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Signal transduction: Hair brains in bacterial chemotaxis
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Signal transduction: Hair brains in bacterial chemotaxis Jeff Stock and Mikhail Levit
The conserved cytoplasmic domains of bacterial chemotaxis receptors are a fibrous arrangement of α-helical coiled coils that look a lot like hair. Such bundles of α-helical filaments mediate sensory-motor responses in all prokaryotic cells. How do they work? Very nearly perfectly is probably as good an answer as any. Address: Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA. Current Biology 2000, 10:R11–R14 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Bacterial chemotaxis receptors are transmembrane proteins composed of an outside periplasmic sensing domain connected via a single transmembrane α helix to an inside signaling domain at the membrane–cytosol interface (Figure 1). A few years ago, crystal structures of a fragment of a chemotaxis receptor corresponding to just its outside sensing domain were determined [1,2], revealing a homodimer of two ‘up–down–up–down’ four helix bundles which, as part of the receptor, would be pinned to the membrane by transmembrane helices contiguous with the first and last helices of each monomeric unit.
of CheW, the conserved residues in the neighborhood of the reverse turn appear to be involved in specific monomer–monomer, dimer–dimer, and tetramer–tetramer contacts that serve to organize the crystal [3]. How do the component parts of the system — the receptors, CheW and the protein kinase CheA — come together to form a functional membrane receptor signaling unit? The structures of the components are now mostly established. The kinase CheA [8,9] forms a homodimer through interactions between antiparallel helical dimerization domains, which lead into opposing ATP-binding phosphotransfer domains that are in turn connected to tandem SH3-like domains homologous to CheW (Figure 2). The protein has two additional domains, H and YB, attached by flexible linker sequences to the amino-terminal helix of the dimerization domain [10,11].
A crystal structure of the inside signaling portion of a chemotaxis receptor has recently been solved [3]. This is the part that connects the transmembrane helices from the sensory domains on the outside of the membrane to the Che proteins of the chemotaxis system on the inside. The structure is composed of antiparallel, coiled-coil hairpins, which are supercoiled into four-helical bundles organized in triangular clusters. It is a remarkable structure, insofar as it even exists in a crystal, considering all of the possible irregularities and asymmetries in the numerous coiled-coil interactions that contribute to each higher and higher ordered assembly. Thus, on the one hand the receptor forms a hairy looking structure, but on the other hand this apparently fibrous assemblage is a crystal at atomic resolution.
The receptor H and YB domains function in the phosphorelay by which CheA signals motor regulation. The H domain is an up-down-up-down four-helix bundle with a highly conserved sequence at the surface of one helix, centering on a histidine residue that is the site of kinase autophosphorylation [12,13]. The YB domain is located in the CheA sequence between the H domain and the dimerization domain [14,15]. It binds the chemotaxis response regulator protein, CheY, which transfers the phosphoryl group from the phosphohistidine on phosphoCheA to one of its own aspartyl groups. Phosphorylation of CheY induces a conformational change that causes its release from the YB domain [16]. Phospho-CheY is then free to diffuse to the motor, where it acts as an allosteric regulator of the flagellar motor switch [17, 18]. Attractant stimuli, such as amino acids and sugars, interact with portions of the receptors outside the cell to inhibit CheA autophosphorylation inside the cell. This leads to a decrease in CheY phosphorylation so that motor switching is suppressed and the cell tends to keep moving toward nutrient-rich environments.
