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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Curr Opin Struct Biol. Author manuscript; available in PMC 2010 July 13.
Published in final edited form as:
PMCID: PMC2902786

Structure of a conserved receptor domain that regulates kinase activity: the cytoplasmic domain of bacterial taxis receptors


Many bacteria are motile and use a conserved class of transmembrane sensory receptor to regulate cellular taxis toward an optimal living environment. These conserved receptors are typically stimulated by extracellular signals, but also undergo adaptation via covalent modification at specific sites on their cytoplasmic domains. The function of the cytoplasmic domain is to integrate the extracellular and adaptive signals, and to use this integrated information to regulate an associated histidine kinase. The kinase, in turn, triggers a cytoplasmic phosphorylation pathway of the two-component class. The high-resolution structure of a receptor cytoplasmic domain has recently been determined by crystallographic methods and is largely consistent with a structural model independently generated by chemical studies of the domain in the full-length, membrane-bound receptor. These results represent an important step toward a mechanistic understanding of receptor-to-kinase information transfer.


Both prokaryotic and eukaryotic cells possess cell-surface sensory receptors that detect specific extracellular cues, generate a transmembrane signal and regulate intracellular pathways. In many cases, the receptor is activated by the binding of a ligand molecule and is also modulated by covalent modification at specific adaptation sites on its cytoplasmic surface. The cytoplasmic domain, in turn, transmits information to one or more cytoplasmic signaling proteins. Thus, the cytoplasmic domain plays a central role as an information integrator and relay system, and is essential to both signal transduction and adaptation to constant background signals.

This review summarizes recent structural advances that have shed light on the conserved cytoplasmic domains of the sensory receptors that regulate bacterial taxis pathways. Two independent structural studies using crystallographic and chemical approaches have characterized the cytoplasmic domains of the closely related Escherichia coli serine receptor and Salmonella typhimurium aspartate receptor, respectively [1••,2••]. In both proteins, the cytoplasmic domain is a dimer of two identical helical hairpins, yielding a highly extended four-helix bundle. In general, the new structures are in excellent agreement and, together, demonstrate that chemical and crystallographic approaches provide complementary information about structure and mechanism. The integration of the new structures with genetic and biochemical data yields novel insights into the mechanisms of receptor adaptation and kinase regulation.

Methyl-accepting taxis protein superfamily of bacterial taxis receptors

In prokaryotes, the most common type of signaling pathway is the ubiquitous two-component pathway found in all prokaryotes to date, as well as in lower eukaryotes. Two-component pathways begin with a sensor protein, often a transmembrane receptor, which regulates a cytoplasmic histidine kinase, which, in turn, modulates an aspartate kinase. Together, the histidine and aspartate kinases comprise a conserved phosphorelay system [36].

A large superfamily of two-component sensor proteins is composed of receptors that regulate bacterial motility pathways [4,7,8]. These receptors, termed the methyl-accepting taxis proteins (MTPs), control cellular taxis and possess conserved cytoplasmic domains that are covalently methylated and demethylated by adaptation enzymes. MTP receptors direct a diversity of bacterial behavior, including chemotaxis, phototaxis, osmotaxis, thermotaxis, pH taxis and aerotaxis. Currently, 146 members of the MTP superfamily have been identified by sequence analysis [9].

The MTP superfamily can be divided into at least four subfamilies on the basis of the disposition of hydrophobic regions that are predicted to form transmembrane helices by hydropathy analysis [10]. Family A is the largest and is characterized by two putative transmembrane helices separated by a substantial periplasmic domain. This family includes the enterobacterial chemotaxis receptors for aspartate, serine, ribose and galactose, peptides, citrate and osmolarity [7,8]. Family B members also possess two distinct putative transmembrane helices but, in this case, the hydrophobic regions are connected by only a short hydrophilic loop, so that a substantial periplasmic domain does not exist. Such an arrangement is illustrated by the receptors that associate with sensory rhodopsin I or II [11]. Family C includes proteins that contain just one segment with a high transmembrane helix potential, such as the aerotaxis receptor [12,13]. Family D members appear to lack transmembrane helices altogether and are predicted to be peripheral membrane or soluble proteins [14].

All of the MTP receptors for which structural information exists are family A chemoreceptors from the homologous chemotaxis pathways of E. coli and S. typhimurium [1••,2••,4,7,1527]. These chemoreceptors are homodimers and possess a large periplasmic domain formed by the association of two identical, antiparallel four-helix bundles, one from each subunit [1519]. The transmembrane region is a single antiparallel four-helix bundle formed by the pairing of two membrane-spanning α helices from each subunit [2027]. Thus, the periplasmic and transmembrane architectures of selected family A chemoreceptors are well understood and are likely to be representative of many family A members. In general, however, the structural details of the periplasmic and transmembrane regions are variable, as these regions are specialized to recognize different stimuli. For example, the sequence conservation among distantly related periplasmic domains is low and often this domain is missing entirely (families B and C), whereas the total number of transmembrane helices per dimeric receptor is predicted to range from four (families A and B, perhaps C) to two (perhaps family C) and to zero (family D).

