Maltose transport in Escherichia coli K12. A comparison of transport kinetics in wild-type and lambda-resistant mutants as measured by fluorescence quenching

A carboxymethyl cellulase from the yeast Cryptococcus gattii WM276: Expression, purification and characterisation

Cryptococcus gattii and its medical implications have been extensively studied. There is, however, a significant knowledge gap regarding cryptococcal survival in its environmental niche, namely woody material, which is glaring given that infection is linked to environmental populations. A gene from C. gattii (WM276), the predominant global molecular type (VGI), has been sequenced and annotated as a putative cellulase. It is therefore, of both medical and industrial intertest to delineate the structure and function of this enzyme. A homology model of the enzyme was constructed as a fusion protein to a maltose binding protein (MBP). The CGB_E4160W gene was overexpressed as an MBP fusion enzyme in Escherichia coli T7 cells and purified to homogeneity using amylose affinity chromatography. The structural and functional character of the enzyme was investigated using fluorescence spectroscopy and enzyme activity assays, respectively. The optimal enzyme pH and temperature were found to be 6.0 and 50 °C, respectively, with an optimal salt concentration of 500 mM. Secondary structure analysis using Far-UV CD reveals that the MBP fusion protein is primarily α-helical with some β-sheets. Intrinsic tryptophan fluorescence illustrates that the MBP-cellulase undergoes a conformational change in the presence of its substrate, CMC-Na+. The thermotolerant and halotolerant nature of this particular cellulase, makes it useful for industrial applications, and adds to our understanding of the pathogen's environmental physiology.

Microbial membrane transport proteins and their biotechnological applications

Because of the hydrophobic nature of the membrane lipid bilayer, the majority of the hydrophilic solutes require special transportation mechanisms for passing through the cell membrane. Integral membrane transport proteins (MTPs), which belong to the Major Intrinsic Protein Family, facilitate the transport of these solutes across cell membranes. MTPs including aquaporins and carrier proteins are transmembrane proteins spanning across the cell membrane. The easy handling of microorganisms enabled the discovery of a remarkable number of transport proteins specific to different substances. It has been realized that these transporters have very important roles in the survival of microorganisms, their pathogenesis, and antimicrobial resistance. Astonishing features related to the solute specificity of these proteins have led to the acceleration of the research on the discovery of their properties and the development of innovative products in which these unique properties are used or imitated. Studies on microbial MTPs range from the discovery and characterization of a novel transporter protein to the mining and screening of them in a large transporter library for particular functions, from simulations and modeling of specific transporters to the preparation of biomimetic synthetic materials for different purposes such as biosensors or filtration membranes. This review presents recent discoveries on microbial membrane transport proteins and focuses especially on formate nitrite transport proteins and aquaporins, and advances in their biotechnological applications.

The Distinctive Permutated Domain Structure of Periplasmic α-Amylase (MalS) from Glycoside Hydrolase Family 13 Subfamily 19

Periplasmic α-amylase MalS (EC. 3.2.1.1), which belongs to glycoside hydrolase (GH) family 13 subfamily 19, is an integral component of the maltose utilization pathway in Escherichia coli K12 and used among Ecnterobacteriaceae for the effective utilization of maltodextrin. We present the crystal structure of MalS from E. coli and reveal that it has unique structural features of circularly permutated domains and a possible CBM69. The conventional C-domain of amylase consists of amino acids 120-180 (N-terminal) and 646-676 (C-terminal) in MalS, and the whole domain architecture shows the complete circular permutation of C-A-B-A-C in domain order. Regarding substrate interaction, the enzyme has a 6-glucosyl unit pocket binding it to the non-reducing end of the cleavage site. Our study found that residues D385 and F367 play important roles in the preference of MalS for maltohexaose as an initial product. At the active site of MalS, β-CD binds more weakly than the linear substrate, possibly due to the positioning of A402. MalS has two Ca²⁺ binding sites that contribute significantly to the thermostability of the enzyme. Intriguingly, the study found that MalS exhibits a high binding affinity for polysaccharides such as glycogen and amylopectin. The N domain, of which the electron density map was not observed, was predicted to be CBM69 by AlphaFold2 and might have a binding site for the polysaccharides. Structural analysis of MalS provides new insight into the structure-evolution relationship in GH13 subfamily 19 enzymes and a molecular basis for understanding the details of catalytic function and substrate binding of MalS.

Determination of dissociation constants of protein ligands by thermal shift assay

The thermal shift assay (TSA) is a powerful tool used to detect molecular interactions between proteins and ligands. Using temperature as a physical denaturant and an extrinsic fluorescent dye, the TSA tracks protein unfolding. This method precisely determines the midpoint of the unfolding transition (T_m), which can shift upon the addition of a ligand. Though experimental protocols have been well developed, the thermal shift assay data traditionally yielded qualitative results. Quantitative methods for K_d determination relied either on empirical and inaccurate usage of T_m or on isothermal approaches, which do not take full advantage of the melting point precision provided by the TSA. We present a new analysis method based on a model that relies on the equilibrium system between the native and molten globule state of the protein using the van't Hoff equation. We propose the K_d can be determined by plotting T_m values versus the logarithm of ligand concentrations and fitting the data to an equation we derived. After testing this procedure with the monomeric maltose-binding protein and an allosterically regulated homotetrameric enzyme (ADP-glucose pyrophosphorylase), we observed that binding results correlated very well with previously established parameters. We demonstrate how this method could potentially offer a broad applicability to a wide range of protein classes and the ability to detect both active and allosteric site binding compounds.

The Whole Is Bigger than the Sum of Its Parts: Drug Transport in the Context of Two Membranes with Active Efflux

Cell envelope plays a dual role in the life of bacteria by simultaneously protecting it from a hostile environment and facilitating access to beneficial molecules. At the heart of this ability lie the restrictive properties of the cellular membrane augmented by efflux transporters, which preclude intracellular penetration of most molecules except with the help of specialized uptake mediators. Recently, kinetic properties of the cell envelope came into focus driven on one hand by the urgent need in new antibiotics and, on the other hand, by experimental and theoretical advances in studies of transmembrane transport. A notable result from these studies is the development of a kinetic formalism that integrates the Michaelis-Menten behavior of individual transporters with transmembrane diffusion and offers a quantitative basis for the analysis of intracellular penetration of bioactive compounds. This review surveys key experimental and computational approaches to the investigation of transport by individual translocators and in whole cells, summarizes key findings from these studies and outlines implications for antibiotic discovery. Special emphasis is placed on Gram-negative bacteria, whose envelope contains two separate membranes. This feature sets these organisms apart from Gram-positive bacteria and eukaryotic cells by providing them with full benefits of the synergy between slow transmembrane diffusion and active efflux.

