Chimeric chemoreceptors in Escherichia coli: signaling properties of Tar-Tap and Tap-Tar hybrids

The Unconventional Cytoplasmic Sensing Mechanism for Ethanol Chemotaxis in Bacillus subtilis

Ethanol is a chemoattractant for Bacillus subtilis even though it is not metabolized and inhibits growth. B. subtilis likely uses ethanol to find ethanol-fermenting microorganisms to utilize as prey. Two chemoreceptors sense ethanol: HemAT and McpB. HemAT’s myoglobin-like sensing domain directly binds ethanol, but the heme group is not involved. McpB is a transmembrane receptor consisting of an extracellular sensing domain and a cytoplasmic signaling domain. While most attractants bind the extracellular sensing domain, we found that ethanol directly binds between intermonomer helices of the cytoplasmic signaling domain of McpB, using a mechanism akin to those identified in many mammalian ethanol-binding proteins. Our results indicate that the sensory repertoire of chemoreceptors extends beyond the sensing domain and can directly involve the signaling domain.

Motile bacteria sense chemical gradients using chemoreceptors, which consist of distinct sensing and signaling domains. The general model is that the sensing domain binds the chemical and the signaling domain induces the tactic response. Here, we investigated the unconventional sensing mechanism for ethanol taxis in Bacillus subtilis. Ethanol and other short-chain alcohols are attractants for B. subtilis. Two chemoreceptors, McpB and HemAT, sense these alcohols. In the case of McpB, the signaling domain directly binds ethanol. We were further able to identify a single amino acid residue, Ala⁴³¹, on the cytoplasmic signaling domain of McpB that, when mutated to serine, reduces taxis to alcohols. Molecular dynamics simulations suggest that the conversion of Ala⁴³¹ to serine increases coiled-coil packing within the signaling domain, thereby reducing the ability of ethanol to bind between the helices of the signaling domain. In the case of HemAT, the myoglobin-like sensing domain binds ethanol, likely between the helices encapsulating the heme group. Aside from being sensed by an unconventional mechanism, ethanol also differs from many other chemoattractants because it is not metabolized by B. subtilis and is toxic. We propose that B. subtilis uses ethanol and other short-chain alcohols to locate prey, namely, alcohol-producing microorganisms.

Hybrid Two-Component Sensors for Identification of Bacterial Chemoreceptor Function

Soil bacteria adapt to diverse and rapidly changing environmental conditions by sensing and responding to environmental cues using a variety of sensory systems. Two-component systems are a widespread type of signal transduction system present in all three domains of life and typically are comprised of a sensor kinase and a response regulator. Many two-component systems function by regulating gene expression in response to environmental stimuli. The bacterial chemotaxis system is a modified two-component system with additional protein components and a response that, rather than regulating gene expression, involves behavioral adaptation and results in net movement toward or away from a chemical stimulus. Soil bacteria generally have 20 to 40 or more chemoreceptors encoded in their genomes. To simplify the identification of chemoeffectors (ligands) sensed by bacterial chemoreceptors, we constructed hybrid sensor proteins by fusing the sensor domains of Pseudomonas putida chemoreceptors to the signaling domains of the Escherichia coli NarX/NarQ nitrate sensors. Responses to potential attractants were monitored by β-galactosidase assays using an E. coli reporter strain in which the nitrate-responsive narG promoter was fused to lacZ Hybrid receptors constructed from PcaY, McfR, and NahY, which are chemoreceptors for aromatic acids, tricarboxylic acid cycle intermediates, and naphthalene, respectively, were sensitive and specific for detecting known attractants, and the β-galactosidase activities measured in E. coli correlated well with results of chemotaxis assays in the native P. putida strain. In addition, a screen of the hybrid receptors successfully identified new ligands for chemoreceptor proteins and resulted in the identification of six receptors that detect propionate.IMPORTANCE Relatively few of the thousands of chemoreceptors encoded in bacterial genomes have been functionally characterized. More importantly, although methyl-accepting chemotaxis proteins, the major type of chemoreceptors present in bacteria, are easily identified bioinformatically, it is not currently possible to predict what chemicals will bind to a particular chemoreceptor. Chemotaxis is known to play roles in biodegradation as well as in host-pathogen and host-symbiont interactions, but many studies are currently limited by the inability to identify relevant chemoreceptor ligands. The use of hybrid receptors and this simple E. coli reporter system allowed rapid and sensitive screening for potential chemoeffectors. The fusion site chosen for this study resulted in a high percentage of functional hybrids, indicating that it could be used to broadly test chemoreceptor responses from phylogenetically diverse samples. Considering the wide range of chemical attractants detected by soil bacteria, hybrid receptors may also be useful as sensitive biosensors.

Engineering Hybrid Chemotaxis Receptors in Bacteria

Most bacteria use transmembrane sensors to detect a wide range of environmental stimuli. A large class of such sensors are the chemotaxis receptors used by motile bacteria to follow environmental chemical gradients. In Escherichia coli, chemotaxis receptors are known to mediate highly sensitive responses to ligands, making them potentially useful for biosensory applications. However, with only four ligand-binding chemotaxis receptors, the natural ligand spectrum of E. coli is limited. The design of novel chemoreceptors to extend the sensing capabilities of E. coli is therefore a critical aspect of chemotaxis-based biosensor development. One path for novel sensor design is to harvest the large natural diversity of chemosensory functions found in bacteria by creating hybrids that have the signaling domain from E. coli chemotaxis receptors and sensory domains from other species. In this work, we demonstrate that the E. coli receptor Tar can be successfully combined with most typical sensory domains found in chemotaxis receptors and in evolutionary-related two-component histidine kinases. We show that such functional hybrids can be generated using several different fusion points. Our work further illustrates how hybrid receptors could be used to quantitatively characterize ligand specificity of chemotaxis receptors and histidine kinases using standardized assays in E. coli.

In silico and proteomic analysis of protein methyltransferase CheR from Bacillus subtilis

Protein methyltransferase (CheR) catalyzes the methylation of the cytosolic domain of the membrane bound chemotaxis receptors, and plays a pivotal role in the chemotactic signal transduction pathway in bacteria. Crystal structure of CheR is available only from the gram-negative bacterium Salmonella typhimurium (StCheR), which contain a catalytic C-terminal domain, encompassing a β-subdomain, connected via a linker to the N-terminal domain. The structural-functional similitude between CheR of the gram-negative and the gram-positive bacteria remains obscure. We investigated CheR, from a gram-positive bacterium, Bacillus subtilis (BsCheR), and have identified the functional roles of its N-terminal domain, by using the in silico molecular modeling and docking approach along with mass spectrophotometry and sequence analysis. The structural studies established that the N-terminal domain directly bound to S-Adenosyl-l-homocysteine (SAH). Structural and sequence analyses revealed that the α2 helix of the N-terminal domain was involved in the recognition of the methylation site of the chemotactic receptor. Additionally, immunoblot analysis showed that the purified BsCheR was phosphorylated. Further, mass spectrometry studies detected the phosphorylation at Thr3 position in the N-terminal domain of BsCheR. Phosphorylation of BsCheR suggested a regulatory role of the N-terminal domain, analogous to its antagonistic enzyme, the chemotaxis-specific methylesterase (CheB).

Bacterial chemoreceptors and chemoeffectors

Bacteria use chemotaxis signaling pathways to sense environmental changes. Escherichia coli chemotaxis system represents an ideal model that illustrates fundamental principles of biological signaling processes. Chemoreceptors are crucial signaling proteins that mediate taxis toward a wide range of chemoeffectors. Recently, in deep study of the biochemical and structural features of chemoreceptors, the organization of higher-order clusters in native cells, and the signal transduction mechanisms related to the on-off signal output provides us with general insights to understand how chemotaxis performs high sensitivity, precise adaptation, signal amplification, and wide dynamic range. Along with the increasing knowledge, bacterial chemoreceptors can be engineered to sense novel chemoeffectors, which has extensive applications in therapeutics and industry. Here we mainly review recent advances in the E. coli chemotaxis system involving structure and organization of chemoreceptors, discovery, design, and characterization of chemoeffectors, and signal recognition and transduction mechanisms. Possible strategies for changing the specificity of bacterial chemoreceptors to sense novel chemoeffectors are also discussed.

