Hypertrophic cardiomyopathies (HCMs) are the leading cause of acute cardiac failure in young individuals. Over 300 mutations throughout β-cardiac myosin, including in the motor domain, are associated with HCM. A β-cardiac myosin motor mutation (R712L) leads to a severe form of HCM. Actin-gliding motility of R712L-myosin is inhibited, despite near-normal ATPase kinetics. By optical trapping, the working stroke of R712L-myosin was decreased 4-fold, but actin-attachment durations were normal. A prevalent hypothesis that HCM mutants are hypercontractile is thus not universal. R712 is adjacent to the binding site of the heart failure drug omecamtiv mecarbil (OM). OM suppresses the working stroke of normal β-cardiac myosin, but remarkably, OM rescues the R712L-myosin working stroke. Using a flow chamber to interrogate a single molecule during buffer exchange, we found OM rescue to be reversible. Thus, the R712L mutation uncouples lever arm rotation from ATPase activity and this inhibition is rescued by OM.

Hypertrophic cardiomyopathies (HCMs) are the leading cause of acute cardiac failure in young individuals. Over 300 mutations throughout β-cardiac myosin, including in the motor domain, are associated with HCM. A β-cardiac myosin motor mutation (R712L) leads to a severe form of HCM. Actin-gliding motility of R712L-myosin is inhibited, despite near normal ATPase kinetics. By optical trapping, the working stroke of R712L-myosin was decreased 4-fold, but actin-attachment durations were normal. A prevalent hypothesis that HCM mutants are hypercontractile is thus not universal. R712 is adjacent to the binding site of the heart failure drug omecamtiv mecarbil (OM). OM suppresses the working stroke of normal β-cardiac myosin, but remarkably, OM rescues the R712L-myosin working stroke. Using a flow chamber to interrogate a single molecule during buffer exchange, we found OM rescue to be reversible. Thus, the R712L mutation uncouples lever arm rotation from ATPase activity and this inhibition is rescued by OM.

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and participates in a salt-bridge between the switch I and switch II nucleotide binding regions. We observed very minimal actin-activated ATPase activity for the mutant compared to WT (2-fold compared to 100-fold activation, respectively), while the basal ATPase was unchanged. R243H bound to actin in an ATP-dependent manner based on actin-cosedimentation assays. We also observed no actin-sliding by R243H in the in vitro motility assay. In the mixed motility assay, low ratios (%50%) of R243H did not change or only slightly decreased sliding velocities. At higher ratios of R243H (R50%) the velocity decreased dramatically. We speculate that at low ratios of R243H/WT the mutant myosin may cooperatively activate the actin thin filaments to allow more opportunity for WT myosin heads to bind actin without slowing shortening velocity, which would correlate with HCM. At higher ratios of R243/ WT, the inhibitory action of R243H may dominate and slow muscle shortening, which would correlate with DCM.