The simplicity of the reverse turn formed by each monomeric hairpin is noteworthy. The turn is centered on a sequence of about 30 amino acids that constitutes a binding site for the tandem-SH3 adaptor protein CheW [4,5]. CheW is required for the assembly of receptor–kinase signaling complexes in most motile bacteria, and the reverse turn sequences are highly conserved with over 60% sequence identity even between the most distantly related eubacterial and archaeal species [6,7]. Yet, in the crystal structure of the cytoplasmic signaling domain, this highly conserved region looks to be a simple reverse turn; and in the absence
How does transmembrane signaling work? The most thoroughly investigated examples are the closely related receptors for aspartate, Tar, and serine, Tsr, from Escherichia coli and Salmonella. The structure of the sensory domain of Tar has been determined in the presence and absence of aspartate [1,19]. Aspartate binds at an interface between the four helical bundles that form the sensory domains of each monomeric receptor unit. This breaks the symmetry of monomer–monomer interactions, and induces small shifts in the conformation of one of the subunits which substantially reduce the affinity for ligand binding to
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Figure 1
Figure 2
Domain structure of the histidine protein kinase CheA. The CheA protein is a dimer composed of five distinct domains. The H domain at the amino terminus contains the site of histidine autophosphorylation. The YB domain functions to bind the chemotaxis response regulator CheY. The D (dimerization) and C (catalytic) domains comprise the kinase catalytic core; both are required for the phosphorylation of H-domain histidines. The R domain is an adaptor module that, like CheW, is composed of tandem SH3-like domains that mediate the assembly of receptor signaling complexes.
a second, symmetrically opposed aspartate binding site [20]. As to the conformational changes, there are several ways one helix can move with respect to another: it can rotate, swing, piston or some combination of these. This has led to considerable speculation as to the precise nature of the ligand-induced changes in receptor structure that are responsible for transmembrane signaling and inhibition of CheA autophosphorylation [19,21,22].
Structural organization of a bacterial chemotaxis receptor. The chemotaxis receptors are hair-like bundles of α helices that pass through the cytoplasmic membrane. Attractant ligands, such as aspartate and serine, bind to their respective receptors at a dimer interface between the helical bundles, at the outside surface of the membrane. The intracellular portion of the receptor bundle is characterized by a zone of glutamate (E) and glutamine (Q) residues that are subject to modification by an amidase/esterase, CheB, and a methyltransferase, CheR. An adaptor protein, CheW, and histidine protein kinase, CheA, bind via tandem SH3-like domains to highly conserved sequences in the region of the tight turn between antiparallel receptor helices.
Any straightforward attempt to relate stimulus-induced conformational changes in sensory domain structure to transmembrane signaling is complicated by the finding that chemotaxis receptor dimers function within the context of extended complexes, which may contain thousands of receptor, adaptor and kinase subunits arranged in an architecturally dynamic array [23,24]. The receptor bundles are apparently bound together by adaptor modules and assembled on a larger scale by the adaptors that are part of the kinase CheA (Figure 3). It is as if the receptor fibers, and the CheA and CheW adaptor elements were bricks and mortar for the construction of a nanoscale sensory scaffold [25]. In their ‘ON’ state, in the absence of attractants, several receptors bind to CheA in such a way that CheA is activated over 100-fold [24], and conversely CheA binding to the receptors appears to be required for long-range structural interactions that serve to organize the array. In the ‘OFF’ state, in the presence of attractants, it is as if the CheA dimer is torn apart by binding to the receptor network. It seems likely that the major determinant of
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CheA kinase activity is the tension on the two tandem SH3 domains, bound to receptor bundles at least 70 Å apart. The bundles are part of a network, and the interaction between all the bundles in the network controls the distance between neighbouring receptor bundles. It could be as simple as that.
