Correlating Conformational Equilibria with Catalysis in the Electron Bifurcating EtfABCX of Thermotoga maritima

Electron bifurcation (BF) is an evolutionarily ancient energy coupling mechanism in anaerobes, whose associated enzymatic machinery remains enigmatic. In BF-flavoenzymes, a chemically high-potential electron forms in a thermodynamically favorable fashion by simultaneously dropping the potential of a second electron before its donation to physiological acceptors. The cryo-EM and spectroscopic analyses of the BF-enzyme Fix/EtfABCX from Thermotoga maritima suggest that the BF-site contains a special flavin-adenine dinucleotide and, upon its reduction with NADH, a low-potential electron transfers to ferredoxin and a high-potential electron reduces menaquinone. The transfer of energy from high-energy intermediates must be carefully orchestrated conformationally to avoid equilibration. Herein, anaerobic size exclusion-coupled small-angle X-ray scattering (SEC-SAXS) shows that the Fix/EtfAB heterodimer subcomplex, which houses BF- and electron transfer (ET)-flavins, exists in a conformational equilibrium of compacted and extended states between flavin-binding domains, the abundance of which is impacted by reduction and NAD(H) binding. The conformations identify dynamics associated with the T. maritima enzyme and also recapitulate states identified in static structures of homologous BF-flavoenzymes. Reduction of Fix/EtfABCX's flavins alone is insufficient to elicit domain movements conducive to ET but requires a structural "trigger" induced by NAD(H) binding. Models show that Fix/EtfABCX's superdimer exists in a combination of states with respect to its BF-subcomplexes, suggesting a cooperative mechanism between supermonomers for optimizing catalysis. The correlation of conformational states with pathway steps suggests a structural means with which Fix/EtfABCX may progress through its catalytic cycle. Collectively, these observations provide a structural framework for tracing Fix/EtfABCX's catalysis.

Characterization of the Membrane-Associated Electron-Bifurcating Flavoenzyme EtfABCX from the Hyperthermophilic Bacterium Thermotoga maritima

Electron bifurcation is an energy-conservation mechanism in which a single enzyme couples an exergonic reaction with an endergonic one. Heterotetrameric EtfABCX drives the reduction of low-potential ferredoxin (E°' ∼ -450 mV) by oxidation of the midpotential NADH (E°' = -320 mV) by simultaneously coupling the reaction to reduction of the high-potential menaquinone (E°' = -74 mV). Electron bifurcation occurs at the NADH-oxidizing bifurcating-flavin adenine dinucleotide (BF-FAD) in EtfA, which has extremely crossed half-potentials and passes the first, high-potential electron to an electron-transferring FAD and via two iron-sulfur clusters eventually to menaquinone. The low-potential electron on the BF-FAD semiquinone simultaneously reduces ferredoxin. We have expressed the genes encodingThermotoga maritimaEtfABCX in E. coli and purified the EtfABCX holoenzyme and the EtfAB subcomplex. The bifurcation activity of EtfABCX was demonstrated by using electron paramagnetic resonance (EPR) to follow accumulation of reduced ferredoxin. To elucidate structural factors that impart the bifurcating ability, EPR and NADH titrations monitored by visible spectroscopy and dye-linked enzyme assays have been employed to characterize four conserved residues, R38, P239, and V242 in EtfA and R140 in EtfB, in the immediate vicinity of the BF-FAD. The R38, P239, and V242 variants showed diminished but still significant bifurcation activity. Despite still being partially reduced by NADH, the R140 variant had no bifurcation activity, and electron transfer to its two [4Fe-4S] clusters was prevented. The role of R140 is discussed in terms of the bifurcation mechanism in EtfABCX and in the other three families of bifurcating enzymes.

The Role of a Loop in the Non-catalytic Domain B on the Hydrolysis/Transglycosylation Specificity of the 4-α-Glucanotransferase from Thermotoga maritima

The mechanism by which glycoside hydrolases control the reaction specificity through hydrolysis or transglycosylation is a key element embedded in their chemical structures. The determinants of reaction specificity seem to be complex. We looked for structural differences in domain B between the 4-α-glucanotransferase from Thermotoga maritima (TmGTase) and the α-amylase from Thermotoga petrophila (TpAmylase) and found a longer loop in the former that extends towards the active site carrying a W residue at its tip. Based on these differences we constructed the variants W131G and the partial deletion of the loop at residues 120-124/128-131, which showed a 11.6 and 11.4-fold increased hydrolysis/transglycosylation (H/T) ratio relative to WT protein, respectively. These variants had a reduction in the maximum velocity of the transglycosylation reaction, while their affinity for maltose as the acceptor was not substantially affected. Molecular dynamics simulations allow us to rationalize the increase in H/T ratio in terms of the flexibility near the active site and the conformations of the catalytic acid residues and their associated pKas.

