Purified125I-labeled 33-kDa protein binds to calcium-washed photosystem II preparations at high-affinity and low-affinity binding sites. Filling 70% of the high-affinity site with 33-kDa protein induces 63% of the maximum achievable reconstitution of 02-evolving activity. When N-succinimidyl [(4-azidophenyl)dithio]propionate modified 33-kDa protein was reconstituted into Ca(II)-washed membranes under conditions that primarily filled the high-affinity site and then cross-linked to adjacent proteins by illumination of the photoaffinity label, a cross-linked protein complex was formed that could be solubilized from the membranes with sodium dodecyl sulfate. The protein complex consisted of 22-, 24-, 26-, 28-, 29-, and 31-kDa proteins cross-linked to the 33-kDa protein and contained about 3–4 mol of Mn/mol of protein.
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