Residues at the cytoplasmic end of transmembrane helix 2 determine the signal output of the TarEc chemoreceptor

Baseline signal output and communication between the periplasmic and cytoplasmic domains of the Escherichia coli aspartate chemoreceptor Tar(Ec) are both strongly influenced by residues at the C-terminus of transmembrane helix 2 (TM2). In particular, the cytoplasmic aromatic anchor, composed of residues Trp-209 and Tyr-210 in wild-type Tar(Ec), is important for determining the CheA kinase-stimulating activity of the receptor and its ability to respond to chemoeffector-induced stimuli. Here, we have studied the effect on Tar(Ec) function of the six-residue sequence at positions 207-212. Moving various combinations of aromatic residues among these positions generates substantial changes in receptor activity. Trp has the largest effect on function, both in maintaining normal activity and in altering activity when it is moved. Tyr has a weaker effect, and Phe has the weakest; however, all three aromatic residues can alter signal output when they are placed in novel positions. We also find that Gly-211 plays an important role in receptor function, perhaps because of the flexibility it introduces into the TM2-HAMP domain connector. The conservation of this Gly residue in the high-abundance chemoreceptors of E. coli and Salmonella enterica suggests that it may be important for the nuanced, bidirectional transmembrane signaling that occurs in these proteins.

Salmonella chemoreceptors McpB and McpC mediate a repellent response to L-cystine: a potential mechanism to avoid oxidative conditions

Chemoreceptors McpB and McpC in Salmonella enterica have been reported to promote chemotaxis in LB motility-plate assays. Of the chemicals tested as potential effectors of these receptors, the only response was towards L-cysteine and its oxidized form, L-cystine. Although enhanced radial migration in plates suggested positive chemotaxis to both amino acids, capillary assays failed to show an attractant response to either, in cells expressing only these two chemoreceptors. In vivo fluorescence resonance energy transfer (FRET) measurements of kinase activity revealed that in wild-type bacteria, cysteine and cystine are chemoeffectors of opposing sign, the reduced form being a chemoattractant and the oxidized form a repellent. The attractant response to cysteine was mediated primarily by Tsr, as reported earlier for Escherichia coli. The repellent response to cystine was mediated by McpB/C. Adaptive recovery upon cystine exposure required the methyl-transferase/-esterase pair, CheR/CheB, but restoration of kinase activity was never complete (i.e. imperfect adaptation). We provide a plausible explanation for the attractant-like responses to both cystine and cysteine in motility plates, and speculate that the opposing signs of response to this redox pair might afford Salmonella a mechanism to gauge and avoid oxidative environments.

The residue composition of the aromatic anchor of the second transmembrane helix determines the signaling properties of the aspartate/maltose chemoreceptor Tar of Escherichia coli

Repositioning of the tandem aromatic residues (Trp-209 and Tyr-210) at the cytoplasmic end of the second transmembrane helix (TM2) modulates the signal output of the aspartate/maltose chemoreceptor of Escherichia coli (Tar(Ec)). Here, we directly assessed the effect of the residue composition of the aromatic anchor by studying the function of a library of Tar(Ec) variants that possess all possible combinations of Ala, Phe, Tyr, and Trp at positions 209 and 210. We identified three important properties of the aromatic anchor. First, a Trp residue at position 209 was required to maintain clockwise (CW) signal output in the absence of adaptive methylation, but adaptive methylation restored the ability of all of the mutant receptors to generate CW rotation. Second, when the aromatic anchor was replaced with tandem Ala residues, signaling was less compromised than when an Ala residue occupied position 209 and an aromatic residue occupied position 210. Finally, when Trp was present at position 209, the identity of the residue at position 210 had little effect on baseline signal output or aspartate chemotaxis, although maltose taxis was significantly affected by some substitutions at position 210. All of the mutant receptors we constructed supported some level of aspartate and maltose taxis in semisolid agar swim plates, but those without Trp at position 209 were overmethylated in their baseline signaling state. These results show the importance of the cytoplasmic aromatic anchor of TM2 in maintaining the baseline Tar(Ec) signal output and responsiveness to attractant signaling.

Direct evidence that the carboxyl-terminal sequence of a bacterial chemoreceptor is an unstructured linker and enzyme tether

Sensory adaptation in bacterial chemotaxis involves reversible methylation of specific glutamyl residues on chemoreceptors. The reactions are catalyzed by a dedicated methyltransferase and dedicated methylesterase. In Escherichia coli and related organisms, control of these enzymes includes an evolutionarily recent addition of interaction with a pentapeptide activator located at the carboxyl terminus of the receptor polypeptide chain. Effective enzyme activation requires not only the pentapeptide but also a segment of the receptor polypeptide chain between that sequence and the coiled-coil body of the chemoreceptor. This segment has features consistent with a role as a flexible and presumably unstructured linker and enzyme tether, but there has been no direct information about its structure. We used site-directed spin labeling and electron paramagnetic resonance spectroscopy to characterize structural features of the carboxyl-terminal 40 residues of E. coli chemoreceptor Tar. Beginning ∼ 35 residues from the carboxyl terminus and continuing to the end of the protein, spectra of spin-labeled Tar embedded in native membranes or in reconstituted proteoliposomes, exhibited mobilities characteristic of unstructured, disordered segments. Binding of methyltransferase substantially reduced mobility for positions in or near the pentapeptide but mobility for the linker sequence remained high, being only modestly reduced in a gradient of decreasing effects for 10-15 residues, a pattern consistent with the linker providing a flexible arm that would allow enzyme diffusion within defined limits. Thus, our data identify that the carboxyl-terminal linker between the receptor body and the pentapeptide is an unstructured, disordered segment that can serve as a flexible arm and enzyme tether.

The mechanisms of HAMP-mediated signaling in transmembrane receptors

HAMP domains mediate signal transduction in over 7500 enzyme-coupled receptors represented in all kingdoms of life. The HAMP domain of the putative archaeal receptor Af1503 has a parallel, dimeric, four-helical coiled coil structure, but with unusual core packing, related to canonical packing by concerted axial rotation of the helices. This has led to the gearbox model for signal transduction, whereby the alternate packing modes correspond to signaling states. Here we present structures of a series of Af1503 HAMP variants. We show that substitution of a conserved small side chain within the domain core (A291) for larger residues induces a gradual transition in packing mode, involving both changes in helix rotation and bundle shape, which are most prominent at the C-terminal, output end of the domain. These are correlated with activity and ligand response in vitro and in vivo by incorporating Af1503 HAMP into mycobacterial adenylyl cyclase assay systems.

Oligopeptide permease A5 modulates vertebrate host-specific adaptation of Borrelia burgdorferi

Borrelia burgdorferi, the agent of Lyme disease, undergoes rapid adaptive gene expression in response to signals unique to its arthropod vector or vertebrate hosts. Among the upregulated genes under vertebrate host conditions is one of the five annotated homologs of oligopeptide permease A (OppA5, BBA34). A mutant lacking oppA5 was constructed in an lp25-deficient isolate of B. burgdorferi strain B31, and the minimal regions of infectivity were restored via a shuttle vector pBBE22 with or without an intact copy of bba34. Immunoblot analysis of the bba34 mutant revealed a reduction in the levels of RpoS, BosR, and CsrA(Bb) with a concomitant reduction in the levels of OspC, DbpA, BBK32, and BBA64. There were no changes in the levels of OspA, NapA, P66, and three other OppA orthologs. Quantitative transcriptional analysis correlated with the changes in the protein levels. However, the bba34 mutant displayed comparable infectivities in the C3H/HeN mice and the wild-type strain, despite the reduction in several pathogenesis-related proteins. Supplementation of the growth medium with increased levels of select components, notably sodium acetate and sodium bicarbonate, restored the levels of several proteins in the bba34 mutant to wild-type levels. We speculate that the transport of acetate appears to contribute to the accumulation of key metabolites, like acetyl phosphate, that facilitate the adaptation of B. burgdorferi to the vertebrate host by the activation of the Rrp2-RpoN-RpoS pathway. These studies underscore the importance of solute transport to host-specific adaptation of B. burgdorferi.

