Regulation of myocardial cross-bridge cyling kinetics by myosin binding protein C: Structural insights from x-ray diffraction studies

Mutations in the gene encoding cardiac myosin binding protein-C (cMyBP-C) comprise the most common cause of hypertrophic cardiomyopathies (HCM), a disease that affects 1 in 500 individuals and is the leading cause of sudden death among athletes and young people. In addition, altered phosphorylation of cMyBP-C in heart muscle cells contributes to beat-to-beat regulation of myocardial function and may contribute to compensatory mechanisms and contractile dysfunction in heritable and acquired myocardial disease. In myocardium, the phosphorylation status of myofibrillar proteins affects protein function, which modulates Ca2+ activated force and the rate at which force is developed, presumably as a consequence of alterations in myofilament structure. We hypothesize that cMyBP-C normally acts to tether myosin cross-bridges near to the thick filament backbone, thereby reducing the likelihood of cross-bridge binding to actin and limiting the cooperative activation of the thin filament; upon protein kinase A-mediated (PKA) phosphorylation of cMyBP-C, the tether-like constraint of myosin heads due to interactions with cMyBP-C is relieved by disruption of its binding to myosin, resulting in movement of the heads toward actin, increased likelihood of myosin head binding to actin, and acceleration of the kinetics of cross-bridge cycling.;To explore the structural basis for cMyBP-C function, we used synchrotron low-angle x-ray diffraction to measure inter-thick filament spacing and the equatorial intensity ratio, I1,1/ I1,0, in skinned myocardium. Trabeculae were isolated from wild-type and genetically-modified mice, including the cMyBP-C knockout (cMyBP-C-/-) mouse which mimics human hypertrophic cardiomyopathy, exhibiting both septal hypertrophy and contractile dysfunction. The main finding of our studies is that I1,1/I 1,0 increases upon ablation or phosphorylation of cMyBP-C, which we interpret as a radial or aziumuthal displacement of cross-bridges away from the thick filament backbone and closer to actin. Our studies are consistent with a mechanism in which cMyBP-C modulates cross-bridge cycling kinetics by regulating the proximity and therefore the probability of interaction of myosin with actin. Such a mechanism is likely mediated by dynamic interactions between the S2 region in myosin and the PKA phosphorylation motif in cMyBP-C.