Connected Component Analysis of Dynamical Perturbation Contact Networks

Describing protein dynamical networks through amino acid contacts is a powerful way to analyze complex biomolecular systems. However, due to the size of the systems, identifying the relevant features of protein-weighted graphs can be a difficult task. To address this issue, we present the connected component analysis (CCA) approach that allows for fast, robust, and unbiased analysis of dynamical perturbation contact networks (DPCNs). We first illustrate the CCA method as applied to a prototypical allosteric enzyme, the imidazoleglycerol phosphate synthase (IGPS) enzyme from Thermotoga maritima bacteria. This approach was shown to outperform the clustering methods applied to DPCNs, which could not capture the propagation of the allosteric signal within the protein graph. On the other hand, CCA reduced the DPCN size, providing connected components that nicely describe the allosteric propagation of the signal from the effector to the active sites of the protein. By applying the CCA to the IGPS enzyme in different conditions, i.e., at high temperature and from another organism (yeast IGPS), and to a different enzyme, i.e., a protein kinase, we demonstrated how CCA of DPCNs is an effective and transferable tool that facilitates the analysis of protein-weighted networks.

A simple method to determine changes in the affinity between HisF and HisH in the Imidazole Glycerol Phosphate Synthase heterodimer

The bi-enzyme HisF-HisH heterodimer is part of the pathway that produces histidine and purines in bacteria and lower eukaryotes, but it is absent in mammals. This heterodimer has been largely studied probing the basis of the allosteric effects and the structural stability in proteins. It is also a potential target for antibacterial drugs. In this work, we developed a simple method to evaluate changes in the affinity between HisF and HisH in the heterodimer of the bacteria Thermotoga maritima. HisH contains a single tryptophan residue, which is exposed in the free protein, but buried in the heterodimer interface. Hence, the intrinsic fluorescence maximum of this residue changes to shorter wavelengths upon dimerization. Thus, we used the fluorescence intensity at this shorter wavelength to monitor heterodimer accumulation when HisH was combined with sub-stoichiometric HisF. Under conditions where the HisF-HisH heterodimer is in equilibrium with the free states of these enzymes, when [HisH] > [HisF], we deduced a linear function connecting [HisF-HisH] to [HisF], in which the slope depends on the heterodimer dissociation constant (Kd). Based on this equation, taking fluorescence intensities as proxies of the heterodimer and HisF concentrations, we experimentally determined the Kd at four different temperatures. These Kd values were compared to those evaluated using ITC. Both methods revealed an increase in the HisF and HisH binding affinity as the temperature increases. In spite of differences in their absolute values, the Kd determined using these methods presented an evident linear correlation. To demonstrate the effectiveness of the fluorescence method we determined the effect on the Kd caused by 12 single mutations in HisF. Coherently, this test singled out the only mutation in the binding interface. In brief, the method described here effectively probes qualitative effects on the Kd, can be carried out using common laboratory equipment and is scalable.

Characterisation of a hyperthermophilic transketolase from Thermotoga maritima DSM3109 as a biocatalyst for 7-keto-octuronic acid synthesis

Transketolase (TK) is a fundamentally important enzyme in industrial biocatalysis which carries out a stereospecific carbon-carbon bond formation, and is widely used in the synthesis of prochiral ketones. This study describes the biochemical and molecular characterisation of a novel and unusual hyperthermophilic TK from Thermotoga maritima DSM3109 (TKtmar). TKtmar has a low protein sequence homology compared to the already described TKs, with key amino acid residues in the active site highly conserved. TKtmar has a very high optimum temperature (>90 °C) and shows pronounced stability at high temperature (e.g. t1/2 99 and 9.3 h at 50 and 80 °C, respectively) and in presence of organic solvents commonly used in industry (DMSO, acetonitrile and methanol). Substrate screening showed activity towards several monosaccharides and aliphatic aldehydes. In addition, for the first time, TK specificity towards uronic acids was achieved with TKtmar catalysing the efficient conversion of d-galacturonic acid and lithium hydroxypyruvate into 7-keto-octuronic acid, a very rare C8 uronic acid, in high yields (98%, 49 mM).

