Investigation of the shape and size of myosin subfragment 1 using small-angle X-ray scattering

Measurement of x-ray scattering at very small angles by solutions of myosin subfragment 1 (S1) yields a radius of gyration of 3.24 nm (mean) +/- 0.03 nm (standard error for N = 9). If S1 is assumed to be ellipsoid of revolution, of uniform electron density, with a molecular weight of 1.15 X 10(5) and a partial specific volume of 0.73 cm3 g-1, then the axial ratio of the ellipsoid is 2.89 +/- 0.06 (prolate) or 0.26 +/- 0.01 (oblate), and the major axis is 13.0 +/- 0.2 nm (prolate) or 10.1 +/- 0.1 nm (oblate). Measurements at larger angles allow models of S1 morphology to be tested; theoretical scattering curves for various ellipsoids of revolution were calculated. The observed scatterina can be approximated by the scattering from ellipsoids with axial ratio 2.0 to 3.0 (prolate), or 0.25 to 0.4 (oblate). Models that fit the data over the range of scattering angles from 0 to 30 mrad are: prolate ellipsoid with axial ratio 2.3, major axis 12 nm; and oblate ellipsoid with axial ratio 0.4 and major axis 10 nm. Thus analyses of two parts of the scattering curve, by different methods, indicate that S1 is fae from spherical, and that its morphology may be approximated by ellipsoids of the aforesaid dimensions.