Pseudomonas aeruginosa delivers the toxin ExoU to eukaryotic cells via a type III secretion system. Intoxication with ExoU is associated with lung injury, bacterial dissemination and sepsis in animal model and human infections. To search for ExoU targets in a genetically tractable system, we used controlled expression of the toxin in Saccharomyces cerevisiae. ExoU was cytotoxic for yeast and caused a vacuolar fragmentation phenotype. Inhibitors of human calcium-independent (iPLA 2 ) and cytosolic phospholipase A 2 (cPLA 2 ) lipase activity reduce the cytotoxicity of ExoU. The catalytic domains of patatin, iPLA 2 and cPLA 2 align or are similar to ExoU sequences. Sitespeci®c mutagenesis of predicted catalytic residues (ExoUS142A or ExoUD344A) eliminated toxicity. ExoU expression in yeast resulted in an accumulation of free palmitic acid, changes in the phospholipid pro®les and reduction of radiolabeled neutral lipids. ExoUS142A and ExoUD344A expressed in yeast failed to release palmitic acid. Recombinant ExoU demonstrated lipase activity in vitro, but only in the presence of a yeast extract. From these data we conclude that ExoU is a lipase that requires activation or modi®cation by eukaryotic factors.

The virus responsible for Middle Eastern respiratory syndrome, MERS-CoV is a lineage C betacoronavirus similar to the mouse hepatitis virus type A59 (MHV-A59). The first reported case of MERS occurred in Saudi Arabia in 2012 and resulted in 76 deaths1. Outbreaks of MERS have since occurred not only in the Middle East but South Korea as well2. Rapid, efficient, and automated methods of disinfecting surfaces contaminated with the MERS-CoV virus may prevent the spread of the virus in the healthcare setting. Here we report on the use of an automated triple-emitter whole room disinfection system to inactivate the MHV-A59 and the MERS-CoV viruses on surfaces with a greater than 5 log10 reduction on MERS in 5 minutes of UV-C exposure.

The ExoU type III secretion enzyme is a potent phospholipase A secreted by the Gram-negative opportunistic pathogen, Activation of phospholipase activity is induced by protein-protein interactions with ubiquitin in the cytosol of a targeted eukaryotic cell, leading to destruction of host cell membranes. Previous work in our laboratory suggested that conformational changes within a C-terminal domain of the toxin might be involved in the activation mechanism. In this study, we use site-directed spin-labeling electron paramagnetic resonance spectroscopy to investigate conformational changes in a C-terminal four-helical bundle region of ExoU as it interacts with lipid substrates and ubiquitin, and to examine the localization of this domain with respect to the lipid bilayer. In the absence of ubiquitin or substrate liposomes, the overall structure of the C-terminal domain is in good agreement with crystallographic models derived from ExoU in complex with its chaperone, SpcU. Significant conformational changes are observed throughout the domain in the presence of ubiquitin and liposomes combined that are not observed with either liposomes or ubiquitin alone. In the presence of ubiquitin, two interhelical loops of the C-terminal four-helix bundle appear to penetrate the membrane bilayer, stabilizing ExoU-membrane association. Thus, ubiquitin and the substrate lipid bilayer act synergistically to induce a conformational rearrangement in the C-terminal domain of ExoU.

MsbA is an ABC transporter that transports lipid A across the inner membrane of Gram-negative bacteria such as Escherichia coli. Without functional MsbA present, bacterial cells accumulate a toxic amount of lipid A within their inner membranes. A crystal structure of MsbA was recently obtained that provides an excellent starting point for functional dynamics studies in membranes [Chang and Roth (2001) Science 293, 1793-1800]. Although a structure of MsbA is now available, several functionally important motifs common to ABC transporters are unresolved in the crystal structure. The Walker A domain, one of the ABC transporter consensus motifs that is directly involved in ATP binding, is located within a large unresolved region of the MsbA ATPase domain. Site-directed spin labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy is a powerful technique for characterizing local areas within a large protein structure in addition to detecting and following changes in local structure due to dynamic interactions. MsbA reconstituted into lipid membranes has been evaluated by EPR spectroscopy, and it has been determined that the Walker A domain forms an alpha-helical structure, which is consistent with the structure of this motif observed in other crystallized ABC transporters. In addition, the interaction of the Walker A residues with ATP before, during, and after hydrolysis was followed using SDSL EPR spectroscopy in order to identify the residues directly involved in substrate binding and hydrolysis.

ATP-binding cassette (ABC) transporters make up one of the largest classes of proteins found in nature, and their ability to move a variety of substrates across the membrane using energy from the binding or hydrolysis of ATP is essential to an array of human pathologies and to bacterial viability. MsbA is an essential ABC transporter that specifically transports lipid A across the inner membranes of Gram-negative organisms such as Escherichia coli. The exact mechanisms of function during the binding and hydrolysis of ATP at the molecular level remain unclear. The studies presented and summarized in this work directly address the role and local dynamics of specific residues within the conserved ABC motifs in E. coli MsbA using in vivo growth and biochemical activity assays coupled with site-directed spin labeling electron paramagnetic resonance (EPR) spectroscopy motional and accessibility analysis. This first comprehensive analysis of the specific residues in these motifs within MsbA indicates that closure of the dimer interface does not occur upon ATP binding in this transporter.ATP-binding cassette (ABC) 1 transporters constitute one of the largest and most important families of integral membrane proteins. They specifically contribute to drug resistance by using ATP hydrolysis to export delivered drugs back across the cell membrane, and the function or dysfunction of other known ABC transporters results in serious genetic disorders such as cystic fibrosis (1). ABC transporters are typically comprised of two membrane-spanning domains and two intracellular ATP binding domains [or nucleotide binding domains (NBDs)].MsbA is an ABC transporter that functions as a homodimer with a molecular mass of 130 kDa and is found in the inner membranes of Gram-negative bacteria such as Escherichia coli, Salmonella typhimurium, Vibrio cholera, and Pseudomonas aeruginosa (2). Its role is to transport the negatively charged lipid A across the hydrophobic bacterial inner membrane. Because lipid A is the major component of the outer leaflet of the outer membrane of Gramnegative bacteria, its synthesis and transport are essential for cell growth. Thus, the functional loss of MsbA from the bacterium results in a toxic accumulation of lipid A within the inner membrane (3), making MsbA the only ABC transporter currently know to be required for bacterial viability (2).A reanalyzed crystal structure of the E. coli MsbA homodimer is now available at a resolution of 5.3 Å, along with the structures of MsbA from S. typhimurium and V. cholera and homologues such as SAV1866 (4-6). These structures reveal that the MsbA monomer contains † This work was supported by the National Institutes of Health (GM070642).*To whom correspondence should be addressed. Phone: (414) . E-mail: candice@mcw.edu. 1 Abbreviations: ABC, ATP-binding cassette; EPR, electron paramagnetic resonance; SDSL, site-directed spin labeling; CW, continuous wave; NBD, nucleotide binding domain; MTSL, 2,2,5,5-tetramethylpyrroline-3-yl-methanethiosulfonate spin label; WT,...