A single residue can modulate nanocage assembly in salt dependent ferritin

Cage forming proteins have numerous potential applications in biomedicine and biotechnology, where the iron storage ferritin is a widely used example. However, controlling ferritin cage assembly/disassembly remains challenging, typically requiring extreme conditions incompatible with many desirable cargoes, particularly for more fragile biopharmaceuticals. Recently, a ferritin from the hyperthermophile bacterium Thermotoga maritima (TmFtn) has been shown to have reversible assembly under mild conditions, offering greater potential biocompatibility in terms of cargo access and encapsulation. Like Archeoglobus fulgidus ferritin (AfFtn), TmFtn forms 24mer cages mediated by metal ions (Mg2+). We have solved the crystal structure of the wild type TmFtn and several mutants displaying different assembly/disassembly properties. These data combined with other biophysical studies allow us to suggest candidate interfacial amino acids crucial in controlling assembly. This work deepens our understanding of how these ferritin complexes assemble and is a useful step towards production of triggerable ferritins in which these properties can be finely designed and controlled.

Extracting the Hidden Distributions Underlying the Mean Transition State Structures in Protein Folding

The inherent conflict between noncovalent interactions and the large conformational entropy of the polypeptide chain forces folding reactions and their mechanisms to deviate significantly from chemical reactions. Accordingly, measures of structure in the transition state ensemble (TSE) are strongly influenced by the underlying distributions of microscopic folding pathways that are challenging to discern experimentally. Here, we present a detailed analysis of 150,000 folding transition paths of five proteins at three different thermodynamic conditions from an experimentally consistent statistical mechanical model. We find that the underlying TSE structural distributions are rarely unimodal, and the average experimental measures arise from complex underlying distributions. Unfolding pathways also exhibit subtle differences from folding counterparts due to a combination of Hammond behavior and native-state movements. Local interactions and topological complexity, to a lesser extent, are found to determine pathway heterogeneity, underscoring the importance of the balance between local and nonlocal energetics in protein folding.

Biohydrogen production from hyperthermophilic anaerobic digestion of fruit and vegetable wastes in seawater: Simplification of the culture medium of Thermotoga maritima

Biohydrogen production by the hyperthermophilic and halophilic bacterium T. maritima, using fruit and vegetable wastes as the carbon and energy sources was studied. Batch fermentation cultures showed that the use of a culture medium containing natural seawater and fruit and vegetable wastes can replace certain components (CaCl₂, MgCl₂, Balch's oligo-elements, yeast extract, KH₂PO₄ and K₂HPO₄) present in basal medium. However, a source of nitrogen and sulfur remained necessary for biohydrogen production. When fruit and vegetable waste collected from a wholesale market landfill was used, no decreases in total H₂ production (139 mmol L^-1) or H₂ yield (3.46 mol mol^-1) was observed.

A fast approximate method of identifying paths of allosteric communication in proteins

Fluctuations of the distance between a pair of residues i and j may be correlated with the fluctuations of the distance between another pair k and l. In this case, information may be transmitted among these four residues. Allosteric activity is postulated to proceed through such correlated paths. In this short communication a fast method for calculating correlations among all possible pairs ij and kl leading to a pathway of correlated residues of a protein is proposed. The method is based on the alpha carbon centered Gaussian Network Model. The model is applied to Glutamine Amidotransferase and pathways of allosteric activity are identified and compared with literature.

On the variability of experimental data in macromolecular crystallography

Experimental errors as determined by data-processing algorithms in macromolecular crystallography are compared with the direct error estimates obtained by a multiple crystal data-collection protocol. It is found that several-fold error inflation is necessary to account for crystal-to-crystal variation. It is shown that similar error inflation is observed for data collected from multiple sections of the same crystal, indicating non-uniform crystal growth as one of the likely sources of additional data variation. Other potential sources of error inflation include differential X-ray absorption for different reflections and variation of unit-cell parameters. The underestimation of the experimental errors is more severe in lower resolution shells and for reflections characterized by a higher signal-to-noise ratio. These observations partially account for the gap between the expected and the observed R values in macromolecular crystallography.

