Interaction of the 33 kDa Extrinsic Protein with Photosystem II:  Rebinding of the 33 kDa Extrinsic Protein to Photosystem II Membranes Which Contain Four, Two, or Zero Manganese per Photosystem II Reaction Center

The 33 kDa extrinsic protein of photosystem II acts to enhance oxygen evolution and to stabilize the manganese cluster at low chloride concentrations. Due to controversies concerning the stoichiometry of this protein [Miyao, M., & Murata, N. (1989) Biochim. Biophys. Acta 977, 315−321, versus Xu, Q., & Bricker, T. M. (1992) J. Biol. Chem. 267, 25816−25821] we have examined the rebinding of this protein to PS II membrane preparations which contain four, two, or zero manganese per photosystem II reaction center. After rebinding, immunoquantification of the 33 kDa extrinsic protein demonstrated that each of these photosystem II membrane preparations strongly bound two copies of the 33 kDa extrinsic protein per photosystem II reaction center. The first and second stoichiometric binding constants (K a1 and K a2) for the binding of the 33 kDa protein to PS II centers containing four manganese were 0.42 and 0.67 nM-1, respectively. Disruption of the manganese cluster either by removal of the chloride-sensitive manganese or extraction of the manganese cluster by alkaline Tris led to a 5−6-fold decrease in K a1 and about a 3-fold decrease in K a2. In all cases the binding of the two copies of the 33 kDa extrinsic protein exhibited positive cooperativity with Hill coefficients ranging from 1.6 to 2.2. These findings demonstrate that damage to the manganese cluster alters the binding affinity of the 33 kDa extrinsic protein to photosystem II but does not alter the molecularity of the binding reaction.