Part of the ABC transporter complex CmpABCD involved in bicarbonate transport. Responsible for energy coupling to the transport system (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 ABC transporter superfamily. Nitrate/nitrite/cyanate uptake transporter (NitT) (TC 3.A.1.16) family.