SPECIFIC DEGRADATION OF THE D1 PROTEIN OF PHOTOSYSTEM-II BY TREATMENTWITH HYDROGEN-PEROXIDE IN DARKNESS - IMPLICATIONS FOR THE MECHANISM OF DEGRADATION OF THE D1 PROTEIN UNDER ILLUMINATION

The D1 protein of the photosystem II (PSII) reaction center has a rapi d turnover and is specifically degraded under illumination in vivo. Wh en isolated PSII membranes were treated in darkness with 10 mM hydroge n peroxide (H2O2), an active form of oxygen that is generated at the a cceptor side of PSII under illumination, proteins of the PSII reaction center were specifically damaged in almost the same way as observed u nder illumination with strong light. The D1 protein and, to a lesser e xtent, the D2 protein were degraded to specific fragments, and cross-l inked products (the covalently linked adduct of the D1 protein and the alpha subunit of cytochrome b(559) and the heterodimer of the D1 and D2 proteins) were generated concomitantly. The site of cleavage of the D1 protein that gave rise to a major fragment of 22 kDa was located i n the loop that connects membrane-spanning helixes IV and V. Treatment with H2O2 caused the same damage to proteins in isolated thylakoids a nd in core complexes that contained the non-heme iron at the acceptor side, but not in isolated reaction centers depleted of the iron. From these observations and the effects of reagents that are known to inter act with the non-heme iron, it is suggested that the damage to protein s is caused by oxygen radicals generated by the non-heme iron in the F e(II) state in a reaction with H2O2. It is proposed, moreover, that a similar mechanism is operative during the selective and specific degra dation of the D1 protein under illumination.