The envelope of Gram-negative bacteria is delimited by two lipid bilayers, the inner and outer membranes (IM and OM, respectively). The external leaflet of the OM contains densely packed lipopolysaccharides (LPS) that confer unusually high impermeability towards small hydrophobic molecules. As a result, Gram-negative bacteria are naturally resistant to many antibiotics. The IM and OM are separated by the aqueous compartment known as the periplasm where a cell wall composed of peptidoglycan resides. The peptidoglycan cell wall is an essential polymeric rigid structure that protects cells from osmotic lysis. Given the structural and protective functions of the cell envelope, proper envelope biogenesis is crucial for the survival of bacteria in many environments. Underscoring this is the fact that many antibiotics target envelope biogenesis pathways. Our long-term goal is to understand at the molecular level how Gram-negative bacteria build their cell envelope. Here, we propose to primarily use a combination of genetic and biochemical approaches to investigate two highly conserved systems that transport glycolipids across the cell envelope from their site of synthesis to the cellular compartment where they function: 1) MurJ, a polytopic IM protein that facilitates the most poorly understood step in peptidoglycan biosynthesis, the translocation of the lipid-linked peptidoglycan precursor lipid II across the IM; and 2) Lpt (LPS transport), a mult-protein bridge that spans the envelope and that functions to transport LPS from the IM to the cell surface. Both of these systems are essential for the viability of many bacteria including our model organism Escherichia coli.
In aim 1, we propose studies to understand the mechanism that MurJ uses to flip lipid II by: a) conducting structure-function studies on MurJ; b) determinin how MurJ interacts with lipid II; c) probing conformational changes that MurJ undergoes during the transport cycle; and, d) studying how MurJ is powered.
In aim 2, we will investigate the most poorly understood step in LPS transport by focusing our studies on the LptFGB2C sub-complex, a unique ATP- binding cassette transporter that powers the extraction of LPS from the IM and its transport along the Lpt bridge to the cell surface. Specifically, in aim 2, we will: a) determie the topology of the membrane components LptF and LptG with respect to the IM; b) define protein-protein interactions in the LptFGB2C sub- complex; and c) elucidate how LptFGB2C couples ATP binding and hydrolysis in the cytoplasm to the extraction of LPS from the outer leaflet of the IM. Because inhibition of MurJ function leads to cell lysis and defects in the Lpt system can either increase OM permeability to many antibiotics or even cause death, knowledge gained from the proposed work will help in developing novel antimicrobial therapies. Studies on Lpt are especially needed to understand how we can overcome the innate resistance to antibiotics that Gram- negative have because of the barrier imposed by the presence of LPS at the cell surface.

Public Health Relevance

One of the most serious threats to global health is the combination of the rise in antibiotic resistance and the lack of new antibiotics effective against Gram-negative bacteria. This project seeks to understand how Gram-negative bacteria build their cell envelope, including studies of novel antibiotic targets and the barrier that protects these microorganisms against the entry of many antibiotics. The knowledge gained from these studies will help develop new antimicrobial therapies against Gram-negative pathogens.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Prokaryotic Cell and Molecular Biology Study Section (PCMB)
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Marino, Pamela
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Ohio State University
Schools of Arts and Sciences
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Bertani, Blake; Ruiz, Natividad (2018) Function and Biogenesis of Lipopolysaccharides. EcoSal Plus 8:
Rubino, Frederick A; Kumar, Sujeet; Ruiz, Natividad et al. (2018) Membrane Potential Is Required for MurJ Function. J Am Chem Soc 140:4481-4484
Bertani, Blake R; Taylor, Rebecca J; Nagy, Emma et al. (2018) A cluster of residues in the lipopolysaccharide exporter that selects substrate variants for transport to the outer membrane. Mol Microbiol 109:541-554
May, Janine M; Owens, Tristan W; Mandler, Michael D et al. (2017) The Antibiotic Novobiocin Binds and Activates the ATPase That Powers Lipopolysaccharide Transport. J Am Chem Soc 139:17221-17224
Elhenawy, Wael; Davis, Rebecca M; Fero, Jutta et al. (2017) Correction: The O-Antigen Flippase Wzk Can Substitute for MurJ in Peptidoglycan Synthesis in Helicobacter pylori and Escherichia coli. PLoS One 12:e0170518
Chamakura, Karthik R; Sham, Lok-To; Davis, Rebecca M et al. (2017) A viral protein antibiotic inhibits lipid II flippase activity. Nat Microbiol 2:1480-1484
Qiao, Yuan; Srisuknimit, Veerasak; Rubino, Frederick et al. (2017) Lipid II overproduction allows direct assay of transpeptidase inhibition by ?-lactams. Nat Chem Biol 13:793-798
Simpson, Brent W; Owens, Tristan W; Orabella, Matthew J et al. (2016) Identification of Residues in the Lipopolysaccharide ABC Transporter That Coordinate ATPase Activity with Extractor Function. MBio 7:
Okuda, Suguru; Sherman, David J; Silhavy, Thomas J et al. (2016) Lipopolysaccharide transport and assembly at the outer membrane: the PEZ model. Nat Rev Microbiol 14:337-45
Ruiz, Natividad (2016) Filling holes in peptidoglycan biogenesis of Escherichia coli. Curr Opin Microbiol 34:1-6

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