Initial binding of ions to the interhelical loops of divalent ion transporter CorA: replica exchange molecular dynamics simulation study

Crystal structures of Thermotoga maritima magnesium transporter CorA, reported in 2006, revealed its homo-pentameric constructions. However, the structure of the highly conserved extracellular interhelical loops remains unsolved, due to its high flexibility. We have explored the configurations of the loops through extensive replica exchange molecular dynamics simulations in explicit solvent model with the presence of either Co(III) Hexamine ions or Mg(2+) ions. We found that there are multiple binding sites available on the interhelical loops in which the negatively charged residues, E316 and E320, are located notably close to the positively charged ions during the simulations. Our simulations resolved the distinct binding patterns of the two kinds of ions: Co(III) Hexamine ions were found to bind stronger with the loop than Mg(2+) ions with binding free energy -7.3 kJ/mol lower, which is nicely consistent with the previous data. Our study provides an atomic basis description of the initial binding process of Mg(2+) ions on the extracellular interhelical loops of CorA and the detailed inhibition mechanism of Co(III) Hexamine ions on CorA ions transportation.

Kissing G domains of MnmE monitored by X-ray crystallography and pulse electron paramagnetic resonance spectroscopy

The authors of this research article demonstrate the nature of the conformational changes MnmE was previously suggested to undergo during its GTPase cycle, and show the nucleotide-dependent dynamic movements of the G domains around two swivel positions relative to the rest of the protein. These movements are of crucial importance for understanding the mechanistic principles of this GAD.

MnmE, which is involved in the modification of the wobble position of certain tRNAs, belongs to the expanding class of G proteins activated by nucleotide-dependent dimerization (GADs). Previous models suggested the protein to be a multidomain protein whose G domains contact each other in a nucleotide dependent manner. Here we employ a combined approach of X-ray crystallography and pulse electron paramagnetic resonance (EPR) spectroscopy to show that large domain movements are coupled to the G protein cycle of MnmE. The X-ray structures show MnmE to be a constitutive homodimer where the highly mobile G domains face each other in various orientations but are not in close contact as suggested by the GDP-AlF_x structure of the isolated domains. Distance measurements by pulse double electron-electron resonance (DEER) spectroscopy show that the G domains adopt an open conformation in the nucleotide free/GDP-bound and an open/closed two-state equilibrium in the GTP-bound state, with maximal distance variations of 18 Å. With GDP and AlF_x, which mimic the transition state of the phosphoryl transfer reaction, only the closed conformation is observed. Dimerization of the active sites with GDP-AlF_x requires the presence of specific monovalent cations, thus reflecting the requirements for the GTPase reaction of MnmE. Our results directly demonstrate the nature of the conformational changes MnmE was previously suggested to undergo during its GTPase cycle. They show the nucleotide-dependent dynamic movements of the G domains around two swivel positions relative to the rest of the protein, and they are of crucial importance for understanding the mechanistic principles of this GAD.

MnmE is an evolutionary conserved G protein that is involved in modification of the wobble U position of certain tRNAs to suppress translational wobbling. Despite high homology between its G domain and the small G protein Ras, MnmE displays entirely different regulatory properties to that of many molecular switch-type G proteins of the Ras superfamily, as its GTPase is activated by nucleotide-dependent homodimerization across the nucleotide-binding site. Here we explore the unusual G domain cycle of the MnmE protein by combining X-ray crystallography with pulse electron paramagnetic resonance (EPR) spectroscopy, which enables distance determinations between spin markers introduced at specific sites within the G domain. We determined the structures of the full-length MnmE dimer in the diphosphate and triphosphate states, which represent distinct steps of the G domain cycle, and demonstrate that the G domain cycle of MnmE comprises large conformational changes and domain movements of up to 18 Å, in which the G domains of the dimeric protein traverse from a GDP-bound open state through an open/closed equilibrium in the triphosphate state to a closed conformation in the transition state, so as to assemble the catalytic machinery.

Crystallization and preliminary X-ray diffraction analysis of the truncated cytosolic domain of the iron transporter FeoB

FeoB-family proteins are widely distributed in bacteria and archaea and are involved in high-affinity Fe(2+) uptake through the plasma membrane. FeoB consists of an N-terminal cytosolic region followed by a C-terminal transmembrane region. The cytosolic region contains small GTPase and GDP dissociation inhibitor-like domains, which serve a regulatory function. The truncated cytosolic region of the iron transporter FeoB from Thermotoga maritima was overexpressed, purified and crystallized. Four native or SeMet crystal forms in a nucleotide-free state or in complex with either GDP or GMPPNP diffracted to resolutions of between 1.5 and 2.1 A.