Distinct patterns of mutational sensitivity for λ resistance and maltodextrin transport in Escherichia coli LamB

Bacteria can evade cohabiting phages through mutations in phage receptors, but these mutations may come at a cost if they disrupt the receptor's native cellular function. To investigate the relationship between these two conflicting activities, we generated sequence-function maps of Escherichia coli LamB with respect to sensitivity to phage λ and transport of maltodextrin. By comparing 413 missense mutations whose effect on both traits could be analysed, we find that these two phenotypes were correlated, implying that most mutations affect these phenotypes through a common mechanism such as loss of protein stability. However, individual mutations could be found that specifically disrupt λ-sensitivity without affecting maltodextrin transport. We identify and individually assay nine such mutations, whose spatial positions implicate loop L6 of LamB in λ binding. Although missense mutations that lead to λ-resistance are rare, they were approximately as likely to be maltodextrin-utilizing (Mal+) as not (Mal-), implying that E. coli can adapt to λ while conserving the receptor's native function. We propose that in order for E. coli and λ to stably cohabitate, selection for λ-resistance and maltose transport must be spatially or temporally separated.

Modelling bacterial chemotaxis for indirectly binding attractants

In bacterial chemotaxis, chemoattractant molecules may bind either directly or indirectly with receptors within the cell periplasmic space. The indirect binding mechanism, which involves an intermediate periplasmic binding protein, has been reported to increase sensitivity to dilute attractant concentrations as well as range of response. Current mathematical models for bacterial chemotaxis at the population scale do not appear to take the periplasmic binding protein (BP) concentration or the indirect binding mechanics into account. We formulate an indirect binding extension to the existing Rivero equation for chemotactic velocity based on fundamental reversible enzyme kinetics. The formulated indirect binding expression accounts for the periplasmic BP concentration and the dissociation constants for binding between attractant and periplasmic BP, as well as between BP and chemoreceptor. We validate the indirect-binding model using capillary assay simulations of the chemotactic responses of E. coli to the indirectly-binding attractants maltose and AI-2. The predicted response agrees well with experimental data from a number of maltose capillary assay studies conducted in previous literature. The model is also able to achieve good agreement with AI-2 capillary assay data of one study out of two tested. The chemotactic response of E. coli towards AI-2 appears to be of higher complexity due to reports of variable periplasmic BP concentration as well as the low concentration of periplasmic BP relative to the total receptor concentration. Our current model is thus suitable for indirect binding chemotactic response systems with constant periplasmic BP concentration that is significantly larger than the total receptor concentration, such as the response of E. coli towards maltose. Further considerations may be taken into account to model the chemotactic response towards AI-2 with greater accuracy.

Rise and rise of the ABC transporter families

This review will inevitably be influenced by my personal experience and personal view of the progression of this amazing family of proteins. This has generated a huge literature in over nearly five decades, some ideas have bloomed and faded while others have persisted, other contributions simply become redundant, overtaken by better techniques. At the outset, the pioneers had no idea of the magnitude of the topic they were working on, then a very rough idea of the significance emerged and, progressively, the picture becomes sharper and finally extraordinary. I have tried to produce at least an outline of that progression. My apologies for the also inevitable omissions, especially relating to the mass of biochemical and spectroscopy and genetical studies. I decided to prioritise structural biology because structures when successful are definitive and of course provide a 'visual' image. However, I tried to limit the structural aspects to the proteins that reflected the main advances.

Periplasmic solute-binding proteins: Structure classification and chitooligosaccharide recognition

Periplasmic solute-binding proteins (SBPs) serve as molecular shuttles that assist the transport of small solutes from the outer membrane to the inner membrane of all Gram-negative bacteria. Based on the available crystal structures, SBPs are classified into seven clusters, A-G, and are further divided into subclusters, IV. This minireview is focused on the classification, structure and substrate specificity of a distinct class of SBPs specific for chitooligosaccharides (CBPs). To date, only two structures of CBP homologues, VhCBP and VcCBP, have been reported in the marine Vibrio species, with exposition of their limited function. The Vibrio CBPs are structurally classified as cluster C/subcluster IV SBPs that exclusively recognize β-1,4- or β-1,3-linked linear oligosaccharides. The overall structural feature of the Vibrios CBPs is most similar to the cellobiose-binding orthologue from the hyperthermophilic bacterium Thermotoga maritima. This similarity provides an opportunity to engineer the substrate specificity of the proteins and to control the uptake of chitinous and cellulosic nutrients in marine bacteria.

Dynamical persistence of active sites identified in maltose-binding protein

This study identifies dynamical properties of maltose-binding protein (MBP) useful in unveiling active site residues susceptible to ligand binding. The described methodology has been previously used in support of novel topological techniques of persistent homology and statistical inference in complex, multi-scale, high-dimensional data often encountered in computational biophysics. Here we outline a computational protocol that is based on the anisotropic elastic network models of 14 all-atom three-dimensional protein structures. We introduce the notion of dynamical distance matrices as a measure of correlated interactions among 370 amino acid residues that constitute a single protein. The dynamical distance matrices serve as an input for a persistent homology suite of codes to further distinguish a small subset of residues with high affinity for ligand binding and allosteric activity. In addition, we show that ligand-free closed MBP structures require lower deformation energies than open MBP structures, which may be used in categorization of time-evolving molecular dynamics structures. Analysis of the most probable allosteric coupling pathways between active site residues and the protein exterior is also presented.

Polar flagella rotation in Vibrio parahaemolyticus confers resistance to bacteriophage infection

Bacteriophage has been recognized as a novel approach to treat bacterial infectious diseases. However, phage resistance may reduce the efficacy of phage therapy. Here, we described a mechanism of bacterial resistance to phage infections. In Gram-negative enteric pathogen Vibrio parahaemolyticus, we found that polar flagella can reduce the phage infectivity. Deletion of polar flagella, but not the lateral flagella, can dramatically promote the adsorption of phage to the bacteria and enhances the phage infectivity to V. parahaemolyticus, indicating that polar flagella play an inhibitory role in the phage infection. Notably, it is the rotation, not the physical presence, of polar flagella that inhibits the phage infection of V. parahaemolyticus. Strikingly, phage dramatically reduces the virulence of V. parahaemolyticus only when polar flagella were absent both in vitro and in vivo. These results indicated that polar flagella rotation is a previously unidentified mechanism that confers bacteriophage resistance.

