IN-VIVO AND IN-VITRO PHOTOINHIBITION REACTIONS GENERATE SIMILAR DEGRADATION FRAGMENTS OF D1 AND D2 PHOTOSYSTEM-II REACTION-CENTER PROTEINS

Isolation of photosystem-II reaction centres from pea leaves after pho toinhibitory treatment at low temperature (0-1 degrees C) has provided evidence for the mechanism of degradation of the D1 protein in vivo. These isolated reaction centres did not appear to be spectrally distin ct from preparations obtained from control leaves that had not been ph otoinhibited. Breakdown fragments of both the D1 and D2 proteins were, however, found in preparations isolated from photoinhibited leaves, a nd showed similarities with those detected when isolated reaction cent res were exposed to acceptor-side photoinhibition. Analyses of the ori gin of D1 fragments indicated that the primary cleavage site of this p rotein was between transmembrane helices IV and V indicative of the ac ceptor-side mechanism for photoinhibition. The origins of other D1 pro tein fragments indicate that some donor-side photoinhibition may also have occurred in vivo under the conditions employed. We have shown tha t the spectral and functional integrating of the isolated photosystem II reaction centre complex is resistant to proteolytic cleavage by try psin. Use of a more non-specific protease (subtilisin), however, cause d significant destabilisation of the special pair of chlorophylls cons tituting the primary electron donor, P680, with a consequential loss o f functional activity. Thus, it is possible that specific cleavage of photosystem-II reaction-centre proteins may occur in vivo following ph otoinhibitory damage without a significant change in structural integr ity, a conclusion supported by the finding that photodamaged and norma l reaction centres were isolated together.