Determination of the molecular weight and the hydrodynamic properties of a polypeptide from the thylakoid membrane by sedimentation, diffusion and binding measurements in dodecyl sulphate solutions.

The molecular weight and hydrodynamic properties of a polypeptide isolated from the lamellar system of Antirrhinum chloroplasts were determined in sodium dodecyl sulphate solution by measurement of sedimentation velocity, diffusion and effective partial specific volume. The polypeptide fraction exhibits a molecular weight of 25000 which agrees with the apparent molecular weight found by polyacrylamide gel electrophoresis. The molecular weight of the polypeptide-sodium dodecyl sulphate micelle was 54 000, with a friction ratio of 1.6 which indicates an effective asymmetric hydrodynamic shape. For binding measurements self-diffusion equilibrium dialysis with dodecyl [35S] sulphate was used. In this case, dialysis equilibrium was reached within about 10 hours, in contrast to the dialysis with initial concentration differences which requires much longer times. A binding value of deltaD=1.15 g sodium dodecyl sulphate per g polypeptide was obtained which corresponds to a molar binding ratio of 100 mol dodecyl sulphate bound per mol of polypeptide. After the removal of dodecyl sulphate the polypeptide is present in an aggregated state. In phosphate buffers of pH 6.8 and 7.5 the aggregates preponderantly have sedimentation coefficients of 11.7 and 6.8 Svedberg units respectively. Assuming equivalent spheres the molecular weights were calculated to be 340 000 and 150 000.