Bacterial chemotaxis: a field in motion

S- and N-Oxide Reductases

Escherichia coli is a versatile facultative anaerobe that can respire on a number of terminal electron acceptors, including oxygen, fumarate, nitrate, and S- and N-oxides. Anaerobic respiration using S- and N-oxides is accomplished by enzymatic reduction of these substrates by dimethyl sulfoxide reductase (DmsABC) and trimethylamine N-oxide reductase (TorCA). Both DmsABC and TorCA are membrane-associated redox enzymes that couple the oxidation of menaquinol to the reduction of S- and N-oxides in the periplasm. DmsABC is membrane bound and is composed of a membrane-extrinsic dimer with a 90.4-kDa catalytic subunit (DmsA) and a 23.1-kDa electron transfer subunit (DmsB). These subunits face the periplasm and are held to the membrane by a 30.8-kDa membrane anchor subunit (DmsC). The enzyme provides the scaffold for an electron transfer relay composed of a quinol binding site, five [4Fe-4S] clusters, and a molybdo-bis(molybdopterin guanine dinucleotide) (present nomenclature: Mo-bis-pyranopterin) (Mo-bisMGD) cofactor. TorCA is composed of a soluble periplasmic subunit (TorA, 92.5 kDa) containing a Mo-bis-MGD. TorA is coupled to the quinone pool via a pentaheme c subunit (TorC, 40.4 kDa) in the membrane. Both DmsABC and TorCA require system-specific chaperones (DmsD or TorD) for assembly, cofactor insertion, and/or targeting to the Tat translocon. In this chapter, we discuss the complex regulation of the dmsABC and torCAD operons, the poorly understood paralogues, and what is known about the assembly and translocation to the periplasmic space by the Tat translocon.

Impact of fluorochrome stains used to study bacterial transport in shallow aquifers on motility and chemotaxis of Pseudomonas species

One of the most common methods of tracking movement of bacteria in groundwater environments involves a priori fluorescent staining. A major concern in using these stains to label bacteria in subsurface injection-and-recovery studies is the effect they may have on the bacterium's transport properties. Previous studies investigated the impact of fluorophores on bacterial surface properties (e.g. zeta potential). However, no previous study has looked at the impact of fluorescent staining on swimming speed and chemotaxis. It was found that DAPI lowered the mean population swimming speed of Pseudomonas putida F1 by 46% and Pseudomonas stutzeri by 55%. DAPI also inhibited the chemotaxis in both strains. The swimming speeds of P. putida F1 and P. stutzeri were diminished slightly by CFDA/SE, but not to a statistically significant extent. CFDA/SE had no effect on chemotaxis of either strain to acetate. SYBR(®) Gold had no effect on swimming speed or the chemotactic response to acetate for either strain. This research indicates that although DAPI may not affect sorption to grain surfaces, it adversely affects other potentially important transport properties such as swimming and chemotaxis. Consequently, bacterial transport studies conducted using DAPI are biased to nonchemotactic conditions and do not appear to be suitable for monitoring the effect of chemotaxis on bacterial transport in shallow aquifers.

Transport genes and chemotaxis in Laribacter hongkongensis: a genome-wide analysis

Background

Laribacter hongkongensis is a Gram-negative, sea gull-shaped rod associated with community-acquired gastroenteritis. The bacterium has been found in diverse freshwater environments including fish, frogs and drinking water reservoirs. Using the complete genome sequence data of L. hongkongensis, we performed a comprehensive analysis of putative transport-related genes and genes related to chemotaxis, motility and quorum sensing, which may help the bacterium adapt to the changing environments and combat harmful substances.

Results

A genome-wide analysis using Transport Classification Database TCDB, similarity and keyword searches revealed the presence of a large diversity of transporters (n = 457) and genes related to chemotaxis (n = 52) and flagellar biosynthesis (n = 40) in the L. hongkongensis genome. The transporters included those from all seven major transporter categories, which may allow the uptake of essential nutrients or ions, and extrusion of metabolic end products and hazardous substances. L. hongkongensis is unique among closely related members of Neisseriaceae family in possessing higher number of proteins related to transport of ammonium, urea and dicarboxylate, which may reflect the importance of nitrogen and dicarboxylate metabolism in this assacharolytic bacterium. Structural modeling of two C^₄-dicarboxylate transporters showed that they possessed similar structures to the determined structures of other DctP-TRAP transporters, with one having an unusual disulfide bond. Diverse mechanisms for iron transport, including hemin transporters for iron acquisition from host proteins, were also identified. In addition to the chemotaxis and flagella-related genes, the L. hongkongensis genome also contained two copies of qseB/qseC homologues of the AI-3 quorum sensing system.

