Crystal structure of the bacterial nucleoside transporter Tsx

Substrate Uptake by TonB-Dependent Outer Membrane Transporters

TonB is an essential component of an energy-generating system that powers active transport across the outer membrane (OM) of compounds that are too large or too scarce to diffuse through porins. The TonB-dependent OM transport proteins (TBDTs) consist of β barrels forming pores that are closed by plugs. The binding of TonB to TBDTs elicits plug movement, which opens the pores and enables nutrient translocation from the cell surface into the periplasm. TonB is also involved in the uptake of certain proteins, particularly toxins, through OM proteins that differ structurally from TBDTs. TonB binds to a sequence of five residues, designated as the TonB box, which is conserved in all TBDTs. Energy from the proton motive force (pmf) of the cytoplasmic membrane is transmitted to TonB by two proteins, ExbB and ExbD. These proteins form an energy-transmitting protein complex consisting of five ExbB proteins, forming a pore that encloses the ExbD dimer. This review discusses the structural changes that occur in TBDTs upon interaction with TonB, as well as the interaction of ExbB-ExbD with TonB, which is required to transmit the energy of the pmf and thereby open TBDT pores. TonB facilitates import of a wide range of substrates.

Structural bioinformatics studies of bacterial outer membrane beta-barrel transporters and their AlphaFold2 predicted water-soluble QTY variants

Beta-barrel outer membrane proteins (OMP) are integral components of Gram-negative bacteria, eukaryotic mitochondria, and chloroplasts. They play essential roles in various cellular processes including nutrient transport, membrane stability, host-pathogen interactions, antibiotic resistance and more. The advent of AlphaFold2 for accurate protein structure predictions transformed structural bioinformatic studies. We previously used a QTY code to convert hydrophobic alpha-helices to hydrophilic alpha-helices in over 50 membrane proteins with all alpha-helices. The QTY code systematically replaces hydrophobic leucine (L), isoleucine (I), valine (V), and phenylalanine (F) with hydrophilic glutamine (Q), threonine (T), and tyrosine (Y). We here present a structural bioinformatic analysis of five outer membrane beta-barrel proteins with known molecular structures, including a) BamA, b) Omp85 (also called Sam50), c) FecA, d) Tsx, and e) OmpC. We superposed the structures of five native beta-barrel outer membrane proteins and their AlphaFold2-predicted corresponding QTY variant structures. The superposed structures of OMPs and their QTY variants exhibit remarkable structural similarity, as evidenced by residue mean square distance (RMSD) values between 0.206Å to 0.414Å despite the replacement of at least 22% (Transmembrane variation) of the amino acids in the transmembrane regions. We also show that native outer membrane proteins and QTY variants have different hydrophobicity patches. Our study provides important insights into the differences between hydrophobic and hydrophilic beta-barrels and validates the QTY code for studying beta-barrel membrane proteins and perhaps other hydrophobic aggregated proteins. Our findings demonstrate that the QTY code can be used as a simple tool for designing hydrophobic proteins in various biological contexts.

Infection leaves a genetic and functional mark on the gut population of a commensal bacterium

Gastrointestinal infection changes microbiome composition and gene expression. In this study, we demonstrate that enteric infection also promotes rapid genetic adaptation in a gut commensal. Measurements of Bacteroides thetaiotaomicron population dynamics within gnotobiotic mice reveal that these populations are relatively stable in the absence of infection, and the introduction of the enteropathogen Citrobacter rodentium reproducibly promotes rapid selection for a single-nucleotide variant with increased fitness. This mutation promotes resistance to oxidative stress by altering the sequence of a protein, IctA, that is essential for fitness during infection. We identified commensals from multiple phyla that attenuate the selection of this variant during infection. These species increase the levels of vitamin B6 in the gut lumen. Direct administration of this vitamin is sufficient to significantly reduce variant expansion in infected mice. Our work demonstrates that a self-limited enteric infection can leave a stable mark on resident commensal populations that increase fitness during infection.

The Expanded Central Dogma: Genome Resynthesis, Orthogonal Biosystems, Synthetic Genetics

Synthetic biology seeks to probe fundamental aspects of biological form and function by construction [i.e., (re)synthesis] rather than deconstruction (analysis). In this sense, biological sciences now follow the lead given by the chemical sciences. Synthesis can complement analytic studies but also allows novel approaches to answering fundamental biological questions and opens up vast opportunities for the exploitation of biological processes to provide solutions for global problems. In this review, we explore aspects of this synthesis paradigm as applied to the chemistry and function of nucleic acids in biological systems and beyond, specifically, in genome resynthesis, synthetic genetics (i.e., the expansion of the genetic alphabet, of the genetic code, and of the chemical make-up of genetic systems), and the elaboration of orthogonal biosystems and components.

Transmembrane β-barrel proteins of bacteria: From structure to function

The outer membrane of Gram-negative bacteria is a specialized organelle conferring protection to the cell against various environmental stresses and resistance to many harmful compounds. The outer membrane has a number of unique features, including an asymmetric lipid bilayer, the presence of lipopolysaccharides and an individual proteome. The vast majority of the integral transmembrane proteins in the outer membrane belongs to the family of β-barrel proteins. These evolutionarily related proteins share a cylindrical, anti-parallel β-sheet core fold spanning the outer membrane. The loops and accessory domains attached to the β-barrel allow for a remarkable versatility in function for these proteins, ranging from diffusion pores and transporters to enzymes and adhesins. We summarize the current knowledge on β-barrel structure and folding and give an overview of their functions, evolution, and potential as drug targets.

Improvement of the antimicrobial potency, pharmacokinetic and pharmacodynamic properties of albicidin by incorporation of nitrogen atoms

The worrisome development and spread of multidrug-resistant bacteria demands new antibacterial agents with strong bioactivities particularly against Gram-negative bacteria. Albicidins were recently structurally characterized as highly active antibacterial natural products from the bacterium Xanthomonas albilineans. Albicidin, which effectively targets the bacterial DNA-gyrase, is a lipophilic hexapeptide mostly consisting of para amino benzoic acid units and only one α-amino acid. In this study, we report on the design and synthesis of new albicidins, containing N-atoms on each of the 5 different phenyl rings. We systematically introduced N-atoms into the aromatic backbone to monitor intramolecular H-bonds and for one derivative correlated them with a significant enhancement of the antibacterial activity and activity spectrum, particularly also towards Gram-positive bacteria. In parallel we conducted DFT calculations to find the most stable conformation of each derivative. A drastic angle-change was observed for the lead compound and shows a preferred planarity through H-bonding with the introduced N-atom at the D-fragment of albicidin. Finally, we went to the next level and conducted the first in vivo experiments with an albicidin analogue. Our lead compound was evaluated in two different mouse experiments: In the first we show a promising PK profile and the absence of toxicity and in the second very good efficiency and reduction of the bacterial titre in an E. coli infection model with FQ-resistant clinically relevant strains. These results qualify albicidins as active antibacterial substances with the potential to be developed as a drug for treatment of infections caused by Gram-negative and Gram-positive bacteria.

