SLIDING DISTANCE BETWEEN ACTIN AND MYOSIN-FILAMENTS PER ATP MOLECULE HYDROLYZED IN SKINNED MUSCLE-FIBERS

MUSCLE contraction is generally thought to be driven by tilting 1,2 of the 19-nm-long myosin head 3, part of the thick filament, while attached to actin, part of the thin filament. This motion would produce about 12 nm of filament sliding 4,5. Recent estimates of the sliding distance per ATP molecule hydrolysed by actomyosin in vitro vary widely from 8 nm (ref. 6) to greater-than-or-equal-to 200 nm (ref. 7). The latter value is incompatible with a power stroke incorporating a single tilting motion of the head. We have measured the isotonic sliding distance per ATP molecule hydrolysed during the interaction between myosin and actin in skinned muscle fibres. We directly estimated the proportion of simultaneously attached actomyosin complexes and their ATP use. We report here that at low loads the interaction distance is at least 40 nm. This distance corresponds to the length of the power stroke plus the filament sliding while actomyosin crossbridges bear negative drag forces 5,8. If the power stroke is 12 nm, then our results indicate the drag distance to be at least 28 nm. Our results could also be explained by multiple power strokes per ATP molecule hydrolysed.