Receptors and transmembrane signaling

A ligand-induced switch in the periplasmic domain of sensor histidine kinase CitA

Sensor histidine kinases of two-component signal-transduction systems are essential for bacteria to adapt to variable environmental conditions. However, despite their prevalence, it is not well understood how extracellular signals such as ligand binding regulate the activity of these sensor kinases. CitA is the sensor histidine kinase in Klebsiella pneumoniae that regulates the transport and anaerobic metabolism of citrate in response to its extracellular concentration. We report here the X-ray structures of the periplasmic sensor domain of CitA in the citrate-free and citrate-bound states. A comparison of the two structures shows that ligand binding causes a considerable contraction of the sensor domain. This contraction may represent the molecular switch that activates transmembrane signaling in the receptor.

A new paradigm for hormone recognition and allosteric receptor activation revealed from structural studies of NPR-C

The natriuretic peptide system of hormones and receptors poses an abundance of interesting biophysical questions regarding receptor structure, hormone recognition, and receptor activation. Functional and biochemical data have implicated a series of conformational changes as the mechanism by which NP receptor activation is achieved. We have explored the structural basis of hormone recognition by the NP clearance receptor, termed NPR-C. While NPR-C does not contain the classical guanylyl-cyclase activity in its intracellular domains, its extracellular domain is highly similar to the GC-coupled members of this family. The 1:2 stoichiometry of hormone binding to NPR-C is also used by NPR-A and -B to bind hormones. The structure of NPR-C in both quiescent and hormone-bound forms reveals the hormone intercalates within the interface of a receptor dimer, inducing a large-scale conformational change in the membrane proximal regions. This mechanism of hormone recognition will be conserved across the entire NPR family. The allosteric response of the NPR-C ectodomain to ligand binding is likely a glimpse of the general activation signal of these receptors, despite their differing downstream signaling cascades. In this review, we discuss our results on NPR-C and their relevance to the NPR family as a whole, as well as its place as a basic new paradigm for receptor activation.

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

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

Agonist-independent desensitization and internalization of the human platelet-activating factor receptor by coumermycin-gyrase B-induced dimerization

Platelet-activating factor (PAF) is a phospholipid with potent and diverse physiological actions, particularly as a mediator of inflammation. We have reported previously that mutant G protein-coupled receptors (GPCRs) affect the functional properties of coexpressed wild-type human PAF receptor (hPAFR) (Le Gouill, C., Parent, J. L., Caron, C. A., Gaudreau, R., Volkov, L., Rola-Pleszczynski, M., and Stankova, J. (1999) J. Biol. Chem. 274, 12548-12554). Increasing evidence suggests that dimerization of GPCRs may play an important role in the regulation of their biological activity. Additional data have also suggested that dimerization may be important in the subsequent internalization of the delta-opioid receptor. To investigate the specific role of dimerization in the internalization process of GPCRs, we generated a fusion protein of hPAFR and bacterial DNA gyrase B (GyrB), dimerized through the addition of coumermycin. We found that dimerization potentiates PAF-induced internalization of hPAFR-GyrB in Chinese hamster ovary cells stably expressing c-Myc-hPAFR-GyrB. Coumermycin-driven dimerization was also sufficient to induce an agonist-independent sequestration process in an arrestin- and clathrin-independent manner. Moreover, the protein kinase C inhibitors staurosporine and GF109203X blocked the coumermycin-induced desensitization of hPAFR-GyrB, suggesting the implication of protein kinase C in the molecular mechanism mediating the agonist-independent desensitization of the receptor. Taken together, these findings suggest a novel mechanism of GPCR desensitization and internalization triggered by dimerization.

Agonist-specific structural rearrangements of integrin alpha IIbbeta 3. Confirmation of the bent conformation in platelets at rest and after activation