Genomic insights into diverse bacterial taxa that degrade extracellular DNA in marine sediments

Extracellular DNA is a major macromolecule in global element cycles, and is a particularly crucial phosphorus, nitrogen and carbon source for microorganisms in the seafloor. Nevertheless, the identities, ecophysiology and genetic features of DNA-foraging microorganisms in marine sediments are largely unknown. Here, we combined microcosm experiments, DNA stable isotope probing (SIP), single-cell SIP using nano-scale secondary isotope mass spectrometry (NanoSIMS) and genome-centric metagenomics to study microbial catabolism of DNA and its subcomponents in marine sediments. 13C-DNA added to sediment microcosms was largely degraded within 10 d and mineralized to 13CO2. SIP probing of DNA revealed diverse 'Candidatus Izemoplasma', Lutibacter, Shewanella and Fusibacteraceae incorporated DNA-derived 13C-carbon. NanoSIMS confirmed incorporation of 13C into individual bacterial cells of Fusibacteraceae sorted from microcosms. Genomes of the 13C-labelled taxa all encoded enzymatic repertoires for catabolism of DNA or subcomponents of DNA. Comparative genomics indicated that diverse 'Candidatus Izemoplasmatales' (former Tenericutes) are exceptional because they encode multiple (up to five) predicted extracellular nucleases and are probably specialized DNA-degraders. Analyses of additional sediment metagenomes revealed extracellular nuclease genes are prevalent among Bacteroidota at diverse sites. Together, our results reveal the identities and functional properties of microorganisms that may contribute to the key ecosystem function of degrading and recycling DNA in the seabed.

Drug repurposing: phosphate prodrugs of anticancer and antiviral FDA-approved nucleosides as novel antimicrobials

OBJECTIVES

Following a drug repurposing approach, we aimed to investigate and compare the antibacterial and antibiofilm activities of different classes of phosphate prodrugs (HepDirect, cycloSal, SATE and mix SATE) of antiviral and anticancer FDA-approved nucleoside drugs [zidovudine (AZT), floxouridine (FUDR) and gemcitabine (GEM)] against a variety of pathogenic Gram-positive and -negative bacteria.

METHODS

Ten prodrugs were synthesized and screened for antibacterial activity against seven Gram-negative and two Gram-positive isolates fully susceptible to traditional antibiotics, alongside six Gram-negative and five Gram-positive isolates with resistance mechanisms. Their ability to prevent and eradicate biofilms of different bacterial pathogens in relation to planktonic growth inhibition was also evaluated, together with their effect on proliferation, viability and apoptosis of different eukaryotic cells.

RESULTS

The prodrugs showed decreased antibacterial activity compared with the parent nucleosides. cycloSal-GEM-monophosphate (MP) prodrugs 20a and 20b were the most active agents against Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and retained their activity against antibiotic-resistant isolates. cycloSal-FUDR-MP 21a partially retained good activity against the Gram-positive bacteria E. faecalis, Enterococcus faecium and S. aureus. Most of the prodrugs tested displayed very potent preventive antibiofilm specific activity, but not curative. In terms of cytotoxicity, AZT prodrugs did not affect apoptosis or cell viability at the highest concentration tested, and only weak effects on apoptosis and/or cell viability were observed for GEM and FUDR prodrugs.

CONCLUSIONS

Among the different prodrug approaches, the cycloSal prodrugs appeared the most effective. In particular, cycloSal (17a) and mix SATE (26) AZT prodrugs combine the lowest cytotoxicity with high and broad antibacterial and antibiofilm activity against Gram-negative bacteria.

Evolution of Bacterial Cross-Resistance to Lytic Phages and Albicidin Antibiotic

Due to concerns over the global increase of antibiotic-resistant bacteria, alternative antibacterial strategies, such as phage therapy, are increasingly being considered. However, evolution of bacterial resistance to new therapeutics is almost a certainty; indeed, it is possible that resistance to alternative treatments might result in an evolved trade-up such as enhanced antibiotic resistance. Here, we hypothesize that selection for Escherichia coli bacteria to resist phage T6, phage U115, or albicidin, a DNA gyrase inhibitor, should often result in a pleiotropic trade-up in the form of cross-resistance, because all three antibacterial agents interact with the Tsx porin. Selection imposed by any one of the antibacterials resulted in cross-resistance to all three of them, in each of the 29 spontaneous bacterial mutants examined in this study. Furthermore, cross-resistance did not cause measurable fitness (growth) deficiencies for any of the bacterial mutants, when competed against wild-type E. coli in both low-resource and high-resource environments. A combination of whole-genome and targeted sequencing confirmed that mutants differed from wild-type E. coli via change(s) in the tsx gene. Our results indicate that evolution of cross-resistance occurs frequently in E. coli subjected to independent selection by phage T6, phage U115 or albicidin. This study cautions that deployment of new antibacterial therapies such as phage therapy, should be preceded by a thorough investigation of evolutionary consequences of the treatment, to avoid the potential for evolved trade-ups.

Engineering of a new Escherichia coli strain efficiently metabolizing cellobiose with promising perspectives for plant biomass-based application design

The necessity to decrease our fossil energy dependence requests bioprocesses based on biomass degradation. Cellobiose is the main product released by cellulases when acting on the major plant cell wall polysaccharide constituent, the cellulose. Escherichia coli, one of the most common model organisms for the academy and the industry, is unable to metabolize this disaccharide. In this context, the remodeling of E. coli to catabolize cellobiose should thus constitute an important progress for the design of such applications. Here, we developed a robust E. coli strain able to metabolize cellobiose by integration of a small set of modifications in its genome. Contrary to previous studies that use adaptative evolution to achieve some growth on this sugar by reactivating E. coli cryptic operons coding for cellobiose metabolism, we identified easily insertable modifications impacting the cellobiose import (expression of a gene coding a truncated variant of the maltoporin LamB, modification of the expression of lacY encoding the lactose permease) and its intracellular degradation (genomic insertion of a gene encoding either a cytosolic β-glucosidase or a cellobiose phosphorylase). Taken together, our results provide an easily transferable set of mutations that confers to E. coli an efficient growth phenotype on cellobiose (doubling time of 2.2 ​h in aerobiosis) without any prior adaptation.

