Structural and functional modulation of the manganese cluster in Ca(2+)-depleted photosystem II induced by binding of the 24-kilodalton extrinsic protein.

Depletion of functional Ca2+ from photosystem (PS) II membranes impairs O2 evolution. Redox properties of the Mn cluster as probed by thermoluminescence were modified differently in Ca(2+)-depleted PSII depending on the procedure for Ca2+ extraction. Ca2+ depletion by low-pH treatment gave rise to an abnormally modified S2 state exhibiting a thermoluminescence band with elevated peak temperature accompanied by a marked upshift in threshold temperature for its formation, whereas Ca2+ depletion by NaCl washing in the light followed by the addition of EDTA could generate a similarly modified S2 state only when the Ca(2+)-depleted PSII was reconstituted with the 24-kDa extrinsic proteins. These results indicated that manifestation of the abnormal properties of the Ca(2+)-depleted S2 state is significantly contributed by the association of the 24-kDa extrinsic protein to PSII. It was inferred that the 24-kDa extrinsic protein regulates the structure and function of the Mn cluster in the absence of functional Ca2+ through a conformational modulation of the intrinsic protein(s) that bind(s) both Mn and Ca. Features of the extrinsic protein-dependent modulation of the Mn cluster were discussed in relation to the function of Ca2+ in O2 evolution.