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