Structural Insights into the Coupling Mechanism of Vectorial CO ₂ Uptake by DAB1

CO ₂ transporters enable bacterial carbon-concentrating mechanisms by catalyzing directional hydration of CO ₂ , yet the basis of this vectorial carbonic anhydrase (CA) activity remains an open question. We used cryo-EM to determine the structure of the DAB1 complex from Thermocrinis albus to 2.13 Å, revealing a heterotrimer in which a deeply buried β-CA active site in DabA is structurally coupled to the proton-translocating subunit DabB. Two conformational states define distinct solvent channels for substrate entry and product exit. A suppressor screen identifies mutations that disrupt coupling while retaining CA activity, underlying the importance of conserved residues that link proton translocation to active-site remodeling. These results support a model in which proton-driven conformational changes regulate substrate access to the active site, enabling vectorial CO₂ hydration.