How to Enter a Bacterium: Bacterial Porins and the Permeation of Antibiotics

Despite tremendous successes in the field of antibiotic discovery seen in the previous century, infectious diseases have remained a leading cause of death. More specifically, pathogenic Gram-negative bacteria have become a global threat due to their extraordinary ability to acquire resistance against any clinically available antibiotic, thus urging for the discovery of novel antibacterial agents. One major challenge is to design new antibiotics molecules able to rapidly penetrate Gram-negative bacteria in order to achieve a lethal intracellular drug accumulation. Protein channels in the outer membrane are known to form an entry route for many antibiotics into bacterial cells. Up until today, there has been a lack of simple experimental techniques to measure the antibiotic uptake and the local concentration in subcellular compartments. Hence, rules for translocation directly into the various Gram-negative bacteria via the outer membrane or via channels have remained elusive, hindering the design of new or the improvement of existing antibiotics. In this review, we will discuss the recent progress, both experimentally as well as computationally, in understanding the structure-function relationship of outer-membrane channels of Gram-negative pathogens, mainly focusing on the transport of antibiotics.

Molecular basis for substrate recognition by the bacterial nucleoside transporter NupG

Nucleoside homeostasis, which is mediated by transporters and channels, is essential for all life on Earth. In Escherichia coli, NupG mediates the transport of nucleosides and was deemed to be the prototype of the nucleoside proton symporter (NHS) family and the major facilitator superfamily. To date, the substrate recognition and transport mechanisms of NHS transporters are still elusive. Here, we report two crystal structures of NupG (WT and D323A NupG) resolved at 3.0 Å. Both structures reveal an identical inward-open conformation. Together with molecular docking and molecular dynamics simulations and in vitro uridine-binding assays, we found that the uridine binding site, which locates in the central cavity between N and C domains of NupG, is constituted by R136, T140, F143, Q225, N228, Q261, E264, Y318, and F322. Moreover, we found that D323 is very important for substrate binding via in vitro uridine-binding assays using D323 mutations, although it does not have a direct contact with uridine. Our structural and biochemical data therefore provide an important framework for the mechanistic understanding of nucleoside transporters of the NHS family.

Towards defining the outer membrane proteome of Porphyromonas gingivalis

Porphyromonas gingivalis is a Gram-negative anaerobic pathogen found in subgingival plaque associated with progressive periodontitis. Proteins associated with the outer membrane (OM) of Gram-negative pathogens are particularly important for understanding virulence and for developing vaccines. The aim of this study was to establish a reliable list of outer membrane associated proteins (Omps) for this organism. Starting with a list of 99 experimentally determined Omps, several bioinformatics tools were used to predict a further 52 proteins, leading to a predicted OM proteome of 151 proteins. The tools used included databases of protein families, prediction of OM β-barrels and structural homology. The list includes 33 T9SS cargo proteins, 43 lipoproteins and 66 OM β-barrel proteins with some overlap between categories. The proteins are discussed both in these structural categories as well as their various functions in OM biogenesis, nutrient acquisition, protein secretion, adhesion and efflux. Proteins that were previously shown to be part of large complexes are highlighted and cross reference is provided to a previous major study of protein localization in P. gingivalis. Finally, proteins were also scored according to their level of conservation within the Bacteroidales taxon. Low scores were shown to correlate with virulence factors and may be predictive of novel virulence factors.

A holin/peptidoglycan hydrolase-dependent protein secretion system

Gram-negative bacteria have evolved numerous pathways to secrete proteins across their complex cell envelopes. Here, we describe a protein secretion system that uses a holin membrane protein in tandem with a cell wall-editing enzyme to mediate the secretion of substrate proteins from the periplasm to the cell exterior. The identity of the cell wall-editing enzymes involved was found to vary across biological systems. For instance, the chitinase secretion pathway of Serratia marcescens uses an endopeptidase to facilitate secretion, whereas the secretion of Typhoid toxin in Salmonella enterica serovar Typhi relies on a muramidase. Various families of holins are also predicted to be involved. Genomic analysis indicates that this pathway is conserved and implicated in the secretion of hydrolytic enzymes and toxins for a range of bacteria. The pairing of holins from different families with various types of peptidoglycan hydrolases suggests that this secretion pathway evolved multiple times. We suggest that the complementary bodies of evidence presented is sufficient to propose that the pathway be named the Type 10 Secretion System (TXSS).

Role of the lipid bilayer in outer membrane protein folding in Gram-negative bacteria

β-Barrel outer membrane proteins (OMPs) represent the major proteinaceous component of the outer membrane (OM) of Gram-negative bacteria. These proteins perform key roles in cell structure and morphology, nutrient acquisition, colonization and invasion, and protection against external toxic threats such as antibiotics. To become functional, OMPs must fold and insert into a crowded and asymmetric OM that lacks much freely accessible lipid. This feat is accomplished in the absence of an external energy source and is thought to be driven by the high thermodynamic stability of folded OMPs in the OM. With such a stable fold, the challenge that bacteria face in assembling OMPs into the OM is how to overcome the initial energy barrier of membrane insertion. In this review, we highlight the roles of the lipid environment and the OM in modulating the OMP-folding landscape and discuss the factors that guide folding in vitro and in vivo We particularly focus on the composition, architecture, and physical properties of the OM and how an understanding of the folding properties of OMPs in vitro can help explain the challenges they encounter during folding in vivo Current models of OMP biogenesis in the cellular environment are still in flux, but the stakes for improving the accuracy of these models are high. OMP folding is an essential process in all Gram-negative bacteria, and considering the looming crisis of widespread microbial drug resistance it is an attractive target. To bring down this vital OMP-supported barrier to antibiotics, we must first understand how bacterial cells build it.

Improvement of the dP-nucleoside-mediated herpes simplex virus thymidine kinase negative-selection system by manipulating dP metabolism genes

A variety of positive/negative selection systems have been exploited as genome engineering tools and screening platforms for genetic switches. While numerous positive-selection systems are available, only a handful of negative-selection systems are useful for such applications. We previously reported a powerful negative-selection system using herpes simplex virus thymidine kinase (HsvTK) and the mutagenic nucleoside analog 6-(β-d-2-deoxyribofuranosyl)-3,4-dihydro-8H-pyrimido [4,5-c][1,2] oxazin-7-one (dP). Upon addition of 1000 nM dP, cells expressing HsvTK quickly die, with unprecedented efficacy. However, this selection procedure elevates the spontaneous mutation rate of the host cells by 10-fold due to the mutagenic nature of dP. To decrease the operative concentration of dP required for negative selection, we systematically created the strains of Escherichia coli either by removing or overexpressing genes involved in DNA/RNA metabolism. We found that over-expression of NupC and NupG (nucleoside uptake-related inner membrane transporters), Tsx (outer membrane transporter), NdK (nucleotide kinase) sensitized E. coli cells to dP. Simultaneous overexpression of these three genes (ndk-nupC-tsx) significantly improved the dP-sensitivity of E. coli, lowering the necessary operative concentration of dP for negative selection by 10-fold. This enabled robust and selective elimination of strains harboring chromosomally-encoded hsvtk simply by adding as low as 100 nM dP, which causes only a modest increase in the spontaneous mutation frequency as compared to the cells without hsvtk.

