Cardiac myosin's mechanics and load dependent kinetics are not altered by the thin-filament proteins

The force necessary to power human heart contraction is generated by beta cardiac myosin pulling on regulated thin filaments. Myosin's force generation is a tightly regulated process, and its disruption causes diseases including genetic cardiomyopathies. Deciphering the details of force generation at the molecular scale is critical for our understanding of both cardiac physiology and disease. While excellent studies have elucidated the mechanics and kinetics underlying the interactions between cardiac myosin and actin at the single molecule scale, the majority of these studies have been conducted in the absence of regulatory proteins. In the case of several other myosin isoforms, regulatory proteins alter the mechanics and load-dependent kinetics of the myosin working stroke, and it is not clear what role, if any they play in tuning the cardiac myosin working stroke and load-dependent kinetics. Here, we used a single molecule optical trapping assay to measure myosin's mechanics in the presence and absence of reconstituted thin-filaments (RTFs). We observed that regulatory proteins do not affect the myosin working stroke or its substeps. Interestingly, at low ATP, we observed that regulatory proteins slow the rate of actomyosin dissociation, and we showed using stopped flow kinetics that this is due to tuning of the rate of ATP-induced dissociation. The difference in the dissociation rate disappears at physiologically relevant ATP concentrations. We also used an isometric force clamp to investigate the load dependence of the myosin working stroke at physiologically relevant ATP concentrations, and this was also unchanged with RTFs. Taken together, these results show that cardiac myosin's mechanics are not altered by regulatory proteins at physiologically relevant ATP concentrations, and they provide important insights into the molecular mechanisms underlying heart contraction.