Myosin essential light chain 1sa decelerates actin and thin filament gliding on β-myosin molecules

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. Summary Compared 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.