulation of ATPase activity by C-protein was due to an increase in the Vmax value (from 0.25/second to 0.62/second) and a decrease in the Km value (from 11.9 microM to 6.7 microM). The addition of C-protein to actomyosin solutions produced an increase in the light-scattering of the actomyosin solution and a distinct precipitation of the actomyosin with time. Phosphorylated C-protein had a smaller effect on light-scattering than dephosphorylated C-protein. C-protein had a negligible effect on Ca-ATPase, EDTA-K-ATPase, or Mg-ATPase activities in the absence of actin. C-protein had only small effects on the actin-activated ATPase of heavy meromyosin. These results suggest that C-protein stimulates actin-activated myosin ATPase activity by enhancing the formation of stable aggregates between actin and myosin filaments.
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins; D002645:Chickens; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D009124:Muscle Proteins / Q000378:metabolism*; D009132:Muscles / Q000032:analysis; D009206:Myocardium / Q000378:metabolism; D015879:Myosin Subfragments / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D010766:Phosphorylation; D013053:Spectrophotometry; D000099163:Myosin Binding Protein C
chemical_list: D000199:Actins; D000251:Adenosine Triphosphatases; D002352:Carrier Proteins; D009124:Muscle Proteins; D015879:Myosin Subfragments; D009218:Myosins; D000099163:Myosin Binding Protein C