CHANGES IN MUSCLE CROSSBRIDGES WHEN BETA, GAMMA-IMIDO-ATP BINDS TO MYOSIN

The binding of .beta.,.gamma.-imido-ATP to glycerol-extracted insect [Lethocerus cordofanus and L. maximus] flight and rabbit back muscle fibers was studied. The binding was at relatively high affinity, of the same quantity as that of other nucleotides, and was inhibited by the presence of ATP. Imido-ATP bound, without hydrolysis, at the enzymic site of myosin. The mechanical effects of imido-ATP on the glycerol-extracted fibers were measured. Concentrations sufficient to bind to myosin caused a small increase in the length of the rigor muscle for a given tension without alteration in the shape of the length-tension diagram. The magnitude of the length change paralleled the binding curve of imido-ATP to the fiber. Binding caused some change in myosin without its detachment from actin. EM and X-ray diffraction studies of glycerol-extracted flight muscle fibers showed an increase in the angle of attachment of myosin to actin when imido-ATP was added. The results are discussed in relation to current concepts of force generation in active muscle.