The structure of the ε-subunit from the chloroplast coupling factor (CF 1) studied by means of small angle X-ray scattering and inelastic light scattering

Abstract

The ε-subunit from the chloroplast coupling factor (CF 1) was purified on a BioGel A 0.5 m column, and the size, shape and radius of gyration were determined. The diffusion coefficient, D 20, w o, was determined by means of inelastic light scattering and was found to be (11.3 ± 0.05) × 10 −7 cm 2·sec −1 and independent of pH and ionic strength. From the sedimentation coefficient (1.70S) and D 20, w o we obtained a molecular weight of 11,900 with a partial specific volume of 0.740 ± 0.003 ml · g −1. The radius of gyration of ε in 0.05 M TRIS-HCl, pH 7.0, was found to be 11.8 ± 0.04 Å, the volume 17.0 × 10 3 (Å 3) and the specific inner surface 0.19 A −1, indicating a spherical molecule with overall dimensions of D = 31.8 Å with a given axial ratio of 1:1:2.4. The description of the ε-subunit in solution as a prolate ellipsoid of revolution with half axes a = b = 12.7 Å and c = 25.4 Å was obtained from a comparison of the theoretical and the experimental scattering curves. The degree of hydration was determined to be 0.15 g H 2O/g protein.