Photosystem II (PSII) from a red alga, Cyanidium caldarium, contains four extrinsic proteins of 33, 20, and 12 kDa and cytochrome (cyt) c550 [Enami, I., et al., (1995) Biochim. Biophys. Acta 1232, 208-216]. The binding and functional properties of these four proteins in the red algal PSII were studied by release-reconstitution experiments. Of the four components, the 33 kDa protein binds to PSII completely by itself, and the 20 kDa protein binds to a level 61% of that in native PSII in the absence of other proteins. In contrast, cyt c550 and the 12 kDa protein cannot bind to PSII efficiently by themselves; their effective binding requires the other three extrinsic proteins. In particular, a strong interaction was observed between cyt c550 and the 12 kDa protein, and a weaker interaction was observed between cyt c550 and the 20 kDa protein. While binding of the 33 kDa protein alone or cyt c550 and the 12 kDa protein in the presence of the 33 and/or the 20 kDa protein generally enhanced oxygen evolution, binding of the 20 kDa protein did not. Oxygen evolution was strongly dependent on Ca2+ and Cl- in the absence of cyt c550 and the 12 kDa protein, suggesting that these two proteins have functions similar to those of the 23 and 17 kDa proteins in higher plant PSII. From these results, we propose that the unique 20 kDa extrinsic protein found only in the red algal PSII functions in maintaining the proper binding of cyt c550 and the 12 kDa protein but is not involved directly in oxygen evolution. The binding and functional properties of these four proteins were compared with those of the three extrinsic proteins found in cyanobacterial and higher plant PSII in an evolutionary point of view.
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