Distinct distributions of myosin motor conformations during contraction of slow and fast skeletal muscle

Slow skeletal muscles maintain posture and produce graded movement at low metabolic cost. Force development and ATP utilisation during fixed-end contractions are typically five times slower in slow than fast muscles from the same species. Mechanical measurements previously suggested that more myosins are attached to thin filaments during contraction of slow muscle, which seems incompatible with its high efficiency. We therefore used small-angle X-ray diffraction to provide a structural estimate of the fraction of myosins attached to thin filaments in slow muscle. X-ray signals associated with myosin binding to actin indicate that only about 10% of myosin motors are actin-bound during fixed-end tetani of rat soleus slow muscles, compared with about 25% in mouse EDL fast muscle. Moreover, X-ray signals associated with the helical organisation of OFF myosin motors in the thick filaments show that about 70% of myosin motors remain in the OFF conformation during tetanic contraction of slow muscle, compared with only 30% in fast muscle. The much slower force development in soleus muscle also allowed clear separation of early structural changes in thick filaments on activation, some of which are distinct from those reported previously in fast muscles. Moreover, the early structural changes in soleus muscle have about the same amplitude in a twitch and a tetanus, suggesting that they are triggered by thin filament activation rather than thick filament stress, and implying a fast signalling pathway between thin and thick filaments. Key PointsO_LIThe interaction between myosin motors and actin filaments in slow skeletal muscles maintain posture and produce graded movement at low metabolic cost. C_LIO_LIMechanical studies have suggested that more myosins are attached to actin filaments in slow than in fast muscle, but this seems incompatible with its high efficiency. C_LIO_LIWe used X-ray diffraction to show that there are fewer myosin motors attached to actin in slow muscle than in fast muscle because more motors are sequestered on the myosin filament. C_LIO_LIThe slower force development in slow muscle also allowed us to isolate and characterise fast changes in myosin motor conformation associated with activation of the actin filaments. C_LIO_LIThe results reveal a distinct pathway of inter-filament signalling in slow muscle that could help the development of novel therapies for muscle weakness. C_LI