Thiol Redox Regulation of Plant β-Carbonic Anhydrase

beta-carbonic anhydrases (beta CA) accelerate the equilibrium formation between CO(2)and carbonate. Two plant beta CA isoforms are targeted to the chloroplast and represent abundant proteins in the range of >1% of chloroplast protein. While their function in gas exchange and photosynthesis is well-characterized in carbon concentrating mechanisms of cyanobacteria and plants with C4-photosynthesis, their function in plants with C3-photosynthesis is less clear. The presence of conserved and surface-exposed cysteinyl residues in the beta CA-structure urged to the question whether beta CA is subject to redox regulation. Activity measurements revealed reductive activation of beta CA1, whereas oxidized beta CA1 was inactive. Mutation of cysteinyl residues decreased beta CA1 activity, in particular C280S, C167S, C230S, and C257S. High concentrations of dithiothreitol or low amounts of reduced thioredoxins (TRXs) activated oxidized beta CA1. TRX-y1 and TRX-y2 most efficiently activated beta CA1, followed by TRX-f1 and f2 and NADPH-dependent TRX reductase C (NTRC). High light irradiation did not enhance beta CA activity in wildtype Arabidopsis, but surprisingly in beta ca1knockout plants, indicating light-dependent regulation. The results assign a role of beta CA within the thiol redox regulatory network of the chloroplast.