Conserved cytoplasmic domain of the methyl-accepting taxis protein receptors

In contrast to the specialized periplasmic and transmembrane regions, the cytoplasmic domain of MTP receptors (MTPCD) exhibits large regions of homology, even between the most distantly related sequences, indicating a highly conserved domain architecture [8,10]. These conserved regions are presumed to play ubiquitous roles in receptor adaptation and histidine kinase regulation. Indeed, functional chimaeric receptors have been generated by swapping the cytoplasmic domains of closely or distantly related MTP receptors, indicating that the cytoplasmic domain is a fully interchangeable signaling module [2831]. Early circular dichroism, hydrodynamic and NMR studies of isolated MTPCDs have suggested that the domain structure is predominantly α-helical and highly elongated [3234]. Moreover, sequence alignments have revealed a pattern of heptad repeats throughout large regions of the domain, in which the first and fourth positions of the heptad are typically hydrophobic, as expected for bundles of α helices in a coiled-coil arrangement [8,10]. Within the putative α-helical regions lie two distinct groups of conserved adaptation sites; these are specific glutamate sidechains targeted for methyl esterification and demethylation by adaptation enzymes [48]. Furthermore, the two putative methylation helices of each subunit have been proposed to associate to form an antiparallel four-helix bundle in the homodimer [4,8,35]. The most highly conserved region of the MTPCD is that which has been shown to be required for histidine kinase coupling and regulation [8,10]. This region, termed the signaling domain, has been found to fold independently and to regulate kinase activity when generated as an isolated fragment [3639]. Another conserved region is the linker, which contains the HAMP motif and bridges the C terminus of the transmembrane domain and the N terminus of the cytoplasmic domain [40,41••,42].

Cysteine scanning and solvent exposure studies have probed the secondary structure of the cytoplasmic domain and have identified six regions of α helix, some of which could represent different sections of a longer continuous helix ([4345,46••]; also S Winston, JJ Falke, unpublished data). These chemical studies analyzed a representative family A chemoreceptor, the S. typhimurium aspartate receptor, in its full-length, membrane-bound state. The solvent exposures of consecutive engineered cysteines were estimated from their chemical reactivities toward an aqueous, fluorescent alkylating agent. In many (but not all) regions examined, when cysteine was scanned through the sequence, the relative solvent exposures of consecutive positions displayed a helical periodicity, indicating the presence of α-helical structures with distinct exposed and buried faces.

Chemical and crystallographic analyses of the cytoplasmic domain architecture

The recent structural elucidation of the MTPCD illustrates the complementary information provided by chemical and crystallographic approaches. The primary advantages of the chemical approach, which employs site-directed sulfhydryl chemistry and spectroscopy to map out secondary structure and tertiary contacts, include the ability to probe structure in the full-length, membrane-bound protein in vitro or even in vivo [2027,4750]. This approach begins with a cysteine scanning and solvent exposure analysis of the targeted region, which, as described above, has identified extensive helical regions within the cytoplasmic domain. Subsequently, tertiary contacts are identified by measuring relative disulfide bond formation rates between pairs of cysteines, although considerable caution must be used in this analysis as the disulfide reaction is sensitive to local structure, electrostatics and dynamics, as well as to the cysteine–cysteine separation. Finally, the effects of the engineered cysteines and disulfides on protein activity are quantitated. This last step provides a wealth of functional and mechanistic information, as essential sidechains are identified by the activity loss that occurs upon cysteine substitution, whereas the effects of disulfide bonds on activity provide additional data regarding tertiary contacts. Activity-retaining disulfide bonds directly identify contacts between structural elements that are proximal in the working structure and, in a signaling protein, certain disulfide bonds can covalently lock the protein in the on or off state. The latter lock-on and lock-off disulfide bonds provide unique mechanistic insights and place strong constraints on models for on-off switching. The chief disadvantage of the chemical approach, however, is its relatively low resolution. By contrast, the spatial resolution of the crystallographic approach is unmatched, yielding essential structural details that are not accessible to other methods. Although it remains difficult to crystallize many transmembrane proteins, the approach can be applied to smaller independent folding units, including cytoplasmic domains, which can often be generated as soluble fragments.

