CAPK-phosphorylation controls the interaction of the regulatory domain of cardiac myosin binding protein C with myosin-S2 in an on-off fashion

Myosin binding protein C is a protein of the myosin filaments of striated m uscle which is expressed in isoforms specific for cardiac and skeletal musc le. The cardiac isoform is phosphorylated rapidly upon adrenergic stimulati on of myocardium by cAMP-dependent protein kinase, and together with the ph osphorylation of troponin-I and phospholamban contributes to the positive i notropy that results from adrenergic stimulation of the heart, Cardiac myos in binding protein C is phosphorylated by cAMP-dependent protein kinase on three sites in a myosin binding protein C specific N-terminal domain which binds to myosin-S2. This interaction with myosin close to the motor domain is likely to mediate the regulatory function of the protein. Cardiac myosin binding protein C is a common target gene of familial hypertrophic cardiom yopathy and most mutations encode N-terminal subfragments of myosin binding protein C. The understanding of the signalling interactions of the N-termi nal region is therefore important for understanding the pathophysiology of myosin binding protein C associated cardiomyopathy. We demonstrate here by cosedimentation assays and isothermal titration calorimetry that the myosin -S2 binding properties of the myosin binding protein C motif are abolished by cAMP-dependent protein kinase-mediated trisphosphorylation, decreasing t he S2 affinity from a K-d of approximate to 5 mu M to undetectable levels. We show that the slow and fast skeletal muscle isoforms are no cAMP-depende nt protein kinase substrates and that the S2 interaction of these myosin bi nding protein C isoforms is therefore constitutively on, The regulation of cardiac contractility by myosin binding protein C therefore appears to be a 'brake-off mechanism that will free a specific subset of myosin heads from sterical constraints imposed by the binding to the myosin binding protein C motif. (C) 1999 Federation of European Biochemical Societies.