The R403Q mutation in human β-cardiac myosin resulted in a ~15% lower intrinsic force and ~30% lower actin-activated ATPase activity with regulated actin filaments compared to wild type.
The R403Q mutation in human β-cardiac myosin results in a loss of motor function and contractility in vitro, providing mechanistic insight into hypertrophic cardiomyopathy.
Hypertrophic cardiomyopathy (HCM) is the most frequently occurring inherited cardiovascular disease. It is caused by mutations in genes encoding the force-generating machinery of the cardiac sarcomere, including human β-cardiac myosin. We present a detailed characterization of the most debated HCM-causing mutation in human β-cardiac myosin, R403Q. Despite numerous studies, most performed with nonhuman or noncardiac myosin, there is no consensus about the mechanism of action of this mutation on the function of the enzyme. We use recombinant human β-cardiac myosin and new methodologies to characterize in vitro contractility parameters of the R403Q myosin compared to wild type. We extend our studies beyond pure actin filaments to include the interaction of myosin with regulated actin filaments containing tropomyosin and troponin. We find that, with pure actin, the intrinsic force generated by R403Q is ~15% lower than that generated by wild type. The unloaded velocity is, however, ~10% higher for R403Q myosin, resulting in a load-dependent velocity curve that has the characteristics of lower contractility at higher external loads compared to wild type. With regulated actin filaments, there is no increase in the unloaded velocity and the contractility of the R403Q myosin is lower than that of wild type at all loads. Unlike that with pure actin, the actin-activated adenosine triphosphatase activity for R403Q myosin with Ca(2+)-regulated actin filaments is ~30% lower than that for wild type, predicting a lower unloaded duty ratio of the motor. Overall, the contractility parameters studied fit with a loss of human β-cardiac myosin contractility as a result of the R403Q mutation.
Nag et al. (Fri,) conducted a other in Hypertrophic cardiomyopathy (HCM). R403Q mutation in human β-cardiac myosin vs. Wild type human β-cardiac myosin was evaluated on In vitro contractility parameters (intrinsic force, unloaded velocity, ATPase activity). The R403Q mutation in human β-cardiac myosin resulted in a ~15% lower intrinsic force and ~30% lower actin-activated ATPase activity with regulated actin filaments compared to wild type.