Muscle contraction results from cyclic attachment and detachment between
myosin heads and actin filaments, coupled with
ATP hydrolysis. Despite extensive studies, however, the amplitude of
myosin head power
stroke still remains to be a mystery. Using the gas environmental chamber, we have succeeded in recording the power
stroke of position-marked
myosin heads in hydrated mixture of actin and
myosin filaments in a nearly isometric condition, in which
myosin heads do not produce gross myofilament sliding, but only stretch adjacent elastic structures. On application of
ATP, individual
myosin heads move by ~3.3 nm at the distal region, and by ~2.5 nm at the proximal region of
myosin head catalytic domain. After exhaustion of applied
ATP, individual
myosin heads return towards their initial position. At low ionic strength, the amplitude of
myosin head power
stroke increases to >4 nm at both distal and proximal regions of
myosin heads catalytic domain, being consistent with the report that the force generated by individual
myosin heads in muscle fibers is enhanced at low ionic strength. The advantages of the present study over other in vitro motility assay systems, using
myosin heads detached from
myosin filaments, are discussed.