Part of the ABC transporter complex CmpABCD involved in bicarbonate transport. Binds bicarbonate with high affinity. 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. Also induced by growth under high light intensities. Repressed by iron-deficient conditions. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. The N-terminus is blocked. Belongs to the CmpA/NrtA family. Was originally thought to be a carotenoid-binding protein.