Assembly and Regulation of the Lipopolysaccharide Transporter

The hallmark of Gram-negative bacteria is the presence of an outer membrane (OM) surrounding the cytoplasmic membrane (here called the inner membrane [IM]) and the cell wall. The OM is a unique asymmetric bilayer with an inner leaflet consisting of phospholipid and an outer leaflet consisting of lipopolysaccharide (LPS). LPS is a large anionic molecule that typically contains six fatty acyl chains and up to several hundred sugar residues. This chemical structure explains why the OM is relatively impermeable to large hydrophobic molecules, such as detergents, bile salts, and high molecular weight antibiotics, which readily cross a normal phospholipid bilayer. LPS and the OM are essential to the viability of most Gram-negative organisms, including major human pathogens. LPS molecules are biosynthesized at the IM and subsequently exported out of the IM, across the intermembrane space (the periplasm) and through the OM to their final position at the cell surface. In Escherichia coli, the essential LPS transport proteins, LptA-G, are required for this process. This Lpt pathway includes an IM adenosine triphosphate binding cassette (ABC) transporter, LptBFG, which is associated with an additional IM protein, LptC; a periplasmic protein, LptA; and an OM complex consisting of the lipoprotein LptE and the transmembrane β-barrel protein LptD. All seven Lpt proteins associate as a single complex that spans the cell envelope. However, little is known about how these proteins work together to transport LPS. Here, we use in vivo and in vitro biochemical studies to probe the organization, function, and assembly of the Lpt machine. In Chapter 2, we show that LptE forms a plug within the LptD β-barrel and present a model for how this unusual structure can move LPS from the periplasm directly into the outer leaflet of the OM. In Chapter 3, we demonstrate that the Lpt transenvelope bridge consists of a series of structurally homologous domains – LptC, LptA, and the N-terminal domain of LptD – stacked in a head-to-tail orientation, providing a route for LPS from the IM to the OM. Finally, in Chapter 4, we connect these two sets of results by showing how the assembly of the Lpt transenvelope bridge is regulated by that of the LptD/E complex in the OM. Together, these findings explain how the functions of the Lpt proteins are coordinated to ensure delivery of LPS to the correct cellular compartment. A fundamental understanding of LPS biogenesis will contribute to the development of new therapies against Gram-negative infections.