For many years, it has been known that
myosin binds to actin tightly, but it had not been possible to devise a muscle fiber experiment to determine whether this binding energy is directly coupled to the working
stroke of the
actomyosin crossbridge cycle. Addressing the question at the single-molecule level with optical tweezers allows the problem to be resolved. We have compared the working
stroke on the binding of four
myosin complexes (
myosin,
myosin-
ADP,
myosin-
pyrophosphate, and
myosin-adenyl-5'yl
imidodiphosphate) with that observed while hydrolyzing
ATP. None of the four was observed to give a working
stroke significantly different from zero. A working
stroke (5.4 nm) was observed only with
ATP, which indicates that the other states bind to actin in a rigor-like conformation and that
myosin products (M.
ADP.Pi), the state that binds to actin during
ATPase activity, binds in a different, prestroke conformation. We conclude that
myosin, while dissociated from actin, must be able to take up at least two mechanical conformations and show that our results are consistent with these conformations corresponding to the two states characterized at high resolution, which are commonly referred to in terms of having open and closed
nucleotide binding pockets.