We have shown for the first time that the myosin head (subfragment-1, S1), the energy-transducing componentin the actomyosin motor system undergoes a distinct shape change during hydrolysis of ATP using x-ray solution scatteringtechniques. Among various analogs for intermediate states of the S1 ATPase cycle, the complexes with MgADP and vanadate(S1.ADP.V1), MgADP and beryllium fluoride (S1.ADP.BeF3), or MgADP and aluminum fluoride (S1.ADP.AIF4) showed a shapechange similar to that in the presence of MgATP, but the complexes with ATPγS (S1.ADPγS) and MgADP trapped by crosslinking with pPDM (S1.ADP-pPDM) seemed to have a shape similar to that of nucleotide-free S1. These results indicate that the shape of an S1*.ADP.P1 state is more rounded or bent than in other intermediate states of the S1 ATPase cycle. Such changes occur in light chain 2-deficient S1 and also in smooth muscle S1. However, MgADP-fluoride complexes with smoothmuscle S1 (without phosphorylation of a regulatory light chain) seemed to have a structure similar to that of nucleotide-free S1. Analysis of x-ray scattering data indicated that a conformational change of S1 in the presence of MgATP might be caused bya hinge-like bending movement between the catalytic and regulatory domains. The global change of S1 is correlated with some specific changes of a nucleotide-binding moiety.
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