Carboxylate groups on the manganese-stabilizing protein are required for its efficient binding to photosystem II

The effects of the modification of carboxylate groups on the manganese-stab ilizing protein of photosystem II were investigated. Carboxylate groups (in cluding possibly the C-terminus) on the manganese-stabilizing protein were modified with glycine methyl ester in a reaction facilitated by 1-ethyl-3- [3-(dimethylamino)propyl]carbodiimide. The manganese-stabilizing protein th at was modified while associated with NaCl-washed photosystem II membranes contained 1-2 modified carboxylates, whereas the protein that was modified while free in solution contained 4 modified carboxylates. Both types of mod ified protein could reconstitute oxygen evolution at high manganese-stabili zing protein to photosystem II reaction center ratios. However, the protein that had been modified in solution exhibited a dramatically altered bindin g affinity for photosystem LT. No such alteration in binding affinity was o bserved for the protein that had been modified while associated with the ph otosystem. Mapping of the sites of modification was carried out by trypsin and Staphylococcus V8 protease digestion of the modified proteins and analy sis by matrix-assisted laser desorption/ionization mass spectrometry. These studies indicated that the domains D-157-D-168 and E-212-(247)Q (C-terminu s) are labeled only when the manganese-stabilizing protein is modified in s olution. Modified carboxylates in these domains are responsible for the alt ered binding affinity of this protein for the photosystem.