The chemical and crystallographic methods have independently elucidated the structures of two MTPCDs from the closely related chemotaxis pathways of E. coli and S. typhimurium [1••,2••]. Each of these transmembrane receptors forms a stable ternary complex with the chemotaxis coupling protein CheW and the chemotaxis histidine kinase CheA, yielding a cytoplasmic receptor–kinase signaling complex that is regulated by periplasmic or cytoplasmic signals [4,5]. The chemical approach has elucidated the architecture of the cytoplasmic domain in the full-length, membrane-bound S. typhimurium aspartate receptor, whereas the crystallographic approach has solved the high-resolution structure of a major 227-residue fragment from the cytoplasmic domain of the E. coli serine receptor. Overall, the cytoplasmic domains of these receptors are 341 and 337 residues in length, respectively, and exhibit 71% sequence identity, including 86% identity in the signaling subdomain [8,10]. Position numbers are offset by two residues between these two cytoplasmic domains, such that homologous position numbers in the serine receptor are two larger than in the aspartate receptor (except for the C-terminal 10 residues, where a deletion has occurred in the serine receptor). Both new structures reveal that the cytoplasmic domain is a homodimeric, four-helix bundle formed by the intimate association of two long α-helical hairpins, one from each subunit. Schematic Figure 1 displays the cytoplasmic domain in the context of the full-length receptor and identifies the functionally important regions.

Figure 1
Schematic model of the chemotaxis receptor structure, including the cytoplasmic domain architecture established by recent chemical and crystallographic studies [1••,2••]. The different structural and functional regions ...

The cytoplasmic domain is a dimeric, extended, four-helix bundle

The chemical analysis of the cytoplasmic domain architecture, carried out in the intact, membrane-bound aspartate receptor [2••], began with the prior knowledge of large helical regions defined by the aforementioned cysteine scanning and solvent exposure studies ([4345,46••]; also S Winston, JJ Falke, unpublished data). The packing of these helical regions was elucidated by measuring disulfide bond formation rates between a set of 44 cysteine pairs engineered at buried positions to test for specific helix–helix contacts within and between different subunits [2••]. Altogether, 21 of the 44 cysteine pairs rapidly formed disulfide bonds, whereas 23 formed slowly or not at all. This strategy tested only a small subset of the combinatorial possibilities, but the observed 21 rapidly forming cross-links provided sufficient information to define the relative helix positions. Moreover, the 44 cysteine pairs selected for the study were carefully designed to resolve the two conflicting published models of the cytoplasmic domain structure [4,7]. Indeed, the observed pattern of disulfide bond formation excludes a compact architecture formed by the side-to-side association of two short helical bundles; instead, the results establish the architecture of the domain as the extended four-helix bundle displayed in the schematic Figure 2.

Figure 2
Schematic structure of the dimeric aspartate receptor cytoplasmic domain determined by chemical methods (adapted from [2••]), illustrating the 14 functional, symmetric, intersubunit disulfide bonds (dashed lines). The architecture is a ...

Further confirmation of the extended four-helix bundle structure was provided by an analysis of the effects of 188 intersubunit disulfide bonds on receptor activity in the reconstituted receptor–CheW–CheA signaling complex [2••]. Figure 2 displays the seven disulfide bonds found to retain receptor-regulated kinase activity, as well as the seven disulfides that lock the receptor in the kinase-activating state, in both the absence and the presence of saturating aspartate ligand. The signal-retaining and lock-on disulfide bonds bridging the two subunits at the hairpin turn (A387C–A387C′ and F394C–F394C′, respectively) further exclude the compact tertiary model, while supporting the extended structure. Moreover, the presence of the cluster of intersubunit lock-on disulfides in the vicinity of the adaptation sites highlights the importance of this interfacial region to kinase regulation, as discussed further below.

The crystal structure of the serine receptor fragment, corresponding to residues 294–520, provides a very similar picture of the cytoplasmic domain [1••]. The crystal structure, determined to 2.6 Å resolution and illustrated in Figure 3, also reveals an extended four-helix bundle in which the hairpin turn of each subunit lies at the same location identified chemically in the full-length receptor. Moreover, the four helices of the bundle show the same exposed and buried faces detected by the chemical approach. The only significant differences between the two structures are observed near the ends of the fragment, where, during crystal formation, the helical ends of one dimer form coiled coils with two other dimers, causing the four-helix bundle to splay apart in the vicinity of the adaptation sites. This separation of the bundle is not observed in the full-length receptor, wherein multiple signal-retaining and lock-on disulfide bonds are formed between the four helices in the adaptation site region [2••]. Future crystallographic studies of longer cytoplasmic fragments or of type D soluble cytoplasmic domains will ultimately yield a high-resolution view of the adaptation sites.

Figure 3
Ribbon diagram illustrating the dimer structure of the serine receptor cytoplasmic domain (adapted from [1••]). Major adaptive methylation sites are shown as yellow spheres in one subunit and blue spheres in the other. One subunit is in ...

