Attractant signaling by an aspartate chemoreceptor dimer with a single cytoplasmic domain

Sequence-related behaviour of transmembrane domains from class I receptor tyrosine kinases

2H NMR spectroscopy and freeze-fracture electron microscopy were used to compare the transmembrane domains of two Class I protein receptor tyrosine kinases (the EGF receptor and Neu/erbB-2) regarding overall behaviour in fluid lipid bilayer membranes. The 34-residue peptide, EGFRtm, was synthesised to contain the 23 amino acid hydrophobic stretch (Ile622 to Met644) thought to span the membrane of the human EGF receptor, plus the first 10 amino acids (Arg645 to Thr654) of the cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at sites corresponding to Ala623, Met644, and Val650. The 38-residue peptide, Neutm, was synthesised having the 21 residue hydrophobic stretch (Ile660 to Ile680) calculated to span the membrane in rat Neu/erbB-2, plus residues Lys681 to Thr691 of the contiguous cytoplasmic domain. Deuterium probes replaced selected 1H nuclei at Ala661, Leu667, and Val676. A third peptide, Neutm*, was also prepared, corresponding to the transmembrane domain of a constitutively-activating Neu/erbB-2 transformant in which Val664 is replaced by Glu: it was deuterated in a manner identical to Neutm. Peptides were studied by 2H NMR spectroscopy at 1 mol% and 6 mol% in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and in POPC containing 33 mol% cholesterol, over the range 12 degrees to 65 degreesC. Overall motion was found to be different for each of the three peptides under a given set of conditions. EGFRtm spectra were characteristic of axially symmetric motion in membranes of POPC alone, and in POPC/cholesterol at 35 degreesC and above. In contrast, spectra of the transmembrane peptides, Neutm and Neutm*, were characteristic of significantly axially asymmetric motion under all conditions studied (and regardless of sample preparation method). Addition of 33% cholesterol to membranes was accompanied by spectral changes consistent with increased formation of peptide dimers/oligomers in all cases. The transformant peptide, Neutm*, showed greater spectral evidence of immobilisation than did the wild type - probably reflecting a greater tendency to form large oligomers. Sequence-related details within the transmembrane domains of Class I receptor tyrosine kinases appear to exert important control over their associations within membranes. Freeze-fracture electron microscopy of the NMR samples demonstrated their liposomal nature. Peptide-related intramembranous particles (IMPs) were present which likely represent oligomers of the transmembrane peptide. IMP size and distribution were similar under a given set of conditions for all three peptides, suggesting that the differences seen by NMR spectroscopy reflect structures smaller than the 2 nm resolution limit of freeze-fracture EM and peptide relationships within its 20 nm accuracy of identifying lateral position.

Receptor clustering as a cellular mechanism to control sensitivity

Chemotactic bacteria such as Escherichia coli can detect and respond to extremely low concentrations of attractants, concentrations of less than 5 nM in the case of aspartate. They also sense gradients of attractants extending over five orders of magnitude in concentration (up to 1 mM aspartate). Here we consider the possibility that this combination of sensitivity and range of response depends on the clustering of chemotactic receptors on the surface of the bacterium. We examine what will happen if ligand binding changes the activity of a receptor, propagating this change in activity to neighbouring receptors in a cluster. Calculations based on these assumptions show that sensitivity to extracellular ligands increases with the extent of spread of activity through an array of receptors, but that the range of concentrations over which the array works is severely diminished. However, a combination of low threshold of response and wide dynamic range can be attained if the cell has both clusters and single receptors on its surface, particularly if the extent of activity spread can adapt to external conditions. A mechanism of this kind can account quantitatively for the sensitivity and response range of E. coli to aspartate.

