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The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal soleus muscle type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between soleus and ventricular myosin, which could influence functional parameters such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules we made use of the in vitro motility assay.We found that ventricular myosin moved actin filaments with approx. 0.9 μm/s significantly faster than soleus myosin (0.3 μm/s). Unregulated actin filaments are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from soleus or ventricular tissue, the gliding velocity of soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and soleus β-myosin a difference became evident. Soleus myosin contains not only the “ventricular” essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding.We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between soleus and ventricular myosin persisted when native thin filaments were used.SummaryCompared to the “ventricular” essential myosin light chain MLC1sb/v, the longer and more positively charged MLC1sa present in slow-twitch soleus muscle fibers decelerates actin filament gliding on β-myosin molecules presumably by a decreased dissociation rate from actin filaments.

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the “ventricular” essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

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 nearly 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

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