The structural detail provided by the crystallographic approach reveals extensive supercoiling of the helices in the four-helix bundle [1••], as seen in Figure 3. This supercoiling explains the heptad repeating pattern of exposed and buried positions observed in the aspartate receptor using cysteine scanning and solvent exposure measurements, and is a conserved feature of MTPCDs, as indicated by sequence alignment [8,10,4345,46••]. Furthermore, the crystal structure reveals a second heptad repeat superimposed on the first to yield a ‘double-heptad repeat’, arising from the fact that each helix is supercoiled with two other helices [1••]. Overall, assuming that the supercoiled helices extend at least from the adaptation sites to the hairpin turn, the length of the cytoplasmic domain exceeds 130 Å. Thus, as the periplasmic and transmembrane regions are approximately 80 and 40 Å in length, respectively, both the serine and aspartate receptors are highly extended, rod-like molecules with lengths exceeding 250 Å. In fact, if the linker region is also helical, these receptors could be 380 Å in length, as in the model shown in Figure 4. Such extended structures may allow a small ligand-induced conformational or dynamic change in the periplasmic domain to be amplified into a larger change at the regulatory receptor–kinase interface in the cytoplasm [23,26,51].

Figure 4
Model of an intact serine receptor dimer spanning the membrane (adapted from [1••]). (a) Ribbon diagram of the model, scaled to match the dimensions in (b). One subunit is in purple and the other is in cyan. Methylation sites are marked ...

One interesting feature of both the chemical and the crystallographic structures is a group of hydrophobic residues near the hairpin turn of each subunit [1••,2••]. In the crystal, these hydrophobic sidechains, as well as hydrophilic sidechains from the same dimer, interact with residues from two other dimers to form a trimer of dimers, as shown in Figure 5. This observation may help explain the clustering of the E. coli chemoreceptors that is known to occur under certain conditions in vitro and in vivo [5254,55••]. Moreover, the exposed hydrophobic surface could represent a docking site for CheA or CheW, as previously proposed [46••], such that the trimer would separate into individual dimers upon formation of the receptor–CheA–CheW ternary complex. Additional studies are needed to ascertain the physiological roles of the hydrophobic surface and the trimer.

Figure 5
Trimer formation by three dimeric cytoplasmic domains of the serine receptor (adapted from [1••]). Stereo diagram of the trimer of dimers, in which each monomer is colored differently. The methylation sites are shown as small spheres.

Genetic and biochemical data identify functionally important regions of the domain

Genetic and biochemical studies carried out in several laboratories have highlighted the functional importance of specific regions of the cytoplasmic domain. Firstly, studies of aspartate receptor heterodimers in which one cytoplasmic domain is truncated have shown that the linker region is essential for function, suggesting its importance to critical intersubunit or intrasubunit interactions [56,57]. In addition, cysteine scanning and other studies have revealed a high density of mutations in this region that block receptor accumulation or prevent function [42,44]. The poorly characterized structure of the linker region lies outside the region characterized by crystallographic analysis, but is partially defined as α-helical by cysteine scanning and solvent exposure measurements, and by the occurrence of heptad repeat patterns in its primary structure [8,10,40,44]. Secondly, the new structural evidence reviewed herein indicates that the adaptive methylation sites of the dimer lie in close proximity within a narrow band wrapping around the exposed face of the four-helix bundle [1••,2••]. Numerous studies have revealed evidence for the functional importance of the adaptation site region, especially the region containing the three adaptation sites on helix HP1. Mutagenic analysis of the region has identified it as a key sensor of not only adaptation signals in chemotaxis, but also temperature in thermotaxis [58]. Moreover, site-directed mutations and engineered intersubunit disulfide bonds demonstrate that the region is tightly coupled in an unknown way to CheA kinase activation, so that modifications of buried positions within the region often lock the kinase in its activated state [43,59•]. These findings suggest that the helix–helix packing within the adaptation region of the four-helix bundle is critical for kinase regulation. Thirdly, the signaling subdomain retains at least partial kinase regulation as an isolated fragment and mutagenesis studies have implicated this region in the docking of CheA and CheW to form the ternary complex [3639,45,46••,60]. The new structures are in excellent agreement within the signaling subdomain; both reveal a four-helix bundle composed of two helical hairpins [1••,2••]. The exposed hydrophobic residues lying on the surface of the dimer, near the hairpin turn, are probably involved in the intermolecular interactions that stabilize either the ternary complex or receptor clustering, as noted above [46••,53,60].

