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 (PubMed:19383799, PubMed:27529189). MlaB plays critical roles in both the assembly and activity of the complex. May act by modulating MlaF structure and stability (PubMed:27529189). 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). Mutation leads to accumulation of phospholipid in the outer leaflet of the outer membrane and increased outer membrane permeability. It confers sensitivity to SDS-EDTA. Extended N-terminus.