Ribonucleoside triphosphates promote T7 DNA replication and the lysis of T7-Infected Escherichia coli

rNTPs are structurally similar to dNTPs, but their concentrations are much higher than those of dNTPs in cells. rNTPs in solutions or rNMP at the primer terminus or embedded in template always inhibit or block DNA replication, due to the reduced Mg²⁺ apparent concentration, competition of rNTPs with dNTPs, and the extra repulsive interaction of rNTP or rNMP with polymerase active site. In this work, unexpectedly, we found rNTPs can promote T7 DNA replication with the maximal promotion at rNTPs/dNTPs concentration ratio of 20. This promotion was not due to the optimized Mg²⁺ apparent concentration or the direct incorporation of extra rNMPs into DNA. This promotion was dependent on the concentrations and types of rNTPs. Kinetic analysis showed that this promotion was originated from the increased fraction of polymerase-DNA productive complex and the accelerated DNA polymerization. Further evidence showed that more polymerase-DNA complex was formed and their binding affinity was also enhanced in the presence of extra rNTPs. Moreover, this promotion in T7 DNA replication also accelerated the lysis of T7-infected host Escherichia coli. This work discovered that rNTPs could promote DNA replication, completely different from the traditional concept that rNTPs always inhibit DNA replication.

Programmed Secretion Arrest and Receptor-Triggered Toxin Export during Antibacterial Contact-Dependent Growth Inhibition

Contact-dependent growth inhibition (CDI) entails receptor-mediated delivery of CdiA-derived toxins into Gram-negative target bacteria. Using electron cryotomography, we show that each CdiA effector protein forms a filament extending ∼33 nm from the cell surface. Remarkably, the extracellular filament represents only the N-terminal half of the effector. A programmed secretion arrest sequesters the C-terminal half of CdiA, including the toxin domain, in the periplasm prior to target-cell recognition. Upon binding receptor, CdiA secretion resumes, and the periplasmic FHA-2 domain is transferred to the target-cell outer membrane. The C-terminal toxin region of CdiA then penetrates into the target-cell periplasm, where it is cleaved for subsequent translocation into the cytoplasm. Our findings suggest that the FHA-2 domain assembles into a transmembrane conduit for toxin transport into the periplasm of target bacteria. We propose that receptor-triggered secretion ensures that FHA-2 export is closely coordinated with integration into the target-cell outer membrane. VIDEO ABSTRACT.

Screening of Polyvalent Phage-Resistant Escherichia coli Strains Based on Phage Receptor Analysis

Bacteria-based biotechnology processes are constantly under threat from bacteriophage infection, with phage contamination being a non-neglectable problem for microbial fermentation. The essence of this problem is the complex co-evolutionary relationship between phages and bacteria. The development of phage control strategies requires further knowledge about phage-host interactions, while the widespread use of Escherichia coli strain BL21 (DE3) in biotechnological processes makes the study of phage receptors in this strain particularly important. Here, eight phages infecting E. coli BL21 (DE3) via different receptors were isolated and subsequently identified as members of the genera T4virus, Js98virus, Felix01virus, T1virus, and Rtpvirus. Phage receptors were identified by whole-genome sequencing of phage-resistant E. coli strains and sequence comparison with wild-type BL21 (DE3). Results showed that the receptors for the isolated phages, designated vB_EcoS_IME18, vB_EcoS_IME253, vB_EcoM_IME281, vB_EcoM_IME338, vB_EcoM_IME339, vB_EcoM_IME340, vB_EcoM_IME341, and vB_EcoS_IME347 were FhuA, FepA, OmpF, lipopolysaccharide, Tsx, OmpA, FadL, and YncD, respectively. A polyvalent phage-resistant BL21 (DE3)-derived strain, designated PR8, was then identified by screening with a phage cocktail consisting of the eight phages. Strain PR8 is resistant to 23 of 32 tested phages including Myoviridae and Siphoviridae phages. Strains BL21 (DE3) and PR8 showed similar expression levels of enhanced green fluorescent protein. Thus, PR8 may be used as a phage resistant strain for fermentation processes. The findings of this study contribute significantly to our knowledge of phage-host interactions and may help prevent phage contamination in fermentation.

Nanomaterials and molecular transporters to overcome the bacterial envelope barrier: Towards advanced delivery of antibiotics

With the dramatic consequences of bacterial resistance to antibiotics, nanomaterials and molecular transporters have started to be investigated as alternative antibacterials or anti-infective carrier systems to improve the internalization of bactericidal drugs. However, the capability of nanomaterials/molecular transporters to overcome the bacterial cell envelope is poorly understood. It is critical to consider the sophisticated architecture of bacterial envelopes and reflect how nanomaterials/molecular transporters can interact with these envelopes, being the major aim of this review. The first part of this manuscript overviews the permeability of bacterial envelopes and how it limits the internalization of common antibiotic and novel oligonucleotide drugs. Subsequently we critically discuss the mechanisms that allow nanomaterials/molecular transporters to overcome the bacterial envelopes, focusing on the most promising ones to this end - siderophores, cyclodextrins, metal nanoparticles, antimicrobial/cell-penetrating peptides and fusogenic liposomes. This review may stimulate drug delivery and microbiology scientists in designing effective nanomaterials/molecular transporters against bacterial infections.

Understanding antibiotic resistance via outer membrane permeability

Collective antibiotic drug resistance is a global threat, especially with respect to Gram-negative bacteria. The low permeability of the bacterial outer cell wall has been identified as a challenging barrier that prevents a sufficient antibiotic effect to be attained at low doses of the antibiotic. The Gram-negative bacterial cell envelope comprises an outer membrane that delimits the periplasm from the exterior milieu. The crucial mechanisms of antibiotic entry via outer membrane includes general diffusion porins (Omps) responsible for hydrophilic antibiotics and lipid-mediated pathway for hydrophobic antibiotics. The protein and lipid arrangements of the outer membrane have had a strong impact on the understanding of bacteria and their resistance to many types of antibiotics. Thus, one of the current challenges is effective interpretation at the molecular basis of the outer membrane permeability. This review attempts to develop a state of knowledge pertinent to Omps and their effective role in solute influx. Moreover, it aims toward further understanding and exploration of prospects to improve our knowledge of physicochemical limitations that direct the translocation of antibiotics via bacterial outer membrane.

Protein-enriched outer membrane vesicles as a native platform for outer membrane protein studies

Most studies characterizing the folding, structure, and function of membrane proteins rely on solubilized or reconstituted samples. Whereas solubilized membrane proteins lack the functionally important lipid membrane, reconstitution embeds them into artificial lipid bilayers, which lack characteristic features of cellular membranes including lipid diversity, composition and asymmetry. Here, we utilize outer membrane vesicles (OMVs) released from Escherichia coli to study outer membrane proteins (Omps) in the native membrane environment. Enriched in the native membrane of the OMV we characterize the assembly, folding, and structure of OmpG, FhuA, Tsx, and BamA. Comparing Omps in OMVs to those reconstituted into artificial lipid membranes, we observe different unfolding pathways for some Omps. This observation highlights the importance of the native membrane environment to maintain the native structure and function relationship of Omps. Our fast and easy approach paves the way for functional and structural studies of Omps in the native membrane.