Remaining structural and mechanistic questions

Although the recently determined structures of MTPCDs represent a first step toward a structural understanding of receptor adaptation and kinase regulation, a number of important pieces are still missing from the mechanistic puzzle. The conserved linker region connecting the second transmembrane helix to the cytoplasmic domain is widely observed in MTP receptors and certain other proteins as well, and is known to be essential for effective signal transduction by chemotaxis receptors. The structure of this linker remains unknown, although it is bordered by α helices and appears to include helical elements [40,41••,4244]. Assuming that these helical regions extend throughout the linker, the simplest working model for the linker structure is an α-helical coiled coil formed by the association of two helices, one from each subunit of the dimer (Figure 4). As the linker appears to be essential for the transmission of information from the transmembrane domain to the cytoplasmic domain, its structural elucidation remains a high priority. Another priority is a high-resolution view of the cytoplasmic four-helix bundle in the vicinity of the adaptation sites, which is crucial to obtaining a molecular understanding of adaptation and the coupling of this region to kinase regulation [59•]. Moreover, although three-dimensional structures are now available for the cytoplasmic domain, CheA and CheW ([61••,6264]; FW Dahlquist, personal communication), the structure of the assembled ternary complex remains unknown. Further structural studies will probe the architecture of the assembled complex using the cytoplasmic domain fragment or a soluble family D receptor as the framework for CheA and CheW docking. Together, high-resolution views of the adaptation sites and the ternary complex will shed light on the mechanism by which the transmembrane and adaptation signals are transmitted by the cytoplasmic domain to the histidine kinase. Finally, recent biochemical studies have revealed evidence of limited cooperative interactions between chemotaxis receptors during kinase regulation, and chemoreceptors are known to cluster at the poles of E. coli in living cells [53,54,55••,65••,66••]. Such clustering is critical to the adaptation of certain receptors that lack the C-terminal docking site for adaptation enzymes that is present on other receptors, including the aspartate and serine receptors [6769]. It is not yet clear, however, whether clustering plays an important role in transmembrane kinase regulation. The trimer of dimers observed in the crystal structure of the cytoplasmic domain may be involved in such higher order interactions, but elucidating the functional role and mechanism of receptor clustering requires further study.


We thank our collaborators at the University of Colorado, University of California and Lawrence Berkeley National Laboratories for their essential contributions, including Randal Bass, Scott Butler, Steve Chervitz, Matthew Coleman, Mark Danielson, Kyeong Kyu Kim, Matthew Trammel, Susanna Winston and Hisao Yokota. This work was supported by National Institutes of Health grants R01 GM-40731 (JJF) and R01 CA-78406 (S-HK), and by funds from the Director, Office of Science, Office of Biological and Environmental Research, US Department of Energy under contract number DE-AC03-76SF00098 (S-HK).


chemotaxis histidine kinase
chemotaxis coupling protein
methyl-accepting taxis protein
MTP cytoplasmic domain