Concrete structural features of integrin alpha(IIb)beta(3) on the surface of platelets (at rest and after activation) have been obtained from epitope maps based on cross-competition among monoclonal antibodies directed against the alpha(IIb) subunit calf-2 domain and the beta(3) subunit betaA domain of alpha(IIb)beta(3). At rest, the observed intersubunit interface is formed by the sequence stretches beta(3)-(150-216), alpha(IIb) light chain-(1-92), and alpha(IIb) heavy chain-(826-856); and the alpha(IIb) interchain interface is formed by the two latter sequence stretches, disulfide-bonded between alpha(IIb) heavy chain Cys(826) and alpha(IIb) light chain Cys(9). These structural features agree with those observed in the alpha(IIb)beta(3) rudimentary connectivity map in solution and with the alpha(v)beta(3) V-shaped crystal structure (Xiong, J.-P., Zhang, R., Dunker, R., Scott, D. L., Joachimiak, A., Goodman, S. L., and Arnaout, M. A. (2001) Science 294, 339-345), but they disagree with the domain disposition suggested by the actual ultrastructural model. The epitope maps in platelets activated by ADP, thrombin receptor activation peptide, and arachidonic acid differ not only from those in platelets at rest, but also among themselves. The structural rearrangements observed confirm the presence in activated platelets of the crystallographically observed knee and argue against the switchblade mechanism proposed for activation (Beglova, N., Blacklow, S. C., Takagi, J., and Springer, T. A. (2002) Nat. Struct. Biol. 9, 282-287), demonstrate the existence of alpha(IIb)beta(3) agonist-specific activation states, explain the specificity for ligand binding and functional inhibition for some agonists, and predict the existence of agonist-specific final effectors and receptor activation mechanisms. The distinct non-reciprocal competition patterns observed at rest and after activation support the agonist-specific activation states and the existence of intrasubunit and intersubunit allosteric effects, previously proposed as the mechanism for alpha(IIb)beta(3) transmembrane activation.

A piston model for transmembrane signaling of the aspartate receptor

To characterize the mechanism by which receptors propagate conformational changes across membranes, nitroxide spin labels were attached at strategic positions in the bacterial aspartate receptor. By collecting the electron paramagnetic resonance spectra of these labeled receptors in the presence and absence of the ligand aspartate, ligand binding was shown to generate an approximately 1 angstrom intrasubunit piston-type movement of one transmembrane helix downward relative to the other transmembrane helix. The receptor-associated phosphorylation cascade proteins CheA and CheW did not alter the ligand-induced movement. Because the piston movement is very small, the ability of receptors to produce large outcomes in response to stimuli is caused by the ability of the receptor-coupled enzymes to detect small changes in the conformation of the receptor.

HtrI is a dimer whose interface is sensitive to receptor photoactivation and His-166 replacements in sensory rhodopsin I

Single cysteine substitutions were introduced into three positions of otherwise cysteineless HtrI, a phototaxis transducer found in Halobacterium salinarum that transmits signals from the photoreceptor sensory rhodopsin I (SRI) to a cytoplasmic pathway controlling the cell's motility. Oxidative cross-linking of the monocysteine HtrI mutants in membrane suspensions resulted in dimer forms evident in SDS-polyacrylamide gels. The rate of cross-linking of I64C on the cytoplasmic side of HtrI was accelerated by SRI binding in the dark and further increased by SRI photoactivation. Several residue replacements of His-166 in SRI accelerated the cross-linking rate of I64C in the dark and His-166 mutants that exhibit "inverted signaling" (mediating repellent instead of the normally attractant response to orange light) inverted the light effect on the cross-linking rate of I64C. Secondary structure prediction of HtrI indicates a coiled coil structure in the cytoplasmic region following TM2, a dimerization domain found in a diverse group of proteins. We conclude that 1) HtrI exists as a dimer both in the absence of SRI and in the SRI-HtrI complex, 2) binding of SRI in the dark increases reactivity of the two cysteines at position 64 in the dimer by increasing their proximity or mobility, 3) light activation of wild-type SRI further increases their reactivity, 4) His-166 replacements in the SRI receptor have conformational effects on the structure of HtrI at position 64, and 5) inverted signaling by His-166 mutants likely results from an inverted conformational change at this region induced by SRI photoactivation.

Functional rescue of a constitutively desensitized beta2AR through receptor dimerization

