Hydrodynamic studies on the manganese-stabilizing protein of photosystem II

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The solution conformation of the manganese-stabilizing protein of photosystem II was examined by analytical ultracentrifugation. Sedimentation velocity and sedimentation equilibrium studies were performed. These experiments yielded values for S(20,w)/0 of 2.26 S with a diffusion constant, D, of 7.7 x 10-7 cm2 s-1. This s value is significantly 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 cm2 s-1. This result was somewhat lower than that observed by analytical ultracentrifugation due to the presence of slowly diffusing components in the sample. A two-component exponential fit of the dynamic light scattering data, however, gave D = 7.52 x 10-7 cm2 s-1 for the major 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 concentration 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 axial ratio of 4.2.

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