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest

•• of outstanding interest

1••. Kim KK, Yokota H, Kim SH. Four-helical-bundle structure of the cytoplasmic domain of a serine chemotaxis receptor. Nature. 1999;400:787–792. The authors describe the crystal structure of the isolated serine receptor cytoplasmic domain, providing the first high-resolution view of this conserved signaling motif, particularly the functionally critical signaling subdomain. [PubMed]
2••. Bass RB, Falke JJ. The aspartate receptor cytoplasmic domain: in situ chemical analysis of structure, mechanism and dynamics. Structure. 1999;7:829–840. The chemically derived structure of the dimeric aspartate receptor cytoplasmic domain reveals architectural, dynamical and functional features of the four-helix bundle in the full-length, membrane-bound receptor. [PMC free article] [PubMed]
3. Armitage JP. Bacterial tactic responses. Adv Microb Physiol. 1999;41:229–289. [PubMed]
4. 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. [PMC free article] [PubMed]
5. Stock JB, Surette M. Chemotaxis. In: Neidhardt RC, editor. Escherichia Coli and Salmonella Typhimurium: Cellular and Molecular Biology. 2. Washington, DC: ASM Press; 1996. pp. 123–145.
6. Blair DF. How bacteria sense and swim. Annu Rev Microbiol. 1995;49:489–522. [PubMed]
7. Mowbray SL. Chemotaxis receptors: a progress report on structure and function. J Struct Biol. 1998;124:257–275. [PubMed]
8. Le Moual H, Koshland DE. Molecular evolution of the C-terminal cytoplasmic domain of a superfamily of bacterial receptors involved in taxis. J Mol Biol. 1996;261:568–585. [PubMed]
9. Schultz J, Copley RR, Doerks T, Ponting CP, Bork P. SMART: a web-based tool for the study of genetically mobile domains. Nucleic Acids Res. 2000;28:231–234. [PMC free article] [PubMed]
10. Danielson MA. PhD Thesis. Boulder, CO: University of Colorado; 1997. Molecular mechanism of transmembrane signaling and kinase regulation by the aspartate receptor of bacterial chemotaxis.
11. Hoff WD, Jung KH, Spudich JL. Molecular mechanism of photosignaling by archaeal sensory rhodopsins. Annu Rev Biophys Biomol Struct. 1997;26:223–258. [PubMed]
12. Bibikov SI, Biran R, Rudd KE, Parkinson JS. A signal transducer for aerotaxis in Escherichia coli. J Bacteriol. 1997;179:4075–4079. [PMC free article] [PubMed]
13. Rebbapragada A, Johnson MS, Harding GP, Zuccarelli AJ, Fletcher HM, Zhulin IB, Taylor BL. The Aer protein and the serine chemoreceptor Tsr independently sense intracellular energy levels and transduce oxygen, redox, and energy signals for Escherichia coli behavior. Proc Natl Acad Sci USA. 1997;94:10541–10546. [PubMed]
14. Brooun A, Zhang WS, Alam M. Primary structure and functional analysis of the soluble transducer protein HtrXI in the archaeon Halobacterium salinarium. J Bacteriol. 1997;179:2963–2968. [PMC free article] [PubMed]
15. Milburn MV, Prive GG, Milligan DL, Scott WG, Yeh J, Jancarik J, Koshland DE, Kim SH. Three-dimensional structures of the ligand-binding domain of the bacterial aspartate receptor with and without a ligand. Science. 1991;254:1342–1347. [PubMed]
16. Yeh JI, Biemann HP, Pandit J, Koshland DE, Kim SH. The three-dimensional structure of the ligand binding domain of a wild type bacterial chemotaxis receptor: structural comparison to the cross-linked mutant forms and conformational changes upon ligand binding. J Biol Chem. 1993;268:9787–9792. [PubMed]
17. Bowie JU, Pakula AA, Simon MI. The three-dimensional structure of the aspartate receptor from Escherichia coli. Acta Crystallogr. 1995;51:145–154. [PubMed]
18. Yeh JI, Biemann HP, Prive GG, Pandit J, Koshland DE, Kim SH. High resolution structures of the ligand binding domain of the wild type bacterial aspartate receptor. J Mol Biol. 1996;262:186–201. [PubMed]
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. [PubMed]
20. Falke JJ, Dernburg AF, Sternburg DA, Zalkin N, Milligan DL, Koshland DE. Structure of a bacterial sensory receptor: a site directed sulfhydryl study. J Biol Chem. 1988;263:14850–14858. [PubMed]
21. Chervitz SA, Lin CM, Falke JJ. Transmembrane signaling by the aspartate receptor: engineered disulfides reveal static regions of the subunit interface. Biochemistry. 1995;34:9722–9733. [PMC free article] [PubMed]
22. Chervitz SA, Falke JJ. Lock on/off disulfides identify the transmembrane signaling helix of the aspartate receptor. J Biol Chem. 1995;270:24043–24053. [PMC free article] [PubMed]
23. Chervitz SA, Falke JJ. Molecular mechanism of transmembrane signaling by the aspartate receptor: a model. Proc Natl Acad Sci USA. 1996;93:2545–2550. [PubMed]
24. Lee GF, Burrows GG, Lebert MR, Dutton DP, Hazelbauer GL. Deducing the organization of a transmembrane domain by disulfide cross-linking: the bacterial chemoreceptor Trg. J Biol Chem. 1994;269:29920–29927. [PubMed]
