Conformations of myosin subfragment 1 ATPase intermediates from neutron and X-ray scattering

In order to elucidate the structural changes that occur during the hydrolysis of ATP by myosin, low-angle neutron and X-ray scattering have been used to investigate the shape of the myosin head (S1) with various bound nucleotides and nucleotide analogs. It was found that the radius of gyration (Rg) of S1.MgADP.AlF4 and of S1MgADP.Vi were similar and significantly smaller (approximately 3%) than the similar Rg values of nucleotide-free S1, S1.MgADP and S1.MgADP.BeFx. In addition, S1 in the presence of MgATP, which is predominantly in the S1.MgADP.Pi state under the experimental conditions employed, showed a change in Rg comparable with that of S1.MgADP.AlF4 and S1.MgADP.Vi. The results obtained here with BeFx and AlF4 are in close harmony with crystallographic results on truncated S1 bearing MgADP.BeFx and MgADP.AlF4. A. Fisher and co-workers have postulated that these two systems, which exhibit some structural differences, represent the pre-hydrolysis state and the transition state of ATP hydrolysis, respectively. It was postulated that this structural difference might alter the orientation of the light-chain-binding domain (tail) of intact S1 relative to the remainder of the molecule. Since this orientation is the major determinant of the Rg of S1, the current data support the hypothesis that a unitary large-scale conformational cocking of S1 for subsequent force production occurs just before or during ATP hydrolysis. Modeling changes in Rg by rigid-body rotations indicates that the longitudinal component of the force-producing throw is likely to be less than 6 nm.