Chloroplast ATP synthase is reduced by both f-type and m-type thioredoxins.

The activity of the molecular motor enzyme, chloroplast ATP synthase, is regulated in a redox-dependent manner. The γ subunit, CF 1 -γ, is the central shaft of this enzyme complex and possesses the redox-active cysteine pair, which is reduced by thioredoxin (Trx). In light conditions, Trx transfers the reducing equivalent obtained from the photosynthetic electron transfer system to the CF 1 -γ. Previous studies showed that the light-dependent reduction of CF 1 -γ is more rapid than those of other Trx target proteins in the stroma. Although there are multiple Trx isoforms in chloroplasts, it is not well understood as to which chloroplast Trx isoform primarily contributes to the reduction of CF 1 -γ, especially under physiological conditions. We therefore performed direct assessment of the CF 1 -γ reduction capacity of each of the Trx isoforms. The kinetic analysis of the reduction process showed no significant difference in the reduction efficiency between two major chloroplast Trxs, namely Trx- f and Trx- m. Based on the thorough analyses of the CF 1 -γ redox dynamics in Arabidopsis thaliana Trx mutant plants, we found that lack of Trx- f or Trx- m had no significant impact on the in vivo light-dependent reduction of CF 1 -γ. The results showed that CF 1 -γ can accept the reducing power from both Trx- f and Trx- m in chloroplasts. • The light-dependent rapid reduction on CF 1 -γ is the unique molecular mechanism of this regulation. • The CF 1 -γ reduction capacity of each of the Trx isoforms was directly examined in vivo. • CF 1 -γ shows the modest specificity on thioredoxin isoforms in chloroplasts. [ABSTRACT FROM AUTHOR]