Mutational analysis of the transmembrane helix 2-HAMP domain connection in the Escherichia coli aspartate chemoreceptor tar

Transmembrane helix 2 (TM2) of the Tar chemoreceptor undergoes an inward piston-like displacement of 1 to 3 Å upon binding aspartate. This signal is transmitted to the kinase-control module via the HAMP domain. Within Tar, the HAMP domain forms a parallel four-helix bundle consisting of a dimer of two amphipathic helices connected by a flexible linker. In the nuclear magnetic resonance structure of an archaeal HAMP domain, residues corresponding to the MLLT sequence between Arg-214 at the end of TM2 and Pro-219 of Tar are an N-terminal helical extension of AS1. We modified this region to test whether it behaves as a continuous helical connection between TM2 and HAMP. First, one to four Gly residues were inserted between Thr-218 and Pro-219. Second, the MLLT sequence was replaced with one to nine Gly residues. Third, the sequence was shortened or extended with residues compatible with helix formation. Cells expressing receptors in which the MLLT sequence was shortened to MLL or in which the MLLT sequence was replaced by four Gly residues performed good aspartate chemotaxis. Other mutant receptors supported diminished aspartate taxis. Most mutant receptors had biased signal outputs and/or abnormal patterns of adaptive methylation. We interpret these results to indicate that a strong, permanent helical connection between TM2 and the HAMP domain is not necessary for normal transmembrane signaling.

Molecular modeling of flexible arm-mediated interactions between bacterial chemoreceptors and their modification enzyme

Sensory adaptation in bacterial chemotaxis is mediated by methylation and demethylation of specific glutamyl residues in the cytoplasmic domain of chemoreceptors. Methylation is catalyzed by methyltransferase CheR. In E. coli and related organisms, methylation sufficiently rapid to be physiologically effective requires a carboxyl terminal pentapeptide sequence on the receptor being modified or, via adaptational assistance, on a neighboring homodimer in a receptor cluster. Pentapeptide-enhanced methylation is thought to be mediated by a approximately 30 residue, potentially disordered sequence that serves as a flexible arm connecting the receptor body and pentapeptide-bound methyltransferase, thus allowing diffusionally restricted enzyme to reach methyl-accepting sites. However, it was not known how many or which sites on the same or neighboring receptors were accessible to the tethered enzyme. We investigated using molecular modeling and found that, in a hexagonal array of trimers of receptor dimers, CheR tethered to a dimer of chemoreceptor Tar by its native 30-residue flexible-arm sequence could reach all methyl-accepting sites on the dimer to which it was tethered plus 48 methyl-accepting sites distributed among nine neighboring dimers, equivalent to the total sites carried by six receptors. This modeling-determined methylation neighborhood of one enzyme-binding dimer and six neighbors corresponds precisely with the experimentally identified neighborhood of seven. Thus, the experimentally observed adaptational assistance can occur by docking of pentapeptide-bound, diffusionally restricted enzyme to methyl-accepting sites on neighboring receptors. Our analysis introduces the notion that physiologically relevant adaptational assistance could occur even if only a subset of sites on a particular receptor are within reach.

The region preceding the C-terminal NWETF pentapeptide modulates baseline activity and aspartate inhibition of Escherichia coli Tar

The Tar chemoreceptor-CheA-CheW ternary complex of Escherichia coli is a transmembrane allosteric enzyme in which binding of ligands to the periplasmic domain modulates the activity of CheA kinase. Kinase activity is also affected by reversible methylation of four glutamyl residues in the cytoplasmic domain of the receptor. E. coli Tar contains 553 residues. Residues 549-553 comprise the NWETF pentapeptide that binds the CheR methyltransferase and CheB methylesterase. The crystal structure of the similar Tsr chemoreceptor predicts that residues 263-289 and 490-515 of Tar form the most membrane-proximal portion of the extended CD1-CD2 four-helix bundle of the cytoplasmic domain. The last methylation site, Glu-491, is in the C19 heptad, and the N22-19 and C22-19 heptads are present in all classes of bacterial transmembrane chemoreceptors. Residues 516-548 probably serve as a flexible tether for the NWETF pentapeptide. Here, we present a mutational analysis of residues 505-548. The more of this region that is deleted, the less sensitive Tar is to inhibition by aspartate. Tar deleted from residue 505 through the NWETF sequence stimulates CheA in vitro but is not inhibited by aspartate. Thus, interaction of the last two heptads (C21 and C22) of CD2 with the first two heptads (N22 and N21) of CD1 must be important for transmitting an inhibitory signal from the HAMP domain to the four-helix bundle. The R514A, K523A, R529A, R540A, and R542A substitutions, singly or together, increase the level of activation of CheA in vitro, whereas the R505A substitution decreases the level of CheA stimulation by 40% and lowers the aspartate K(i) 7-fold. The R505E substitution completely abolishes stimulation of CheA in vitro. Glu-505 may interact electrostatically with Asp-273 to destabilize the "on" signaling state by loosening the four-helix bundle.

Chemotaxis of Escherichia coli to pyrimidines: a new role for the signal transducer tap

Escherichia coli exhibits chemotactic responses to sugars, amino acids, and dipeptides, and the responses are mediated by methyl-accepting chemotaxis proteins (MCPs). Using capillary assays, we demonstrated that Escherichia coli RP437 is attracted to the pyrimidines thymine and uracil and the response was constitutively expressed under all tested growth conditions. All MCP mutants lacking the MCP Tap protein showed no response to pyrimidines, suggesting that Tap, which is known to mediate dipeptide chemotaxis, is required for pyrimidine chemotaxis. In order to confirm the role of Tap in pyrimidine chemotaxis, we constructed chimeric chemoreceptors (Tapsr and Tsrap), in which the periplasmic and cytoplasmic domains of Tap and Tsr were switched. When Tapsr and Tsrap were individually expressed in an E. coli strain lacking all four native MCPs, Tapsr mediated chemotaxis toward pyrimidines and dipeptides, but Tsrap did not complement the chemotaxis defect. The addition of the C-terminal 19 amino acids from Tsr to the C terminus of Tsrap resulted in a functional chemoreceptor that mediated chemotaxis to serine but not pyrimidines or dipeptides. These results indicate that the periplasmic domain of Tap is responsible for detecting pyrimidines and the Tsr signaling domain confers on Tapsr the ability to mediate efficient chemotaxis. A mutant lacking dipeptide binding protein (DBP) was wild type for pyrimidine taxis, indicating that DBP, which is the primary chemoreceptor for dipeptides, is not responsible for detecting pyrimidines. It is not yet known whether Tap detects pyrimidines directly or via an additional chemoreceptor protein.

