Journal Article

Lipopolysaccharide transport and assembly at the outer membrane: the PEZ model

Ruiz, Natividad, Kahne, Daniel, Okuda, Suguru, Sherman, David J., Silhavy, Thomas J.

2016

Overview

Key Points The outer membrane (OM) of most Gram-negative bacteria contains lipopolysaccharide (LPS), a large molecule that contains several fatty acyl chains and up to hundreds of sugars, in its outer leaflet, which creates a barrier that prevents the entry of large polar molecules and small hydrophobic molecules. The transport of millions of LPS molecules from the inner membrane (IM), across the aqueous periplasmic compartment, and across the OM to the cell surface was not well understood, except that the process is mediated by seven essential and conserved LPS transport (Lpt) proteins. The extraction of LPS from the IM is mediated by an ATP binding cassette (ABC) transporter, LptB 2 FG, and an associated membrane protein, LptC. These proteins couple ATP hydrolysis in the cytoplasm by LptB to movement to LptC; the LptB 2 FG and LptB 2 FGC protein complexes have been purified and demonstrate ATPase activity in vitro . LPS is thought to transit the periplasm by a bridge between LptC and the OM mediated by the periplasmic protein LptA. The bridge is formed by structurally homologous domains of LptC, LptA and the OM protein LptD, and it helps to mediate the transit of the hydrophobic acyl chains of LPS through an aqueous compartment. The OM β-barrel protein LptD and the OM lipoprotein LptE form a two-protein plug-and-barrel complex that is responsible for transporting LPS from the periplasmic bridge across the OM to the cell surface. A current model proposes that the OM translocon changes its conformation, enabling LPS molecules to enter the barrel of LptD and move to the cell surface through lateral openings without ever residing in the inner leaflet of the OM. LptD is a large β-barrel protein that contains two non-consecutive disulfide bonds, either of which is sufficient for the function of LptD. Correct rearrangement of the disulfide bonds to the final configuration is required for LptA to interact with LptD, which prevents mislocalization of LPS when the OM translocon is not properly assembled. Identification of LPS transport intermediates in Escherichia coli cells has enabled the development of a system to study the ATP requirement for LPS transport out of membrane vesicles to soluble LptA. Using this system, the PEZ model was developed to describe how ATP hydrolysis by LptB in the cytoplasm 'pushes' LPS molecules in a continuous stream out of the IM toward the cell surface through the periplasmic bridge comprised of LptC, LptA and LptD. In this Review, Kahne and colleagues discuss how lipopolysaccharide (LPS) is transported across the cellular envelope and inserted into the outer membrane of Gram-negative bacteria. They propose a new model, which explains how energy from the cytoplasm is used to power LPS transport to the cell surface. Gram-negative bacteria have a double-membrane cellular envelope that enables them to colonize harsh environments and prevents the entry of many clinically available antibiotics. A main component of most outer membranes is lipopolysaccharide (LPS), a glycolipid containing several fatty acyl chains and up to hundreds of sugars that is synthesized in the cytoplasm. In the past two decades, the proteins that are responsible for transporting LPS across the cellular envelope and assembling it at the cell surface in Escherichia coli have been identified, but it remains unclear how they function. In this Review, we discuss recent advances in this area and present a model that explains how energy from the cytoplasm is used to power LPS transport across the cellular envelope to the cell surface.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/326/22 / 631/326/41/1969 / 631/326/41/2536 / Antibiotics / Antigens / Bacteria / Bacterial Outer Membrane Proteins - metabolism / Biological Transport / Cell Membrane - chemistry / Cell Membrane - metabolism / Chemistry / Cytoplasm / E coli / Escherichia coli - metabolism / Escherichia coli Proteins - metabolism / Gram-negative bacteria / Infectious Diseases / Life Sciences / Lipids / Lipopolysaccharides - chemistry / Lipopolysaccharides - metabolism / Medical Microbiology / Metabolic Networks and Pathways / Microbiology / Models, Biological / Parasitology / Proteins / review-article / Virology