Selective and specific degradation of the D 1 protein induced by binding of a novel Photosystem II inhibitor to the Q B site

Abstract

When Photosystem (PS) II membranes were incubated with PNO8 ( N-octyl-3-nitro-2,4,6-trihydroxybenzamide) classified as a phenol-type PS II inhibitor, the D1 protein of PS II reaction center was degraded into two fragments of 23 and 9 kDa in complete darkness, while the D2 protein was not affected at all by incubation with PNO8. Other typical PS II inhibitors, including DCMU, atrazine, ioxynil and dinoseb, showed no degradation activity. Occupation by another PS II inhibitor, DCMU, of the binding site of the secondary quinone acceptor, Q B, prevented the D1 protein from PNO8-induced degradation. The degradation was inhibited at low temperature, but occurred even in the absence of oxygen. Moreover, an inhibitor of serine-type proteinase, PMSF, was effective in suppressing the D1 protein degradation. Photoinhibitory treatment of the membranes also induced a degradation product with an apparent molecular mass of 23 kDa that corresponds to the PNO8-induced 23 kDa fragment. The data were interpreted as indicating a selective and specific cleavage of the D1 protein triggered by binding of PNO8 to the Q B site, and the results were discussed in relation to the features of proteolytic degradation of the D1 protein in photoinhibition.