Structural and Functional Studies of the HAMP Domain of EnvZ, an Osmosensing Transmembrane Histidine Kinase in Escherichia coli

The HAMP domain plays an essential role in signal transduction not only in histidine kinase but also in a number of other signal-transducing receptor proteins. Here we expressed the EnvZ HAMP domain (Arg(180)-Thr(235)) with the R218K mutation (termed L(RK)) or with L(RK) connected with domain A (Arg(180)-Arg(289)) (termed LA(RK)) of EnvZ, an osmosensing transmembrane histidine kinase in Escherichia coli, by fusing it with protein S. The L(RK) and LA(RK) proteins were purified after removing protein S. The CD analysis of the isolated L protein revealed that it consists of a random structure or is unstructured. This suggests that the EnvZ HAMP domain by itself is unable to form a stable structure and that this structural fragility may be important for its role in signal transduction. Interestingly the substitution of Ala(193) in the EnvZ HAMP domain with valine or leucine in Tez1A1, a chimeric protein of Tar and EnvZ, caused a constitutive OmpC phenotype. The CD analysis of LA(RK)(A193L) revealed that this mutated HAMP domain possesses considerable secondary structures and that the thermostability of this entire LA(RK)(A193L) became substantially lower than that of LA(RK) or just domain A, indicating that the structure of the HAMP domain with the A193L mutation affects the stability of downstream domain A. This results in cooperative thermodenaturation of domain A with the mutated HAMP domain. These results are discussed in light of the recently solved NMR structure of the HAMP domain from a thermophilic bacterium (Hulko, M., Berndt, F., Gruber, M., Linder, J. U., Truffault, V., Schultz, A., Martin, J., Schultz, J. E., Lupas, A. N., and Coles, M. (2006) Cell 126, 929-940).

Tuning a bacterial chemoreceptor with protein-membrane interactions

Chemoreceptors in Escherichia coli are homodimeric transmembrane proteins that convert environmental stimuli into intracellular signals controlling flagellar motion. Chemoeffectors bind to the extracellular (periplasmic) domain of the receptors, whereas their cytoplasmic domain mediates signaling and adaptation. The second transmembrane helix (TM2) connects these two domains. TM2 contains an aliphatic core flanked by amphipathic aromatic residues that have specific affinity for polar-hydrophobic membrane interfaces. We previously showed that Trp-209, near the cytoplasmic end of TM2, helps maintain the normal baseline-signaling state of the aspartate chemoreceptor (Tar) and that Tyr-210 plays an auxiliary role in this control. We have now repositioned the Trp-209/Tyr-210 pair in single-residue increments about the cytoplasmic polar-hydrophobic interface. Changes from WY-2 to WY+1 modulate the baseline-signaling state of the receptor in predictable and incremental steps that can be compensated by adaptive methylation/demethylation. Greater displacements, as in WY-3, WY+2, and WY+3, bias the receptor to the off kinase-inhibiting state or the on kinase-stimulating state, respectively, to a degree that cannot be fully compensated by the adaptation system. Aromatic residues analogous to Trp-209/Tyr-210 are present in other chemoreceptors and many transmembrane sensor kinases, where they may serve a similar function.

The Escherichia coli MotAB Proton Channel Unplugged

The MotA and MotB proteins of Escherichia coli serve two functions. The MotA4MotB2 complex attaches to the cell wall via MotB to form the stator of the flagellar motor. The complex also couples the flow of hydrogen ions across the cell membrane to movement of the rotor. The TM3 and TM4 transmembrane helices of MotA and the single TM of MotB comprise the proton channel, which is inactive until the complex assembles into a motor. Here, we identify a segment of the MotB protein that acts as a plug to prevent premature proton flow. The plug is in the periplasm just C-terminal to the MotB TM. It consists of an amphipathic alpha helix flanked by Pro52 and Pro65. When MotA is over-expressed with MotB deleted for residues 51-70, a massive influx of protons acidifies the cytoplasm without significantly depleting the proton motive force. Either that acidification or some sequela thereof, such as potassium or water efflux from the cells, inhibits growth. The Pro residues and Ile58, Tyr61, and Phe62 are essential for plug function. Cys-substituted MotB proteins form a disulfide bond between the two plugs that hold the channels open, and the plugs function intrans within the MotA4MotB2 complex. We present a model in which the MotA4MotB2 complex forms in the bulk membrane. Before association with a motor, we propose the plugs insert into the cell membrane parallel with its periplasmic face and interfere with channel formation. When a complex incorporates into a motor, the plugs leave the membrane and associate with each other via their hydrophobic faces to hold the proton channel open.

Mutationally altered signal output in the Nart (NarX-Tar) hybrid chemoreceptor

Signal-transducing proteins that span the cytoplasmic membrane transmit information about the environment to the interior of the cell. In bacteria, these signal transducers include sensor kinases, which typically control gene expression via response regulators, and methyl-accepting chemoreceptor proteins, which control flagellar rotation via the CheA kinase and CheY response regulator. We previously reported that a chimeric protein (Nart) that joins the ligand-binding, transmembrane, and linker regions of the NarX sensor kinase to the signaling and adaptation domains of the Tar chemoreceptor elicits a repellent response to nitrate and nitrite. As with NarX, nitrate evokes a stronger response than nitrite. Here we show that mutations targeting a highly conserved sequence (the P box) in the periplasmic domain alter chemoreception by Nart and signaling by NarX similarly. In particular, the G51R substitution converts Nart from a repellent receptor into an attractant receptor for nitrate. Our results underscore the conclusion that the fundamental mechanism of transmembrane signaling is conserved between homodimeric sensor kinases and chemoreceptors. They also highlight the plasticity of the coupling between ligand binding and signal output in these systems.

The carboxyl-terminal linker is important for chemoreceptor function

Sensory adaptation in bacterial chemotaxis is mediated by chemoreceptor methylation and demethylation. In Escherichia coli, methyltransferase CheR and methylesterase CheB bind both substrate sites and a carboxyl-terminal pentapeptide sequence carried by certain receptors. Pentapeptide binding enhances enzyme action, an enhancement required for effective adaptation and chemotaxis. Pentapeptides are linked to the conserved body of chemoreceptors through a notably variable sequence of 30-35 residues. We created nested deletions from the distal end of this linker in chemoreceptor Tar. Chemotaxis was eliminated by deletion of 20-40 residues and reduced by shorter deletions. This did not reflect generalized disruption, because all but the most extremely truncated receptors activated kinase, were substrates for adaptational modification and performed transmembrane signalling. In contrast, linker truncations reduced rates of adaptational modification in parallel with chemotaxis. We concluded the linker is important for chemotaxis because of its role in adaptational modification. Effects of linker truncations on CheR binding to receptor-borne pentapeptide implied linker (i) makes pentapeptide available to modification enzymes by separation from the helical receptor body, and (ii) is a flexible arm allowing dual binding of enzyme to pentapeptide and modification site. The data suggest linker and the helix from which it emerges are structurally dynamic.

Cooperative signaling among bacterial chemoreceptors

Four chemoreceptors in Escherichia coli mediate responses to chemicals in the environment. The receptors self-associate and localize to the cell poles. This aggregation implies that interactions among receptors are important parameters of signal processing during chemotaxis. We examined this phenomenon using a receptor-coupled in vitro assay of CheA kinase activity. The ability of homogeneous populations of the serine receptor Tsr and the aspartate receptor Tar to stimulate CheA was directly proportional to the ratio of the receptor to total protein in cell membranes up to a fraction of 50%. Membranes containing mixed populations of Tar and Tsr supported an up to 4-fold greater stimulation of CheA than expected on the basis of the contributions of the individual receptors. Peak activity was seen at a Tar:Tsr ratio of 1:4. This synergy was observed only when the two proteins were expressed simultaneously, suggesting that, under our conditions, the fundamental "cooperative receptor unit" is relatively static, even in the absence of CheA and CheW. Finally, we observed that inhibition of receptor-stimulated CheA activity by serine or aspartate required significantly higher concentrations of ligand for membranes containing mixed Tsr and Tar populations than for membranes containing only Tsr (up to 10(2)-fold more serine) or Tar (up to 10(4)-fold more aspartate). Together with recent analyses of the interactions of Tsr and Tar in vivo, our results reveal the emergent properties of mixed receptor populations and emphasize their importance in the integrated signal processing that underlies bacterial chemotaxis.