Full engagement of liganded maltose-binding protein stabilizes a semi-open ATP-binding cassette dimer in the maltose transporter

MalFGK2 is an ATP-binding cassette (ABC) transporter that mediates the uptake of maltose/maltodextrins into Escherichia coli. A periplasmic maltose-binding protein (MBP) delivers maltose to the transmembrane subunits (MalFG) and stimulates the ATPase activity of the cytoplasmic nucleotide-binding subunits (MalK dimer). This MBP-stimulated ATPase activity is independent of maltose for purified transporter in detergent micelles. However, when the transporter is reconstituted in membrane bilayers, only the liganded form of MBP efficiently stimulates its activity. To investigate the mechanism of maltose stimulation, electron paramagnetic resonance spectroscopy was used to study the interactions between the transporter and MBP in nanodiscs and in detergent. We found that full engagement of both lobes of maltose-bound MBP unto MalFGK2 is facilitated by nucleotides and stabilizes a semi-open MalK dimer. Maltose-bound MBP promotes the transition to the semi-open state of MalK when the transporter is in the membrane, whereas such regulation does not require maltose in detergent. We suggest that stabilization of the semi-open MalK2 conformation by maltose-bound MBP is key to the coupling of maltose transport to ATP hydrolysis in vivo, because it facilitates the progression of the MalK dimer from the open to the semi-open conformation, from which it can proceed to hydrolyze ATP.

Role of water in ligand binding to maltose-binding protein: insight from a new docking protocol based on the 3D-RISM-KH molecular theory of solvation

Maltose-binding protein is a periplasmic binding protein responsible for transport of maltooligosaccarides through the periplasmic space of Gram-negative bacteria, as a part of the ABC transport system. The molecular mechanisms of the initial ligand binding and induced large scale motion of the protein's domains still remain elusive. In this study, we use a new docking protocol that combines a recently proposed explicit water placement algorithm based on the 3D-RISM-KH molecular theory of solvation and conventional docking software (AutoDock Vina) to explain the mechanisms of maltotriose binding to the apo-open state of a maltose-binding protein. We confirm the predictions of previous NMR spectroscopic experiments on binding modes of the ligand. We provide the molecular details on the binding mode that was not previously observed in the X-ray experiments. We show that this mode, which is defined by the fine balance between the protein-ligand direct interactions and solvation effects, can trigger the protein's domain motion resulting in the holo-closed structure of the maltose-binding protein with the maltotriose ligand in excellent agreement with the experimental data. We also discuss the role of water in blocking unfavorable binding sites and water-mediated interactions contributing to the stability of observable binding modes of maltotriose.

Microwave assisted solubilization of inclusion bodies

Background

Production of recombinant proteins in bacterial hosts often produces insoluble intracellular particles called inclusion bodies. Recovery of active protein from inclusion bodies generally requires their solubilization in chemical denaturants followed by a refolding strategy. The solubilization is carried out with shaking/stirring and takes several hours.

Results

Using inclusion bodies of seven diverse kinds of recombinant proteins [mutants of controller of cell division or death protein B (CcdB), human CD4D12, thioredoxin fusion protein (malETrx), mutants of maltose binding protein (MBP), single chain variable fragment (ScFv) b12 and single chain antigen binding fragment (ScFab) b12 (anti-HIV-1)], it is shown that exposure to microwave irradiation (200 W) for 2 min, solubilized these inclusion bodies completely. This was confirmed by data based upon turbidity measurements at 400 nm and dynamic light scattering studies. These solubilized inclusion bodies could be refolded correctly in all the cases by known methods. The refolding was confirmed by fluorescence emission spectra and biological activity studies.

Conclusion

Solubilization of the inclusion bodies before refolding is a part of protein production processes for several recombinant proteins which are overexpressed in the bacterial host systems. Our results show that microwave assistance can considerably shorten the process time.

WITHDRAWN: Smart polymer-coated microplate wells: Applications in protein purification, protein refolding, and sensing of analytes

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Advantages of substituting bioluminescence for fluorescence in a resonance energy transfer-based periplasmic binding protein biosensor

A genetically encoded maltose biosensor was constructed, comprising maltose binding protein (MBP) flanked by a green fluorescent protein (GFP(2)) at the N-terminus and a Renilla luciferase variant (RLuc2) at the C-terminus. This Bioluminescence resonance energy transfer(2) (BRET(2)) system showed a 30% increase in the BRET ratio upon maltose binding, compared with a 10% increase with an equivalent fluorescence resonance energy transfer (FRET) biosensor. BRET(2) provides a better matched Förster distance to the known separation of the N and C termini of MBP than FRET. The sensor responded to maltose and maltotriose and the response was completely abolished by introduction of a single point mutation in the BRET(2) tagged MBP protein. The half maximal effective concentration (EC(50)) was 0.37 μM for maltose and the response was linear over almost three log units ranging from 10nM to 3.16 μM maltose for the BRET(2) system compared to an EC(50) of 2.3 μM and a linear response ranging from 0.3 μM to 21.1 μM for the equivalent FRET-based biosensor. The biosensor's estimate of maltose in beer matched that of a commercial enzyme-linked assay but was quicker and more precise, demonstrating its applicability to real-world samples. A similar BRET(2)-based transduction scheme approach would likely be applicable to other binding proteins that have a "venus-fly-trap" mechanism.

Trehalose-recycling ABC transporter LpqY-SugA-SugB-SugC is essential for virulence of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an exclusively human pathogen that proliferates within phagosomes of host phagocytes. Host lipids are believed to provide the major carbon and energy sources for Mtb, with only limited availability of carbohydrates. There is an apparent paradox because five putative carbohydrate uptake permeases are present in Mtb, but there are essentially no host carbohydrates inside phagosomes. Nevertheless, carbohydrate transporters have been implicated in Mtb pathogenesis, suggesting that acquisition of host sugars is important during some stages of infection. Here we show, however, that the LpqY-SugA-SugB-SugC ATP-binding cassette transporter is highly specific for uptake of the disaccharide trehalose, a sugar not present in mammals, thus refuting a role in nutrient acquisition from the host. Trehalose release is known to occur as a byproduct of the biosynthesis of the mycolic acid cell envelope by Mtb's antigen 85 complex. The antigen 85 complex constitutes a group of extracellular mycolyl transferases, which transfer the lipid moiety of the glycolipid trehalose monomycolate (TMM) to arabinogalactan or another molecule of TMM, yielding trehalose dimycolate. These reactions also lead to the concomitant extracellular release of the trehalose moiety of TMM. We found that the LpqY-SugA-SugB-SugC ATP-binding cassette transporter is a recycling system mediating the retrograde transport of released trehalose. Perturbations in trehalose recycling strongly impaired virulence of Mtb. This study reveals an unexpected accessory component involved in the formation of the mycolic acid cell envelope in mycobacteria and provides a previously unknown role for sugar transporters in bacterial pathogenesis.

