Lipoprotein LptE is required for the assembly of LptD by the β-barrel assembly machine in the outer membrane of Escherichia coli

Most Gram-negative bacteria contain lipopolysaccharide (LPS), a glucosamine-based phospholipid, in the outer leaflet of the outer membrane (OM). LPS is unique to the bacterial OM and, in most cases, essential for cell viability. Transport of LPS from its site of synthesis to the cell surface requires eight essential proteins, MsbA and LptABCDEFG. Although the key players have been identified, the mechanism of LPS transport and assembly is not clear. The stable LptD/E complex is present at the OM and functions in the final stages of LPS assembly. Here, we have identified the mutant allele lptE6, which causes a two-amino-acid deletion in the lipoprotein LptE that affects its interaction with LptD. Highly specific suppressor mutations were isolated not only in lptD but also in bamA, which encodes the central component of the beta-barrel assembly machine. We show that lptE6 and both suppressor mutations affect the assembly of the LptD/E complex and suggest that the lipoprotein LptE interacts with LptD while this protein is being assembled by the beta-barrel assembly machine.