84 research outputs found
Are immigrant children admitted to intensive care at increased risk?
These findings indicate that disparities may exist at a lower level of illness severity, due to many possible reasons (for example shortcomings in primary health care). However, once a child enters tertiary health care, nationality and socio-economic factors no longer influence quality of health care delivery
Personal and environmental factors associated with active commuting to school in Switzerland
Objective. To assess whether prevalence of active commuting and regular car trips to school varies across communities and language regions in Switzerland and to determine personal and environmental correlates. Methods. During the school year 2004/2005, 1345 parental questionnaires (response rate 65%) of children attending 1st, 4th and 8th grades were completed, 1031 could be linked to a GIS environmental database. A German-speaking, a French-speaking and a bilingual study area were included. Usual mode of transportation and frequency of regular car trips to school were assessed. Associations with personal and environmental factors were evaluated with multivariate regression models. Results. Seventy-eight percent of the children actively traveled to school. Twelve percent were regularly driven at least once a week by car. Major road crossings and distance were significantly related to usual mode of transportation, but not to regular car trips. Age, daycare attendance, parental safety concerns, number of cars in the household and belonging to French-speaking population were significantly associated with increased regular car trips. Conclusion. Objective predictors are main deciding factors for active commuting to school as main mode of transport whereas personal and lifestyle factors are important factors associated with frequency of car use. Not only objective but also differing cultural attitudes should be considered when promoting non-motorized travel
Crystal structure of the CusBA heavy-metal efflux complex of Escherichia coli
Gram-negative bacteria, such as Escherichia coli, expel toxic chemicals via tripartite efflux pumps spanning both the inner and outer membranes. The three parts are: 1) a membrane fusion protein connecting 2) a substrate-binding inner membrane transporter to 3) an outer membrane-anchored channel in the periplasmic space. A crystallographic model of this tripartite efflux complex has been unavailable simply because co-crystallization of different components of the system has proven to be extremely difficult. We previously described the crystal structures of both the inner membrane transporter CusA1 and membrane fusion protein CusB2 of the CusCBA efflux system3,4 from E. coli. We here report the co-crystal structure of the CusBA efflux complex, revealing the trimeric CusA efflux pump interacts with six CusB protomers at the upper half of the periplasmic domain. These six CusB molecules form a channel extending contiguously from the top of the pump. The affinity of the CusA and CusB interaction was found to be in the micromolar range. Finally, we predicted a three-dimensional structure of the trimeric CusC outer membrane channel, and develop a model of the tripartite efflux assemblage. This CusC3-CusB6-CusA3 model presents a 750 kDa efflux complex spanning the entire bacterial cell envelope to export Cu(I)/Ag(I) ions
Crystal structures of the CusA efflux pump suggest methionine-mediated metal transport
Gram-negative bacteria, such as Escherichia coli, frequently utilize tripartite efflux complexes in the resistance-nodulation-cell division (RND) family to expel diverse toxic compounds from the cell.1,2 The efflux system CusCBA is responsible for extruding biocidal Cu(I) and Ag(I) ions.3,4 No prior structural information was available for the heavy-metal efflux (HME) subfamily of the RND efflux pumps. Here we describe the crystal structures of the inner membrane transporter CusA in the absence and presence of bound Cu(I) or Ag(I). These CusA structures provide important new structural information about the HME sub-family of RND efflux pumps. The structures suggest that the metal binding sites, formed by a three-methionine cluster, are located within the cleft region of the periplasmic domain. Intriguingly, this cleft is closed in the apo-CusA form but open in the CusA-Cu(I) and CusA-Ag(I) structures, which directly suggests a plausible pathway for ion export. Binding of Cu(I) and Ag(I) triggers significant conformational changes in both the periplasmic and transmembrane domains. The crystal structure indicates that CusA has, in addition to the three-methionine metal binding site, four methionine pairs - three located in the transmembrane region and one in the periplasmic domain. Genetic analysis and transport assays suggest that CusA is capable of actively picking up metal ions from the cytosol, utilizing these methionine pairs/clusters to bind and export metal ions. These structures suggest a stepwise shuttle mechanism for transport between these sites