25. Lee GF, Hazelbauer GL. Quantitative approaches to utilizing mutational analysis and disulfide cross-linking for modeling a transmembrane domain. Protein Sci. 1995;4:1100–1107. [PubMed]
26. Hughson AG, Hazelbauer GL. Detecting the conformational change of transmembrane signaling in a bacterial chemoreceptor by measuring effects on disulfide cross- linking in vivo. Proc Natl Acad Sci USA. 1996;93:11546–11551. [PubMed]
27. Hughson AG, Lee GF, Hazelbauer GL. Analysis of protein structure in intact cells: crosslinking in vivo between introduced cysteines in the transmembrane domain of a bacterial chemoreceptor. Protein Sci. 1997;6:315–322. [PubMed]
28. Krikos A, Conley MP, Boyd A, Berg HC, Simon MI. Chimeric chemosensory transducers of E. coli. Proc Natl Acad Sci USA. 1985;82:1326–1330. [PubMed]
29. Baumgartner JW, Kim C, Brissette RE, Inouye M, Park C, Hazelbauer GL. Transmembrane signaling by a hybrid protein: communication from the domain of chemoreceptor Trg that recognizes sugar-binding proteins to the kinase/phosphatase domain of osmosensor Envz. J Bacteriol. 1994;176:1157–1163. [PMC free article] [PubMed]
30. Tatsuno I, Lee L, Kawagishi I, Homma M, Imae Y. Transmembrane signaling by the chimeric chemosensory receptors of E. coli Tsr and Tar with heterologous membrane-spanning regions. Mol Microbiol. 1994;14:755–762. [PubMed]
31. Zhang XN, Zhu JY, Spudich JL. The specificity of interaction of archaeal transducers with their cognate sensory rhodopsins is determined by their transmembrane helices. Proc Natl Acad Sci USA. 1999;96:857–862. [PubMed]
32. Mowbray SL, Foster DL, Koshland DE. Proteolytic fragments identified with domains of the aspartate chemoreceptor. J Biol Chem. 1985;260:11711–11718. [PubMed]
33. Long DG, Weis RM. Oligomerization of the cytoplasmic fragment from the aspartate receptor of Escherichia coli. Biochemistry. 1992;31:9904–9911. [PubMed]
34. Seeley SK, Weis RM, Thompson LK. The cytoplasmic fragment of the aspartate receptor displays globally dynamic behavior. Biochemistry. 1996;35:5199–5206. [PubMed]
35. Stock JB, Lukat GS, Stock AM. Bacterial chemotaxis and the molecular logic of intracellular signal transduction networks. Annu Rev Biophys Chem. 1991;20:109–136. [PubMed]
36. Ames P, Yu YA, Parkinson JS. Methylation segments are not required for chemotactic signaling by cytoplasmic fragments of Tsr, the methyl-accepting serine chemoreceptor of E. coli. Mol Microbiol. 1996;19:737–746. [PubMed]
37. Ames P, Parkinson JS. Constitutively signaling fragments of Tsr, the E. coli serine chemoreceptor. J Bacteriol. 1994;176:6340–6348. [PMC free article] [PubMed]
38. 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. [PubMed]
39. Cochran AG, Kim PS. Imitation of E. coli aspartate receptor signaling in engineered dimers of the cytoplasmic domain. Science. 1996;271:1113–1116. [PubMed]
40. Singh M, Berger B, Kim PS, Berger JM, Cochran AG. Computational learning reveals coiled coil-like motifs in histidine kinase linker domains. Proc Natl Acad Sci USA. 1998;95:2738–2743. [PubMed]
41••. Aravind L, Ponting CP. The cytoplasmic helical linker domain of receptor histidine kinase and methyl-accepting proteins is common to many prokaryotic signalling proteins. FEMS Microbiol Lett. 1999;176:111–116. This paper defines the conserved HAMP domain shared by various signaling proteins, including the linker regions of methyl-accepting taxis proteins. [PubMed]
42. Williams SB, Stewart V. Functional similarities among two-component sensors and methyl-accepting chemotaxis proteins suggest a role for linker region amphipathic helices in transmembrane signal transduction. Mol Microbiol. 1999;33:1093–1102. [PubMed]
43. Danielson MA, Bass RB, Falke JJ. Cysteine and disulfide scanning reveals a regulatory alpha-helix in the cytoplasmic domain of the aspartate receptor. J Biol Chem. 1997;272:32878–32888. [PMC free article] [PubMed]
44. Butler SL, Falke JJ. Cysteine and disulfide scanning reveals two amphiphilic helices in the linker region of the aspartate chemoreceptor. Biochemistry. 1998;37:10746–10756. [PMC free article] [PubMed]
45. Bass RB, Falke JJ. Detection of a conserved alpha-helix in the kinase-docking region of the aspartate receptor by cysteine and disulfide scanning. J Biol Chem. 1998;273:25006–25014. [PMC free article] [PubMed]
46••. Bass RB, Coleman MD, Falke JJ. Signaling domain of the aspartate receptor is a helical hairpin with a localized kinase docking surface: cysteine and disulfide scanning studies. Biochemistry. 1999;38:9317–9327. Chemical evidence reveals the dimerization of two helical hairpins, one from each subunit of the dimer, to form a four-helix bundle in the cytoplasmic domain of the membrane-bound aspartate receptor. [PMC free article] [PubMed]