Stabilization of polar localization of a chemoreceptor via its covalent modifications and its communication with a different chemoreceptor

In the chemotaxis of Escherichia coli, polar clustering of the chemoreceptors, the histidine kinase CheA, and the adaptor protein CheW is thought to be involved in signal amplification and adaptation. However, the mechanism that leads to the polar localization of the receptor is still largely unknown. In this study, we examined the effect of receptor covalent modification on the polar localization of the aspartate chemoreceptor Tar fused to green fluorescent protein (GFP). Amidation (and presumably methylation) of Tar-GFP enhanced its own polar localization, although the effect was small. The slight but significant effect of amidation on receptor localization was reinforced by the fact that localization of a noncatalytic mutant version of GFP-CheR that targets to the C-terminal pentapeptide sequence of Tar was similarly facilitated by receptor amidation. Polar localization of the demethylated version of Tar-GFP was also enhanced by increasing levels of the serine chemoreceptor Tsr. The effect of covalent modification on receptor localization by itself may be too small to account for chemotactic adaptation, but receptor modification is suggested to contribute to the molecular assembly of the chemoreceptor/histidine kinase array at a cell pole, presumably by stabilizing the receptor dimer-to-dimer interaction.

Clustering requires modified methyl-accepting sites in low-abundance but not high-abundance chemoreceptors of Escherichia coli

Chemotaxis signalling complexes of Escherichia coli, composed of chemoreceptors, CheA and CheW, form clusters located predominantly at cell poles. As the only kind of receptor in a cell, high-abundance receptors are polar and clustered whereas low-abundance chemoreceptors are polar but largely unclustered. We found that clustering was a function of the cytoplasmic, carboxyl-terminal domain and that effective clustering was conferred on low-abundance receptors by addition of the approximately 20-residue sequence from the carboxyl terminus of either high-abundance receptor. These sequences are different but share a carboxyl-terminal pentapeptide that enhances adaptational covalent modification and allows a physiological balance between modified and unmodified methyl-accepting sites, implying that receptor modification might influence clustering. Thus we investigated directly effects of modification state on chemoreceptor clustering. As the sole receptor type in a cell, low-abundance receptors were clustered only if modified, but high-abundance receptors were clustered independent of extent of modification. This difference could mean that the two receptor types are fundamentally different or that they are poised at different positions in the same conformational equilibrium. Notably, no receptor perturbation we tested altered a predominant location at cell poles, emphasizing a distinction between determinants of clustering and polar localization.

Adaptational assistance in clusters of bacterial chemoreceptors

Sensory adaptation of low-abundance chemoreceptors in Escherichia coli requires assistance from high-abundance receptors, because only high-abundance receptors carry the carboxyl-terminal pentapeptide sequence NWETF that enhances adaptational covalent modification. Using membrane vesicles containing both high-abundance receptor Tar and low-abundance receptor Trg, we observed effective assistance in vitro for all three adaptational modifications: methylation, demethylation and deamidation. These results demonstrated that adaptational assistance involves not only the previously documented assistance for methylation but also assistance for the two CheB-catalysed reactions. We determined rates of assisted methylation and demethylation at many ratios of assisting to assisted receptor. Analysis by a model of assistance indicated one Tar dimer could assist seven Trg dimers in methylation or five in demethylation, defining assistance neighbourhoods. These neighbourhoods were larger than a trimer of homodimers, required only receptors and were minimally affected by formation of signalling complexes. Time courses of assisted Trg methylation in membranes with low amounts of Tar showed that assisting receptors did not diffuse beyond initial neighbourhoods for at least two hours. Taken together, these observations indicate that chemoreceptors can form stable neighbourhoods larger than trimers in the absence of other chemotaxis proteins. Such interactions are likely to occur in natural receptor clusters in vivo.

Carboxyl-terminal extensions beyond the conserved pentapeptide reduce rates of chemoreceptor adaptational modification

Sensory adaptation in bacterial chemotaxis is mediated by covalent modification of chemoreceptors. Specific glutamyl residues are methylated and demethylated in reactions catalyzed by methyltransferase CheR and methylesterase CheB. In the well-characterized chemosensory systems of Escherichia coli and Salmonella spp., efficient modification by either enzyme is dependent on a conserved pentapeptide sequence, NWETF or NWESF, present at the extreme carboxyl terminus of high-abundance chemoreceptors. To what extent is position at the extreme carboxyl terminus important for pentapeptide-mediated enhancement of adaptational modification? Is this position equally important for enhancement of both enzyme activities? To address these questions, we created forms of high-abundance receptor Tsr or Tar carrying one, six, or eight additional amino acids extending beyond the pentapeptide at their carboxyl termini and assayed methylation, demethylation, deamidation, and ability to mediate chemotaxis. In vitro and in vivo, all three carboxyl-terminal extensions reduced pentapeptide-mediated enhancement of rates of adaptational modification. CheB-catalyzed reactions were more affected than CheR-catalyzed reactions. Effects were less severe for the complete sensory system in vivo than for the minimal system of receptor and modification enzymes in vitro. Notably, extended receptors mediated chemotaxis as efficiently as wild-type receptors, providing a striking example of robustness in chemotactic systems. This could reflect compensatory reductions of rates for both modification reactions, mitigation of effects of slower reactions by the intertwined circuitry of signaling and adaptation, or tolerance of a range of reactions rates for adaptational modification. No matter what the mechanism, the observations provide a challenging test for mathematical models of chemotaxis.

Collaborative signaling by bacterial chemoreceptors

Motile bacteria seek optimal living habitats by following gradients of attractant and repellent chemicals in their environment. The signaling machinery for these chemotactic behaviors, although assembled from just a few protein components, has extraordinary information-processing capabilities. Escherichia coli, the best-studied model, employs a networked cluster of transmembrane receptors to detect minute chemical stimuli, to integrate multiple and conflicting inputs, and to generate an amplified output signal that controls the cell's flagellar motors. Signal gain arises through cooperative action of chemoreceptors of different types. The signaling-teams within a receptor cluster may be built from trimers of receptor dimers that communicate through shared connections to their partner signaling proteins.

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.

Tryptophan residues flanking the second transmembrane helix (TM2) set the signaling state of the Tar chemoreceptor

The chemoreceptors of Escherichia coli are homodimeric membrane proteins that cluster in patches near the cell poles. They convert environmental stimuli into intracellular signals that control flagellar rotation. The functional domains of a receptor are physically separated by the cell membrane. Chemoeffectors bind to the extracellular (periplasmic) domain, and the cytoplasmic domain mediates signaling and adaptation. These two domains communicate through the second transmembrane helix (TM2) that connects them. In the high-abundance receptors Tar and Tsr, TM2 is flanked by tryptophan residues, which should localize preferentially to the interfacial zone between the polar and hydrophobic layers of the phospholipid bilayer. To investigate the functional significance of the Trp residues that flank TM2 of Tar, we used site-directed mutagenesis to generate the W192A and W209A substitutions. The W192A protein retains full activity in vivo and in vitro, but it increases the K(i) for aspartate in the in vitro assay 3-fold. The W209A replacement eliminates receptor-mediated stimulation of CheA in vitro, and it leads to an increased level of adaptive methylation in vivo. This phenotype in some respects mimics the changes seen upon binding aspartate. Since the W209A substitution may cause the C-terminus of TM2 to protrude farther into the cytoplasm, these results reinforce the hypothesis that aspartate binding causes a similar displacement. Moving Trp to each position from residue 206 to residue 212 generated a wide variety of Tar signaling states that are generally consistent with the predictions of the piston model of transmembrane signaling. None of these receptors was completely locked in one signaling mode, although most showed pronounced signaling biases. Our findings suggest that the Trp residues flanking TM2, especially Trp-209, are important in setting the baseline activity and ligand sensitivity of the Tar receptor. We also conclude that the Tyr-210 residue plays at least an auxiliary role in this control.

