Radial Displacement of Myosin Cross-bridges in Mouse Myocardium due to Ablation of Myosin Binding Protein-C

Myosin binding protein-C (cMyBP-C) is a thick filament accessory protein, which in cardiac muscle functions to regulate the kinetics of cross-bridge interaction with actin; however, the underlying mechanism is not yet understood. To explore the structural basis for cMyBP-C function, we used synchrotron low-angle X-ray diffraction to measure interfilament lattice spacing and the equatorial intensity ratio, I₁₁/I₁₀, in skinned myocardial preparations isolated from wild-type (WT) and cMyBP-C null (cMyBP-C^-/-). In relaxed myocardium, ablation of cMyBP-C appeared to result in radial displacement of cross-bridges away from the thick filaments, as there was a significant increase (∼30%) in the I₁₁/I₁₀ ratio for cMyBP-C^-/- (0.37 ± 0.03) myocardium as compared to WT (0.28 ± 0.01). While lattice spacing tended to be greater in cMyBP-C^-/- myocardium (44.18 ± 0.68 nm) when compared to WT (42.95 ± 0.43 nm), the difference was not statistically significant. Furthermore, liquid-like disorder in the myofilament lattice was significantly greater (∼40% greater) in cMyBP-C^-/- myocardium as compared to WT. These results are consistent with our working hypothesis that cMyBP-C normally acts to tether myosin cross-bridges nearer to the thick filament backbone, thereby reducing the likelihood of cross-bridge binding to actin and limiting cooperative activation of the thin filament.