In vitro evidence for the involvement of activated oxygen in light‐induced aggregation of thylakoid proteins

Protein aggregation in thylakoids incurred in situ during light‐induced heat shock damage can be simulated in vitro by illuminating isolated thylakoids at normal temperatures. Aggregation is detectable in the in vitro model system by fluorography of [35‐S]‐methionine‐labelled thylakoids fractionated by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS‐PAGE) and also by Coomassie staining after SDS‐PAGE of unlabelled thylakoids. As in the case of light‐induced heat shock damage, protein aggregation in the in vitro system is completely light dependent, and the D‐1 protein of PS][is present in the protein aggregate. The model system has also provided evidence for the involvement of activated oxygen in aggregation of thylakoid proteins. Histidine, which scavenges singlet oxygen, and n‐propylgallate; a non‐specific scavenger of activated oxygen, both provided complete protection against light induced protein aggregation in isolated thylakoids. These compounds also strongly reduced the levels of activated oxygen by illuminated thylakoids as measured by electron spin resonance. The involvement of activated oxygen is further supported by the finding that protein aggregation in the model system proved to be oxygen dependent. The herbicide dichlorophenyldimethyl urea, which binds to the QB site of the D‐1 protein of PSII and provides protection against photoinhibition and light dependent degradation of the D‐1 protein, also provided partial protection against protein aggregation in the in vitro system. Protein continues to aggregate after PSII activity has reached undetectable levels suggesting that aggregation is a consequence rather than a cause of photoinhibition. The observations collectively indicate that aggregation of thylakoid proteins is attributable to activated oxygen.