Suppressor mutation analysis of the sensory rhodopsin I-transducer complex: insights into the color-sensing mechanism

The molecular complex containing the phototaxis receptor sensory rhodopsin I (SRI) and transducer protein HtrI (halobacterial transducer for SRI) mediates color-sensitive phototaxis responses in the archaeon Halobacterium salinarum. One-photon excitation of the complex by orange light elicits attractant responses, while two-photon excitation (orange followed by near-UV light) elicits repellent responses in swimming cells. Several mutations in SRI and HtrI cause an unusual mutant phenotype, called orange-light-inverted signaling, in which the cell produces a repellent response to normally attractant light. We applied a selection procedure for intragenic and extragenic suppressors of orange-light-inverted mutants and identified 15 distinct second-site mutations that restore the attractant response. Two of the 3 suppressor mutations in SRI are positioned at the cytoplasmic ends of helices F and G, and 12 suppressor mutations in HtrI cluster at the cytoplasmic end of the second HtrI transmembrane helix (TM2). Nearly all suppressors invert the normally repellent response to two-photon stimulation to an attractant response when they are expressed with their suppressible mutant alleles or in an otherwise wild-type strain. The results lead to a model for control of flagellar reversal by the SRI-HtrI complex. The model invokes an equilibrium between the A (reversal-inhibiting) and R (reversal-stimulating) conformers of the signaling complex. Attractant light and repellent light shift the equilibrium toward the A and R conformers, respectively, and mutations are proposed to cause intrinsic shifts in the equilibrium in the dark form of the complex. Differences in the strength of the two-photon signal inversion and in the allele specificity of suppression are correlated, and this correlation can be explained in terms of different values of the equilibrium constant (Keq) for the conformational transition in different mutants and mutant-suppressor pairs.

Signal transduction in bacteria: molecular mechanisms of stimulus-response coupling

In bacteria, adaptive responses to changing environmental conditions are mediated by signal transduction systems that involve modular protein domains. Despite great diversity in the integration of domains into different systems, studies of individual components have revealed molecular strategies that are widely applicable. Studies of receptors have advanced our understanding of how information is transmitted across membranes, the determination of three-dimensional structures of domains of histidine protein kinase domains and response regulator proteins has begun to reveal the molecular basis of signaling via two-component phosphoryltransfer pathways, and the description of 'eukaryotic-like' protein domains involved in bacterial signaling has emphasized the universality of intracellular signaling mechanisms.

Chemotactic adaptation is altered by changes in the carboxy-terminal sequence conserved among the major methyl-accepting chemoreceptors

In Escherichia coli and Salmonella typhimurium, methylation and demethylation of receptors are responsible for chemotactic adaptation and are catalyzed by the methyltransferase CheR and the methylesterase CheB, respectively. Among the chemoreceptors of these species, Tsr, Tar, and Tcp have a well-conserved carboxy-terminal motif (NWET/SF) that is absent in Trg and Tap. When they are expressed as sole chemoreceptors, Tsr, Tar, and Tcp support good adaptation, but Trg and Tap are poorly methylated and supported only weak adaptation. It was recently discovered that CheR binds to the NWETF sequence of Tsr in vitro. To examine the physiological significance of this binding, we characterized mutant receptors in which this pentapeptide sequence was altered. C-terminally-mutated Tar and Tcp expressed in a receptorless E. coli strain mediated responses to aspartate and citrate, respectively, but their adaptation abilities were severely impaired. Their expression levels and attractant-sensing abilities were similar to those of the wild-type receptors, but the methylation levels of the mutant receptors increased only slightly upon addition of attractants. When CheR was overproduced, both the adaptation and methylation profiles of the mutant Tar receptor became comparable to those of wild-type Tar. Furthermore, overproduction of CheR also enhanced adaptive methylation of wild-type Trg, which lacks the NWETF sequence, in the absence of any other chemoreceptor. These results suggest that the pentapeptide sequence facilitates effective adaptation and methylation by recruiting CheR.