Conclusions

The large number of diverse transporters and genes involved in chemotaxis, motility and quorum sensing suggested that the bacterium may utilize a complex system to adapt to different environments. Structural modeling will provide useful insights on the transporters in L. hongkongensis.

Change in rigidity in the activated form of the glucose/galactose receptor from Escherichia coli: a phenomenon that will be key to the development of biosensors

Recently a periplasmic glucose/galactose binding protein, GGRQ26C, immobilized on a gold surface has been used as an active part of a glucose biosensor based on quartz microbalance technique. However the nature of the glucose detection was not clear. Here we have found that the receptor protein film immobilized on the gold surface increases its rigidity when glucose is added, which explains the unexpected detection signal. To study the rigidity change, we developed a new fast and simple method based on using atomic force microscopy (AFM) in tapping mode. The method was verified by explicit measurements of the Young's modulus of the protein film by conventional AFM methods. Since there are a host of receptors that undergo structural change when activated by ligand, AFM can play a key role in the development and/or optimization of biosensors based on rigidity changes in biomolecules.

Conformational equilibria and free energy profiles for the allosteric transition of the ribose-binding protein

The ribose-binding protein (RBP) is a sugar-binding bacterial periplasmic protein whose function is associated with a large allosteric conformational change from an open to a closed conformation upon binding to ribose. The crystal structures of RBP in open and closed conformations have been solved. It has been hypothesized that the open and closed conformations exist in a dynamic equilibrium in solution, and that sugar binding shifts the population from open conformations to closed conformations. Here, we study by computer simulations the thermodynamic changes that accompany this conformational change, and model the structural changes that accompany the allosteric transition, using umbrella sampling molecular dynamics and the weighted histogram analysis method. The open state is comprised of a diverse ensemble of conformations; the open ribose-free X-ray crystal conformations being representative of this ensemble. The unligated open form of RBP is stabilized by conformational entropy. The simulations predict detectable populations of closed ribose-free conformations in solution. Additional interdomain hydrogen bonds stabilize this state. The predicted shift in equilibrium from the open to the closed state on binding to ribose is in agreement with experiments. This is driven by the energetic stabilization of the closed conformation due to ribose-protein interactions. We also observe a significant population of a hitherto unobserved ribose-bound partially open state. We believe that this state is the one that has been suggested to play a role in the transfer of ribose to the membrane-bound permease complex.

Biophysical and kinetic characterization of HemAT, an aerotaxis receptor from Bacillus subtilis

HemAT from Bacillus subtilis is a new type of heme protein responsible for sensing oxygen. The structural and functional properties of the full-length HemAT protein, the sensor domain (1-178), and Tyr-70 mutants have been characterized. Kinetic and equilibrium measurements reveal that both full-length HemAT and the sensor domain show two distinct O(2) binding components. The high-affinity component has a K(dissociation) approximately 1-2 microM and a normal O(2) dissociation rate constant, k(O2) = 50-80 s(-1). The low-affinity component has a K(dissociation) approximately 50-100 microM and a large O(2) dissociation rate constant equal to approximately 2000 s(-1). The low n-value and biphasic character of the equilibrium curve indicate that O(2) binding to HemAT involves either independent binding to high- and low-affinity subunits in the dimer or negative cooperativity. Replacement of Tyr-70(B10) with Phe, Leu, or Trp in the sensor domain causes dramatic increases in k(O2) for both the high- and low-affinity components. In contrast, the rates and affinity for CO binding are little affected by loss of the Tyr-70 hydroxyl group. These results suggest highly dynamic behavior for the Tyr-70 side chain and the fraction of the "up" versus "down" conformation is strongly influenced by the nature of the iron-ligand complex. As a result of having both high- and low-affinity components, HemAT can respond to oxygen concentration gradients under both hypoxic (0-10 microM) and aerobic (50-250 microM) conditions, a property which could, in principle, be important for a robust sensing system. The unusual ligand-binding properties of HemAT suggest that asymmetry and apparent negative cooperativity play an important role in the signal transduction pathway.

Chemotaxis: signalling the way forward

During random locomotion, human neutrophils and Dictyostelium discoideum amoebae repeatedly extend and retract cytoplasmic processes. During directed cell migration--chemotaxis--these pseudopodia form predominantly at the leading edge in response to the local accumulation of certain signalling molecules. Concurrent changes in actin and myosin enable the cell to move towards the stimulus. Recent studies are beginning to identify an intricate network of signalling molecules that mediate these processes, and how these molecules become localized in the cell is now becoming clear.

