Extrinsic proteins of photosystem II (PSII) play an important role in optimizing oxygen-evolving reactions in all oxyphototrophs. The currently available crystal structures of cyanobacterial PSII core complexes show the binding structures of the extrinsic proteins, PsbO, PsbV, and PsbU; however, how the individual extrinsic proteins affect the structure and the function of the oxygen-evolving center (OEC) in cyanobacterial PSII remains unknown. In this study, we have investigated the effects of the binding of the extrinsic proteins on the protein conformation of the OEC in PSII core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. Upon removal of the three extrinsic proteins, an S2-minus-S1 FTIR difference spectrum measured in the presence of a high CaCl2 concentration showed a drastic change in amide I bands, reflecting perturbation of the secondary structures of polypeptides, whereas the overall spectral intensity was lost at a low CaCl2 concentration, indicative of inactivation of the Mn4CaO5 cluster. The amide I features as well as the overall intensity were recovered mainly by binding of PsbO, while complete amide I recovery was achieved by further binding of PsbV and PsbU. We thus concluded that PsbO, together with smaller contributions of PsbV and PsbU, plays a role in the maintenance of the proper protein conformation of the OEC in cyanobacterial PSII, which provides the stability of the Mn4CaO5 cluster via the enhanced retention capability of Ca2+ and Cl- ions.
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