Dimerization of the polymeric immunoglobulin receptor controls its transcytotic trafficking

Binding of dimeric immunoglobulin (Ig)A to the polymeric Ig receptor (pIgR) stimulates transcytosis of pIgR across epithelial cells. Through the generation of a series of pIgR chimeric constructs, we have tested the ability of ligand to promote receptor dimerization and the subsequent role of receptor dimerization on its intracellular trafficking. Using the cytoplasmic domain of the T cell receptor-zeta chain as a sensitive indicator of receptor oligomerization, we show that a pIgR:zeta chimeric receptor expressed in Jurkat cells initiates a zeta-specific signal transduction cascade when exposed to dimeric or tetrameric IgA, but not when exposed to monomeric IgA. In addition, we replaced the pIgR's transmembrane domain with that of glycophorin A to force dimerization or with a mutant glycophorin transmembrane domain to prevent dimerization. Forcing dimerization stimulated transcytosis of the chimera, whereas preventing dimerization abolished ligand-stimulated transcytosis. We conclude that binding of dimeric IgA to the pIgR induces its dimerization and that this dimerization is necessary and sufficient to stimulate pIgR transcytosis.

Bacterial chemotaxis: the five sensors of a bacterium

The components of the Escherichia coli chemosensory system have been identified and their activities characterized, but how sensory information is processed to give an integrated response remains an open question.

Differential use of the betaL subunit of the type I interferon (IFN) receptor determines signaling specificity for IFNalpha2 and IFNbeta

The signaling specificity for cytokines that have common receptor subunits is achieved by the presence of additional cytokine-specific receptor components. In the type I interferon (IFN) family, all 14 subtypes of IFNalpha, IFNbeta, and IFNomega bind to the same alpha and betaL subunits of the type I IFN-R, yet differences in signaling and biological effects exist among them. Our data demonstrate that IFNalpha2 and IFNbeta utilize different regions of the betaL subunit for signaling. Thus, in contrast to other cytokine systems, signal diversity in the type I IFN system can be accomplished within the same receptor complex by utilizing different regions of the same receptor subunits.

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

The Tap (taxis toward peptides) receptor and the periplasmic dipeptide-binding protein (DBP) of Escherichia coli together mediate chemotactic responses to dipeptides. Tap is a low-abundance receptor. It is present in 5- to 10-fold-fewer copies than high-abundance receptors like Tar and Tsr. Cells expressing Tap as the sole receptor, even from a multicopy plasmid at 5- to 10-fold-overexpressed levels, do not generate sufficient clockwise (CW) signal to tumble and thus swim exclusively smoothly (run). To study the signaling properties of Tap in detail, we constructed reciprocal hybrids between Tap and Tar fused in the linker region between the periplasmic and cytoplasmic domains. The Tapr hybrid senses dipeptides and is a good CW-signal generator, whereas the Tarp hybrid senses aspartate but is a poor CW-signal generator. Thus, the poor CW signaling of Tap is a property of its cytoplasmic domain. Eighteen residues at the carboxyl terminus of high-abundance receptors, including the NWETF sequence that binds the CheR methylesterase, are missing in Tap. The Tart protein, created by removing these 18 residues from Tar, has diminished CW-signaling ability. The Tapl protein, made by adding the last 18 residues of Tar to the carboxyl terminus of Tap, also does not support CW flagellar rotation. However, Tart and Tapl cross-react well with antibody directed against the conserved cytoplasmic region of Tsr, whereas Tap does not cross-react with this antibody. Tap does cross-react, however, with antibody directed against the low-abundance chemoreceptor Trg. The hybrid, truncated, and extended receptors exhibit various levels of methylation. However, Tar and Tapl, which contain a consensus CheR-binding motif (NWETF) at their carboxyl termini, exhibit the highest basal levels of methylation, as expected. We conclude that no simple correlation exists between the abundance of a receptor, its methylation level, and its CW-signaling ability.

