Cardiac ventricular myosin and slow skeletal myosin exhibit dissimilar chemo-mechanical properties despite the same myosin heavy chain isoform.

The myosin II motors are ATP-powered, force-generating machines driving cardiac and muscle contraction. Myosin II heavy chain isoform-beta ({beta}-MyHC) is primarily expressed in the ventricular myocardium and slow-twitch muscle fibers, such as in M. soleus. M. soleus-derived myosin II (SolM-II) is often used as an alternative to the ventricular {beta}-cardiac myosin ({beta}M-II); however, the direct assessment of detailed biochemical and mechanical features of the native myosins is limited. By employing the optical trapping method, we examined the mechanochemical properties of the native myosins isolated from rabbit heart ventricle and M. soleus muscles at the single-molecule level. Contrary to previous reports, the purified motors from the two tissue sources, despite the same MyHC isoform, displayed distinct motile and ATPase kinetic properties. {beta}M-II was [~]threefold faster in the actin filament-gliding assay than SolM-II. The maximum acto-myosin (AM) detachment rate derived in single-molecule assays was [~]threefold higher in {beta}M-II. The stroke size for both myosins was comparable. The stiffness of the AM rigor cross-bridge was also similar for both the motor forms. The stiffness of {beta}M-II was found to be determined by the nucleotide state of the actin-bound myosin. Our analysis revealed distinct kinetic differences, i.e., a higher AM detachment rate for the {beta}M-II, corresponding to the ADP release rates from the cross-bridge, thus elucidating the observed differences in the motility driven by {beta}M-II and SolM-II. These studies have important implications for the future choice of tissue sources to gain insights into cardiomyopathies