Reinvestigation of the shape and state of hydration of the skeletal myosin subfragment 1 monomer in solution

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Abstract
Hydrodynamic calculations lead to the conclusion that chymotryptic (or ethylenediaminetetraacetic acid) [rabbit] myosin S1 in solution (hydrated), at 1-5.degree. C, can be modeled as a prolate ellipsoid, with an axial ratio lying between P = 1.0 and 2.5 (major axis between 100.5 .ANG., for P = 1.0, and 162.5 .ANG. for P = 2.5). The degree of hydration is considerable (1.24 g/g for P = 2.5 and 2.02 g/g for P = 1.0). The dehydrated myosin head is pear-shaped under the EM, and its narrowest head is pear-shaped under the EM, and its narrowest part is located near the junction with the tail. The pear-shaped molecule does not predict the experimental X-ray scattering curve. Nor is the model able to predict the hydrodynamic values. The 3-dimensional model for S1 used by Mendelson and Kretzschmar gives a rather good fit to the experimental X-ray scattering curve, but it does not predict the hydrodynamic values. To try to reconcile the 3 models and to fit the X-ray scattering curve and the hydrodynamic data, in solution, the S1 monomer has the shape of a prolate ellipsoid and that an inclusion of bound water exists at 1 extremity of the protein. The rest of bound water surrounds the protein. As first approximation, the dry protein and the hole are assumed to have the same shape as the hydrated molecule (prolate ellipsoid; P). The long axis of the hole is assumed to lie along the long axis of the protein. The calculation of the radius of gyration and the intensity of X-ray scattering for such a model and the comparison with the available data lead to the conclusion that the S1 monomer in solution can be modeled as a prolate ellipsoid of moderate axial ratio P = 2.3 (major axis 156.5 .ANG.; minor axis 68.0 .ANG.), with an usually high degree of hydration (1.37 g/g). The dry protein has a major axis of 119.0 .ANG. and a minor axis of 51.8 .ANG.. The major axis of the hole is 78.0 .ANG. and its minor axis, 34.0 .ANG.. The hole is located at 20.5 .ANG. from the center of the protein; i.e., it is completely at 1 extremity of the protein, although it includes the center of this protein. It is possible that there is an opening between the pocket of water and the outside. The presence of this large hole explains the pear shape of the dehydrated myosin head and maybe the complicated shape of dehydrated S1 attached to actin. The differences between the shape of the myosin heads and that of S1 in solution cannot be accounted for by a noticeable digestion. The maximum length of S1 found here (119 .ANG.) and by Mendelsor and Kretzschmar (120 .ANG.) is well below the maximum length of the dehydrated head found by Elliott and Offer (190 .ANG.). This difference is likely attributable to a stretching of the myosin head on dehydration. As concerns the dehydrated S1 attached to actin, the large hole in the ellipsoid could explain part of its shape, but it is probable that attachment to actin leads to further distortions.