Mycobacterium tuberculosis secretes ESAT-6, a virulence factor that triggers cell-mediated immune responses and IFN-␥ production during tuberculosis. ESAT-6 is transported across the bacterial envelope by a specialized secretion system with a FSD (FtsK-SpoIIIE domain) membrane protein. Although the presence of ESAT-6-like genes has been identified in the genomes of other microbes, the possibility that they may encode general virulence functions has hitherto not been addressed. Herein we show that the human pathogen Staphylococcus aureus secretes EsxA and EsxB, ESAT-6-like proteins, across the bacterial envelope. Staphylococcal esxA and esxB are clustered with six other genes and some of these are required for synthesis or secretion of EsxA and EsxB. Mutants that failed to secrete EsxA and EsxB displayed defects in the pathogenesis of S. aureus murine abscesses, suggesting that this specialized secretion system may be a general strategy of human bacterial pathogenesis.specialized secretion ͉ Gram-positive ͉ exoprotein ͉ ess

Staphylococcus aureus is a human pathogen responsible for most wound and hospital-acquired infections. The protein MgrA is both an important virulence determinant during infection and a regulator of antibiotic resistance in S. aureus. The crystal structure of the MgrA homodimer, solved at 2.86 A, indicates the presence of a unique cysteine residue located at the interface of the protein dimer. We discovered that this cysteine residue can be oxidized by various reactive oxygen species, such as hydrogen peroxide and organic hydroperoxide. Cysteine oxidation leads to dissociation of MgrA from DNA and initiation of signaling pathways that turn on antibiotic resistance in S. aureus. The oxidation-sensing mechanism is typically used by bacteria to counter challenges of reactive oxygen and nitrogen species. Our study reveals that in S. aureus, MgrA adopts a similar mechanism but uses it to globally regulate different defensive pathways.

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Bacterial dipeptide ABC transporters function to import a wide range of dipeptide substrates. This ability to transport a wide variety of dipeptides is conferred by the cognate substrate binding protein (SBP) of these transporters. SBPs bind dipeptides with little regard for their amino acid content. Here, we report the 1.7 Å resolution structure of lipoprotein-9 (SA0422) of Staphylococcus aureus in complex with the dipeptide glycylmethionine. Experimental characterization of the subcellular location of the protein confirmed that SA0422 is an acylated, peripheral membrane protein. This is the first structure determined for an SBP of a Gram-positive dipeptide ABC transporter. Usually, binding of dipeptides occurs in a binding pocket that is largely hydrated and able to accommodate the side chains of several different amino acid residues. Unlike any other known SBP, lipoprotein-9 binds the side chains of the glycylmethionine dipeptide through very specific interactions. Lipoprotein-9 shares significant structural and sequence homology with the MetQ family of methionine SBP. Sequence comparisons between MetQ-like proteins and lipoprotein-9 suggest that the residues forming the tight interactions with the methionine side chains of the ligand are highly conserved between lipoprotein-9 and MetQ homologues, while the residues involved in coordinating the glycine residue are not. Modeling of the Vibrio cholerae MetQ and lipoprotein-9 binding pockets can account for lipoprotein-9 substrate specificity toward glycylmethionine. For this reason, we have designated lipoprotein-9 GmpC, for glycylmethionine binding protein. † The uptake of peptides from the environment by bacteria is an important process that not only supplies bacteria with nutrients but also allows them to sense environmental conditions and to initiate appropriate signaling cascades (1-3). These peptide uptake systems are usually members of the ATP-binding cassette (ABC) 1 family of transporters. 2 The uptake ABC transporters are multisubunit complexes composed of integral membrane proteins that function as a permease, peripheral membrane ATP binding proteins that hydrolyze ATP, and extracellular substrate binding proteins (SBPs) that act as receptors for the substrate to be transported (1,4). Although structurally conserved, these transporters function in the uptake of a very diverse range of molecules. The SBP components of these systems to a large extent determine the substrate specificity of the ABC transporters with which they are associated. In Gram-negative bacteria, these SBPs are secreted into the periplasm and retained in this compartment by the outer membrane (1). However, in Gram-positive bacteria, the lack of an outer membrane necessitates the tethering of these proteins to the plasma membrane by a lipid anchor or by fusion to an integral membrane component of the transporter (1,5). All SBPs studied to date are structurally similar and bind their substrate through a conserved mechanism, termed the Venus' flytrap mechanism (6). The unlig...

Opioid medications are used extensively as potent analgesics for treating moderate to severe pain. Although opioids offer reliable pain relief, their use is associated with a number of adverse effects, especially constipation. Conventional measures for ameliorating opioid-induced adverse effects are often insufficient. Thus, reducing the severity of these adverse effects is of utmost importance for patients who require the benefits of opioid analgesics. Since opioids mediate pain-relieving and adverse effects through the same classes of receptors, i.e., mu, delta, and kappa, it has been challenging to dissociate beneficial effects from detrimental ones. Methylnaltrexone is the first peripherally acting opioid receptor antagonist to receive FDA approval. This compound offers the therapeutic potential to block or reverse the undesired side effects of opioids that are mediated by receptors located in the periphery (e.g., in the gastrointestinal tract), without affecting analgesia or precipitating opioid withdrawal symptoms, which are mediated predominantly by receptors in the central nervous system. This review provides a history of the development of methylnaltrexone, discusses studies that relate to opioid bowel dysfunction, and explores other uses of this compound. Drug Dev Res 70 : 403-416, 2009.