Uncoupling substrate transport from ATP hydrolysis in the Escherichia coli maltose transporter

Members of the ATP-binding cassette superfamily couple the energy from ATP hydrolysis to the active transport of substrates across the membrane. The maltose transporter, a well characterized model system, consists of a periplasmic maltose-binding protein (MBP) and a multisubunit membrane transporter, MalFGK(2). On the basis of the structure of the MBP-MalFGK(2) complex in an outward-facing conformation (Oldham, M. L., Khare, D., Quiocho, F. A., Davidson, A. L., and Chen, J. (2007) Nature 450, 515-521), we identified two mutants in transmembrane domains MalF and MalG that generated futile cycling; although interaction with MBP stimulated the ATPase activity of the transporter, maltose was not transported. Both mutants appeared to disrupt the normal transfer of maltose from MBP to MalFGK(2). In the first case, substitution of aspartate for glycine in the maltose-binding site of MalF likely generated a futile cycle by preventing maltose from binding to MalFGK(2) during the catalytic cycle. In the second case, a four-residue deletion of a periplasmic loop of MalG limited its reach into the maltose-binding pocket of MBP, allowing maltose to remain associated with MBP during the catalytic cycle. Retention of maltose in the MBP binding site in the deletion mutant, as well as insertion of this loop into the binding site in the wild type, was detected by EPR as a change in mobility of a nitroxide spin label positioned near the maltose-binding pocket of MBP.

Glucose- and glucokinase-controlled mal gene expression in Escherichia coli

MalT is the central transcriptional activator of all mal genes in Escherichia coli. Its activity is controlled by the inducer maltotriose. It can be inhibited by the interaction with certain proteins, and its expression can be controlled. We report here a novel aspect of mal gene regulation: the effect of cytoplasmic glucose and glucokinase (Glk) on the activity and the expression of MalT. Amylomaltase (MalQ) is essential for the metabolism of maltose. It forms maltodextrins and glucose from maltose or maltodextrins. We found that glucose above a concentration of 0.1 mM blocked the activity of the enzyme. malQ mutants when grown in the absence of maltodextrins are endogenously induced by maltotriose that is derived from the degradation of glycogen. Therefore, the fact that glk malQ(+) mutants showed elevated mal gene expression finds its explanation in the reduced ability to remove glucose from MalQ-catalyzed maltodextrin formation and is caused by a metabolically induced MalQ(-) phenotype. However, even in mutants lacking glycogen, Glk controls endogenous induction. We found that overexpressed Glk due to its structural similarity with Mlc, the repressor of malT, binds to the glucose transporter (PtsG), releasing Mlc and thus increasing malT repression. In addition, even in mutants lacking Mlc (and glycogen), the overexpression of glk leads to a reduction in mal gene expression. We interpret this repression by a direct interaction of Glk with MalT concomitant with MalT inhibition. This repression was dependent on the presence of either maltodextrin phosphorylase or amylomaltase and led to the inactivation of MalT.

Ligand-induced structural changes in maltose binding proteins measured by atomic force microscopy

We use atomic force microscopy (AFM) based force-compression measurements to probe the ligand-induced functional conformational changes in surface-immobilized dicysteine-terminated maltose binding proteins (dicys-MBPs). The proteins are immobilized at well-defined locations directly on Au substrates using the previously reported technique of nanografting. By measuring the difference between the ligand-free and ligand-bound mechanical work performed by the AFM-tip during the protein compression, we determine the open-closed transition energy for dicys-MBPs to be DeltaE0 = (8 +/- 4) Kcal/mol. We also compare the binding kinetics of two different ligands (maltose and maltotriose) to dicys-MBPs by performing AFM-friction measurements. We show that our results are consistent with a simple model for the surface-immobilized dicys-MBPs: the protein consists of two rigid lateral lobes connected by a hinge-loaded spring.

Glycogen and maltose utilization by Escherichia coli O157:H7 in the mouse intestine

Mutant screens and transcriptome studies led us to consider whether the metabolism of glucose polymers, i.e., maltose, maltodextrin, and glycogen, is important for Escherichia coli colonization of the intestine. By using the streptomycin-treated mouse model, we found that catabolism of the disaccharide maltose provides a competitive advantage in vivo to pathogenic E. coli O157:H7 and commensal E. coli K-12, whereas degradation of exogenous forms of the more complex glucose polymer, maltodextrin, does not. The endogenous glucose polymer, glycogen, appears to play an important role in colonization, since mutants that are unable to synthesize or degrade glycogen have significant colonization defects. In support of the hypothesis that E. coli relies on internal carbon stores to maintain colonization during periods of famine, we found that by providing a constant supply of a readily metabolized sugar, i.e., gluconate, in the animal's drinking water, the competitive disadvantage of E. coli glycogen metabolism mutants is rescued. The results suggest that glycogen storage may be widespread in enteric bacteria because it is necessary for maintaining rapid growth in the intestine, where there is intense competition for resources and occasional famine. An important implication of this study is that the sugars used by E. coli are present in limited quantities in the intestine, making endogenous carbon stores valuable. Thus, there may be merit to combating enteric infections by using probiotics or prebiotics to manipulate the intestinal microbiota in such a way as to limit the availability of sugars preferred by E. coli O157:H7 and perhaps other pathogens.

Structure, function, and evolution of bacterial ATP-binding cassette systems

SUMMARY

ATP-binding cassette (ABC) systems are universally distributed among living organisms and function in many different aspects of bacterial physiology. ABC transporters are best known for their role in the import of essential nutrients and the export of toxic molecules, but they can also mediate the transport of many other physiological substrates. In a classical transport reaction, two highly conserved ATP-binding domains or subunits couple the binding/hydrolysis of ATP to the translocation of particular substrates across the membrane, through interactions with membrane-spanning domains of the transporter. Variations on this basic theme involve soluble ABC ATP-binding proteins that couple ATP hydrolysis to nontransport processes, such as DNA repair and gene expression regulation. Insights into the structure, function, and mechanism of action of bacterial ABC proteins are reported, based on phylogenetic comparisons as well as classic biochemical and genetic approaches. The availability of an increasing number of high-resolution structures has provided a valuable framework for interpretation of recent studies, and realistic models have been proposed to explain how these fascinating molecular machines use complex dynamic processes to fulfill their numerous biological functions. These advances are also important for elucidating the mechanism of action of eukaryotic ABC proteins, because functional defects in many of them are responsible for severe human inherited diseases.