The Tp38 (TpMglB-2) lipoprotein binds glucose in a manner consistent with receptor function in Treponema pallidum

A 38-kDa lipoprotein of Treponema pallidum (Tp38) was predicted to be a periplasmic sugar-binding protein based on its sequence similarity to the glucose/galactose-binding (MglB) protein of Escherichia coli (P. S. Becker, D. R. Akins, J. D. Radolf, and M. V. Norgard, Infect. Immun. 62:1381-1391, 1994). Inasmuch as glucose is believed to be the principal, if not sole, carbon and energy source for T. pallidum and is readily available to the spirochete during its obligate infection of humans, we hypothesized that Tp38 may serve as the organism's requisite glucose receptor. For the present study, a nonacylated recombinant form of Tp38 was coexpressed with GroES and GroEL in E. coli to facilitate the isolation of soluble, properly folded Tp38. The highly sensitive method of intrinsic fluorescence spectroscopy, predicated on the manner in which tryptophan residues reside and move within protein microenvironments, was then used to assess sugar binding to Tp38. The intrinsic fluorescence of Tp38 was essentially unaltered when it was exposed to D-mannose, D-fucose, D-ribose, L-glucose, or L-galactose, but it changed markedly in the presence of D-glucose, and to a lesser extent, D-galactose, indicating binding. The K(d) values for D-glucose and D-galactose binding to Tp38 were 152.2 +/- 20.73 nM and 251.2 +/- 55.25 nM, respectively. Site-directed mutagenesis of Trp-145, a residue postulated to contribute to the sugar-binding pocket in a manner akin to the essential Trp-183 in E. coli MglB, abolished Tp38's conformational change in response to D-glucose. The combined data are consistent with Tp38 serving as a glucose receptor for T. pallidum. These findings potentially have important implications for syphilis pathogenesis, particularly as they may pertain to glucose-mediated chemotactic responses by T. pallidum.

Learning and evolution in bacterial taxis: an operational amplifier circuit modeling the computational dynamics of the prokaryotic ‘two component system’ protein network

Adaptive behavior in unicellular organisms (i.e., bacteria) depends on highly organized networks of proteins governing purposefully the myriad of molecular processes occurring within the cellular system. For instance, bacteria are able to explore the environment within which they develop by utilizing the motility of their flagellar system as well as a sophisticated biochemical navigation system that samples the environmental conditions surrounding the cell, searching for nutrients or moving away from toxic substances or dangerous physical conditions. In this paper we discuss how proteins of the intervening signal transduction network could be modeled as artificial neurons, simulating the dynamical aspects of the bacterial taxis. The model is based on the assumption that, in some important aspects, proteins can be considered as processing elements or McCulloch-Pitts artificial neurons that transfer and process information from the bacterium's membrane surface to the flagellar motor. This simulation of bacterial taxis has been carried out on a hardware realization of a McCulloch-Pitts artificial neuron using an operational amplifier. Based on the behavior of the operational amplifier we produce a model of the interaction between CheY and FliM, elements of the prokaryotic two component system controlling chemotaxis, as well as a simulation of learning and evolution processes in bacterial taxis. On the one side, our simulation results indicate that, computationally, these protein 'switches' are similar to McCulloch-Pitts artificial neurons, suggesting a bridge between evolution and learning in dynamical systems at cellular and molecular levels and the evolutive hardware approach. On the other side, important protein 'tactilizing' properties are not tapped by the model, and this suggests further complexity steps to explore in the approach to biological molecular computing.

A diffusion-translocation model for gradient sensing by chemotactic cells

Small chemotactic cells like Dictyostelium and neutrophils transduce shallow spatial chemoattractant gradients into strongly localized intracellular responses. We show that the capacity of a second messenger to establish and maintain localized signals, is mainly determined by its dispersion range, lambda = the square root of D(m)/k(-1), which must be small compared to the cell's length. Therefore, short-living second messengers (high k(-1)) with diffusion coefficients D(m) in the range of 0-5 microm(2) s(-1) are most suitable. Additional to short dispersion ranges, gradient sensing may include positive feedback mechanisms that lead to local activation and global inhibition of second-messenger production. To introduce the essential nonlinear amplification, we have investigated models in which one or more components of the signal transduction cascade translocate from the cytosol to the second messenger in the plasma membrane. A one-component model is able to amplify a 1.5-fold difference of receptor activity over the cell length into a 15-fold difference of second-messenger concentration. Amplification can be improved considerably by introducing an additional activating component that translocates to the membrane. In both models, communication between the front and the back of the cell is mediated by partial depletion of cytosolic components, which leads to both local activation and global inhibition. The results suggest that a biochemically simple and general mechanism may explain various signal localization phenomena not only in chemotactic cells but also those occurring in morphogenesis and cell differentiation.

