Polypeptides of the Thylakoid Membrane and Their Functional Characterization

From stroma-freed chloroplasts of Antirrhinum m ajus polypeptides with the apparent molecular weights 44 000, 26 000 and 20 000 were isolated. The antiserum to a polypeptide with the moleculair weight 44 000 inhibits the photoreduction of anthraquinone-2-sulfonate with dichlorophenol indophenol/ascorbate when the concentration of the electron donor dichlorophenol indophenol is low. The antiserum enhances the rate of phenazine methosulfate-mediated cyclic photophosphorylation. The variable fluorescence yield is increased by the antiserum . It is assumed that this polypeptide plays a role in electron transport between the two photosystems. From two polypeptides with the apparent molecular weight 26 000 one seems to belong to the reaction center of photosystem II as it inhibits the photooxidation of tetram ethyl benzidine and diphenyl carbazide with suitable electron acceptors and inhibits electron transport between water and silicomolybdate. Variable fluorescence is not or not too strong decreased by the antiserum. The other polypeptide of the apparent molecular weight 26 000 inhibits the photoreduc­ tion of anthraquinone-2-sulfonate with high concentrations of dichlorophenol indophenol as the electron donor. Phenazine methosulfate-mediated cyclic photophosphorylation is also inhibited by the antiserum. Therefore, we should like to associate it with the reaction center of photosystem I. The antiserum to the polypeptide with the apparent molecular weight 20 000 inhibits the photo­ reduction of anthraquinone-2-sulfonate with low and high concentrations of the electron donor dichlorophenol indophenol. I t enhances phenazine methosulfate-mediated cyclic photophosphoryla­ tion. The polypeptide, therefore, should be functionally involved on the acceptor side of photo­ system I. The results obtained up-to-now on the function and localization of the polypeptides in the thylakoid membrane are summarized.