Part of the ABC transporter complex CmpABCD involved in bicarbonate transport. Binds bicarbonate with high affinity (By similarity). The complex is composed of two ATP-binding proteins (CmpC and CmpD), a transmembrane protein (CmpB) and a solute-binding protein (CmpA). By carbon dioxide-limited conditions, probably via CmpR. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the CmpA/NrtA family.