Part of the ABC transporter complex MlaFEDB, which is involved in a phospholipid transport pathway that maintains lipid asymmetry in the outer membrane by retrograde trafficking of phospholipids from the outer membrane to the inner membrane. MlaB plays critical roles in both the assembly and activity of the complex. May act by modulating MlaF structure and stability. The complex is composed of two ATP-binding proteins (MlaF), two transmembrane proteins (MlaE), two cytoplasmic solute-binding proteins (MlaB) and six periplasmic solute-binding proteins (MlaD).