Identification of a site critical for kinase regulation on the central processing unit (CPU) helix of the aspartate receptor

Ligand binding to the homodimeric aspartate receptor of Escherichia coli and Salmonella typhimurium generates a transmembrane signal that regulates the activity of a cytoplasmic histidine kinase, thereby controlling cellular chemotaxis. This receptor also senses intracellular pH and ambient temperature and is covalently modified by an adaptation system. A specific helix in the cytoplasmic domain of the receptor, helix alpha6, has been previously implicated in the processing of these multiple input signals. While the solvent-exposed face of helix alpha6 possesses adaptive methylation sites known to play a role in kinase regulation, the functional significance of its buried face is less clear. This buried region lies at the subunit interface where helix alpha6 packs against its symmetric partner, helix alpha6'. To test the role of the helix alpha6-helix alpha6' interface in kinase regulation, the present study introduces a series of 13 side-chain substitutions at the Gly 278 position on the buried face of helix alpha6. The substitutions are observed to dramatically alter receptor function in vivo and in vitro, yielding effects ranging from kinase superactivation (11 examples) to complete kinase inhibition (one example). Moreover, four hydrophobic, branched side chains (Val, Ile, Phe, and Trp) lock the kinase in the superactivated state regardless of whether the receptor is occupied by ligand. The observation that most side-chain substitutions at position 278 yield kinase superactivation, combined with evidence that such facile superactivation is rare at other receptor positions, identifies the buried Gly 278 residue as a regulatory hotspot where helix packing is tightly coupled to kinase regulation. Together, helix alpha6 and its packing interactions function as a simple central processing unit (CPU) that senses multiple input signals, integrates these signals, and transmits the output to the signaling subdomain where the histidine kinase is bound. Analogous CPU elements may be found in other receptors and signaling proteins.

Chemotaxis receptors: a progress report on structure and function

Recent biochemical and structural studies have provided many new insights into the structure and function of bacterial chemoreceptors. Aspects of their ligand binding, conformational changes, and interactions with other members of the signaling pathway are being defined at the structural level. It is anticipated that the combined effort will soon provide a detailed, unified view of an entire response system.

Detection of a conserved alpha-helix in the kinase-docking region of the aspartate receptor by cysteine and disulfide scanning

The transmembrane aspartate receptor of Escherichia coli and Salmonella typhimurium propagates extracellular signals to the cytoplasm, where its cytoplasmic domain regulates the histidine kinase, CheA. Different signaling states of the cytoplasmic domain modulate the kinase autophosphorylation rate over at least a 100-fold range. Biochemical and genetic studies have implicated a specific region of the cytoplasmic domain, termed the signaling subdomain, as the region that transmits regulation from the receptor to the kinase. Here cysteine and disulfide scanning are applied to the N-terminal half of the signaling subdomain to probe its secondary structure, solvent exposure, and protein-protein interactions. The chemical reactivities of the scanned cysteines exhibit the characteristic periodicity of an alpha-helix with distinct solvent-exposed and buried faces. This helix, termed alpha7, ranges approximately from residue 355 through 386. Activity measurements probing the effects of cysteine substitutions in vivo and in vitro reveal that both faces of helix alpha7 are critical for kinase activation, while the buried face is especially critical for kinase down-regulation. Disulfide scanning of the region suggests that helix alpha7 is not in direct contact with its symmetric partner (alpha7') from the other subunit; presently, the structural element that packs against the buried face of the helix remains unidentified. Finally, a novel approach termed "protein interactions by cysteine modification" indicates that the exposed C-terminal face of helix alpha7 provides an essential docking site for the kinase CheA or for the coupling protein CheW.

