Effects of cholate on Photosystem II: Selective extraction of a 22 kDa polypeptide and modification of QB-site activity

Abstract

A quinone-mediated two-electron gate is shared by Photosystem II (PS II) and the photosystem of purple bacteria. In the bacterial reaction center, electron transfer from the reduced primary quinone acceptor, Q-A, to the secondary quinone, QB, as well as the sensitivity of this electron transfer step to inhibition by terbutryn, are regulated by the H subunit of the reaction center. Sequential removal of three polypeptides (10 and 22, followed by 28 kDa) from O2 evolving PS II reaction center complex preparations impairs QB activity. Removal of the 22 kDa protein does not abolish the herbicide sensitivity of electron transfer mediated by an added p-benzoquinone, but another of these proteins, a species of 28 kDa that binds chlorophyll, appears to regulate the ability of the herbicide DCMU to interfere with PS II electron transfer (Bowlby, N.R. et al. (1990) Curr. Res. Photosynth. I, 539-542). In this communication, we show that exposure of PS II to Na-cholate yields a preparation in which substantial depletion of a 22 kDa intrinsic protein has occurred. In the depleted preparations, a hydrophilic oxidant (Fe(CN)3-6), rather than a lipophilic p-benzoquinone acceptor, supports the highest rates of O2 evolution in a reaction that shows little sensitivity to the QB inhibitor DCMU. Although this observation might indicate a disruption of QB function that is associated with removal of the 22 kDa protein, this is not the case. It is shown here that under some conditions, Na-cholate will modify the response of PS II to Fe(CN)3-6 without release of the 22 kDa protein.