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Figure 3
One important mechanism by which the tension on the network appears to be regulated is through the modification of glutamine and glutamate residues that occur in heptad repeats along opposite solvent exposed faces of each tetramer dyad. The glutamines are subject to deamidation by a specific amidase, CheB [26]. This reaction can produce up to eight excess acidic groups per tetramer [27]. The glutamates are subject to neutralization by a specific S-adenosylmethionine-dependent methyltransferase, CheR, which generates glutamyl methyl esters [28]. The negatively charged glutamates can subsequently be regenerated through the methylesterase activity of the CheB amidase. Thus, each tetrameric stave of the receptor framework has a range of eight possible –/0 (acidic/neutral) values: all eight having the – value is an extreme state in which the kinase is inactive; all eight having the 0 value is a state in which the kinase is maximally active. The former is presumably an extreme state of expansion of the receptor framework; the latter, a relatively compressed state. According to this view of kinase regulation, the question of how transmembrane signaling works reduces to how attractant binding to the sensory domain tends to disperse the receptor bundles so as to inactivate the kinase. The most obvious way for this to occur would be for attractant binding to cause an expansion of the network. This could be done through effects on packing interactions between the sensory domain head groups at the outside surface. Aspartate binding to the Tar sensory domain causes a slight (~1 Å) piston motion [21,22] that could tilt the whole structure by several degrees with respect to the plane of the membrane. This may be one of the changes in sensory domain conformation that causes the expansion of the network and thereby inhibits kinase autophosphorylation. One wonders at the incredible potential for complexity in the molecular details of the chemotaxis receptor signaling structure. Each signaling unit — or organelle — appears to be a field of several thousand helical receptor elements, packed into a bundle about 100 nm across, the architectural dynamics of which must follow a logic determined by the rules of association between CheW, CheA, and the scaffold of α-helical receptor fibers. In E. coli, at least five different receptors — Tar, Tsr, Tap, Trg and Aer — appear to be intermingled within the same complex [29]. Each receptor is subject to the same set of glutamyl modifications, so there are a very large number of possible combinations of differentially modified receptors. The state of receptor glutamyl modification depends on previous exposures to
Receptor signaling complexes. Clusters of receptor α-helical bundles are organized into higher-order structures by the binding of SH3-like adaptor modules in CheW and CheA. Kinase activity may be modulated either by changes in electrostatic repulsion that are introduced by glutamate modifications, or by attractant binding events that cause alterations in packing interactions between the portions of the receptor helices that are at the outside surface of the membrane.
attractant and repellent stimuli, and considerable evidence suggests that the modifications serve a memory function to modulate chemotaxis responses in accord with an individual’s past experience [30]. The signaling organelle is essentially a prokaryotic brain, insofar as it plays a dominant role in coordinating an individual cell’s
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sensory–motor behavior. Furthermore, most cells in a population appear to have only one of these hair brains located in a patch of membrane on the periplasmic bay at one end of the bacterial rod, presumably the head. References 1. Yeh JI, Biemann HP, Prive GG, Pandit J, Koshland DE, Jr, Kim SH: High-resolution structures of the ligand binding domain of the wild-type bacterial aspartate receptor. J Mol Biol 1996, 262:186-201. 2. Falke JJ, Bass RB, Butler SL, Chervitz SA, Danielson MA: The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes. Annu Rev Cell Dev Biol 1997, 13:457-512. 3. Kim KK, Yokota H, Kim SH: Four-helical-bundle structure of the cytoplasmic domain of a serine chemotaxis receptor. Nature 1999, 400:787-792. 4. Surette MG, Stock JB: Role of alpha-helical coiled-coil interactions in receptor dimerization, signaling, and adaptation during bacterial chemotaxis. J Biol Chem 1996, 271:17966-17973. 