| 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 at the cytoplasmic face of the IM to the cell surface requires eight essential proteins, MsbA and LptABCDEFG. While 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 describe the genetic and biochemical characterization of this two-protein complex. Lipid modifications to the N-terminus of the OM lipoprotein LptE are dispensable for LptE function although they are required for maintaining optimal levels of LptD/E at the OM. LptE binds LPS in a specific manner suggesting a role for it in receiving LPS from the periplasm.;Studies presented here have given us new insights into the biogenesis of the LptD/E complex. We have identified the mutant allele lptE6 , which causes a 2-amino acid deletion in 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. |
