Molecular events involved in a single cycle of ligand transfer from an ATP binding cassette transporter, LolCDE, to a molecular chaperone, LolA.

An ATP binding cassette transporter LolCDE complex releases lipoproteins from the inner membrane of Escherichia coli in an ATP-dependent manner, leading to the formation of a complex between a lipoprotein and a periplasmic chaperone, LolA. LolA is proposed to undergo a conformational change upon the lipoprotein binding. The lipoprotein is then transferred from the LolA-lipoprotein complex to the outer membrane via LolB. Unlike most ATP binding cassette transporters mediating the transmembrane flux of substrates, the LolCDE complex catalyzes the extrusion of lipoproteins anchored to the outer leaflet of the inner membrane. Moreover, the LolCDE complex is unique in that it can be purified as a liganded form, which is an intermediate of the lipoprotein release reaction. Taking advantage of these unique properties, we established an assay system that enabled the analysis of a single cycle of lipoprotein transfer reaction from liganded LolCDE to LolA in a detergent solution. The LolA-lipoprotein complex thus formed was physiologically functional and delivered lipoproteins to the outer membrane in a LolB-dependent manner. Vanadate, a potent inhibitor of the lipoprotein release from proteoliposomes, was found to inhibit the release of ADP from LolCDE. However, a single cycle of lipoprotein transfer occurred from vanadate-treated LolCDE to LolA, indicating that vanadate traps LolCDE at the energized state.