A Small Family of Chloroplast Atypical Thioredoxins1[C][W][OA]

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts. Members of the thioredoxin (Trx) superfamily of oxidoreductases play a major role in these processes. We have characterized a small family of plant-specific Trxs in Arabidopsis (Arabidopsis thaliana) that are rich in cysteine and histidine residues and are typified by a variable noncanonical redox active site. We found that the redox midpoint potential of three selected family members is significantly less reducing than that of the classic Trxs. Assays of subcellular localization demonstrated that all proteins are localized to the chloroplast. Selected members showed high activity, contingent on a dithiol electron donor, toward the chloroplast 2-cysteine peroxiredoxin A and poor activity toward the chloroplast NADP-malate dehydrogenase. The expression profile of the family members suggests that they have distinct roles. The intermediate redox midpoint potential value of the atypical Trxs might imply adaptability to function in modulating the redox state of chloroplast proteins with regulatory disulfides.