We have recently demonstrated that wild-type beta2-adrenergic receptors (beta2AR) form homodimers and that disruption of receptor dimerization inhibits signalling via Gs [Hebert, Moffett, Morello, Loisel, Bichet, Barret and Bouvier (1996) J. Biol. Chem. 271, 16384-16392]. Here taking advantage of the altered functional properties of a non-palmitoylated, constitutively desensitized mutant beta2AR (C341Gbeta2AR), we sought to study whether physical interactions between mutant and wild-type beta2AR expressed in Sf9 cells could occur and have functional consequences. Using metabolic labelling with [3H]palmitate and co-immunoprecipitation we demonstrated the existence of heterodimerization between wild-type and C341Gbeta2AR. Furthermore, we show that, in co-expression experiments, wild-type receptors have a dominant positive effect resulting in the functional complementation of C341Gbeta2AR. Indeed, when expressed alone, the mutant C341G receptor displays altered functional characteristics in that (1) the response of the receptor to agonist is reduced as compared to the wild-type receptor and (2) the desensitization of the receptor in response to prolonged exposure to agonist is minimal. In contrast, when C341G and the wild-type beta2AR were expressed together, both the response to agonist and subsequent desensitization (at a constant level of total receptor) were equivalent to the wild-type beta2AR expressed alone. This dominant positive effect was also seen when C341G was co-expressed with a second receptor mutant in which the two protein kinase A phosphorylation sites (S261, 262, 345, 346A beta2AR) were mutated. Taken together these data suggest that intermolecular interactions between receptors may have both functional and structural implications for G-protein-mediated signalling.

Functional importance of platelet-derived growth factor (PDGF) receptor extracellular immunoglobulin-like domains. Identification of PDGF binding site and neutralizing monoclonal antibodies

The biological effects of platelet-derived growth factor (PDGF) are mediated by alpha- and beta-PDGF receptors (PDGFR), which have an intracellular tyrosine kinase domain and an extracellular region comprising five immunoglobulin-like domains (D1-D5). Using deletion mutagenesis we mapped the PDGF binding site in each PDGFR to the D2-D3 region. In the case of alpha-PDGFR, 125I-PDGF AA and 125I-PDGF BB bound to the full-length extracellular domain, D1-D5, and D2-D3 with equal affinity (Kd = 0.21-0.42 nM). Identical results were obtained for 125I-PDGF BB binding to beta-PDGFR mutants D1-D5 and D2-D3, establishing that D1, D4, and D5 do not contribute to PDGF binding. Monoclonal antibodies (mAb) directed against individual PDGFR Ig-like domains were used to extend these observations. The anti-D1 mAb 1E10E2 and anti-D5 mAb 2D4G10 had no effect on alpha- or beta-PDGFR function, respectively. In contrast, mAb 2H7C5 and 2A1E2 directed against D2 of the alpha- and beta-receptor, respectively, blocked PDGF binding, receptor autophosphorylation and mitogenic signaling with IC50 values of 0.1-3.0 nM. An anti-D4 mAb 1C7D5 blocked beta-receptor autophosphorylation and signaling without inhibiting PDGF binding consistent with the observation that D4 is essential for PDGFR dimerization (Omura, T., Heldin, C.-H., and Ostman, A. (1997) J. Biol. Chem. 272, 12676-12682). mAbs identified here act as potent PDGFR antagonists that can be used as research tools and potentially as therapeutic agents for the treatment of diseases involving unwanted PDGFR signaling.

A lectin-like molecule is discharged from mucocysts of Tetrahymena pyriformis in the presence of insulin

By use of a monoclonal antibody directed against purified lectin from the sponge Geodia cydonium it was demonstrated that the mucocysts of Tetrahymena pyriformis contain a substance immunologically similar to that found in G. cydonium. In extracts of T. pyriformis the monoclonal antibody recognizes a 36 kDa protein; binding could be abolished by adsorption of the antibody with (i) crude extract, (ii) purified lectin from G. cydonium and (iii) a 29 aa long peptide. In addition the data show that 10(-6) M of insulin causes first the release of mucocyst material, which reacts with the lectin antibody, and second its subsequent redistribution on the surface of the somatic cilia and the oral field.

The N-terminal cytoplasmic tail of the aspartate receptor is not essential in signal transduction of bacterial chemotaxis

To determine the role in transmembrane signaling of the N-terminal peptide of the first transmembrane region of the aspartate receptor, it was subjected to extensive mutagenesis. Drastic changes did not alter the chemotactic ability of the receptor to aspartate significantly. Thus the cytoplasmic N terminus of the first transmembrane region does not play an essential role in transmembrane signaling, and the entire signal that is transmitted to the cytoplasmic domain must be sent through the second transmembrane region. This eliminates the models requiring an interaction of this N-terminal peptide with the remaining cytoplasmic portion of the receptor.