5. Ames P, Yu YA, Parkinson JS: Methylation segments are not required for chemotactic signalling by cytoplasmic fragments of Tsr, the methyl-accepting serine chemoreceptor of Escherichia coli. Mol Microbiol 1996, 19:737-746. 6. Rudolph J, Nordmann B, Storch KF, Gruenberg H, Rodewald K, Oesterhelt D: A family of halobacterial transducer proteins. FEMS Microbiol Lett 1996, 139:161-168. 7. Le Moual H, Koshland DE, Jr: Molecular evolution of the C-terminal cytoplasmic domain of a superfamily of bacterial receptors involved in taxis. J Mol Biol 1996, 261:568-585. 8. Bilwes AM, Alex LA, Crane BR, Simon MI: Structure of CheA, a signal-transducing histidine kinase. Cell 1999, 96:131-141. 9. Stock J: Signal transduction: Gyrating protein kinases. Curr Biol 1999, 9:R364-R367. 10. Morrison TB, Parkinson JS: Liberation of an interaction domain from the phosphotransfer region of CheA, a signaling kinase of Escherichia coli. Proc Natl Acad Sci USA 1994, 91:5485-5489. 11. Zhou H, McEvoy MM, Lowry DF, Swanson RV, Simon MI, Dahlquist FW: Phosphotransfer and CheY-binding domains of the histidine autokinase CheA are joined by a flexible linker. Biochemistry 1996, 35:433-443. 12. Zhou H, Lowry DF, Swanson RV, Simon MI, Dahlquist FW: NMR studies of the phosphotransfer domain of the histidine kinase CheA from Escherichia coli: assignments, secondary structure, general fold, and backbone dynamics. Biochemistry 1995, 34:13858-13870. 13. Grebe TW, Stock JB: The histidine protein kinase superfamily. Adv Microb Physiol 1999, 41:139-227. 14. McEvoy MM, Hausrath AC, Randolph GB, Remington SJ, Dahlquist FW: Two binding modes reveal flexibility in kinase/response regulator interactions in the bacterial chemotaxis pathway. Proc Natl Acad Sci USA 1998, 95:7333-7338. 15. Welch M, Chinardet N, Mourey L, Birck C, Samama JP: Structure of the CheY-binding domain of histidine kinase CheA in complex with CheY. Nat Struct Biol 1998, 5:25-29. 16. Schuster SC, Swanson RV, Alex LA, Bourret RB, Simon MI: Assembly and function of a quaternary signal transduction complex monitored by surface plasmon resonance. Nature 1993, 365:343-347. 17. Scharf BE, Fahrner KA, Turner L, Berg HC: Control of direction of flagellar rotation in bacterial chemotaxis. Proc Natl Acad Sci USA 1998, 95:201-206. 18. Alon U, Camarena L, Surette MG, Aguera y Arcas B, Liu Y, Liebler S, Stock JB: Response regulator output in bacterial chemotaxis. EMBO J 1998, 17:4238-4248. 19. Chi YI, Yokota H, Kim SH: Apo structure of the ligand-binding domain of aspartate receptor from Escherichia coli and its comparison with ligand-bound or pseudoligand-bound structures. FEBS Lett 1997, 414:327-332. 20. Biemann HP, Koshland DE, Jr: Aspartate receptors of Escherichia coli and Salmonella typhimurium bind ligand with negative and half-of-the-sites cooperativity. Biochemistry 1994, 33:629-634. 21. Chervitz SA, Falke JJ: Molecular mechanism of transmembrane signaling by the aspartate receptor: a model. Proc Natl Acad Sci USA 1996, 93:2545-2550. 22. Ottemann KM, Xiao W, Shin YK, Koshland DE, Jr: A piston model for transmembrane signaling of the aspartate receptor. Science 1999, 285:1751-1754.
23. Maddock JR, Shapiro L: Polar location of the chemoreceptor complex in the Escherichia coli cell. Science 1993, 259:1717-1723. 24. Liu Y, Levit M, Lurz R, Surette M, Stock J: Receptor-mediated protein kinase activation and the mechanism of transmembrane signaling in bacterial chemotaxis. EMBO J 1997, 16:7231-7240. 25. Stock J, Da Re S: A receptor scaffold mediates stimulus-response coupling in bacterial chemotaxis. Cell Calcium 1999, in press. 26. Djordjevic S, Goudreau PN, Xu Q, Stock AM, West AH: Structural basis for methylesterase CheB regulation by a phosphorylationactivated domain. Proc Natl Acad Sci USA 1998, 95:1381-1386. 27. Stock JB, Surette M. Chemotaxis. In Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology, 2nd ed. Edited by Neidhardt FC. Washington, DC: ASM; 1996:1103-1129. 28. Djordjevic S, Stock AM: Crystal structure of the chemotaxis receptor methyltransferase CheR suggests a conserved structural motif for binding S-adenosylmethionine. Structure 1997, 5:545-558. 29. Grebe TW, Stock J: Bacterial chemotaxis: The five sensors of a bacterium. Curr Biol 1998, 8:R154-R157. 30. Stock J, Stock A: What is the role of receptor methylation in bacterial chemotaxis? Trends Biochemical Sci 1987, 12:371-375.