A novel two-zone protein uptake model for affinity chromatography and its application to the description of elution band profiles of proteins fused to a family 9 cellulose binding module affinity tag

A novel two-zone model (TZM) is presented to describe the rate of solute uptake by the stationary phase of a sorption-type chromatography column. The TZM divides the porous stationary-phase particle into an inner protein-free core and an outer protein-containing zone where intraparticle transport is limited by pore diffusion and binding follows Langmuir theory. The TZM and the classic pore-diffusion model (PDM) of chromatography are applied to the prediction of stationary-phase uptake and elution bands within a cellulose-based affinity chromatography column designed to selectively purify proteins genetically labelled with a CBM9 (family 9 cellulose binding module) affinity tag. Under both linear and nonlinear loading conditions, the TZM closely matches rates of protein uptake within the stationary phase particles as measured by confocal laser scanning microscopy, while the PDM deviates from experiment in the linear-binding region. As a result, the TZM is shown to provide improved predictions of product breakthrough, including elution behavior from a bacterial lysate feed.

The contact interface of a 120 kD CheA-CheW complex by methyl TROSY interaction spectroscopy

During bacterial chemotaxis, the histidine autokinase CheA interacts with the chemotaxis receptors with the help of the coupling protein CheW. This interaction is typical of many macromolecular complexes where protein-protein interactions play an important role. In this case, a relatively small protein, CheW, becomes part of a much larger complex. Here we describe a new method to map the residues at a protein-protein interface for macromolecular complexes of molecular weight greater than 100 kD.

NMR structure of the conserved hypothetical protein TM0487 from Thermotoga maritima: implications for 216 homologous DUF59 proteins

The NMR structure of the conserved hypothetical protein TM0487 from Thermotoga maritima represents an alpha/beta-topology formed by the regular secondary structures alpha1-beta1-beta2-alpha2-beta3-beta4-alpha3- beta5-3(10)-alpha4, with a small anti-parallel beta-sheet of beta-strands 1 and 2, and a mixed parallel/anti-parallel beta-sheet of beta-strands 3-5. Similar folds have previously been observed in other proteins, with amino acid sequence identity as low as 3% and a variety of different functions. There are also 216 sequence homologs of TM0487, which all have the signature sequence of domains of unknown function 59 (DUF59), for which no three-dimensional structures have as yet been reported. The TM0487 structure thus presents a platform for homology modeling of this large group of DUF59 proteins. Conserved among most of the DUF59s are 13 hydrophobic residues, which are clustered in the core of TM0487. A putative active site of TM0487 consisting of residues D20, E22, L23, T51, T52, and C55 is conserved in 98 of the 216 DUF59 sequences. Asp20 is buried within the proposed active site without any compensating positive charge, which suggests that its pK(a) value may be perturbed. Furthermore, the DUF59 family includes ORFs that are part of a conserved chromosomal group of proteins predicted to be involved in Fe-S cluster metabolism.

Physics-based protein-structure prediction using a hierarchical protocol based on the UNRES force field: assessment in two blind tests

Recent improvements in the protein-structure prediction method developed in our laboratory, based on the thermodynamic hypothesis, are described. The conformational space is searched extensively at the united-residue level by using our physics-based UNRES energy function and the conformational space annealing method of global optimization. The lowest-energy coarse-grained structures are then converted to an all-atom representation and energy-minimized with the ECEPP/3 force field. The procedure was assessed in two recent blind tests of protein-structure prediction. During the first blind test, we predicted large fragments of alpha and alpha+beta proteins [60-70 residues with C(alpha) rms deviation (rmsd) <6 A]. However, for alpha+beta proteins, significant topological errors occurred despite low rmsd values. In the second exercise, we predicted whole structures of five proteins (two alpha and three alpha+beta, with sizes of 53-235 residues) with remarkably good accuracy. In particular, for the genomic target TM0487 (a 102-residue alpha+beta protein from Thermotoga maritima), we predicted the complete, topologically correct structure with 7.3-A C(alpha) rmsd. So far this protein is the largest alpha+beta protein predicted based solely on the amino acid sequence and a physics-based potential-energy function and search procedure. For target T0198, a phosphate transport system regulator PhoU from T. maritima (a 235-residue mainly alpha-helical protein), we predicted the topology of the whole six-helix bundle correctly within 8 A rmsd, except the 32 C-terminal residues, most of which form a beta-hairpin. These and other examples described in this work demonstrate significant progress in physics-based protein-structure prediction.

