Molecular mechanism of redox regulation of the alpha-carboxysomal carbonic anhydrase CsoSCA

Carboxysomes are protein-based organelles that form the core of the bacterial CO2-concentrating mechanism (CCM) by elevating CO2 levels around Rubisco. They encapsulate Rubisco and carbonic anhydrase (CA) within a protein shell that, after closure, excludes cytosolic reductants. Because cytosolic CA activity would short-circuit the CCM, CA activity must be confined to the carboxysome, yet how -carboxysomes achieve this has remained unknown. Here we show that CsoSCA, the -carboxysomal CA, is redox-regulated: inactive under reducing conditions, active under oxidizing. This regulation is mediated by a conserved vicinal cysteine pair distal from the active site. CryoEM structures of Halothiobacillus neapolitanus CsoSCA under active and inactive conditions, and of an inactive cysteine variant, reveal that redox conditions modulate global conformational dynamics that reorganize the active site for catalysis. These findings advance the understanding of -carboxysome regulation and couple CsoSCA activation to lumenal oxidation during carboxysome maturation.