Part of the ABC transporter complex CmpABCD involved in bicarbonate transport. Probably responsible for the translocation of the substrate across the membrane (Probable). 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. Belongs to the binding-protein-dependent transport system permease family.