The dynamics of the MBP-MalFGK(2) interaction: a prototype for binding protein dependent ABC-transporter systems

This review is focused on the interaction between maltose binding protein (MBP) and the maltose transporter complex, MalFGK(2), which is a member of the ATP Binding Cassette (ABC) superfamily. The interaction between MBP and MalFGK(2) has a critical role in maltose transport, but a coherent description of the interaction is complicated because both MBP and MalFGK(2) can adopt multiple conformations. Drawing on genetic, structural, and biochemical data, the different conformations of MBP and MalFGK(2) are described and incorporated into a model for their interaction. The most important feature of this model is that ligand-bound MBP initiates the process of ATP-dependent maltose transport by stabilizing a high-energy conformation of MalFGK(2). In this model of the MBP-MalFGK(2) interaction, stabilization of a high-energy conformation of MalFGK(2) allows ATP to drive conformational changes in the system - in particular the opening of bound MBP - that leads to formation of a transition state for ATP hydrolysis. Such a role for ligand-bound MBP explains how MBP-independent MalFGK(2) mutants work, and represents a general mechanism for binding-protein dependent ABC import systems. In ABC export systems, which do not use a binding protein, the substrate itself is expected to play a role similar to ligand-bound MBP in the maltose transport system. The mechanistic model for the maltose transporter suggests that ABC-type import systems evolved to make use of a peripheral binding protein so that the transport process is essentially irreversible.

MalE of group A Streptococcus participates in the rapid transport of maltotriose and longer maltodextrins

Study of the maltose/maltodextrin binding protein MalE in Escherichia coli has resulted in fundamental insights into the molecular mechanisms of microbial transport. Whether gram-positive bacteria employ a similar pathway for maltodextrin transport is unclear. The maltodextrin binding protein MalE has previously been shown to be key to the ability of group A Streptococcus (GAS) to colonize the oropharynx, the major site of GAS infection in humans. Here we used a multifaceted approach to elucidate the function and binding characteristics of GAS MalE. We found that GAS MalE is a central part of a highly efficient maltodextrin transport system capable of transporting linear maltodextrins that are up to at least seven glucose molecules long. Of the carbohydrates tested, GAS MalE had the highest affinity for maltotriose, a major breakdown product of starch in the human oropharynx. The thermodynamics and fluorescence changes induced by GAS MalE-maltodextrin binding were essentially opposite those reported for E. coli MalE. Moreover, unlike E. coli MalE, GAS MalE exhibited no specific binding of maltose or cyclic maltodextrins. Our data show that GAS developed a transport system optimized for linear maltodextrins longer than two glucose molecules that has several key differences from its well-studied E. coli counterpart.

Functional Characteristics of TauA Binding Protein from TauABC Escherichia coli System

Although TauA shares few common characteristics with other known periplasmic binding protein, TauA is a putative periplasmic binding protein, part of tauABCD gene cluster involved in sulfonate transport in sulphate starvation condition. This protein was expressed in E. coli BL 21 and purified before to assess its binding functionalities. Measurement of K (d) value (mean 11.3 nM) by binding/dialysis studies revealed high affinity and specificity with taurine and also indicated that TauA possessed a unique binding site for its ligand. Comparisons with other periplasmic binding proteins suggests TauA plays a major role in ABC transport system and could be ideal candidate to serve as taurine catcher in biological fluids.

Proteomic Analysis of the Carbonate Insoluble Outer Membrane Fraction of the Soft-Rot Pathogen Dickeya dadantii (syn. Erwinia chrysanthemi) Strain 3937

We present results of the first comprehensive proteomic analysis of the outer membrane of the bacterial phytopathogen Dickeya dadantii strain 3937 and its response to virulence-contributing factors such as host plant extract, acidic stress, and iron starvation. We analyzed the carbonate-insoluble membrane fractions, which are highly enriched for outer membrane proteins, using two-dimensional electrophoresis and identified the proteins by MALDI-TOF MS. Forty unique proteins were identified, some of which were differentially expressed under the above conditions.

The role of cytosolic alpha-glucan phosphorylase in maltose metabolism and the comparison of amylomaltase in Arabidopsis and Escherichia coli

Transitory starch of leaves is broken down hydrolytically, making maltose the predominant form of carbon exported from chloroplasts at night. Maltose metabolism in the cytoplasm of Escherichia coli requires amylomaltase (MalQ) and maltodextrin phosphorylase (MalP). Possible orthologs of MalQ and MalP in the cytosol of Arabidopsis (Arabidopsis thaliana) were proposed as disproportionating enzyme (DPE2, At2g40840) and alpha-glucan phosphorylase (AtPHS2, At3g46970). In this article, we measured the activities of recombinant DPE2 and AtPHS2 proteins with various substrates; we show that maltose and a highly branched, soluble heteroglycan (SHG) are excellent substrates for DPE2 and propose that a SHG is the in vivo substrate for DPE2 and AtPHS2. In E. coli, MalQ and MalP preferentially use smaller maltodextrins (G(3)-G(7)) and we suggest that MalQ and DPE2 have similar, but nonidentical, roles in maltose metabolism. To study this, we complemented a MalQ(-) E. coli strain with DPE2 and found that the rescue was not complete. To investigate the role of AtPHS2 in maltose metabolism, we characterized a T-DNA insertion line of the AtPHS2 gene. The nighttime maltose level increased 4 times in the Atphs2-1 mutant. The comparison of maltose metabolism in Arabidopsis with that in E. coli and the comparison of the maltose level in plants lacking DPE2 or AtPHS2 indicate that an alternative route to metabolize the glucan residues in SHG exists. Other plant species also contain SHG, DPE2, and alpha-glucan phosphorylase, so this pathway for maltose metabolism may be widespread among plants.

Structure of the monomeric outer-membrane porin OmpG in the open and closed conformation

OmpG, a monomeric pore-forming protein from Escherichia coli outer membranes, was refolded from inclusion bodies and crystallized in two different conformations. The OmpG channel is a 14-stranded beta-barrel, with short periplasmic turns and seven extracellular loops. Crystals grown at neutral pH show the channel in the open state at 2.3 A resolution. In the 2.7 A structure of crystals grown at pH 5.6, the pore is blocked by loop 6, which folds across the channel. The rearrangement of loop 6 appears to be triggered by a pair of histidine residues, which repel one another at acidic pH, resulting in the breakage of neighbouring H-bonds and a lengthening of loop 6 from 10 to 17 residues. A total of 151 ordered LDAO detergent molecules were found in the 2.3 A structure, mostly on the hydrophobic outer surface of OmpG, mimicking the outer membrane lipid bilayer, with three LDAO molecules in the open pore. In the 2.7 A structure, OmpG binds one OG and one glucose molecule as sugar substrates in the closed pore.

