Functional Interaction between the Cytoplasmic ABC Protein LptB and the Inner Membrane LptC Protein, Components of the Lipopolysaccharide Transport Machinery in Escherichia coli

The assembly of lipopolysaccharide (LPS) in the outer leaflet of the outer membrane (OM) requires the transenvelope Lpt (lipopolysaccharide transport) complex, made in Escherichia coli of seven essential proteins located in the inner membrane (IM) (LptBCFG), periplasm (LptA), and OM(LptDE). At the IM, LptBFG constitute an unusual ATP binding cassette (ABC) transporter, composed by the transmembrane LptFG proteins and the cytoplasmic LptB ATPase, which is thought to extract LPS from the IM and to provide the energy for its export across the periplasm to the cell surface. LptC is a small IM bitopic protein that binds to LptBFG and recruits LptA via its N- and C-terminal regions, and its role in LPS export is not completely understood. Here, we show that the expression level of lptB is a critical factor for suppressing lethality of deletions in the C-terminal region of LptC and the functioning of a hybrid Lpt machinery that carries Pa-LptC, the highly divergent LptC orthologue from Pseudomonas aeruginosa. We found that LptB overexpression stabilizes C-terminally truncated LptC mutant proteins, thereby allowing the formation of a sufficient amount of stable IM complexes to support growth. Moreover, the LptB level seems also critical for the assembly of IM complexes carrying Pa-LptC which is otherwise defective in interactions with the E. coli LptFG components. Overall, our data suggest that LptB and LptC functionally interact and support a model whereby LptB plays a key role in the assembly of the Lpt machinery. IMPORTANCE The asymmetric outer membrane (OM) of Gram-negative bacteria contains in its outer leaflet an unusual glycolipid, the lipopolysaccharide (LPS). LPS largely contributes to the peculiar permeability barrier properties of the OM that prevent the entry of many antibiotics, thus making Gram-negative pathogens difficult to treat. In Escherichia coli the LPS transporter (the Lpt machine) is made of seven essential proteins (LptABCDEFG) that form a transenvelope complex. Here, we show that increased expression of the membrane-associated ABC protein LptB can suppress defects of LptC, which participates in the formation of the periplasmic bridge. This reveals functional interactions between these two components and supports a role of LptB in the assembly of the Lpt machine.