Johannes Thoma et al. overexpress outer membrane proteins (Omps) in Escherichia coli and collect the expelled outer membrane vesicles (OMVs) to study Omp assembly, folding and structure. They find that Omps in OMVs show different unfolding pathways compared to Omps reconstituted in artificial lipid membranes.

Recent Progress in Micro/Nanoreactors toward the Creation of Artificial Organelles

Artificial organelles created from a bottom up approach are a new type of engineered materials, which are not designed to be living but, instead, to mimic some specific functions inside cells. By doing so, artificial organelles are expected to become a powerful tool in biomedicine. They can act as nanoreactors to convert a prodrug into a drug inside the cells or as carriers encapsulating therapeutic enzymes to replace malfunctioning organelles in pathological conditions. For the design of artificial organelles, several requirements need to be fulfilled: a compartmentalized structure that can encapsulate the synthetic machinery to perform an enzymatic function, as well as a means to allow for communication between the interior of the artificial organelle and the external environment, so that substrates and products can diffuse in and out the carrier allowing for continuous enzymatic reactions. The most recent and exciting advances in architectures that fulfill the aforementioned requirements are featured in this review. Artificial organelles are classified depending on their constituting materials, being lipid and polymer-based systems the most prominent ones. Finally, special emphasis will be put on the intracellular response of these newly emerging systems.

A Tool for the Import of Natural and Unnatural Nucleoside Triphosphates into Bacteria

Nucleoside triphosphates play a central role in biology, but efforts to study these roles have proven difficult because the levels of triphosphates are tightly regulated in a cell and because individual triphosphates can be difficult to label or modify. In addition, many synthetic biology efforts are focused on the development of unnatural nucleoside triphosphates that perform specific functions in the cellular environment. In general, both of these efforts would be facilitated by a general means to directly introduce desired triphosphates into cells. Previously, we demonstrated that recombinant expression of a nucleoside triphosphate transporter from Phaeodactylum tricornutum (PtNTT2) in Escherichia coli functions to import triphosphates that are added to the media. Here, to explore the generality and utility of this approach, we report a structure-activity relationship study of PtNTT2. Using a conventional competitive uptake inhibition assay, we characterize the effects of nucleobase, sugar, and triphosphate modification, and then develop an LC-MS/MS assay to directly measure the effects of the modifications on import. Lastly, we use the transporter to import radiolabeled or 2'-fluoro-modified triphosphates and quantify their incorporation into DNA and RNA. The results demonstrate the general utility of the PtNTT2-mediated import of natural or modified nucleoside triphosphates for different molecular or synthetic biology applications.

Engineering Enhanced Pore Sizes Using FhuA Δ1-160 from E. coli Outer Membrane as Template

Biological membranes are the perfect example of a molecular filter using membrane channels to control the permeability of small water-soluble molecules. To allow filtering of larger hydrophilic molecules we started from the known mutant channel FhuA Δ1-160 in which the cork domain closing the channel had been removed. Here we further expand the pore diameter by copying the amino acid sequence of two β-strands in a stepwise manner increasing the total number of β-strands from 22 to 34. The pore size of the respective expanded channel protein was characterized by single-channel conductance. Insertion of additional β-strands increased the pore conductance but also induced more ion current flickering on the millisecond scale. Further, polymer exclusion measurements were performed by analyzing single-channel conductance in the presence of differently sized polyethylene glycol of known polymer random coil radii. The conclusion from channel conductance of small channel penetrating polymers versus larger excluded ones suggested an increase in pore radii from 1.6 nm for FhuA Δ1-160 up to a maximum of about 2.7 nm for +8 β insertion. Integration of more β-strand caused instability of the channel and exclusion of smaller sized polymer. FhuA Δ1-160 + 10 β and FhuA Δ1-160 + 12 β effective radius decreased to 1.4 and 1.3 nm, respectively, showing the limitations of this approach.

Topology of membrane proteins-predictions, limitations and variations

Transmembrane proteins perform a variety of important biological functions necessary for the survival and growth of the cells. Membrane proteins are built up by transmembrane segments that span the lipid bilayer. The segments can either be in the form of hydrophobic alpha-helices or beta-sheets which create a barrel. A fundamental aspect of the structure of transmembrane proteins is the membrane topology, that is, the number of transmembrane segments, their position in the protein sequence and their orientation in the membrane. Along these lines, many predictive algorithms for the prediction of the topology of alpha-helical and beta-barrel transmembrane proteins exist. The newest algorithms obtain an accuracy close to 80% both for alpha-helical and beta-barrel transmembrane proteins. However, lately it has been shown that the simplified picture presented when describing a protein family by its topology is limited. To demonstrate this, we highlight examples where the topology is either not conserved in a protein superfamily or where the structure cannot be described solely by the topology of a protein. The prediction of these non-standard features from sequence alone was not successful until the recent revolutionary progress in 3D-structure prediction of proteins.

Conversion of OprO into an OprP-like Channel by Exchanging Key Residues in the Channel Constriction

Under phosphate-limiting conditions, the channels OprP and OprO are induced and expressed in the outer membrane of Pseudomonas aeruginosa. Despite their large homology, the phosphate-specific OprP and the diphosphate-specific OprO pores show structural differences in their binding sites situated in the constriction region. Previously, it was shown that the mutation of amino acids in OprP (Y62F and Y114D) led to an exchange in substrate specificity similar to OprO. To support the role of these key amino acids in the substrate sorting of these specific channels, the reverse mutants for OprO (F62Y, D114Y, and F62Y/D114Y) were created in this study. The phosphate and diphosphate binding of the generated channels was studied in planar lipid bilayers. Our results show that mutations of key residues indeed reverse the substrate specificity of OprO to OprP and support the view that just a few strategically positioned amino acids are mainly responsible for its substrate specificity.

Bifunctional antimicrobial conjugates and hybrid antimicrobials

Covering: up to the end of 2016Novel antimicrobial drugs are continuously needed to counteract bacterial resistance development. An innovative molecular design strategy for novel antibiotic drugs is based on the hybridization of an antibiotic with a second functional entity. Such conjugates can be grouped into two major categories. In the first category (antimicrobial hybrids), both functional elements of the hybrid exert antimicrobial activity. Due to the dual targeting, resistance development can be significantly impaired, the pharmacokinetic properties can be superior compared to combination therapies with the single antibiotics, and the antibacterial potency is often enhanced in a synergistic manner. In the second category (antimicrobial conjugates), one functional moiety controls the accumulation of the other part of the conjugate, e.g. by mediating an active transport into the bacterial cell or blocking the efflux. This approach is mostly applied to translocate compounds across the cell envelope of Gram-negative bacteria through membrane-embedded transporters (e.g. siderophore transporters) that provide nutrition and signalling compounds to the cell. Such 'Trojan Horse' approaches can expand the antibacterial activity of compounds against Gram-negative pathogens, or offer new options for natural products that could not be developed as standalone antibiotics, e.g. due to their toxicity.