The importance of maltose in transitory starch breakdown

Starch content of leaves responds to environmental stresses in various ways. Understanding these environmental effects on starch metabolism has been difficult in the past because the pathways of transitory starch synthesis and degradation are not completely known. Over the past two years there has been a significant increase in our understanding of transitory starch breakdown. The discovery of a maltose transporter (MEX1) and the studies of a cytosolic disproportionating enzyme (D-enzyme, DPE2) confirmed that maltose is the predominant form of carbon exported from chloroplasts at night. Maltose increases in leaves when starch breakdown is induced during the day under photorespiratory conditions. Maltose metabolism is regulated by a circadian clock, day length and temperature. The expression of maltose-metabolizing genes shows a pronounced circadian rhythm indicating maltose metabolism is clock regulated. Indeed, the maltose level oscillates under continuous light. The transcript of a beta-amylase gene (BAM3) peaks during the day in long days and peaks at night in short days. This could provide a mechanism for adjusting starch breakdown rates to day length. Under cold-stress conditions, maltose increases and BAM3 expression is induced. We hypothesize that maltose metabolism is a bridge between transitory starch breakdown and the plants' adaptation to changes in environmental conditions.

The maltodextrin system of Escherichia coli: metabolism and transport

The maltose/maltodextrin regulon of Escherichia coli consists of 10 genes which encode a binding protein-dependent ABC transporter and four enzymes acting on maltodextrins. All mal genes are controlled by MalT, a transcriptional activator that is exclusively activated by maltotriose. By the action of amylomaltase, we prepared uniformly labeled [(14)C]maltodextrins from maltose up to maltoheptaose with identical specific radioactivities with respect to their glucosyl residues, which made it possible to quantitatively follow the rate of transport for each maltodextrin. Isogenic malQ mutants lacking maltodextrin phosphorylase (MalP) or maltodextrin glucosidase (MalZ) or both were constructed. The resulting in vivo pattern of maltodextrin metabolism was determined by analyzing accumulated [(14)C]maltodextrins. MalP(-) MalZ(+) strains degraded all dextrins to maltose, whereas MalP(+) MalZ(-) strains degraded them to maltotriose. The labeled dextrins were used to measure the rate of transport in the absence of cytoplasmic metabolism. Irrespective of the length of the dextrin, the rates of transport at a submicromolar concentration were similar for the maltodextrins when the rate was calculated per glucosyl residue, suggesting a novel mode for substrate translocation. Strains lacking MalQ and maltose transacetylase were tested for their ability to accumulate maltose. At 1.8 nM external maltose, the ratio of internal to external maltose concentration under equilibrium conditions reached 10(6) to 1 but declined at higher external maltose concentrations. The maximal internal level of maltose at increasing external maltose concentrations was around 100 mM. A strain lacking malQ, malP, and malZ as well as glycogen synthesis and in which maltodextrins are not chemically altered could be induced by external maltose as well as by all other maltodextrins, demonstrating the role of transport per se for induction.

A salt-bridge motif involved in ligand binding and large-scale domain motions of the maltose-binding protein

The uptake of nutrients is essential for the survival of bacterial cells. Many specialized systems have evolved, such as the maltose-dependent ABC transport system that transfers oligosaccharides through the cytoplasmic membrane. The maltose/maltodextrin-binding protein (MBP) serves as an initial high-affinity binding component in the periplasm that delivers the bound sugar into the cognate ABC transporter MalFGK(2). We have investigated the domain motions induced by the binding of the ligand maltotriose into the binding cleft using molecular dynamics simulations. We find that MBP is predominantly in the open state without ligand and in the closed state with ligand bound. Oligosaccharide binding induces a closure motion (30.0 degrees rotation), whereas ligand removal leads to domain opening (32.6 degrees rotation) around a well-defined hinge affecting key areas relevant for chemotaxis and transport. Our simulations suggest that a "hook-and-eye" motif is involved in the binding. A salt bridge between Glu-111 and Lys-15 forms that effectively locks the protein-ligand complex in a semiclosed conformation inhibiting any further opening and promoting complete closure. This previously unrecognized feature seems to secure the ligand in the binding site and keeps MBP in the closed conformation and suggests a role in the initial steps of substrate transport.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Site-directed Mutagenesis of the Greasy Slide Aromatic Residues Within the LamB (Maltoporin) Channel of Escherichia coli: Effect on Ion and Maltopentaose Transport

The 3D-structure of the maltooligosaccharide-specific LamB-channel of Escherichia coli (also called maltoporin) is known from X-ray crystallography. The 3D structure suggests that a number of aromatic residues (Y6, Y41, W74, F229, W358 and W420) within the channel lumen are involved in carbohydrate and ion transport. All aromatic residues were replaced by alanine-scanning mutagenesis. Furthermore, LamB mutants were created in which two, three, four, five and all six aromatic residues were replaced to study their effects on ion and maltopentaose transport through LamB. The purified mutant proteins were reconstituted into lipid bilayer membranes and the single-channel conductance of the mutants was studied in conductance experiments. The results suggest that all aromatic residues provide some steric hindrance for ion transport through LamB. Highest impact is provided by Y6 and Y41 that are localized opposite Y118, which form the central constriction of the LamB channel. Stability constants for binding of maltopentaose to the mutant channels were measured using titration experiments with the carbohydrate. The mutation of one or several aromatic residue(s) led to a substantial decrease of the stability constant of binding. The highest effect was observed when all aromatic residues were replaced by alanine because no binding of maltopentaose could be detected in such a case. However, binding was again possible when Y118 was replaced by tryptophan. The carbohydrate-induced block of the channel function could be used also for the study of current noise through the different mutant LamB-channels. The analysis of the power density spectra of some of the mutants allowed the evaluation of the on-rate and off-rate constants (k1 and k(-1)) of carbohydrate binding to the binding site inside the channels. The results suggest that both on-rate and off-rate constants were affected by the mutations. For most mutants, k1 decreased and k(-1) increased. The possible influence of the aromatic residues of the greasy slide on carbohydrate and ion transport through LamB is discussed.

