The Mla system is a bacterial ABC (ATP-binding cassette) transport system proposed to maintain outer membrane (OM) lipid asymmetry by trafficking phospholipids (PLs) between the OM and the inner membrane. The Mla system consists of six components, including the OM lipoprotein, MlaA; the periplasmic lipid chaperone, MlaC; and the inner membrane complex, MlaFEDB composed of the ATP-binding cassette, MlaE and MlaF; and two auxiliary proteins, MlaD and MlaB. MlaD forms a hexameric ring in the MlaFEDB complex, facing towards to the periplasm. Both MlaD and MlaC play an essential role in PLs transport. MlaD accepts and transfers PLs from and to the MlaC in both Escherichia coli and Acinetobacter baumannii. E. coli MlaD binds to PLs and interacts with MlaC via the interface of its hexamer. MlaC binds to MlaA and MlaD, transporting PLs between the two membranes across the periplasm. E. coli MlaC has a hydrophobic pocket and binds to PLs via a pivoting β-sheet mechanism. Recently, the β6-7 loop of E. coli MlaD has been shown to play a key role in the MlaC-MlaD interaction and PL transfer. However, little is known about the interplay between MlaD and MlaC from Acinetobacter baumannii, and their interactions with PLs. This is because A. baumannii MlaD has an additional region of 47-amino acids (47-aa) forming a periplasmic facing, three-helix domain, between the 4th and 5th β-strands. Here we show that PL transfer between MlaC and MlaD in A. baumannii differs from that of E. coli and that both the additional region and the β6-7 loop of A. baumannii MlaD are likely to play a key role in MlaC-MlaD binding and PL transfer in A. baumannii.