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Signal transduction: Hair brains in bacterial chemotaxis Jeff Stock and Mikhail Levit
The conserved cytoplasmic domains of bacterial chemotaxis receptors are a fibrous arrangement of α-helical coiled coils that look a lot like hair. Such bundles of α-helical filaments mediate sensory-motor responses in all prokaryotic cells. How do they work? Very nearly perfectly is probably as good an answer as any. Address: Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA. Current Biology 2000, 10:R11–R14 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Bacterial chemotaxis receptors are transmembrane proteins composed of an outside periplasmic sensing domain connected via a single transmembrane α helix to an inside signaling domain at the membrane–cytosol interface (Figure 1). A few years ago, crystal structures of a fragment of a chemotaxis receptor corresponding to just its outside sensing domain were determined [1,2], revealing a homodimer of two ‘up–down–up–down’ four helix bundles which, as part of the receptor, would be pinned to the membrane by transmembrane helices contiguous with the first and last helices of each monomeric unit.
of CheW, the conserved residues in the neighborhood of the reverse turn appear to be involved in specific monomer–monomer, dimer–dimer, and tetramer–tetramer contacts that serve to organize the crystal [3]. How do the component parts of the system — the receptors, CheW and the protein kinase CheA — come together to form a functional membrane receptor signaling unit? The structures of the components are now mostly established. The kinase CheA [8,9] forms a homodimer through interactions between antiparallel helical dimerization domains, which lead into opposing ATP-binding phosphotransfer domains that are in turn connected to tandem SH3-like domains homologous to CheW (Figure 2). The protein has two additional domains, H and YB, attached by flexible linker sequences to the amino-terminal helix of the dimerization domain [10,11].
A crystal structure of the inside signaling portion of a chemotaxis receptor has recently been solved [3]. This is the part that connects the transmembrane helices from the sensory domains on the outside of the membrane to the Che proteins of the chemotaxis system on the inside. The structure is composed of antiparallel, coiled-coil hairpins, which are supercoiled into four-helical bundles organized in triangular clusters. It is a remarkable structure, insofar as it even exists in a crystal, considering all of the possible irregularities and asymmetries in the numerous coiled-coil interactions that contribute to each higher and higher ordered assembly. Thus, on the one hand the receptor forms a hairy looking structure, but on the other hand this apparently fibrous assemblage is a crystal at atomic resolution.
The receptor H and YB domains function in the phosphorelay by which CheA signals motor regulation. The H domain is an up-down-up-down four-helix bundle with a highly conserved sequence at the surface of one helix, centering on a histidine residue that is the site of kinase autophosphorylation [12,13]. The YB domain is located in the CheA sequence between the H domain and the dimerization domain [14,15]. It binds the chemotaxis response regulator protein, CheY, which transfers the phosphoryl group from the phosphohistidine on phosphoCheA to one of its own aspartyl groups. Phosphorylation of CheY induces a conformational change that causes its release from the YB domain [16]. Phospho-CheY is then free to diffuse to the motor, where it acts as an allosteric regulator of the flagellar motor switch [17, 18]. Attractant stimuli, such as amino acids and sugars, interact with portions of the receptors outside the cell to inhibit CheA autophosphorylation inside the cell. This leads to a decrease in CheY phosphorylation so that motor switching is suppressed and the cell tends to keep moving toward nutrient-rich environments.