Genetic analysis of the HAMP domain of the Aer aerotaxis sensor localizes flavin adenine dinucleotide-binding determinants to the AS-2 helix

HAMP domains are signal transduction domains typically located between the membrane anchor and cytoplasmic signaling domain of the proteins in which they occur. The prototypical structure consists of two helical amphipathic sequences (AS-1 and AS-2) connected by a region of undetermined structure. The Escherichia coli aerotaxis receptor, Aer, has a HAMP domain and a PAS domain with a flavin adenine dinucleotide (FAD) cofactor that senses the intracellular energy level. Previous studies reported mutations in the HAMP domain that abolished FAD binding to the PAS domain. In this study, using random and site-directed mutagenesis, we identified the distal helix, AS-2, as the component of the HAMP domain that stabilizes FAD binding. AS-2 in Aer is not amphipathic and is predicted to be buried. Mutations in the sequence coding for the contiguous proximal signaling domain altered signaling by Aer but did not affect FAD binding. The V264M residue replacement in this region resulted in an inverted response in which E. coli cells expressing the mutant Aer protein were repelled by oxygen. Bioinformatics analysis of aligned HAMP domains indicated that the proximal signaling domain is conserved in other HAMP domains that are not involved in chemotaxis or aerotaxis. Only one null mutation was found in the coding sequence for the HAMP AS-1 and connector regions, suggesting that these are not active signal transduction sites. We consider a model in which the signal from FAD is transmitted across a PAS-HAMP interface to AS-2 or the proximal signaling domain.

Signal transduction in bacterial chemotaxis

Motile bacteria respond to environmental cues to move to more favorable locations. The components of the chemotaxis signal transduction systems that mediate these responses are highly conserved among prokaryotes including both eubacterial and archael species. The best-studied system is that found in Escherichia coli. Attractant and repellant chemicals are sensed through their interactions with transmembrane chemoreceptor proteins that are localized in multimeric assemblies at one or both cell poles together with a histidine protein kinase, CheA, an SH3-like adaptor protein, CheW, and a phosphoprotein phosphatase, CheZ. These multimeric protein assemblies act to control the level of phosphorylation of a response regulator, CheY, which dictates flagellar motion. Bacterial chemotaxis is one of the most-understood signal transduction systems, and many biochemical and structural details of this system have been elucidated. This is an exciting field of study because the depth of knowledge now allows the detailed molecular mechanisms of transmembrane signaling and signal processing to be investigated.

Ligand-specific activation of Escherichia coli chemoreceptor transmethylation

Adaptation in the chemosensory pathways of bacteria like Escherichia coli is mediated by the enzyme-catalyzed methylation (and demethylation) of glutamate residues in the signaling domains of methyl-accepting chemotaxis proteins (MCPs). MCPs can be methylated in trans, where the methyltransferase (CheR) molecule catalyzing methyl group transfer is tethered to the C terminus of a neighboring receptor. Here, it was shown that E. coli cells exhibited adaptation to attractant stimuli mediated through either engineered or naturally occurring MCPs that were unable to tether CheR as long as another MCP capable of tethering CheR was also present, e.g., either the full-length aspartate or serine receptor (Tar or Tsr). Methylation of isolated membrane samples in which engineered tethering and substrate receptors were coexpressed demonstrated that the truncated substrate receptors (trTsr) were efficiently methylated in the presence of tethering receptors (Tar with methylation sites blocked) relative to samples in which none of the MCPs had tethering sites. The effects of ligand binding on methylation were investigated, and an increase in rate was produced only with serine (the ligand specific for the substrate receptor trTsr); no significant change in rate was produced by aspartate (the ligand specific for the tethering receptor Tar). Although the overall efficiency of methylation was lower, receptor-specific effects were also observed in trTar- and trTsr-containing samples, where neither Tar nor Tsr possessed the CheR binding site at the C terminus. Altogether, the results are consistent with a ligand-induced conformational change that is limited to the methylated receptor dimer and does not spread to adjacent receptor dimers.

Analysis of chimeric chemoreceptors in Bacillus subtilis reveals a role for CheD in the function of the McpC HAMP domain

Motile prokaryotes use a sensory circuit for control of the motility apparatus in which ligand-responsive chemoreceptors regulate phosphoryl flux through a modified two-component signal transduction system. The chemoreceptors exhibit a modular architecture, comprising an N-terminal sensory module, a C-terminal output module, and a HAMP domain that connects the N- and C-terminal modules and transmits sensory information between them via an unknown mechanism. The sensory circuits mediated by two chemoreceptors of Bacillus subtilis have been studied in detail. McpB is known to regulate chemotaxis towards the attractant asparagine in a CheD-independent manner, whereas McpC requires CheD to regulate chemotaxis towards the attractant proline. Although CheD is a phylogenetically widespread chemotaxis protein, there exists only a limited understanding of its function. We have constructed chimeras between McpB and McpC to probe the role of CheD in facilitating sensory transduction by McpC. We found that McpC can be converted to a CheD-independent receptor by the replacement of one-half of its HAMP domain with the corresponding sequence from McpB, suggesting that McpC HAMP domain function is complex and may require intermolecular interactions with the CheD protein. When considered in combination with the previous observation that CheD catalyzes covalent modification of the C-terminal modules of B. subtilis receptors, these results suggest that CheD may interact with chemoreceptors at multiple, functionally distinct sites.

The HAMP linker in histidine kinase dimeric receptors is critical for symmetric transmembrane signal transduction

The HAMP linker, a common structural element between a sensor and a transmitter module in various sensor proteins, plays an essential role in signal transduction. Here, by in vivo complementation experiments with Tar-EnvZ hybrid receptor mutants in which the HAMP linker forms a heterodimer with Tar and EnvZ-type subunits, we found that mutations at one linker only affect the function of EnvZ in the same subunit. However, the same mutations affect the EnvZ function of both subunits when only a Tar or EnvZ-type HAMP linker is used. These results suggest that intersubunit interactions in the HAMP linker normally mediate signal transduction through both subunits in a sensor dimer, whereas the signal is asymmetrically transduced through the linker in a heterodimer. This is the first demonstration that two HAMP linkers in a sensor dimer are functionally coupled for normal signal transduction; however, this functional coupling can be reduced when the HAMP linkers lose their symmetric nature.

Methylation-independent aerotaxis mediated by the Escherichia coli Aer protein

Aer is a membrane-associated protein that mediates aerotactic responses in Escherichia coli. Its C-terminal half closely resembles the signaling domains of methyl-accepting chemotaxis proteins (MCPs), which undergo reversible methylation at specific glutamic acid residues to adapt their signaling outputs to homogeneous chemical environments. MCP-mediated behaviors are dependent on two specific enzymes, CheR (methyltransferase) and CheB (methylesterase). The Aer signaling domain contains unorthodox methylation sites that do not conform to the consensus motif for CheR or CheB substrates, suggesting that Aer, unlike conventional MCPs, might be a methylation-independent transducer. Several lines of evidence supported this possibility. (i) The Aer protein was not detectably modified by either CheR or CheB. (ii) Amino acid replacements at the putative Aer methylation sites generally had no deleterious effect on Aer function. (iii) Aer promoted aerotactic migrations on semisolid media in strains that lacked all four of the E. coli MCPs. CheR and CheB function had no influence on the rate of aerotactic movements in those strains. Thus, Aer senses and signals efficiently in the absence of deamidation or methylation, methylation changes, methylation enzymes, and methyl-accepting chemotaxis proteins. We also found that chimeric transducers containing the PAS-HAMP sensing domain of Aer joined to the signaling domain and methylation sites of Tar, an orthodox MCP, exhibited both methylation-dependent and methylation-independent aerotactic behavior. The hybrid Aear transducers demonstrate that methylation independence does not emanate from the Aer signaling domain but rather may be due to transience of the cellular redox changes that are thought to trigger Aer-mediated behavioral responses.

