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The effects of the modification of carboxylate groups on the manganese-stabilizing protein on the binding of the 24 kDa extrinsic protein to Photosystem II were investigated. Carboxylate groups on the manganese-stabilizing protein were modified with glycine methyl ester in a reaction facilitated by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The manganese-stabilizing protein which was modified while associated with NaCl-washed membranes could bind to calcium chloride-washed PS II membranes and reconstitute oxygen evolution in a manner similar to that observed for unmodified manganese-stabilizing protein (Frankel, L,K, Cruz, J. C. and Bricker, T. M. (1999) Biochemistry 38, 14271-14278), However, PS II membranes reconstituted with this modified protein were defective in their ability to bind the extrinsic 24 kDa protein of Photosystem II. Mapping of the sites of modification was carried out by trypsin and Staphylococcus V8 protease digestion of the modified protein and analysis by MALDI mass spectrometry. These studies indicated that the domains 1E-71D, 97D-144D, and 180D-187E are labeled when the manganesestabilizing protein is bound to NaCl-washed Photosystem II membranes. We hypothesize that modified carboxylates, possibly residues 1E, 32E, 139E, and/or 187E, in these domains are responsible for the altered binding affinity of the 24 kDa protein observed.

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