Minimum structural unit required for energy transduction in muscle.

The sliding of actin filaments was directly measured along single-headed myosin filaments on which the density of the heads was widely varied, using video-fluorescence microscopy. The results showed that the double-headed structure of myosin is not essential for inducing the sliding movement of actin filaments. The minimum number of myosin heads required for supporting movement of actin filaments at a maximum velocity of 5 micron/s at 23 degrees C was estimated to be 4, at most 16. This led to the conclusion that the sliding distance of actin filaments induced during a single ATP hydrolysis cycle is probably 160 nm or more, at least 40 nm under unloaded conditions.