Cellular stoichiometry of the components of the chemotaxis signaling complex

The chemotactic sensory system of Escherichia coli comprises membrane-embedded chemoreceptors and six soluble chemotaxis (Che) proteins. These components form signaling complexes that mediate sensory excitation and adaptation. Previous determinations of cellular content of individual components provided differing and apparently conflicting values. We used quantitative immunoblotting to perform comprehensive determinations of cellular amounts of all components in two E. coli strains considered wild type for chemotaxis, grown in rich and minimal media. Cellular amounts varied up to 10-fold, but ratios between proteins varied no more than 30%. Thus, cellular stoichiometries were almost constant as amounts varied substantially. Calculations using those cellular stoichiometries and values for in vivo proportions of core components in complexes yielded an in vivo stoichiometry for core complexes of 3.4 receptor dimers and 1.6 CheW monomers for each CheA dimer and 2.4 CheY, 0.5 CheZ dimers, 0.08 CheB, and 0.05 CheR per complex. The values suggest a core unit of a trimer of chemoreceptor dimers, a dimer (or two monomers) of kinase CheA, and two CheW. These components may interact in extended arrays and, thus, stoichiometries could be nonintegral. In any case, cellular stoichiometries indicate that CheY could be bound to all signaling complexes and this binding would recruit essentially the entire cellular complement of unphosphorylated CheY, and also that phosphatase CheZ, methylesterase CheB, and methyltransferase CheR would be present at 1 per 2, per 14, and per 20 core complexes, respectively. These characteristic ratios will be important in quantitative treatments of chemotaxis, both experimental and theoretical.

Probing conservation of HAMP linker structure and signal transduction mechanism through analysis of hybrid sensor kinases

The HAMP linker, a predicted structural element observed in many sensor kinases and methyl-accepting chemotaxis proteins, transmits signals between sensory input modules and output modules. HAMP linkers are located immediately inside the cytoplasmic membrane and are predicted to form two short amphipathic alpha-helices (AS-1 and AS-2) joined by an unstructured connector. HAMP linkers are found in the Escherichia coli nitrate- and nitrite-responsive sensor kinases NarX and NarQ (which respond to ligand by increasing kinase activity) and the sensor kinase CpxA (which responds to ligand by decreasing kinase activity). We constructed a series of hybrids with fusion points throughout the HAMP linker, in which the sensory modules of NarX or NarQ are fused to the transmitter modules of NarX, NarQ, or CpxA. A hybrid of the NarX sensor module and the CpxA HAMP linker and transmitter module (NarX-CpxA-1) responded to nitrate by decreasing kinase activity, whereas a hybrid in which the HAMP linker of NarX was replaced by that of CpxA (NarX-CpxA-NarX-1) responded to nitrate by increasing kinase activity. However, sequence variations between HAMP linkers do not allow free exchange of HAMP linkers or their components. Certain deletions in the NarX HAMP linker resulted in characteristic abnormal responses to ligand; similar deletions in the NarQ and NarX-CpxA-1 HAMP linkers resulted in responses to ligand generally similar to those seen in NarX. We conclude that the structure and action of the HAMP linker are conserved and that the HAMP linker transmits a signal to the output domain that ligand is bound.

The conserved cytoplasmic module of the transmembrane chemoreceptor McpC mediates carbohydrate chemotaxis in Bacillus subtilis

Escherichia coli cells use two distinct sensory circuits during chemotaxis towards carbohydrates. One circuit requires the phosphoenolpyruvate-dependent phosphotransferase system (PTS) and is independent of any specific chemoreceptor, whereas the other uses a chemoreceptor-dependent sensory mechanism analogous to that used during chemotaxis towards amino acids. Work on the carbohydrate chemotaxis sensory circuit of Bacillus subtilis reported in this article indicates that the B. subtilis circuit is different from either of those used by E. coli. Our chemotactic analysis of B. subtilis strains expressing various chimeric chemoreceptors indicates that the cytoplasmic, C-terminal module of the chemoreceptor McpC acts as a sensory-input element during carbohydrate chemotaxis. Our results also indicate that PTS-mediated carbohydrate transport, but not carbohydrate metabolism, is required for production of a chemotactic signal. We propose a model in which PTS-transport-induced chemotactic signals are transmitted to the C-terminal module of McpC for control of chemotaxis towards PTS carbohydrates.

Allosteric enhancement of adaptational demethylation by a carboxyl-terminal sequence on chemoreceptors

Sensory adaptation in bacterial chemotaxis is mediated by covalent modification of chemoreceptors. Specific glutamyl residues are methylated and demethylated in reactions catalyzed by methyltransferase CheR and methylesterase CheB. In Escherichia coli and Salmonella enterica serovar typhimurium, efficient adaptational modification by either enzyme is dependent on a conserved pentapeptide sequence at the chemoreceptor carboxyl terminus, a position distant from the sites of modification. For CheR-catalyzed methylation, previous work demonstrated that this sequence acts as a high affinity docking site, enhancing methylation by increasing enzyme concentration near methyl-accepting glutamates. We investigated pentapeptide-mediated enhancement of CheB-catalyzed demethylation and found it occurred by a distinctly different mechanism. Assays of binding between CheB and the pentapeptide sequence showed that it was too weak to have a significant effect on local enzyme concentration. Kinetic analyses revealed that interaction of the sequence and the methylesterase enhanced the rate constant of demethylation not the Michaelis constant. This allosteric activation occurred if the sequence was attached to chemoreceptor, but hardly at all if it was present as an isolated peptide. In addition, free peptide inhibited demethylation of the native receptor carrying the pentapeptide sequence at its carboxyl terminus. These observations imply that the allosteric change is transmitted through the protein substrate, not the enzyme.

Intragenic suppressors of a mutation in the aspartate chemoreceptor gene that abolishes binding of the receptor to methyltransferase

In the chemotaxis of Escherichia coli, receptor methylation is the key process of adaptation. The methyltransferase CheR binds to the carboxy-terminal NWETF sequence of major chemoreceptors. The substitution of Ala for Trp of this sequence (W550A) of the aspartate chemoreceptor (Tar) abolishes its CheR-binding ability. In this study, six independent intragenic suppressors of the mutation were isolated. They were divided into two classes. Tar carrying the class I suppressors (G278A-L488M, T334A, G278A, G278C and A398T) showed signal biases toward tumbling, corresponding to increased activities of the receptor-associated histidine kinase CheA. These suppressors further reduced the unstimulated methylation level of Tar-W550A, but allowed slight but significant stimulation of methylation by aspartate. Some other CheA-activating mutations were also found to serve as class I suppressors. These results suggest that the class I suppressors compensate for the signal bias of Tar-W550A caused by its low methylation level and that the NWETF sequence is required primarily to maintain an appropriate level of methylation by increasing the local concentration of CheR around the receptor. The class II suppressor was a mutation in the termination codon (Op554W) resulting in the addition of 11 residues containing an xWxxF motif. This revertant Tar supported chemotaxis and was methylated almost as effectively as wild-type Tar. This effect was reversed by introducing a mutation in the xWxxF motif. These results reinforce the importance of the xWxxF motif and suggest that the motif does not have to be located at the extreme carboxy terminus.

Organization of the receptor-kinase signaling array that regulates Escherichia coli chemotaxis

Motor behavior in prokaryotes is regulated by a phosphorelay network involving a histidine protein kinase, CheA, whose activity is controlled by a family of Type I membrane receptors. In a typical Escherichia coli cell, several thousand receptors are organized together with CheA and an Src homology 3-like protein, CheW, into complexes that tend to be localized at the cell poles. We found that these complexes have at least 6 receptors per CheA. CheW is not required for CheA binding to receptors, but is essential for kinase activation. The kinase activity per mole of bound CheA is proportional to the total bound CheW. Similar results were obtained with the E. coli serine receptor, Tsr, and the Salmonella typhimurium aspartate receptor, Tar. In the case of Tsr, under conditions optimal for kinase activation, the ratio of subunits in complexes is approximately 6 Tsr:4 CheW:1 CheA. Our results indicate that information from numerous receptors is integrated to control the activity of a relatively small number of kinase molecules.