Receptor-mediated protein kinase activation and the mechanism of transmembrane signaling in bacterial chemotaxis

Chemotaxis responses of Escherichia coli and Salmonella are mediated by type I membrane receptors with N-terminal extracytoplasmic sensing domains connected by transmembrane helices to C-terminal signaling domains in the cytoplasm. Receptor signaling involves regulation of an associated protein kinase, CheA. Here we show that kinase activation by a soluble signaling domain construct involves the formation of a large complex, with approximately 14 receptor signaling domains per CheA dimer. Electron microscopic examination of these active complexes indicates a well defined bundle composed of numerous receptor filaments. Our findings suggest a mechanism for transmembrane signaling whereby stimulus-induced changes in lateral packing interactions within an array of receptor-sensing domains at the cell surface perturb an equilibrium between active and inactive receptor-kinase complexes within the cytoplasm.

Expression of dopamine D3 receptor dimers and tetramers in brain and in transfected cells

The expression and characteristics of the dopamine D3 receptor protein were studied in brain and in stably transfected GH3 cells. Monoclonal antibodies were used for immunoprecipitation and immunoblot experiments. Immunoprecipitates obtained from primate and rodent brain tissues contain a low molecular weight D3 protein and one or two larger protein species whose molecular mass are integral multiples of the low molecular weight protein and thus appear to have resulted from dimerization and tetramerization of a D3 monomer. Whereas D3 receptor multimers were found to be abundantly expressed in brain, the major D3 immunoreactivity expressed in stable D3-expressing rat GH3 cells was found to be a monomer. However, multimeric D3 receptor species with electrophoretic mobilities similar to those expressed in brain were also seen in D3-expressing GH3 cells when a truncated D3-like protein (named D3nf) was co-expressed in these cells. Furthermore, results from immunoprecipitation experiments with D3- and D3nf-specific antibodies show that the higher-order D3 proteins extracted from brain and D3/D3nf double transfectants also contain D3nf immunoreactivity, and immunocytochemical studies show that the expression of D3 and D3nf immunoreactivities overlaps substantially in monkey and rat cortical neurons. Altogether, these data show oligomeric D3 receptor protein expression in vivo and they suggest that at least some of these oligomers are heteroligomeric protein complexes containing D3 and the truncated D3nf protein.

Thermosensing properties of mutant aspartate chemoreceptors with methyl-accepting sites replaced singly or multiply by alanine

The aspartate chemoreceptor Tar has a thermosensing function that is modulated by covalent modification of its four methylation sites (Gln295, Glu302, Gln309, and Glu491). Without posttranslational deamidation, Tar has no thermosensing ability. When Gln295 and Gln309 are deamidated to Glu, the unmethylated and heavily methylated forms function as warm and cold sensors, respectively. In this study, we carried out alanine-scanning mutagenesis of the methylation sites. Although alanine substitutions influenced the signaling bias and the methylation level, all of the mutants retained aspartate-sensing function. Those with single substitutions had almost normal thermosensing properties, indicating that substitutions at any particular methylation site do not seriously impair thermosensing function. In the posttranslational modification-defective background, some of the alanine substitutions restored thermosensing ability. Warm sensors were found among mutants retaining two glutamate residues, and cold sensors were found among those with one or no glutamate residue. This result suggests that the negative charge at the methylation sites is one factor that determines thermosensor phenotypes, although the size and shape of the side chain may also be important. The warm, cold, and null thermosensor phenotypes were clearly differentiated, and no intermediate phenotypes were found. Thus, the different thermosensing phenotypes that result from covalent modification of the methylation sites may reflect distinct structural states. Broader implications for the thermosensing mechanism are also discussed.

Converting a transmembrane receptor to a soluble receptor: recognition domain to effector domain signaling after excision of the transmembrane domain

The bacterial aspartate receptor was reconstructed to eliminate the transmembrane domain, thus connecting the recognition domain directly to the effector domain. The resulting soluble receptor folded correctly and was no longer an integral membrane protein. Upon aspartate binding, this soluble receptor was stabilized to a similar extent as that of the native receptor. Of interest, this soluble receptor retained the ability to signal from the recognition to the effector domain. This result defines more clearly the role of the membrane and transmembrane domains in signal transduction and suggests that some ligand-induced motions in receptor proteins do not require the membrane or transmembrane domain for information transmission.