Transmembrane signalling by the chimeric chemosensory receptors of Escherichia coli Tsr and Tar with heterologous membrane-spanning regions

The serine and aspartate chemosensory receptors (Tsr and Tar) of Escherichia coli have two membrane-spanning regions TM1 and TM2. To investigate their roles in transmembrane signalling, we constructed two chimeric receptors from Tsr and Tar with heterologous combinations of TM1 and TM2: the N-terminus of one receptor, including TM1 and the periplasmic domain, was fused to the C-terminus of the other, beginning with TM2. Both of the chimeric receptor genes rescued the chemotactic defect of a receptorless E. coli strain, indicating that the chimeric receptors are functional. Their apparent affinities for the specific ligands were the same as those of Tsr or Tar. Therefore, as far as transmembrane signalling abilities are concerned, the TM2 regions of Tsr and Tar are interchangeable, suggesting that sequence-specific interaction between TM1 and TM2 may not be required for the signal transmission across the membrane. The cells expressing either of the chimeric receptors, however, showed 'smooth', biased, basal swimming patterns. Moreover, they adapted quickly after stimulation with the repellent glycerol. This rapid adaptation was observed even in the methyltransferase-defective strain. Therefore, exchange of TM2 might impose structural constraints on the chimeric receptors that stabilize conformations which elicit smooth swimming.

Transmembrane signalling and the aspartate receptor

BACKGROUND

The aspartate receptor is a transmembrane protein that mediates bacterial chemotaxis. The structures of the periplasmic ligand-binding domain reveal a dimer, each subunit with four alpha-helix bundles, with aspartate binding to one of two sites at the subunit interface. The transmembrane regions of the receptor were not included in these structures.

RESULTS

To investigate the structure of the transmembrane region, we have made a mutant protein with two cross-links, restraining the subunit-subunit interface on both sides of the membrane, and have made an energy-minimized model of the transmembrane region. We demonstrate that the transmembrane helices form a coiled coil which extends from the periplasmic subunit through the membrane. We have constructed a model of the ligand-binding domains with the amino-terminal transmembrane helices.

CONCLUSIONS

We draw three conclusions from our model. Firstly, the interface between receptor subunits in the intact receptor consists of an uninterrupted coiled coil. Secondly, this structure rules out several postulated mechanisms of signalling. Thirdly, side chain packing constraints within the helices dictate that local structural changes must be small, but are propagated over a long distance rather than being dissipated locally. Low energy changes in the conformation of side chains are a probable mechanism of signal transduction in the aspartate receptor.

Identification of a human cDNA encoding a functional high affinity lipoxin A4 receptor

Lipoxin A4 (LXA4) triggers selective responses with human neutrophils that are pertussis toxin sensitive and binds to high affinity receptors (Kd = 0.5 +/- 0.3 nM) that are modulated by stable analogues of guanosine 5'-triphosphate (GTP). Here, we characterized [11,12-(3)]LXA4 specific binding with neutrophil granule and plasma membranes, which each display high affinity binding sites (Kd = 0.7 +/- 0.1 nM) that were regulated by GTP gamma S. Since functional LXA4 receptors are inducible in HL-60 cells, we tested orphan cDNAs encoding 7-transmembrane region receptors cloned from these cells for their ability to bind and signal with LXA4. Chinese hamster ovary (CHO) cells transfected with the orphan receptor cDNA (pINF114) displayed specific 3H-LXA4 high affinity binding (1.7 nM). When displacement of LXA4 binding with pINF114-transfected CHO cells was tested with other eicosanoids, including LXB4, leukotriene D4 (LTD4), LTB4, or prostaglandin E2, only LTD4 competed with LXA4, giving a Ki of 80 nM. In transfected CHO cells, LXA4 also stimulated GTPase activity and provoked the release of esterified arachidonate, which proved to be pertussis toxin sensitive. These results indicate that pINF114 cDNA encodes a 7-transmembrane region-containing protein that displays high affinity for 3H-LXA4 and transmits LXA4-induced signals. Together, they suggest that the encoded protein is a candidate for a LXA4 receptor in myeloid cells.