47. Falke JJ, Sternberg DW, Koshland DE. Site-directed chemistry and spectroscopy: applications in the aspartate receptor system. Biophys J. 1986;49:20.
48. Falke JJ, Koshland DE. Global flexibility in a sensory receptor: a site directed disulfide approach. Science. 1987;237:1596–1600. [PubMed]
49. Careaga CL, Falke JJ. Thermal motions of surface alpha-helices in the D-galactose chemosensory receptor. Detection by disulfide trapping. J Mol Biol. 1992;226:1219–1235. [PMC free article] [PubMed]
50. Sahin-Toth M, Kaback HR. Cysteine scanning mutagenesis of putative transmembrane helices IX and X in the lactose permease of Escherichia coli. Protein Sci. 1993;2:1024–1033. [PubMed]
51. Kim SH. Frozen dynamic dimer model for transmembrane signaling in bacterial chemotaxis receptors. Protein Sci. 1994;3:159–165. [PubMed]
52. Long DG, Weis RM. Oligomerization of the cytoplasmic fragment from the aspartate receptor of E. coli. Biochemistry. 1992;31:9904–9911. [PubMed]
53. Maddock JR, Shapiro L. Polar location of the chemoreceptor complex in the E. coli cell. Science. 1993;259:1717–1723. [PubMed]
54. Skidmore JM, Ellefson DD, McNamara BP, Couto MMP, Wolfe AJ, Maddock JR. Polar clustering of the chemoreceptor complex in E. coli occurs in the absence of complete CheA function. J Bacteriol. 2000;182:967–973. [PMC free article] [PubMed]
55••. Stock J, Levit N. Signal transduction: hair brains in bacterial chemotaxis. Curr Biol. 2000;10:11–14. A recent summary of evidence that the cytoplasmic domain of the E. coli chemoreceptor may form higher order oligomers in vitro and in vivo. [PubMed]
56. Tatsuno I, Homma M, Oosawa K, Kawagishi I. Signaling by the E. coli aspartate chemoreceptor Tar with a single cytoplasmic domain per dimer. Science. 1996;274:423–425. [PubMed]
57. Gardina PJ, Manson MD. Attractant signaling by an aspartate chemoreceptor dimer with a single cytoplasmic domain. Science. 1996;274:425–426. [PubMed]
58. Nishiyama S, Umemura T, Nara T, Homma M, Kawagishi I. Conversion of a bacterial warm sensor to a cold sensor by methylation of a single residue in the presence of an attractant. Mol Microbiol. 1999;32:357–365. [PubMed]
59•. Trammell MA, Falke JJ. Identification of a site critical for kinase regulation on the central processing unit (CPU) helix of the aspartate receptor. Biochemistry. 1999;38:329–336. Evidence that a region of the subunit interface within the aspartate receptor cytoplasmic domain is tightly coupled to kinase regulation is presented. [PMC free article] [PubMed]
60. Liu JD, Parkinson JS. Genetic-evidence for interaction between the Chew and Tsr proteins during chemoreceptor signaling by E. coli. J Bacteriol. 1991;173:4941–4951. [PMC free article] [PubMed]
61••. Bilwes AM, Alex LA, Crane BR, Simon MI. Structure of CheA, a signal-transducing histidine kinase. Cell. 1999;96:131–141. The authors describe the crystal structure of the catalytic and dimerization domains of CheA, the histidine kinase regulated by the serine and aspartate receptor cytoplasmic domains. [PubMed]
62. Zhou HJ, Dahlquist FW. Phosphotransfer site of the chemotaxis-specific protein kinase CheA as revealed by NMR. Biochemistry. 1997;36:699–710. [PubMed]
63. McEvoy MM, Muhandiram DR, Kay LE, Dahlquist FW. Structure and dynamics of a CheY-binding domain of the chemotaxis kinase CheA determined by nuclear-magnetic-resonance spectroscopy. Biochemistry. 1996;35:5633–5640. [PubMed]
64. Djordjevic S, Stock AM. Structural analysis of bacterial chemotaxis proteins: components of a dynamic signaling system. J Struct Biol. 1998;124:189–200. [PubMed]
65••. Li GY, Weis RM. Covalent modification regulates ligand binding to receptor complexes in the chemosensory system of E. coli. Cell. 2000;100:357–365. Positive cooperativity in ligand-induced kinase regulation suggests that two or more serine receptor dimers may interact in cooperative oligomers. [PubMed]
66••. Bornhorst JA, Falke JJ. Attractant regulation of the aspartate receptor-kinase complex: limited cooperative interactions between receptors and effects of receptor modification state. Biochemistry. 2000 in press. Ligand-induced kinase regulation exhibits limited positive cooperativity between a small number of aspartate receptor dimers. [PMC free article] [PubMed]
67. Wu JG, Li JY, Li GY, Long DG, Weis RM. The receptor-binding site for the methyltransferase of bacterial chemotaxis is distinct from the sites of methylation. Biochemistry. 1996;35:4984–4993. [PubMed]
68. Li JY, Li GY, Weis RM. The serine chemoreceptor from E. coli is methylated through an inter-dimer process. Biochemistry. 1997;36:11851–11857. [PubMed]
69. Le Moual H, Quang T, Koshland DE. Methylation of the E. coli chemotaxis receptors: intra- and interdimer mechanisms. Biochemistry. 1997;36:13441–13448. [PubMed]