HYDRODYNAMIC STUDIES ON THE MANGANESE-STABILIZING PROTEIN OF PHOTOSYSTEM-II

The solution conformation of the manganese-stabilizing protein of phot osystem II was examined by analytical ultracentrifugation. Sedimentati on velocity and sedimentation equilibrium studies were performed. Thes e experiments yielded values for s(20,w)(0) of 2.26 S with a diffusion constant, D, of 7.7 x 10(-7) cm(2) s(-1). This s value is significant ly lower than the apparent s value of 2.6 S previously reported [Miyao , M., and Murata, N. (1989) Biochim. Biophys. Acta 977, 315-321]. The molecular mass of the protein, 26.531 kDa, was verified by MALDI mass spectrometry. The diffusion coefficient was also determined by dynamic light scattering. The z-weighted average of D was 6.8 x 10(-7) cm(2) s(-1). This result was somewhat lower than that observed by analytical ultracentrifugation due to the presence of slowly diffusing component s in the sample. A two-component exponential fit of the dynamic light scattering data, however, gave D = 7.52 x 10(-7) cm(2) s(-1) for the m ajor component of the sample, which is in excellent agreement with the value determined by analytical ultracentrifugation. The value of s, the apparent sedimentation coefficient, was found to depend on the con centration of the protein and varied about 4% per milligram of protein . This is a feature of proteins which are asymmetric in solution. This asymmetry was examined using both the v-bar and Teller methods. Both methods indicated a significant degree of asymmetry for the manganese- stabilizing protein. Our findings indicate that the prolate ellipsoid model for the manganese-stabilizing protein is elongated in solution, with approximate dimensions of about 12.6 nm x 3.0 nm, yielding an axi al ratio of 4.2.