Drug export and allosteric coupling in a multidrug transporter revealed by molecular simulations
Multidrug resistance is a serious problem in current chemotherapy. The efflux system largely responsible for resistance in Escherichia coli contains the drug transporter, AcrB. The structures of AcrB were solved in 2002 as the symmetric homo-trimer, and then in 2006 as the asymmetric homo-trimer. The latter suggested a functionally rotating mechanism. Here, by molecular simulations of the AcrB porter domain, we uncovered allosteric coupling and the drug export mechanism in the AcrB trimer. Allosteric coupling stabilized the asymmetric structure with one drug molecule bound, which validated the modelling. Drug dissociation caused a conformational change and stabilized the symmetric structure, providing a unified view of the structures reported in 2002 and 2006. A dynamic study suggested that, among the three potential driving processes, only protonation of the drug-bound protomer can drive the functional rotation and simultaneously export the drug
Functional Rotation of the Transporter AcrB: Insights into Drug Extrusion from Simulations
The tripartite complex AcrAB-TolC is the major efflux system in Escherichia coli. It extrudes a wide spectrum of noxious compounds out of the bacterium, including many antibiotics. Its active part, the homotrimeric transporter AcrB, is responsible for the selective binding of substrates and energy transduction. Based on available crystal structures and biochemical data, the transport of substrates by AcrB has been proposed to take place via a functional rotation, in which each monomer assumes a particular conformation. However, there is no molecular-level description of the conformational changes associated with the rotation and their connection to drug extrusion. To obtain insights thereon, we have performed extensive targeted molecular dynamics simulations mimicking the functional rotation of AcrB containing doxorubicin, one of the two substrates that were co-crystallized so far. The simulations, including almost half a million atoms, have been used to test several hypotheses concerning the structure-dynamics-function relationship of this transporter. Our results indicate that, upon induction of conformational changes, the substrate detaches from the binding pocket and approaches the gate to the central funnel. Furthermore, we provide strong evidence for the proposed peristaltic transport involving a zipper-like closure of the binding pocket, responsible for the displacement of the drug. A concerted opening of the channel between the binding pocket and the gate further favors the displacement of the drug. This microscopically well-funded information allows one to identify the role of specific amino acids during the transitions and to shed light on the functioning of AcrB
AcrB Trimer Stability and Efflux Activity, Insight from Mutagenesis Studies
The multidrug transporter AcrB in Escherichia coli exists and functions as a homo-trimer. The assembly process of obligate membrane protein oligomers, including AcrB, remains poorly understood. In a previous study, we have shown that individual AcrB subunit is capable of folding independently, suggesting that trimerization of AcrB follows a three-stage pathway in which monomers first fold, and then assemble. Here we destabilized the AcrB trimer through mutating a single Pro (P223) in the protruding loop of AcrB, which drastically reduced the protein activity. We replaced P223 separately with five residues, including Ala, Val, Tyr, Asn, and Gly, and found that AcrBP223G was the least active. Detailed characterization of AcrBP223G revealed that the protein existed as a well-folded monomer after purification, but formed a trimer in vivo. The function of the mutant could be partly restored through strengthening the stability of the trimer using an inter-subunit disulfide bond. Our results also suggested that the protruding loop is well structured during AcrB assembly with P223 served as a “wedge” close to the tip to stabilize the AcrB trimer structure. When this wedge is disrupted, the stability of the trimer is reduced, accompanied by a decrease of drug efflux activity
A Re-Evaluation of the nuclear Structure Function Ratios for D, He, Li, C and Ca
We present a re-evaluation of the structure function ratios F2(He)/F2(D),
F2(C)/F2(D) and F2(Ca)/F2(D) measured in deep inelastic muon-nucleus scattering
at an incident muon momentum of 200 GeV. We also present the ratios
F2(C)/F2(Li), F2(Ca)/F2(Li) and F2(Ca)/F2(C) measured at 90 GeV. The results
are based on data already published by NMC; the main difference in the analysis
is a correction for the masses of the deuterium targets and an improvement in
the radiative corrections. The kinematic range covered is 0.0035 < x < 0.65,
0.5 < Q^2 <90 GeV^2 for the He/D, C/D and Ca/D data and 0.0085 < x < 0.6, 0.84
< Q^2 < 17 GeV^2 for the Li/C/Ca ones.Comment: 6 pages, Latex, 3 figures as uuencoded compressed tar file included
at the end, in case of problems contact [email protected] (Antje
Bruell
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