Apo structure of the ligand-binding domain of aspartate receptor from Escherichia coli and its comparison with ligand-bound or pseudoligand-bound structures

The aspartate receptor from E. coli is a dimeric transmembrane-signaling protein that mediates chemotaxis behavior and is the most studied system among the chemotaxis receptors to understand the molecular mechanism for transmembrane signaling. However, there is an unresolved issue for the structural event which initiates the transmembrane signal upon binding to the ligand. Biochemical and genetic evidence implies an intrasubunit mechanism (monomeric model) whereas crystallographic evidence implies an intersubunit mechanism (dimeric model). Crystallographic evidence has been ambiguous because all the apo protein structures contained a pseudoligand sulfate, and a completely ligand-free structure has not been available thus far. Here we present the crystal structure of the ligand binding domain of the aspartate receptor free of the ligand aspartate or pseudoligand sulfate. The structural comparison of this structure with those of ligand-bound and pseudoligand-bound forms revealed that, on ligand or pseudoligand binding, the conformational change in the ligand-binding domain is relatively small, but there is a considerable rotation between two subunits, supporting the dimeric model.

Uncoupling of ligand-binding affinity of the bacterial serine chemoreceptor from methylation- and temperature-modulated signaling states

The Escherichia coli chemoreceptor Tsr mediates tactic responses to serine, repellents, and changes in temperature. We have previously shown that the serine-sensing ability of Tsr-T156C, which has a unique cysteine in place of threonine at residue 156, is specifically inactivated by thiol-modifying reagents and that L-serine protects the receptor from modification. In this study, we demonstrated the correlation between protective effects of various attractants and their potencies to elicit attractant responses. This indirect binding assay was used to monitor the affinity of the receptor for L-serine under various conditions. It has been demonstrated by in vitro assays that the ligand-binding affinities of Tsr and the related chemoreceptor Tar are unaffected by changes in the methylation state of the receptor. Using the serine protection assay, we re-examined this issue both in vitro and in vivo. The methylation levels of Tsr-T156C did not affect its ligand-binding affinity. We also showed both in vitro and in vivo that the ligand-binding affinity was unaffected by temperature. These results suggest that the structure of the periplasmic domain of the receptor is uncoupled from the signaling states of the cytoplasmic domain. This ligand-binding assay system should be applicable to other receptors.

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

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

Molecular mechanism of photosignaling by archaeal sensory rhodopsins

Two sensory rhodopsins (SRI and SRII) mediate color-sensitive phototaxis responses in halobacteria. These seven-helix receptor proteins, structurally and functionally similar to animal visual pigments, couple retinal photoisomerization to receptor activation and are complexed with membrane-embedded transducer proteins (HtrI and HtrII) that modulate a cytoplasmic phosphorylation cascade controlling the flagellar motor. The Htr proteins resemble the chemotaxis transducers from Escherichia coli. The SR-Htr signaling complexes allow studies of the biophysical chemistry of signal generation and relay, from the photobiophysics of initial excitation of the receptors to the final output at the level of the flagellar motor switch, revealing fundamental principles of sensory transduction and more broadly the nature of dynamic interactions between membrane proteins. We review here recent advances that have led to new insights into the molecular mechanism of signaling by these membrane complexes.

Signaling by the Escherichia coli aspartate chemoreceptor Tar with a single cytoplasmic domain per dimer

Many transmembrane receptors are oligomeric proteins. Binding of a ligand may alter the oligomeric state of the receptor, induce structural changes within the oligomer, or both. The bacterial aspartate chemoreceptor Tar forms a homodimer in the presence or absence of ligands. Tar mediates attractant and repellent responses by modulating the activity of the cytoplasmic kinase CheA. In vivo intersubunit suppression was used to show that certain combinations of full-length and truncated mutant Tar proteins complemented each other to restore attractant responses to aspartate. These results suggest that heterodimers with only one intact cytoplasmic domain are functional. The signaling mechanism may require interactions between dimers or conformational changes within a single cytoplasmic domain.