Subunit organization in a soluble complex of tar, CheW, and CheA by electron microscopy

The Salmonella and Escherichia coli aspartate receptor, Tar, is representative of a large class of membrane receptors that generate chemotaxis responses by regulating the activity of an associated histidine protein kinase, CheA. Tar is composed of an NH(2)-terminal periplasmic ligand-binding domain linked through a transmembrane sequence to a COOH-terminal coiled-coil signaling domain in the cytoplasm. The isolated cytoplasmic domain of Tar fused to a leucine zipper sequence forms a soluble complex with CheA and the Src homology 3-like kinase activator, CheW. Activity of the CheA kinase in the soluble complex is essentially the same as in fully active complexes with the intact receptor in the membrane. The soluble complex is composed of approximately 28 receptor cytoplasmic domain chains, 6 CheW chains, and 4 CheA chains. It has a molecular weight of 1,400,000 (Liu, I., Levit, M., Lurz, R., Surette, M.G., and Stock, J.B. (1997) EMBO J. 16, 7231-7240). Electron microscopy reveals an elongated barrel-like structure with a largely hollow center. Immunoelectron microscopy has provided a general picture of the subunit and domain organization of the complex. CheA and CheW appear to be in the middle of the complex with the leucine zippers of the receptor construct at the ends. These findings show that the receptor signaling complex forms higher ordered structures with defined geometric architectures. Coupled with atomic models of the subunits, our results provide insights into the functional architecture by which the receptor regulates CheA kinase activity during bacterial chemotaxis.

LrhA as a new transcriptional key regulator of flagella, motility and chemotaxis genes in Escherichia coli

The function of the LysR-type regulator LrhA of Escherichia coli was defined by comparing whole-genome mRNA profiles from wild-type E. coli and an isogenic lrhA mutant on a DNA microarray. In the lrhA mutant, a large number (48) of genes involved in flagellation, motility and chemotaxis showed relative mRNA abundances increased by factors between 3 and 80. When a representative set of five flagellar, motility and chemotaxis genes was tested in lacZ reporter gene fusions, similar factors for derepression were found in the lrhA mutant. In gel retardation experiments, the LrhA protein bound specifically to flhD and lrhA promoter DNA (apparent K(D) approximately 20 nM), whereas the promoters of fliC, fliA and trg were not bound by LrhA. The expression of flhDC (encoding FlhD(2)C(2)) was derepressed by a factor of 3.5 in the lrhA mutant. FlhD(2)C(2) is known as the master regulator for the expression of flagellar and chemotaxis genes. By DNase I footprinting, LrhA binding sites at the flhDC and lrhA promoters were identified. The lrhA gene was under positive autoregulation by LrhA as shown by gel retardation and lrhA expression studies. It is suggested that LrhA is a key regulator controlling the transcription of flagellar, motility and chemotaxis genes by regulating the synthesis and concentration of FlhD(2)C(2).

Signal transduction: receptor clusters as information processing arrays

The organization of transmembrane receptors into higher-order arrays occurs in cells as different as bacteria, lymphocytes and neurons. What are the implications of receptor clustering for short-term and long-term signaling processes that occur in response to ligand binding?

Collaborative signaling by mixed chemoreceptor teams in Escherichia coli

Chemoreceptors of the methyl-accepting chemotaxis protein family form clusters, typically at the cell pole(s), in both Bacteria and Archaea. To elucidate the architecture and signaling role of receptor clusters, we investigated interactions between the serine (Tsr) and aspartate (Tar) chemoreceptors in Escherichia coli by constructing Tsr mutations at the six hydrophobic and five polar residues implicated in "trimer of dimers" formation. Tsr mutants with proline replacements could not mediate serine chemotaxis, receptor clustering, or clockwise flagellar rotation. Alanine and tryptophan mutants, although also nonchemotactic, formed receptor clusters, and some produced clockwise flagellar rotation, indicating receptor-coupled activation of the signaling CheA kinase. The alanine and tryptophan mutants evidently assemble defective receptor complexes that cannot modulate CheA activity in response to serine stimuli. In cells containing wild-type Tar receptors, tryptophan replacements in Tsr interfered with Tar function, whereas four Tsr mutants with alanine replacements regained Tsr function. These epistatic and rescuable phenotypes imply interactions between Tsr and Tar dimers in higher-order signaling teams. The bulky side chain in tryptophan mutants may prevent stimulus-induced conformational changes in the team, whereas the small side chain in alanine mutants may permit signaling control when teamed with functional receptor molecules. Direct physical interactions between Tsr and Tar molecules were observed by in vivo chemical crosslinking. Wild-type Tsr crosslinked to Tar, whereas a clustering-defective proline replacement mutant did not. These findings indicate that bacterial chemoreceptor clusters are comprised of signaling teams, seemingly based on trimers of dimers, that can contain different receptor types acting collaboratively.

A NarX-Tar chimera mediates repellent chemotaxis to nitrate and nitrite

Membrane receptors communicate between the external world and the cell interior. In bacteria, these receptors include the transmembrane sensor kinases, which control gene expression via their cognate response regulators, and chemoreceptors, which control the direction of flagellar rotation via the CheA kinase and CheY response regulator. Here, we show that a chimeric protein that joins the ligand-binding, transmembrane and linker domains of the NarX sensor kinase to the signalling and adaptation domains of the Tar chemoreceptor of Escherichia coli mediates repellent responses to nitrate and nitrite. Nitrate induces a stronger response than nitrite and is effective at lower concentrations, mirroring the relative sensitivity to these ligands exhibited by NarX itself. We conclude that the NarX-Tar hybrid functions as a bona fide chemoreceptor whose activity can be predicted from its component parts. This observation implies that ligand-dependent activation of a sensor kinase and repellent-initiated activation of receptor-coupled CheA kinase involve a similar transmembrane signal.

The receptor docking segment and S-adenosyl-L-homocysteine bind independently to the methyltransferase of bacterial chemotaxis

To mediate adaptation to stimuli, the methyltransferase (CheR) catalyzes methyl group transfer from S-adenosyl-L-methionine (SAM) to glutamyl residues in the transmembrane receptors of the bacterial chemosensory signaling pathway. The interaction between receptors and CheR occurs at two sites: a methylation site-active site interaction, and a 'docking' site interaction that is separated both from the methylation sites and the CheR active site. It is not certain if the docking site interaction functions merely to localize the transferase in close proximity to the methylation sites, or if it also increases CheR catalytic activity. Isothermal titration calorimetry experiments are conducted to test for allosteric interactions between the docking and active sites on CheR, which are expected to be present if docking activates CheR. The binding parameters (DeltaG, DeltaH, DeltaS) of a substrate analog of SAM, S-adenosyl-L-homocysteine (SAH), are measured both in the absence and presence of saturating concentrations of a pentapeptide (NWETF) that defines the docking receptor docking segment. SAH binding is unaffected by the presence of saturating NWETF, providing evidence that an allosteric activation of CheR does not take place upon docking, and thus supports the idea that the CheR-NWETF interaction merely functions to localize CheR near the sites of methylation.

Binding and diffusion of CheR molecules within a cluster of membrane receptors

Adaptation of the attractant response in Escherichia coli is attributable to the methylation of its transmembrane chemotactic receptors by the methyltransferase CheR. This protein contains two binding domains, one for the sites of methylation themselves and the other for a flexible tether at the C terminus of the receptor. We have explored the theoretical consequences of this binding geometry for a CheR molecule associated with a cluster of chemotactic receptors. Calculations show that the CheR molecule will bind with high net affinity to the receptor lattice, having a high probability of being attached by one or both of its domains at any instant of time. Because of the relatively low affinity of its individual domains and the close proximity of neighboring receptors, it is likely that when one domain unbinds it will reattach to the array before the other domain unbinds. Stochastic simulations show that the enzyme will move through the receptor cluster in a hand-over-hand fashion, like a gibbon swinging through the branches of a tree. We explore the possible consequences of this motion, which we term "molecular brachiation", for chemotactic adaptation and suggest that a similar mechanism may be operative in other large assemblies of protein molecules.