A response regulator of cyanobacteria integrates diverse environmental signals and is critical for survival under extreme conditions

Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently use available resources. We have identified a gene encoding a response regulator, NblR, that complements a cyanobacterial mutant unable to degrade its light-harvesting complex (phycobilisome), in response to nutrient deprivation. Cells of the nblR mutant (i) have more phycobilisomes than wild-type cells during nutrient-replete growth, (ii) do not degrade phycobilisomes during sulfur, nitrogen, or phosphorus limitation, (iii) cannot properly modulate the phycobilisome level during exposure to high light, and (iv) die rapidly when starved for either sulfur or nitrogen, or when exposed to high light. Apart from regulation of phycobilisome degradation, NblR modulates additional functions critical for cell survival during nutrient-limited and high-light conditions. NblR does not appear to be involved in acclimation responses that occur only during a specific nutrient limitation. In contrast, it controls at least some of the general acclimation responses; those that occur during any of a number of different stress conditions. NblR plays a pivotal role in integrating different environmental signals that link the metabolism of the cell to light harvesting capabilities and the activities of the photosynthetic apparatus; this modulation is critical for cell survival.

Cysteine and disulfide scanning reveals two amphiphilic helices in the linker region of the aspartate chemoreceptor

The transmembrane aspartate receptor of E. coli and S. typhimurium mediates cellular chemotaxis toward aspartate by regulating the activity of the cytoplasmic histidine kinase, CheA. Ligand binding results in transduction of a conformational signal through the membrane to the cytoplasmic domain where both kinase regulation and adaptation occur. Of particular interest is the linker region, E213 to Q258, which connects and transduces the conformational signal between the cytoplasmic end of the transmembrane signaling helix (alpha 4/TM2) and the major methylation helix of the cytoplasmic domain (alpha 6). This linker is crucial for stable folding and function of the homodimeric receptor. The present study uses cysteine and disulfide scanning mutagenesis to investigate the secondary structure and packing surfaces within the linker region. Chemical reactivity assays reveal that the linker consists of three distinct subdomains: two alpha-helices termed alpha 4 and alpha 5 and, between them, an ordered region of undetermined secondary structure. When cysteine is scanned through the helices, characteristic repeating patterns of solvent exposure and burial are observed. Activity assays, both in vivo and in vitro, indicate that each helix possesses a buried packing face that is crucial for proper receptor function. The interhelical subdomain is at least partially buried and is also crucial for proper receptor function. Disulfide scanning places helix alpha 4 distal to the central axis of the homodimer, while helix alpha 5 is found to lie at the subunit interface. Finally, sequence alignments suggest that all three linker subdomains are highly conserved among the large subfamily of histidine kinase-coupled sensory receptors that possess methylation sites for use in covalent adaptation.

Mind your B's and R's: bacterial chemotaxis, signal transduction and protein recognition

The crystal structures of two key regulators of the bacterial chemotaxis pathway (CheR and CheB) have been determined. These studies add further detail to the growing picture of signal transduction and attenuation in the bacterial chemotaxis pathway. The recently determined structure of the methyltransferase CheR bound to a peptide of its target receptor, provides a structural model for intermolecular receptor modification during signaling.

Cysteine and disulfide scanning reveals a regulatory alpha-helix in the cytoplasmic domain of the aspartate receptor

The transmembrane, homodimeric aspartate receptor of Escherichia coli and Salmonella typhimurium controls the chemotactic response to aspartate, an attractant, by regulating the activity of a cytoplasmic histidine kinase. The cytoplasmic domain of the receptor plays a central role in both kinase regulation and sensory adaptation, although its structure and regulatory mechanisms are unknown. The present study utilizes cysteine and disulfide scanning to probe residues Leu-250 through Gln-309, a region that contains the first of two adaptive methylation segments within the cytoplasmic domain. Following the introduction of consecutive cysteine residues by scanning mutagenesis, the measurement of sulfhydryl chemical reactivities reveals an alpha-helical pattern of exposed and buried positions spanning residues 270-309. This detected helix, termed the "first methylation helix," is strongly amphiphilic; its exposed face is highly anionic and possesses three methylation sites, while its buried face is hydrophobic. In vivo and in vitro assays of receptor function indicate that inhibitory cysteine substitutions are most prevalent on the buried face of the first methylation helix, demonstrating that this face is involved in a critical packing interaction. The buried face is further analyzed by disulfide scanning, which reveals three "lock-on" disulfides that covalently trap the receptor in its kinase-activating state. Each of the lock-on disulfides cross-links the buried faces of the two symmetric first methylation helices of the dimer, placing these helices in direct contact at the subunit interface. Comparative sequence analysis of 56 related receptors suggests that the identified helix is a conserved feature of this large receptor family, wherein it is likely to play a general role in adaptation and kinase regulation. Interestingly, the rapid rates and promiscuous nature of disulfide formation reactions within the scanned region reveal that the cytoplasmic domain of the full-length, membrane-bound receptor has a highly dynamic structure. Overall, the results demonstrate that cysteine and disulfide scanning can identify secondary structure elements and functionally important packing interfaces, even in proteins that are inaccessible to other structural methods.