The simplicity of the reverse turn formed by each monomeric hairpin is noteworthy. The turn is centered on a sequence of about 30 amino acids that constitutes a binding site for the tandem-SH3 adaptor protein CheW [4,5]. CheW is required for the assembly of receptor–kinase signaling complexes in most motile bacteria, and the reverse turn sequences are highly conserved with over 60% sequence identity even between the most distantly related eubacterial and archaeal species [6,7]. Yet, in the crystal structure of the cytoplasmic signaling domain, this highly conserved region looks to be a simple reverse turn; and in the absence
How does transmembrane signaling work? The most thoroughly investigated examples are the closely related receptors for aspartate, Tar, and serine, Tsr, from Escherichia coli and Salmonella. The structure of the sensory domain of Tar has been determined in the presence and absence of aspartate [1,19]. Aspartate binds at an interface between the four helical bundles that form the sensory domains of each monomeric receptor unit. This breaks the symmetry of monomer–monomer interactions, and induces small shifts in the conformation of one of the subunits which substantially reduce the affinity for ligand binding to
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Figure 1
Figure 2
Domain structure of the histidine protein kinase CheA. The CheA protein is a dimer composed of five distinct domains. The H domain at the amino terminus contains the site of histidine autophosphorylation. The YB domain functions to bind the chemotaxis response regulator CheY. The D (dimerization) and C (catalytic) domains comprise the kinase catalytic core; both are required for the phosphorylation of H-domain histidines. The R domain is an adaptor module that, like CheW, is composed of tandem SH3-like domains that mediate the assembly of receptor signaling complexes.
a second, symmetrically opposed aspartate binding site [20]. As to the conformational changes, there are several ways one helix can move with respect to another: it can rotate, swing, piston or some combination of these. This has led to considerable speculation as to the precise nature of the ligand-induced changes in receptor structure that are responsible for transmembrane signaling and inhibition of CheA autophosphorylation [19,21,22].
Structural organization of a bacterial chemotaxis receptor. The chemotaxis receptors are hair-like bundles of α helices that pass through the cytoplasmic membrane. Attractant ligands, such as aspartate and serine, bind to their respective receptors at a dimer interface between the helical bundles, at the outside surface of the membrane. The intracellular portion of the receptor bundle is characterized by a zone of glutamate (E) and glutamine (Q) residues that are subject to modification by an amidase/esterase, CheB, and a methyltransferase, CheR. An adaptor protein, CheW, and histidine protein kinase, CheA, bind via tandem SH3-like domains to highly conserved sequences in the region of the tight turn between antiparallel receptor helices.
Any straightforward attempt to relate stimulus-induced conformational changes in sensory domain structure to transmembrane signaling is complicated by the finding that chemotaxis receptor dimers function within the context of extended complexes, which may contain thousands of receptor, adaptor and kinase subunits arranged in an architecturally dynamic array [23,24]. The receptor bundles are apparently bound together by adaptor modules and assembled on a larger scale by the adaptors that are part of the kinase CheA (Figure 3). It is as if the receptor fibers, and the CheA and CheW adaptor elements were bricks and mortar for the construction of a nanoscale sensory scaffold [25]. In their ‘ON’ state, in the absence of attractants, several receptors bind to CheA in such a way that CheA is activated over 100-fold [24], and conversely CheA binding to the receptors appears to be required for long-range structural interactions that serve to organize the array. In the ‘OFF’ state, in the presence of attractants, it is as if the CheA dimer is torn apart by binding to the receptor network. It seems likely that the major determinant of
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CheA kinase activity is the tension on the two tandem SH3 domains, bound to receptor bundles at least 70 Å apart. The bundles are part of a network, and the interaction between all the bundles in the network controls the distance between neighbouring receptor bundles. It could be as simple as that.