Nanopores: maltoporin channel as a sensor for maltodextrin and lambda-phage

Background

To harvest nutrition from the outside bacteria e.g. E. coli developed in the outer cell wall a number of sophisticated channels called porins. One of them, maltoporin, is a passive specific channel for the maltodextrin uptake. This channel was also named LamB as the bacterial virus phage Lambda mis-uses this channel to recognise the bacteria. The first step is a reversible binding followed after a lag phase by DNA injection. To date little is known about the binding capacity and less on the DNA injection mechanism. To elucidate the mechanism and to show the sensitivity of our method we reconstituted maltoporin in planar lipid membranes. Application of an external transmembrane electric field causes an ion current across the channel. Maltoporin channel diameter is around a few Angstroem. At this size the ion current is extremely sensitive to any modification of the channels surface. Protein conformational changes, substrate binding etc will cause fluctuations reflecting the molecular interactions with the channel wall. The recent improvement in ion current fluctuation analysis allows now studying the interaction of solutes with the channel on a single molecular level.

Results

We could demonstrate the asymmetry of the bacterial phage Lambda binding to its natural receptor maltoporin.

Conclusion

We suggest that this type of measurement can be used as a new type of biosensors.

Artificial allosteric control of maltose binding protein

We demonstrate the allosteric control of a protein based on mechanical tension. When substrate binding is accompanied by a significant change of conformation of the protein, a mechanical tension favoring one or the other conformation will alter the binding affinity for the substrate. We have constructed a chimera where the two lobes of the maltose-binding protein are covalently coupled to the ends of a DNA oligomer. The mechanical tension on the protein is controlled externally by exploiting the difference in stiffness between single stranded and double stranded DNA. We report that the binding affinity of the protein for its substrates is significantly altered by the tension.

Acidic proteome of growing and restingLactococcus lactismetabolizing maltose

The acidic proteome of Lactococcus lactis grown anaerobically was compared for three different growth conditions: cells growing on maltose, resting cells metabolizing maltose, and cells growing on glucose. In maltose metabolizing cells several proteins were up-regulated compared with glucose metabolizing cells, however only some of the up-regulated proteins had apparent relation to maltose metabolism. Cells growing on maltose produced formate, acetate and ethanol in addition to lactate, whereas resting cells metabolizing maltose and cells growing on glucose produced only lactate. Increased levels of alcohol-acetaldehyde dehydrogenase (ADH) and phosphate acetyltransferase (PTA) in maltose-growing cells compared with glucose-growing cells coincided with formation of mixed acids in maltose-growing cells. The resting cells did not grow due to lack of an amino acid source and fermented maltose with lactate as the sole product, although ADH and PTA were present at high levels. The maltose consumption rate was approximately three times lower in resting cells than in exponentially growing cells. However, the enzyme levels in resting and growing cells metabolizing maltose were similar, which indicates that the difference in product formation in this case is due to regulation at the enzyme level. The levels of 30S ribosomal proteins S1 and S2 increased with increasing growth rate for resting cells metabolizing maltose, maltose-growing cells and glucose-growing cells. A modified form of HPr was synthesized under amino acid starvation. This is suggested to be due to alanine misincorporation for valine, which L. lactis is auxotrophic for. L. lactis conserves the protein profile to a high extent, even after prolonged amino acid starvation, so that the protein expression profile of the bacterium remains almost invariant.

C-Terminus Loop 13 of Na+ Glucose Cotransporter SGLT1 Contains a Binding Site for Alkyl Glucosides

Recently, we identified the extramembranous C-terminus loop 13 of SGLT1 as a binding site for the aromatic glucoside phlorizin, which competitively inhibits sodium D-glucose cotransport. Alkyl glucosides are also competitive inhibitors of the transport. Therefore, in this study, we searched for potential binding sites for alkyl glucosides in loop 13. To this end, we synthesized a photoaffinity label (2'-Azi-n-octyl)-beta-D-glucoside and analyzed the region of attachment using MALDI mass spectrometry, producing wild-type recombinant truncated loop 13. Furthermore, we prepared four single-Trp mutants of the loop and determined their fluorescence, its change in the presence of alkyl glucosides, and their accessibility to acrylamide. Photolabeling of truncated loop 13 with (2'-Azi-n-octyl)-beta-D-glucoside revealed an attachment of the C2 group of the alkyl chain to Gly-Phe-Phe-Arg (amino acid residues 598-601). In the presence of n-hexyl-beta-D-glucoside, all mutants (R601W, D611W, E621W, and L630W) exhibited a significant decrease in Trp fluorescence with an apparent binding affinity of 8-14 microM. Only L630W exhibited a significant blue shift, and only in R601W was a change in acrylamide quenching (protection) observed. No quenching or protection was found for D-glucose; however, 1-hexanol produced the same results as n-hexyl-beta-D-glucoside. The interaction shows stereoselectivity for n-hexyl-beta-D-glucoside binding; the beta-configuration of the sugar moiety at C1, the cis conformation of the unsaturated alkenyl side chain in the C3-C4 bond, and the alkyl chain length of six to eight carbon atoms lead to an optimum interaction. A schematic two-dimensional model was derived in which C2 interacts with the region around residue 601, C3 and C4 interact with the region between residues 614 and 619, and C6-C8 interact with the region between residues 621 and 630. The data demonstrate that loop 13 provides binding sites for alkyl glucosides as well as for phlorizin; thus, loop 13 of SGLT1 seems to be a major binding domain for the aglucone residues of competitive D-glucose transport inhibitors.

Effect of signal peptide on the stability and folding kinetics of maltose binding protein

While the role of the signal sequence in targeting proteins to specific subcellular compartments is well characterized, there are fewer studies that characterize its effects on the stability and folding kinetics of the protein. We report a detailed characterization of the folding kinetics and thermodynamic stabilities of maltose binding protein (MBP) and its precursor form, preMBP. Isothermal GdmCl and urea denaturation as a function of temperature and thermal denaturation studies have been carried out to compare stabilities of the two proteins. preMBP was found to be destabilized by about 2-6 kcal/mol (20-40%) with respect to MBP. Rapid cleavage of the signal peptide by various proteases shows that the signal peptide is accessible in the native form of preMBP. The observed rate constant of the major slow phase in folding was decreased 5-fold in preMBP relative to MBP. The rate constants of unfolding were similar at 25 degrees C, but preMBP also exhibited a large burst phase change in unfolding that was absent in MBP. At 10 degrees C, preMBP exhibited a higher unfolding rate than MBP as well as a large burst phase. The appreciable destabilization of MBP by signal peptide is functionally relevant, because it enhances the likelihood of finding the protein in an unfolded translocation-competent form and may influence the interactions of the protein with the translocation machinery. Destabilization is likely to result from favorable interactions between the hydrophobic signal peptide and other hydrophobic regions that are exposed in the unfolded state.

