Mutation Val235Ala Weakens Binding of the 33-kDa Manganese Stabilizing Protein of Photosystem II to One of Two Sites

The 33-kDa protein of the photosynthetic O2-evolving complex, also known as manganese stabilizing protein, contributes to the structural stability of the photosystem II tetranuclear Mn cluster and stimulates the water-oxidizing activity of this cluster. Quantification of extrinsic polypeptides in photosystem II has yielded data that support stoichiometries of either one or two copies of each protein per photosystem II reaction center. We recently described the cold-sensitive assembly of a mutant 33-kDa protein with a single amino acid replacement (Val235Ala) [Betts, S. D., Ross, J. R., Pichersky, E., & Yocum, C. F. (1996) Biochemistry 35, 6302−6307]. We have extended the characterization of this mutation. When photosystem II membranes depleted of the 33 kDa extrinsic protein are exposed to mixtures of wild type and Val235Ala manganese stabilizing protein, binding of the wild type protein is strongly preferred. If, however, protein containing the Val235Ala mutation is first bound to photosystem II only half of this protein (about 1 mol/mol of photosystem II reaction centers) is susceptible to displacement by the wild type protein, even after multiple exposures to the latter. These results support the conclusion that 2 mol of manganese stabilizing protein are bound per reaction center. Our data show as well that the mutant 33-kDa protein competes with the wild type protein for at least one of two binding sites on photosystem II and that the mutant protein binds tightly to only one of two sites. These results demonstrate that the two binding sites on photosystem II for the 33-kDa protein have different properties with respect to recognition and binding of this protein.