A single carboxy-terminal arginine determines the amino-terminal helix conformation of an alanine-based peptide

Arginine is a stabilizing element in both thermophilic and low molecular weight proteins. Similarly Lys+-->Arg+ substitutions increase the helix content of designed helical peptides. Here we explore this 'arginine effect' by examining how Lys+-->Arg+ substitutions influence the 3(10)-helix-->alpha-helix equilibrium in the helical peptide Ac-(AAAAK)3A-NH2. The unsubstituted sequence contains a significant amount of 3(10)-helix, however, single Lys+-->Arg+ substitutions shift the peptide conformation toward alpha-helix in a position-dependent fashion. The single substitution closest to the carboxy terminus induces the largest conformational change at the helix amino terminus. These findings suggest that a single strategically-placed arginine can exert long range control on helix structure.

Computer simulated evolution of a network of cell-signaling molecules

We have trained a computer model of a simple cell-signaling pathway to give specified responses to a pulse of an extracellular ligand. The pathway consists of two initially identical membrane receptors, each of which relays the concentration of the ligand to the level of phosphorylation of an intracellular molecule. Application of random "mutational" changes to the rate constants of the pathway, followed by selection in favor of certain outputs, generates a variety of wave forms and dose-response curves. The phenotypic effect of mutations and the frequency of selection both affect the efficiency with which the pathway achieves its target. When the pathway is trained to give a maximal response at a specific concentration of the stimulating ligand, it gives a consistent pattern of changes in which the two receptors diverge, producing a high-affinity form with excitatory output and a low-affinity form with inhibitory output. We suggest that some high- and low-affinity forms of receptors found in present-day cells might have originated by a similar process.

Integrin cytoplasmic domains mediate inside-out signal transduction

We analyzed the binding of fibronectin to integrin alpha 5 beta 1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd approximately 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of alpha IIb beta 3 joined to the cytoplasmic domains of alpha 5 beta 1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of alpha 5 beta 1 conferred an energy-dependent high affinity state on alpha IIb beta 3 in CHO but not K562 cells. Three additional alpha cytoplasmic domains (alpha 2, alpha 6A, alpha 6B) conferred PAC1 binding in CHO cells, while three others (alpha M, alpha L, alpha v) did not. In the high affinity alpha chimeras, cotransfection with a truncated (beta 3 delta 724) or mutated (beta 3(S752-->P)) beta 3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved alpha subunit GFFKR motif, resulted in high affinity binding of ligands to alpha IIb beta 3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the beta subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the alpha subunit cytoplasmic domain maintains the default low affinity state.

Aspartate receptors of Escherichia coli and Salmonella typhimurium bind ligand with negative and half-of-the-sites cooperativity

The aspartate receptors of Escherichia coli and Salmonella typhimurium which mediate chemotactic responsiveness to aspartate have 79% amino acid sequence identity but exhibited apparently quite different aspartate binding plots. The Scatchard plot of the Salmonella receptor was concave upward whereas the E. coli receptor gave a straight line. Because the two binding sites in the Salmonella receptor lacking aspartate have a 2-fold crystallographic symmetry axis and do not overlap, the observation of more than one class of binding sites must be due to a ligand-induced conformational change giving negative cooperativity. The closely related E. coli receptor was found to bind with only one class of sites but with a stoichiometry of one aspartate per dimer. The E. coli receptor thus binds with half-of-sites reactivity, an extreme form of negative cooperativity in which the second ligand is not observed to bind at all. Comparison of the X-ray crystal structure of the ligand binding domain with and without bound aspartate revealed ligand-induced conformational changes that explain the two examples of negative cooperativity.

Structure and dynamics of transmembrane signaling by the Escherichia coli aspartate receptor

The structure of the cytosolic extension of the first transmembrane region (TM1) of the Escherichia coli aspartate receptor (residues 3, 4, and 5) and conformational changes within that region have been characterized by targeted cross-linking studies and by measurement of the effect of aspartate binding on cross-linking and methylation rates and compared with the periplasmic extension of the same helix. These experiments show that (1) the cytosolic extension of TM1 is helical, with residues 4 and 4' closest together at the dimer interface; (2) the helix is more solvent-exposed at the cytosolic side of the membrane than on the periplasmic side; and (3) aspartate binding enhances the rate of cross-linking at Cys 4, and the resulting cross-linked receptor displays aspartate-induced transmembrane increases in methylation by the cytoplasmic methylase (the CheR protein). We conclude that aspartate induces a conformational change that does not involve large intersubunit movements that lead to an increase in distance between the cytosolic ends of the first membrane-spanning helices; rather, the motion involved is largely contained within individual subunits, possibly resulting in a small movement between positions 4 and 4'.