Fig. 1: Cryo-EM structures of SbtA and its complex with SbtB.

a Overall structure of a SbtA subunit viewed from the extracellular side with helices shown as cylinders. b Side view of the trimeric structure of SbtA (left) and top view from the intracellular side (right). The gate domains are colored in orange, whereas the core domains are colored in green. The missing residues between Ser166 and Trp212 are indicated as a dashed line. c Cartoon representation of SbtA–SbtB complex structure. Three subunits of SbtB are colored in magentas, red and blue, respectively. The C-terminal disulfide bond between Cys105 and Cys110 is indicated by blue sticks. An AMP molecule in the nucleotide-binding cleft is shown as yellow sticks. d Interfaces between a pair of SbtA and SbtB subunits. The interacting residues are shown as sticks, with the polar interactions indicated by dashed lines. e The putative Na⁺-binding site. The Na⁺ is shown as a violet sphere whereas the Na⁺-binding residues are shown as sticks. The polar interactions are indicated by dashed lines. The cryo-EM density map of Na⁺ is shown in blue mesh. f The putative HCO₃^–-binding site modeled by HADDOCK. The HCO₃^– molecule is shown as sticks and colored by atoms. g The HCO₃^– transport activity assays of the wild-type SbtA and mutants in E. coli membrane vesicles. Three independent experiments were performed for each assay. The means and standard deviations were calculated and the data are presented as means ± SD. Two-tailed Student’s t-test is used for the comparison of statistical significance. The P values of < 0.05 and < 0.01 are indicated with * and **, respectively. h A proposed elevator mechanism of Na⁺-dependent bicarbonate transport of the trimeric SbtA. The Na⁺ and HCO₃^– are shown as red and blue spheres, respectively. SbtA of an outward-open conformation recruits the substrates HCO₃^– and Na⁺ from the periplasm, accompanied by a rigid-body movement of core domains (green) against the immobile gate domains (yellow). Afterwards, the substrates are released into the cytosol followed by the turnover of SbtA into the resting state.