Properties and Phylogeny of 76 Families of Bacterial and Eukaryotic Organellar Outer Membrane Pore-Forming Proteins

We here report statistical analyses of 76 families of integral outer membrane pore-forming proteins (OMPPs) found in bacteria and eukaryotic organelles. 47 of these families fall into one superfamily (SFI) which segregate into fifteen phylogenetic clusters. Families with members of the same protein size, topology and substrate specificities often cluster together. Virtually all OMPP families include only proteins that form transmembrane pores. Nine such families, all of which cluster together in the SFI phylogenetic tree, contain both α- and β-structures, are multi domain, multi subunit systems, and transport macromolecules. Most other SFI OMPPs transport small molecules. SFII and SFV homologues derive from Actinobacteria while SFIII and SFIV proteins derive from chloroplasts. Three families of actinobacterial OMPPs and two families of eukaryotic OMPPs apparently consist primarily of α-helices (α-TMSs). Of the 71 families of (putative) β-barrel OMPPs, only twenty could not be assigned to a superfamily, and these derived primarily from Actinobacteria (1), chloroplasts (1), spirochaetes (8), and proteobacteria (10). Proteins were identified in which two or three full length OMPPs are fused together. Family characteristic are described and evidence agrees with a previous proposal suggesting that many arose by adjacent β-hairpin structural unit duplications.

Nucleotides, Nucleosides, and Nucleobases

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

Solute and Ion Transport: Outer Membrane Pores and Receptors

Two membranes enclose Gram-negative bacteria-an inner membrane consisting of phospholipid and an outer membrane having an asymmetric structure in which the inner leaflet contains phospholipid and the outer leaflet consists primarily of lipopolysaccharide. The impermeable nature of the outer membrane imposes a need for numerous outer membrane pores and transporters to ferry substances in and out of the cell. These outer membrane proteins have structures distinct from their inner membrane counterparts and most often function without any discernable energy source. In this chapter, we review the structures and functions of four classes of outer membrane protein: general and specific porins, specific transporters, TonB-dependent transporters, and export channels. While not an exhaustive list, these classes exemplify small-molecule transport across the outer membrane and illustrate the diversity of structures and functions found in Gram-negative bacteria.

Genetics of Natural Competence in Vibrio cholerae and other Vibrios

Many Gram-positive and Gram-negative bacteria can become naturally competent to take up extracellular DNA from the environment via a dedicated uptake apparatus. The genetic material that is acquired can (i) be used for nutrients, (ii) aid in genome repair, and (iii) promote horizontal gene transfer when incorporated onto the genome by homologous recombination, the process of “transformation.” Recent studies have identified multiple environmental cues sufficient to induce natural transformation in Vibrio cholerae and several other Vibrio species. In V. cholerae , nutrient limitation activates the cAMP receptor protein regulator, quorum-sensing signals promote synthesis of HapR-controlled QstR, chitin stimulates production of TfoX, and low extracellular nucleosides allow CytR to serve as an additional positive regulator. The network of signaling systems that trigger expression of each of these required regulators is well described, but the mechanisms by which each in turn controls competence apparatus genes is poorly understood. Recent work has defined a minimal set of genes that encode apparatus components and begun to characterize the architecture of the machinery by fluorescence microscopy. While studies with a small set of V. cholerae reference isolates have identified regulatory and competence genes required for DNA uptake, future studies may identify additional genes and regulatory connections, as well as revealing how common natural competence is among diverse V. cholerae isolates and other Vibrio species.

Folding of β-barrel membrane proteins in lipid bilayers - Unassisted and assisted folding and insertion

In cells, β-barrel membrane proteins are transported in unfolded form to an outer membrane into which they fold and insert. Model systems have been established to investigate the mechanisms of insertion and folding of these versatile proteins into detergent micelles, lipid bilayers and even synthetic amphipathic polymers. In these experiments, insertion into lipid membranes is initiated from unfolded forms that do not display residual β-sheet secondary structure. These studies therefore have allowed the investigation of membrane protein folding and insertion in great detail. Folding of β-barrel membrane proteins into lipid bilayers has been monitored from unfolded forms by dilution of chaotropic denaturants that keep the protein unfolded as well as from unfolded forms present in complexes with molecular chaperones from cells. This review is aimed to provide an overview of the principles and mechanisms observed for the folding of β-barrel transmembrane proteins into lipid bilayers, the importance of lipid-protein interactions and the function of molecular chaperones and folding assistants. This article is part of a Special Issue entitled: Lipid-protein interactions.

Outer membrane proteomics of kanamycin-resistant Escherichia coli identified MipA as a novel antibiotic resistance-related protein

Antibiotic-resistant bacteria are a great threat to human health and food safety and there is an urgent need to understand the mechanisms of resistance for combating these bacteria. In the current study, comparative proteomic methodologies were applied to identify Escherichia coli K-12 outer membrane (OM) proteins related to kanamycin resistance. Mass spectrometry and western blotting results revealed that OM proteins TolC, Tsx and OstA were up-regulated, whereas MipA, OmpA, FadL and OmpW were down-regulated in kanamycin-resistant E. coli K-12 strain. Genetic deletion of tolC (ΔtolC-Km) led to a 2-fold decrease in the minimum inhibitory concentration (MIC) of kanamycin and deletion of mipA (ΔmipA-Km) resulted in a 4-fold increase in the MIC of kanamycin. Changes in the MICs for genetically modified strains could be completely recovered by gene complementation. Compared with the wild-type strain, the survival capability of ΔompA-Km was significantly increased and that of Δtsx-Km was significantly decreased. We further evaluated the role and expression of MipA in response to four other antibiotics including nalidixic acid, streptomycin, chloramphenicol and aureomycin, which suggested that MipA was a novel OM protein related to antibiotic resistance.