α-Glucan Recognition by a New Family of Carbohydrate-Binding Modules Found Primarily in Bacterial Pathogens

TmPul13, a family 13 glycoside hydrolase from Thermotoga maritima, is a four-module protein having pullulanase activity; the three N-terminal modules are of unknown function while the large C-terminal module is likely the catalytic module. Dissection of the functions of the three unknown modules revealed that the 100 amino acid module at the extreme N-terminus of TmPul13 comprises a new family of carbohydrate-binding modules (CBM) that a bioinformatic analysis shows are most frequently found in pullulanase-like sequences from bacterial pathogens. Detailed binding studies of this isolated CBM, here called TmCBM41, reveals a preference for alpha-(1,4)-linked glucans, but occasional alpha-(1,6)-linked glucose residues, such as those found in pullulan, are tolerated. UV difference, isothermal titration calorimetry, and analytical ultracentrifugation binding studies suggest that maltooligosaccharides longer than four glucose residues are able to bind two TmCBM41 molecules per oligosaccharide when sugar concentrations are below the CBM concentration. This is explained in terms of an equilibrium expression involving the formation of both a 1 to 1 sugar to CBM complex and a 1 to 2 sugar to CBM complex (i.e., a CBM dimer ligated by an oligosaccharide). The presence of an alpha-(1-6) linkage in the oligosaccharide appears to prevent this phenomenon.

Eukaryotic NAD+ synthetase Qns1 contains an essential, obligate intramolecular thiol glutamine amidotransferase domain related to nitrilase

NAD+ is an essential co-enzyme for redox reactions and is consumed in lysine deacetylation and poly(ADP-ribosyl)ation. NAD+ synthetase catalyzes the final step in NAD+ synthesis in the well characterized de novo, salvage, and import pathways. It has been long known that eukaryotic NAD+ synthetases use glutamine to amidate nicotinic acid adenine dinucleotide while many purified prokaryotic NAD+ synthetases are ammonia-dependent. Earlier, we discovered that glutamine-dependent NAD+ synthetases contain N-terminal domains that are members of the nitrilase superfamily and hypothesized that these domains function as glutamine amidotransferases for the associated synthetases. Here we show yeast glutamine-dependent NAD+ synthetase Qns1 requires both the nitrilase-related active-site residues and the NAD+ synthetase active-site residues for function in vivo. Despite failure to complement the lethal phenotype of qns1 disruption, the former mutants retain ammonia-dependent NAD+ synthetase activity in vitro, whereas the latter mutants retain basal glutaminase activity. Moreover, the two classes of mutants fail to trans-complement despite forming a stable heteromultimer in vivo. These data indicate that the nitrilase-related domain in Qns1 is the fourth independently evolved glutamine amidotransferase domain to have been identified in nature and that glutamine-dependence is an obligate phenomenon involving intramolecular transfer of ammonia over a predicted distance of 46 A from one active site to another within Qns1 monomers.

Structural and thermodynamic dissection of specific mannan recognition by a carbohydrate binding module, TmCBM27

The C-terminal 176 amino acids of a Thermotoga maritima mannanase (Man5) constitute a carbohydrate binding module (CBM) that has been classified into CBM family 27. The isolated CBM27 domain, named TmCBM27, binds tightly (K(a)s 10(5)-10(6) M(-1)) to beta-1, 4-mannooligosaccharides, carob galactomannan, and konjac glucomannan, but not to cellulose (insoluble and soluble) or soluble birchwood xylan. The X-ray crystal structures of native TmCBM27, a TmCBM27-mannohexaose complex, and a TmCBM27-6(3),6(4)-alpha-D-galactosyl-mannopentaose complex at 2.0 A, 1.6 A, and 1.35 A, respectively, reveal the basis of TmCBM27's specificity for mannans. In particular, the latter complex, which is the first structure of a CBM in complex with a branched plant cell wall polysaccharide, illustrates how the architecture of the binding site can influence the recognition of naturally substituted polysaccharides.