pH-regulated gene expression of the gastric pathogen Helicobacter pylori

Colonization by the gastric pathogen Helicobacter pylori has been shown to be intricately linked to the development of gastritis, ulcers, and gastric malignancy. Little is known about mechanisms employed by the bacterium that help it adapt to the hostile environment of the human stomach. In an effort to extend our knowledge of these mechanisms, we utilized spotted-DNA microarrays to characterize the response of H. pylori to low pH. Expression of approximately 7% of the bacterial genome was reproducibly altered by shift to low pH. Analysis of the differentially expressed genes led to the discovery that acid exposure leads to profound changes in motility of H. pylori, as a larger percentage of acid-exposed bacterial cells displayed motility and moved at significantly higher speeds. In contrast to previous publications, we found that expression of the bacterial virulence gene cagA was strongly repressed by acid exposure. Furthermore, this transcriptional repression was reflected at the level of protein accumulation in the H. pylori cell.

TrmB, a sugar-specific transcriptional regulator of the trehalose/maltose ABC transporter from the hyperthermophilic archaeon Thermococcus litoralis

We report the characterization of TrmB, a protein of 38,800 apparent molecular weight, that is involved in the maltose-specific regulation of a gene cluster in Thermococcus litoralis, malE malF malG orf trmB malK, encoding a binding protein-dependent ABC transporter for trehalose and maltose. TrmB binds maltose and trehalose half-maximally at 20 microm and 0.5 mm sugar concentration, respectively. Binding of maltose but not of trehalose showed indications of sigmoidality and quenched the intrinsic tryptophan fluorescence by 15%, indicating a conformational change on maltose binding. TrmB causes a shift in electrophoretic mobility of DNA fragments harboring the promoter and upstream regulatory motif identified by footprinting. Band shifting by TrmB can be prevented by maltose. In vitro transcription assays with purified components from Pyrococcus furiosus have been established to show pmalE promoter-dependent transcription at 80 degrees C. TrmB specifically inhibits transcription, and this inhibition is counteracted by maltose and trehalose. These data characterize TrmB as a maltose-specific repressor for the trehalose/maltose transport operon of Thermococcus litoralis.

Isolation and biochemical characterization of hypophosphite/2-oxoglutarate dioxygenase. A novel phosphorus-oxidizing enzyme from Psuedomonas stutzeri WM88

The htxA gene is required for the oxidation of hypophosphite in Pseudomonas stutzeri WM88 (Metcalf, W. W., and Wolfe, R. S. (1998) J. Bacteriol. 180, 5547-5558). Amino acid sequence comparisons suggest that hypophosphite:2-oxoglutarate dioxygenase (HtxA) is a novel member of the 2-oxoglutarate-dependent dioxygenase enzyme family. To provide experimental support for this hypothesis, HtxA was overproduced in Escherichia coli and purified to apparent homogeneity. Recombinant HtxA is identical to the native enzyme based on amino terminus sequencing and mass spectral analysis, and it catalyzes the oxidation of hypophosphite to phosphite in a process strictly dependent on 2-oxoglutarate, ferrous ions, and oxygen. Succinate and phosphite are stoichiometrically produced, indicating a strict coupling of the reaction. Size exclusion analysis suggests that HtxA is active as a homodimer, and maximal activity is observed at pH 7.0 and at 27 degrees C. The apparent K(m) values for hypophosphite and 2-oxoglutarate were 0.58 +/- 0.04 mm and 10.6 +/- 1.4 microm, respectively. V(max) and k(cat) values were determined to be 10.9 +/- 0.30 micromol min(-1) mg(-1) and 355 min(-1), respectively. 2-Oxoadipate and pyruvate substitute poorly for 2-oxoglutarate as a cosubstrate. The highest specific activity is observed with hypophosphite as substrate, but HtxA is also able to oxidize formate and arsenite at significant rates. The substrate analog inhibitors, formate and nitrate, significantly reduce HtxA activity.

Mechanism of maltodextrin transport through LamB

The Gram-negative bacterial outer membrane contains several independent, biochemically distinct transport systems for the acquisition of solutes from the environment. Three or more different classes of membrane proteins exist within the porin superfamily, that facilitate the uptake of sugars, amino acids, nucleotides, vitamins and metals. In spite of crystallographic descriptions of these protein transporters over the past decade, the mechanisms by which porins catalyze solute internalization are controversial, and in some cases still obscure. For many years the research of Maurice Hofnung endeavored to explain the transport of maltose and maltodextrins by LamB, also known as maltoporin. In the shadow of recent crystal structures, his work helped outline a different picture of outer membrane transport physiology, that is a tribute to the powerful genetic approaches Maurice pioneered. These data suggest that the principal determinant of maltodextrin recognition by maltoporin derives from the configuration of aromatic amino acids in its surface loops.

Expression and purification of the extracellular domain of human myelin protein zero

Myelin protein zero (P0), an adhesion protein of the immunoglobulin superfamily, is the major protein of peripheral nervous system myelin in higher vertebrates. Protein zero is required for the formation and maintenance of myelin structure in the internode, likely through homophilic interactions at both the extracellular and the intracellular domains. Mutations and deletions in the P0 gene correlate with hereditary peripheral neuropathies of varying severity. Comparisons between the human and rat isoforms, whose three-dimensional structure has been determined by X-ray crystallography, suggest that these disease-associated genetic alterations lead to structural changes in the protein that alter P0-P0 interactions and hence affect myelin functionality. Knowing the crystal structures of native and altered human P0 isoforms could help to elucidate the structural changes in myelin membrane packing that underlie the altered functionality. Alterations of P0 extracellular domain (P0-ED) are of additional interest as previous X-ray diffraction studies on myelin membrane packing suggest that P0-ED molecules can assume distinct adhesive arrangements. Here, we describe an improved method to express and purify human P0-ED (hP0-ED) suitable for crystallographic analysis. A fusion protein consisting of maltose binding protein fused to hP0-ED was secreted to the periplasm of Escherichia coli to allow an appropriate folding pathway. The fusion protein was extracted via osmotic shock and purified by affinity chromatography. Factor Xa was used to cleave the fusion protein, and a combination of affinity and ion-exchange chromatography was used to further purify hP0-ED. We document several significant improvements to previous protocols, including bacterial growth to approximately 15 OD using orbital shakers and the use of diafiltration, which result in yields of approximately 150 mg highly pure protein per liter of medium.