The secA inhibitor, azide, reversibly blocks the translocation of a subset of proteins across the chloroplast thylakoid membrane.

Abstract

The presence of secA and secY gene homologues in the plastid genomes of red algae and cyanophytes has raised the possibility that the products of these genes are involved in protein translocation across the thylakoid membrane. Bacterial SecA proteins are effectively inhibited by azide, and we have tested the effects of this compound on the transport of lumenal proteins across the thylakoid membrane in pea chloroplasts. Recent studies have shown that lumenal proteins are transported by two different mechanisms, one dependent on the thylakoidal delta pH and the other requiring the presence of a stromal protein factor and ATP. In this report we show that azide inhibits the transport across the thylakoid membrane of the latter group of proteins, which includes plastocyanin and the lumenal 33-kDa protein of photosystem II; translocation of proteins by the delta pH-dependent pathway is unaffected. Following import into isolated chloroplasts in the presence of azide, a substantial proportion of plastocyanin and the 33-kDa protein is found as the stromal intermediate form; the proportion increases with lower ATP concentrations, suggesting that azide and ATP may compete for a single site. The presence of azide completely inhibits the import of the 33-kDa protein by isolated thylakoids, but import is restored if the azide is removed from the stromal extract or thylakoids, prior to the import incubation. The data thus indicate that azide reversibly inhibits the transport of a subset of proteins across the thylakoid membrane, consistent with the involvement of a SecA homolog. The results also indicate that azide is potentially a valuable tool for the future assignment of novel lumenal proteins to one of the thylakoidal protein transport mechanisms.