All-atom 3D structure prediction of transmembrane β-barrel proteins from sequences

Transmembrane β-barrels (TMBs) carry out major functions in substrate transport and protein biogenesis but experimental determination of their 3D structure is challenging. Encouraged by successful de novo 3D structure prediction of globular and α-helical membrane proteins from sequence alignments alone, we developed an approach to predict the 3D structure of TMBs. The approach combines the maximum-entropy evolutionary coupling method for predicting residue contacts (EVfold) with a machine-learning approach (boctopus2) for predicting β-strands in the barrel. In a blinded test for 19 TMB proteins of known structure that have a sufficient number of diverse homologous sequences available, this combined method (EVfold_bb) predicts hydrogen-bonded residue pairs between adjacent β-strands at an accuracy of ∼70%. This accuracy is sufficient for the generation of all-atom 3D models. In the transmembrane barrel region, the average 3D structure accuracy [template-modeling (TM) score] of top-ranked models is 0.54 (ranging from 0.36 to 0.85), with a higher (44%) number of residue pairs in correct strand-strand registration than in earlier methods (18%). Although the nonbarrel regions are predicted less accurately overall, the evolutionary couplings identify some highly constrained loop residues and, for FecA protein, the barrel including the structure of a plug domain can be accurately modeled (TM score = 0.68). Lower prediction accuracy tends to be associated with insufficient sequence information and we therefore expect increasing numbers of β-barrel families to become accessible to accurate 3D structure prediction as the number of available sequences increases.

Small-Molecule Transport by CarO, an Abundant Eight-Stranded β-Barrel Outer Membrane Protein from Acinetobacter baumannii

Outer membrane (OM) β-barrel proteins composed of 12-18 β-strands mediate cellular entry of small molecules in Gram-negative bacteria. Small OM proteins with barrels of 10 strands or less are not known to transport small molecules. CarO (carbapenem-associated outer membrane protein) from Acinetobacter baumannii is a small OM protein that has been implicated in the uptake of ornithine and carbapenem antibiotics. Here we report crystal structures of three isoforms of CarO. The structures are very similar and show a monomeric eight-stranded barrel lacking an open channel. CarO has a substantial extracellular domain resembling a glove that contains all the divergent residues between the different isoforms. Liposome swelling experiments demonstrate that full-length CarO and a "loop-less" truncation mutant mediate small-molecule uptake at low levels but that they are unlikely to mediate passage of carbapenem antibiotics. These results are confirmed by biased molecular dynamics simulations that allowed us to quantitatively model the transport of selected small molecules.

Two nucleoside receptors from Streptomyces coelicolor: expression of the genes and characterization of the recombinant proteins

Streptomyces coelicolor is a soil-dwelling bacterium that undergoes an intricate, saprophytic lifecycle. The bacterium takes up exogenous nucleosides for nucleic acid synthesis or use as carbon and energy sources. However, nucleosides must pass through the membrane with the help of transporters. In the present work, the SCO4884 and SCO4885 genes were cloned into pCOLADuet-1 and overexpressed in Escherichia coli BL21. Each protein was monomeric. Using isothermal titration calorimetry, we determined that SCO4884 and SCO4885 are likely nucleoside receptors with affinity for adenosine and pyrimidine nucleosides. On the basis of bioinformatics analysis and the transporter classification system, we speculate that SCO4884-SCO4888 is an ABC-like transporter responsible for the uptake of adenosine and pyrimidine nucleosides.

Uptake and trans-membrane transport of petroleum hydrocarbons by microorganisms

Petroleum-based products are a primary energy source in the industry and daily life. During the exploration, processing, transport and storage of petroleum and petroleum products, water or soil pollution occurs regularly. Biodegradation of the hydrocarbon pollutants by indigenous microorganisms is one of the primary mechanisms of removal of petroleum compounds from the environment. However, the physical contact between microorganisms and hydrophobic hydrocarbons limits the biodegradation rate. This paper presents an updated review of the petroleum hydrocarbon uptake and transport across the outer membrane of microorganisms with the help of outer membrane proteins.

The major outer sheath protein (Msp) of Treponema denticola has a bipartite domain architecture and exists as periplasmic and outer membrane-spanning conformers

The major outer sheath protein (Msp) is a primary virulence determinant in Treponema denticola, as well as the parental ortholog for the Treponema pallidum repeat (Tpr) family in the syphilis spirochete. The Conserved Domain Database (CDD) server revealed that Msp contains two conserved domains, major outer sheath protein(N) (MOSP(N)) and MOSP(C), spanning residues 77 to 286 and 332 to 543, respectively, within the N- and C-terminal regions of the protein. Circular dichroism (CD) spectroscopy, Triton X-114 (TX-114) phase partitioning, and liposome incorporation demonstrated that full-length, recombinant Msp (Msp(Fl)) and a recombinant protein containing MOSP(C), but not MOSP(N), form amphiphilic, β-sheet-rich structures with channel-forming activity. Immunofluorescence analysis of intact T. denticola revealed that only MOSP(C) contains surface-exposed epitopes. Data obtained using proteinase K accessibility, TX-114 phase partitioning, and cell fractionation revealed that Msp exists as distinct OM-integrated and periplasmic trimers. Msp(Fl) folded in Tris buffer contained slightly less β-sheet structure than detergent-folded Msp(Fl); both forms, however, partitioned into the TX-114 detergent-enriched phase. CDD analysis of the nine Tpr paralogs predicted to be outer membrane proteins (OMPs) revealed that seven have an Msp-like bipartite structure; phylogenetic analysis revealed that the MOSP(N) and MOSP(C) domains of Msp are most closely related to those of TprK. Based upon our collective results, we propose a model whereby a newly exported, partially folded intermediate can be either processed for OM insertion by the β-barrel assembly machinery (BAM) or remain periplasmic, ultimately forming a stable, water-soluble trimer. Extrapolated to T. pallidum, our model enables us to explain how individual Tprs can localize to either the periplasmic (e.g., TprK) or OM (e.g., TprC) compartments.

Substrate specificity within a family of outer membrane carboxylate channels

Characterization of a large family of outer membrane channels from gram-negative bacteria suggest how they can thrive in nutrient-poor environments and how channel inactivation can contribute to antibiotic resistance.

Many Gram-negative bacteria, including human pathogens such as Pseudomonas aeruginosa, do not have large-channel porins. This results in an outer membrane (OM) that is highly impermeable to small polar molecules, making the bacteria intrinsically resistant towards many antibiotics. In such microorganisms, the majority of small molecules are taken up by members of the OprD outer membrane protein family. Here we show that OprD channels require a carboxyl group in the substrate for efficient transport, and based on this we have renamed the family Occ, for outer membrane carboxylate channels. We further show that Occ channels can be divided into two subfamilies, based on their very different substrate specificities. Our results rationalize how certain bacteria can efficiently take up a variety of substrates under nutrient-poor conditions without compromising membrane permeability. In addition, they explain how channel inactivation in response to antibiotics can cause resistance but does not lead to decreased fitness.

