Part of the ABC transporter complex OpuCABCD involved in carnitine uptake. Involved, with BetL and GbuABC, in osmoprotection and cryoprotection of Listeria. The complex is composed of two ATP-binding proteins (OpuCA), two transmembrane proteins (OpuCB and OpuCD) and a solute-binding protein (OpuCC). Belongs to the OsmX family.