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Figure 3
One important mechanism by which the tension on the network appears to be regulated is through the modification of glutamine and glutamate residues that occur in heptad repeats along opposite solvent exposed faces of each tetramer dyad. The glutamines are subject to deamidation by a specific amidase, CheB [26]. This reaction can produce up to eight excess acidic groups per tetramer [27]. The glutamates are subject to neutralization by a specific S-adenosylmethionine-dependent methyltransferase, CheR, which generates glutamyl methyl esters [28]. The negatively charged glutamates can subsequently be regenerated through the methylesterase activity of the CheB amidase. Thus, each tetrameric stave of the receptor framework has a range of eight possible –/0 (acidic/neutral) values: all eight having the – value is an extreme state in which the kinase is inactive; all eight having the 0 value is a state in which the kinase is maximally active. The former is presumably an extreme state of expansion of the receptor framework; the latter, a relatively compressed state. According to this view of kinase regulation, the question of how transmembrane signaling works reduces to how attractant binding to the sensory domain tends to disperse the receptor bundles so as to inactivate the kinase. The most obvious way for this to occur would be for attractant binding to cause an expansion of the network. This could be done through effects on packing interactions between the sensory domain head groups at the outside surface. Aspartate binding to the Tar sensory domain causes a slight (~1 Å) piston motion [21,22] that could tilt the whole structure by several degrees with respect to the plane of the membrane. This may be one of the changes in sensory domain conformation that causes the expansion of the network and thereby inhibits kinase autophosphorylation. One wonders at the incredible potential for complexity in the molecular details of the chemotaxis receptor signaling structure. Each signaling unit — or organelle — appears to be a field of several thousand helical receptor elements, packed into a bundle about 100 nm across, the architectural dynamics of which must follow a logic determined by the rules of association between CheW, CheA, and the scaffold of α-helical receptor fibers. In E. coli, at least five different receptors — Tar, Tsr, Tap, Trg and Aer — appear to be intermingled within the same complex [29]. Each receptor is subject to the same set of glutamyl modifications, so there are a very large number of possible combinations of differentially modified receptors. The state of receptor glutamyl modification depends on previous exposures to
Receptor signaling complexes. Clusters of receptor α-helical bundles are organized into higher-order structures by the binding of SH3-like adaptor modules in CheW and CheA. Kinase activity may be modulated either by changes in electrostatic repulsion that are introduced by glutamate modifications, or by attractant binding events that cause alterations in packing interactions between the portions of the receptor helices that are at the outside surface of the membrane.
attractant and repellent stimuli, and considerable evidence suggests that the modifications serve a memory function to modulate chemotaxis responses in accord with an individual’s past experience [30]. The signaling organelle is essentially a prokaryotic brain, insofar as it plays a dominant role in coordinating an individual cell’s
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sensory–motor behavior. Furthermore, most cells in a population appear to have only one of these hair brains located in a patch of membrane on the periplasmic bay at one end of the bacterial rod, presumably the head. References 1. Yeh JI, Biemann HP, Prive GG, Pandit J, Koshland DE, Jr, Kim SH: High-resolution structures of the ligand binding domain of the wild-type bacterial aspartate receptor. J Mol Biol 1996, 262:186-201. 2. Falke JJ, Bass RB, Butler SL, Chervitz SA, Danielson MA: The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes. Annu Rev Cell Dev Biol 1997, 13:457-512. 3. Kim KK, Yokota H, Kim SH: Four-helical-bundle structure of the cytoplasmic domain of a serine chemotaxis receptor. Nature 1999, 400:787-792. 4. Surette MG, Stock JB: Role of alpha-helical coiled-coil interactions in receptor dimerization, signaling, and adaptation during bacterial chemotaxis. J Biol Chem 1996, 271:17966-17973. 