Emerging antibiotic resistance in the treatment of infectious disease is an increasing problem that urgently requires new drug development. In Gram-negative bacteria, the outer membrane (OM) prevents permeation of small molecules, including antibiotics. A family of channel-forming proteins, called OprD proteins, are present in the OM to enable uptake of nutrients required for growth and cellular function. Since these channels also transport antibiotics, understanding how molecules are recognized and transported by these proteins should enable the design of more effective antibiotics. Here, we have characterized by biophysical and biochemical methods the structures and substrate-specificities of nine members of the OprD channel family of a common multidrug-resistant pathogen, Pseudomonas aeruginosa. Because we demonstrate that efficient passage through these channels requires the presence of a carboxyl group in the substrate, we renamed this channel family outer membrane carboxylate channels, or Occ. This broad substrate specificity suggests that such efficient transport allows bacteria to thrive in nutrient-poor environments. We also show markedly varied substrate specificities among the family members, especially for antibiotics, suggesting that mutation of a single channel can result in antibiotic resistance. These results provide the framework for studying the interaction of antibiotics with OM uptake channels, which will facilitate the development of more permeable and thus effective drugs.

Structural and dynamical analysis of an engineered FhuA channel protein embedded into a lipid bilayer or a detergent belt

Engineered channel proteins are promising nano-components with applications in nanodelivery and nanoreactors technology. Because few of the engineered channel proteins have been crystallized, solution studies based on Neutron Scattering, Circular Dichroism and NMR play a major role. Consequently, the understanding of membrane proteins dynamics in water/detergent solutions or when embedded in a lipid membrane, can clarify how the environment affects protein behavior. In this study, molecular dynamics simulations of the FhuA Escherichia coli outer membrane channel protein and its engineered FhuA Δ1-159 variant have been performed in two different environments: a DNPC (1,2-dinervonyl-sn-glycero-3-phosphocholine) lipid bilayer and a water/OES (N-octyl-2-hydroxyethyl sulfoxide) detergent solution. Furthermore the FhuA Δ1-159 variant has been simulated in the open and closed states, the last induced by the presence of six 3-(2-pyridyldithio)-propionic-acid in the channel inner core. Differences in protein structural and dynamical behavior between the two environments have been found. Considering the FhuA protein characterized by an elliptical-cylindrical symmetry: (a) neither variations on the secondary structure nor axial deformation have been observed in any of the systems; (b) the ellipticity of the channel section (open state) and its fluctuations are enhanced in presence of water/OES, while diminished or suppressed in the DNPC bilayer; (c) the insertion of hydrophobic pyridyl groups into the FhuA Δ1-159 channel (closed state) induces a higher ellipticity in water/OES solution, while shifting to a circular section in the DNPC membrane; (d) the cork domain represented by the first 159 amino acids does not play a major role for protein stability.

Engineering of an E. coli outer membrane protein FhuA with increased channel diameter

Background

Channel proteins like FhuA can be an alternative to artificial chemically synthesized nanopores. To reach such goals, channel proteins must be flexible enough to be modified in their geometry, i.e. length and diameter. As continuation of a previous study in which we addressed the lengthening of the channel, here we report the increasing of the channel diameter by genetic engineering.

Results

The FhuA Δ1-159 diameter increase has been obtained by doubling the amino acid sequence of the first two N-terminal β-strands, resulting in variant FhuA Δ1-159 Exp. The total number of β-strands increased from 22 to 24 and the channel surface area is expected to increase by ~16%. The secondary structure analysis by circular dichroism (CD) spectroscopy shows a high β-sheet content, suggesting the correct folding of FhuA Δ1-159 Exp. To further prove the FhuA Δ1-159 Exp channel functionality, kinetic measurement using the HRP-TMB assay (HRP = Horse Radish Peroxidase, TMB = 3,3',5,5'-tetramethylbenzidine) were conducted. The results indicated a 17% faster diffusion kinetic for FhuA Δ1-159 Exp as compared to FhuA Δ1-159, well correlated to the expected channel surface area increase of ~16%.

Conclusion

In this study using a simple "semi rational" approach the FhuA Δ1-159 diameter was enlarged. By combining the actual results with the previous ones on the FhuA Δ1-159 lengthening a new set of synthetic nanochannels with desired lengths and diameters can be produced, broadening the FhuA Δ1-159 applications. As large scale protein production is possible our approach can give a contribution to nanochannel industrial applications.

The gp38 adhesins of the T4 superfamily: a complex modular determinant of the phage's host specificity

The tail fiber adhesins are the primary determinants of host range in the T4-type bacteriophages. Among the indispensable virion components, the sequences of the long tail fiber genes and their associated adhesins are among the most variable. The predominant form of the adhesin in the T4-type phages is not even the version of the gene encoded by T4, the archetype of the superfamily, but rather a small unrelated protein (gp38) encoded by closely related phages such as T2 and T6. This gp38 adhesin has a modular design: its N-terminal attachment domain binds at the tip of the tail fiber, whereas the C-terminal specificity domain determines its host receptor affinity. This specificity domain has a series of four hypervariable segments (HVSs) that are separated by a set of highly conserved glycine-rich motifs (GRMs) that apparently form the domain’s conserved structural core. The role of gp38’s various components was examined by a comparative analysis of a large series of gp38 adhesins from T-even superfamily phages with differing host specificities. A deletion analysis revealed that the individual HVSs and GRMs are essential to the T6 adhesin’s function and suggests that these different components all act in synergy to mediate adsorption. The evolutionary advantages of the modular design of the adhesin involving both conserved structural elements and multiple independent and easily interchanged specificity determinants are discussed.

Combinations of protein-chemical complex structures reveal new targets for established drugs

Biological networks are powerful tools for predicting undocumented relationships between molecules. The underlying principle is that existing interactions between molecules can be used to predict new interactions. Here we use this principle to suggest new protein-chemical interactions via the network derived from three-dimensional structures. For pairs of proteins sharing a common ligand, we use protein and chemical superimpositions combined with fast structural compatibility screens to predict whether additional compounds bound by one protein would bind the other. The method reproduces 84% of complexes in a benchmark, and we make many predictions that would not be possible using conventional modeling techniques. Within 19,578 novel predicted interactions are 7,793 involving 718 drugs, including filaminast, coumarin, alitretonin and erlotinib. The growth rate of confident predictions is twice that of experimental complexes, meaning that a complete structural drug-protein repertoire will be available at least ten years earlier than by X-ray and NMR techniques alone.

Predicting drug-target interactions is a hot topic, and many efforts have been undertaken to do this, many using large interaction networks. We take a novel approach using protein-chemical interactions derived from 3D structures. The basic premise is that two proteins sharing a common bound chemical will likely share others. We use protein and chemical superimpositions and physical tests of chemical-protein compatibility to identify the most likely candidates among the nearly one million potential interactions. We show for a benchmark that known protein-chemical structures are reconstructed with good accuracy and sometimes via very different proteins and chemicals. We make thousands of confident predictions, including structures for known protein-drug interactions lacking a structure (e.g. topoisomerase-2/radicicol) and many new interactions. The number of confident predictions grows faster than the number of known structures, suggesting that this approach will play a key role in completing the protein-chemical interaction repertoire.