Strategies for structural proteomics of prokaryotes: Quantifying the advantages of studying orthologous proteins and of using both NMR and X-ray crystallography approaches

Only about half of non-membrane-bound proteins encoded by either bacterial or archaeal genomes are soluble when expressed in Escherichia coli (Yee et al., Proc Natl Acad Sci USA 2002;99:1825-1830; Christendat et al., Prog Biophys Mol Biol 200;73:339-345). This property limits genome-scale functional and structural proteomics studies, which depend on having a recombinant, soluble version of each protein. An emerging strategy to increase the probability of deriving a soluble derivative of a protein is to study different sequence homologues of the same protein, including representatives from thermophilic organisms, based on the assumption that the stability of these proteins will facilitate structural analysis. To estimate the relative merits of this strategy, we compared the recombinant expression, solubility, and suitability for structural analysis by NMR and/or X-ray crystallography for 68 pairs of homologous proteins from E. coli and Thermotoga maritima. A sample suitable for structural studies was obtained for 62 of the 68 pairs of homologs under standardized growth and purification procedures. Fourteen (eight E. coli and six T. maritima proteins) samples generated NMR spectra of a quality suitable for structure determination and 30 (14 E. coli and 16 T. maritima proteins) samples formed crystals. Only three (one E. coli and two T. maritima proteins) samples both crystallized and had excellent NMR properties. The conclusions from this work are: (1) The inclusion of even a single ortholog of a target protein increases the number of samples for structural studies almost twofold; (2) there was no clear advantage to the use of thermophilic proteins to generate samples for structural studies; and (3) for the small proteins analyzed here, the use of both NMR and crystallography approaches almost doubled the number of samples for structural studies.

Evidence of intradomain and interdomain flexibility in an OmpR/PhoB homolog from Thermotoga maritima

Two-component systems, the predominant signal transduction strategy used by prokaryotes, involve phosphorelay from a sensor histidine kinase (HK) to an intracellular response regulator protein (RR) that typically acts as a transcription regulator. RRs are modular proteins, usually composed of a conserved regulatory domain, which functions as a phosphorylation-activated switch, and an attached DNA binding effector domain. The crystal structure of a Thermotoga maritima transcription factor, DrrD, has been determined at 1.5 A resolution, providing the first structural information for a full-length member of the OmpR/PhoB subfamily of RRs. A small interdomain interface occurs between alpha 5 of the regulatory domain and an antiparallel sheet of the effector domain. The lack of an extensive interface in the unphosphorylated protein distinguishes DrrD from other structurally characterized multidomain RRs and suggests a different mode of interdomain regulation.

A common evolutionary origin of two elementary enzyme folds

The (beta alpha)(8)-barrel is the most frequent and most versatile fold among enzymes [Höcker et al., Curr. Opin. Biotechnol. 12 (2001) 376-381; Wierenga, FEBS Lett. 492 (2001) 193-198]. Structural and functional evidence suggests that (beta alpha)(8)-barrels evolved from an ancestral half-barrel, which consisted of four (beta alpha) units stabilized by dimerization [Lang et al., Science 289 (2000) 1546-550; Höcker et al., Nat. Struct. Biol. 8 (2001) 32-36; Gerlt and Babbitt, Nat. Struct. Biol. 8 (2001) 5-7]. Here, by performing a comprehensive database search, we detect a striking and unexpected structural and amino acid sequence similarity between (beta alpha)(4) half-barrels and members of the (beta alpha)(5) flavodoxin-like fold. These findings provoke the hypothesis that a large fraction of the modern-day enzymes evolved from a basic structural building block, which can be identified by a combination of sequence and structural analyses.

High-affinity DNA binding of HU protein from the hyperthermophile Thermotoga maritima

Prokaryotic genomes are compacted by association with small basic proteins, generating what has been termed bacterial chromatin. The ubiquitous DNA-binding protein HU serves this function. DNA-binding properties of HU from the hyperthermophilic eubacterium Thermotoga maritima are shown here to differ significantly from those characteristic of previously described HU homologs. Electrophoretic mobility shift analyses show that T. maritima HU (TmHU) binds double-stranded DNA with high affinity (K(d)=5.6(+/-0.7) nM for 37 bp DNA). Equivalent affinity is observed between 4 degrees C and 45 degrees C. TmHU has higher affinity for DNA containing a set of 4 nt loops separated by 9 bp (K(d)=1.4(+/-0.3) nM), consistent with its introduction of two DNA kinks. Using DNA probes of varying length, the optimal binding site for TmHU is estimated at 37 bp, in sharp contrast to the 9-10 bp binding site reported for other HU homologs. Alignment of >60 HU sequences demonstrates significant sequence conservation: A DNA-intercalating proline residue is almost universally conserved, and it is preceded by arginine and asparagine in most sequences, generating a highly conserved RNP motif; V substitutes for R only in HU from Thermotoga, Thermus and Deinococcus. A fivefold increase in DNA-binding affinity is observed for TmHU in which V is replaced with R (TmHU-V61R; K(d)=1.1(+/-0.2) nM), but a change in the trajectory of DNA flanking the sites of DNA intercalation is inferred from analysis of TmHU-V61R binding to DNA modified with 4 nt loops or with substitutions of 5-hydroxymethyluracil for thymine. Survival in extreme environments places unique demands on protection of genomic DNA from thermal destabilization and on access of DNA to the cellular machinery, demands that may be fulfilled by the specific DNA-binding properties of HU and by the fine structure of the bacterial chromatin.