5. Ames P, Yu YA, Parkinson JS: Methylation segments are not required for chemotactic signalling by cytoplasmic fragments of Tsr, the methyl-accepting serine chemoreceptor of Escherichia coli. Mol Microbiol 1996, 19:737-746. 6. Rudolph J, Nordmann B, Storch KF, Gruenberg H, Rodewald K, Oesterhelt D: A family of halobacterial transducer proteins. FEMS Microbiol Lett 1996, 139:161-168. 7. Le Moual H, Koshland DE, Jr: Molecular evolution of the C-terminal cytoplasmic domain of a superfamily of bacterial receptors involved in taxis. J Mol Biol 1996, 261:568-585. 8. Bilwes AM, Alex LA, Crane BR, Simon MI: Structure of CheA, a signal-transducing histidine kinase. Cell 1999, 96:131-141. 9. Stock J: Signal transduction: Gyrating protein kinases. Curr Biol 1999, 9:R364-R367. 10. Morrison TB, Parkinson JS: Liberation of an interaction domain from the phosphotransfer region of CheA, a signaling kinase of Escherichia coli. Proc Natl Acad Sci USA 1994, 91:5485-5489. 11. Zhou H, McEvoy MM, Lowry DF, Swanson RV, Simon MI, Dahlquist FW: Phosphotransfer and CheY-binding domains of the histidine autokinase CheA are joined by a flexible linker. Biochemistry 1996, 35:433-443. 12. Zhou H, Lowry DF, Swanson RV, Simon MI, Dahlquist FW: NMR studies of the phosphotransfer domain of the histidine kinase CheA from Escherichia coli: assignments, secondary structure, general fold, and backbone dynamics. Biochemistry 1995, 34:13858-13870. 13. Grebe TW, Stock JB: The histidine protein kinase superfamily. Adv Microb Physiol 1999, 41:139-227. 14. McEvoy MM, Hausrath AC, Randolph GB, Remington SJ, Dahlquist FW: Two binding modes reveal flexibility in kinase/response regulator interactions in the bacterial chemotaxis pathway. Proc Natl Acad Sci USA 1998, 95:7333-7338. 15. Welch M, Chinardet N, Mourey L, Birck C, Samama JP: Structure of the CheY-binding domain of histidine kinase CheA in complex with CheY. Nat Struct Biol 1998, 5:25-29. 16. Schuster SC, Swanson RV, Alex LA, Bourret RB, Simon MI: Assembly and function of a quaternary signal transduction complex monitored by surface plasmon resonance. Nature 1993, 365:343-347. 17. Scharf BE, Fahrner KA, Turner L, Berg HC: Control of direction of flagellar rotation in bacterial chemotaxis. Proc Natl Acad Sci USA 1998, 95:201-206. 18. Alon U, Camarena L, Surette MG, Aguera y Arcas B, Liu Y, Liebler S, Stock JB: Response regulator output in bacterial chemotaxis. EMBO J 1998, 17:4238-4248. 19. Chi YI, Yokota H, Kim SH: Apo structure of the ligand-binding domain of aspartate receptor from Escherichia coli and its comparison with ligand-bound or pseudoligand-bound structures. FEBS Lett 1997, 414:327-332. 20. Biemann HP, Koshland DE, Jr: Aspartate receptors of Escherichia coli and Salmonella typhimurium bind ligand with negative and half-of-the-sites cooperativity. Biochemistry 1994, 33:629-634. 21. Chervitz SA, Falke JJ: Molecular mechanism of transmembrane signaling by the aspartate receptor: a model. Proc Natl Acad Sci USA 1996, 93:2545-2550. 22. Ottemann KM, Xiao W, Shin YK, Koshland DE, Jr: A piston model for transmembrane signaling of the aspartate receptor. Science 1999, 285:1751-1754.
23. Maddock JR, Shapiro L: Polar location of the chemoreceptor complex in the Escherichia coli cell. Science 1993, 259:1717-1723. 24. Liu Y, Levit M, Lurz R, Surette M, Stock J: Receptor-mediated protein kinase activation and the mechanism of transmembrane signaling in bacterial chemotaxis. EMBO J 1997, 16:7231-7240. 25. Stock J, Da Re S: A receptor scaffold mediates stimulus-response coupling in bacterial chemotaxis. Cell Calcium 1999, in press. 26. Djordjevic S, Goudreau PN, Xu Q, Stock AM, West AH: Structural basis for methylesterase CheB regulation by a phosphorylationactivated domain. Proc Natl Acad Sci USA 1998, 95:1381-1386. 27. Stock JB, Surette M. Chemotaxis. In Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology, 2nd ed. Edited by Neidhardt FC. Washington, DC: ASM; 1996:1103-1129. 28. Djordjevic S, Stock AM: Crystal structure of the chemotaxis receptor methyltransferase CheR suggests a conserved structural motif for binding S-adenosylmethionine. Structure 1997, 5:545-558. 29. Grebe TW, Stock J: Bacterial chemotaxis: The five sensors of a bacterium. Curr Biol 1998, 8:R154-R157. 30. Stock J, Stock A: What is the role of receptor methylation in bacterial chemotaxis? Trends Biochemical Sci 1987, 12:371-375.