The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes

The chemosensory pathway of bacterial chemotaxis has become a paradigm for the two-component superfamily of receptor-regulated phosphorylation pathways. This simple pathway illustrates many of the fundamental principles and unanswered questions in the field of signaling biology. A molecular description of pathway function has progressed rapidly because it is accessible to diverse structural, biochemical, and genetic approaches. As a result, structures are emerging for most of the pathway elements, biochemical studies are elucidating the mechanisms of key signaling events, and genetic methods are revealing the intermolecular interactions that transmit information between components. Recent advances include (a) the first molecular picture of a conformational transmembrane signal in a cell surface receptor, (b) four new structures of kinase domains and adaptation enzymes, and (c) significant new insights into the mechanisms of receptor-mediated kinase regulation, receptor adaptation, and the phospho-activation of signaling proteins. Overall, the chemosensory pathway and the propulsion system it regulates provide an ideal system in which to probe molecular principles underlying complex cellular signaling and behavior.

Temperature-induced switching of the bacterial flagellar motor

Chemotaxis signaling proteins normally control the direction of rotation of the flagellar motor of Escherichia coli. In their absence, a wild-type motor spins exclusively counterclockwise. Although the signaling pathway is well defined, relatively little is known about switching, the mechanism that enables the motor to change direction. We found that switching occurs in the absence of signaling proteins when cells are cooled to temperatures below about 10 degrees C. The forward rate constant (for counterclockwise to clockwise, CCW to CW, switching) increases and the reverse rate constant (for CW to CCW switching) decreases as the temperature is lowered. At about -2 degrees C, most motors spin exclusively CW. At temperatures for which reversals are frequent enough to generate a sizable data set, both CCW and CW interval distributions appear to be exponential. From the rate constants we computed equilibrium constants and standard free energy changes, and from the temperature dependence of the standard free energy changes we determined standard enthalpy and entropy changes. Using transition-state theory, we also calculated the activation free energy, enthalpy, and entropy. We conclude that the CW state is preferred at very low temperatures and that it is relatively more highly bonded and restricted than the CCW state.

Identification of a gene for a chemoreceptor of the methyl-accepting type in the symbiotic plasmid of Rhizobium leguminosarum bv. viciae UPM791

The 4 kb DNA region located immediately upstream of the Rhizobium leguminosarum bv. viciae UPM791 hydrogen structural genes was sequenced and found to encode a chemoreceptor of the methyl-accepting type, the first to be described in a rhizobial symbiotic plasmid. Two additional open reading frames were found. Their protein products showed sequence homology to dehydrogenases and isomerases involved in the metabolism of aromatic compounds. Mutant analysis showed that this region is not required for hydrogenase activity.

The first step in sugar transport: crystal structure of the amino terminal domain of enzyme I of the E. coli PEP: sugar phosphotransferase system and a model of the phosphotransfer complex with HPr

BACKGROUND

The bacterial phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) transports exogenous hexose sugars through the membrane and tightly couples transport with phosphoryl transfer from PEP to the sugar via several phosphoprotein intermediates. The phosphate group is first transferred to enzyme I, second to the histidine-containing phosphocarrier protein HPr, and then to one of a number of sugar-specific enzymes II. The structures of several HPrs and enzymes IIA are known. Here we report the structure of the N-terminal half of enzyme I from Escherichia coli (EIN).

RESULTS

The crystal structure of EIN (MW approximately 30 kDa) has been determined and refined at 2.5 A resolution. It has two distinct structural subdomains; one contains four alpha helices arranged as two hairpins in a claw-like conformation. The other consists of a beta sandwich containing a three-stranded antiparallel beta sheet and a four-stranded parallel beta sheet, together with three short alpha helices. Plausible models of complexes between EIN and HPr can be made without assuming major structural changes in either protein.

CONCLUSIONS

The alpha/beta subdomain of EIN is topologically similar to the phosphohistidine domain of the enzyme pyruvate phosphate dikinase, which is phosphorylated by PEP on a histidyl residue but does not interact with HPr. It is therefore likely that features of this subdomain are important in the autophosphorylation of enzyme I. The helical subdomain of EIN is not found in pyruvate phosphate dikinase; this subdomain is therefore more likely to be involved in phosphoryl transfer to HPr.