An extended RNA binding surface through arrayed S1 and KH domains in transcription factor NusA

The crystal structure of Thermotoga maritima NusA, a transcription factor involved in pausing, termination, and antitermination processes, reveals a four-domain, rod-shaped molecule. An N-terminal alpha/beta portion, a five-stranded beta-barrel (S1 domain), and two K-homology (KH) modules create a continuous spine of positive electrostatic potential, suitable for nonspecific mRNA attraction. Homology models suggest how, in addition, specific mRNA regulatory sequences can be recognized by the S1 and KH motifs. An arrangement of multiple S1 and KH domains mediated by highly conserved residues is seen, creating an extended RNA binding surface, a paradigm for other proteins with similar domain arrays. Structural and mutational analyses indicate that the motifs cooperate, modulating strength and specificity of RNA binding.

A thermostable K(+)-stimulated vacuolar-type pyrophosphatase from the hyperthermophilic bacterium Thermotoga maritima

Current evidence suggests the occurrence of two classes of vacuolar-type H(+)-translocating inorganic pyrophosphatases (V-PPases): K(+)-insensitive proteins, identified in eukaryotes, bacteria and archaea, and K(+)-stimulated V-PPases, identified to date only in eukaryotes. Here, we describe the functional characterization of a thermostable V-PPase from the anaerobic hyperthermophilic bacterium Thermotoga maritima by heterologous expression in Saccharomyces cerevisiae. The activity of this 71-kDa membrane-embedded polypeptide has a near obligate requirement for K(+), like the plant V-PPase, and its thermostability depends on the binding of Mg(2+). Phylogenetic analysis of protein sequences consistently assigned the T. maritima V-PPase to the K(+)-sensitive class of V-PPases so far only known for eukaryotes. The finding of a K(+)-stimulated V-PPase also in a member of a primitive eubacterial lineage strongly supports an ancient evolutionary origin of this group of pyrophosphate-energized proton pumps.

Crystal structure of ribosomal protein L4 shows RNA-binding sites for ribosome incorporation and feedback control of the S10 operon

Ribosomal protein L4 resides near the peptidyl transferase center of the bacterial ribosome and may, together with rRNA and proteins L2 and L3, actively participate in the catalysis of peptide bond formation. Escherichia coli L4 is also an autogenous feedback regulator of transcription and translation of the 11 gene S10 operon. The crystal structure of L4 from Thermotoga maritima at 1.7 A resolution shows the protein with an alternating alpha/beta fold and a large disordered loop region. Two separate binding sites for RNA are discernible. The N-terminal site, responsible for binding to rRNA, consists of the disordered loop with flanking alpha-helices. The C-terminal site, a prime candidate for the interaction with the leader sequence of the S10 mRNA, involves two non-consecutive alpha-helices. The structure also suggests a C-terminal protein-binding interface, through which L4 could be interacting with protein components of the transcriptional and/or translational machineries.

Flexibility, conformational diversity and two dimerization modes in complexes of ribosomal protein L12

Protein L12, the only multicopy component of the ribosome, is presumed to be involved in the binding of translation factors, stimulating factor-dependent GTP hydrolysis. Crystal structures of L12 from Thermotogamaritima have been solved in two space groups by the multiple anomalous dispersion method and refined at 2.4 and 2.0 A resolution. In both crystal forms, an asymmetric unit comprises two full-length L12 molecules and two N-terminal L12 fragments that are associated in a specific, hetero-tetrameric complex with one non-crystallographic 2-fold axis. The two full-length proteins form a tight, symmetric, parallel dimer, mainly through their N-terminal domains. Each monomer of this central dimer additionally associates in a different way with an N-terminal L12 fragment. Both dimerization modes are unlike models proposed previously and suggest that similar complexes may occur in vivo and in situ. The structures also display different L12 monomer conformations, in accord with the suggested dynamic role of the protein in the ribosomal translocation process. The structures have been submitted to the Protein Databank (http://www.rcsb.org/pdb) under accession numbers 1DD3 and 1DD4.