Signal transduction within G-protein coupled receptors via an ion tunnel: a hypothesis

Allosteric sodium in class A GPCR signaling

Despite their functional and structural diversity, G-protein-coupled receptors (GPCRs) share a common mechanism of signal transduction via conformational changes in the seven-transmembrane (7TM) helical domain. New major insights into this mechanism come from the recent crystallographic discoveries of a partially hydrated sodium ion that is specifically bound in the middle of the 7TM bundle of multiple class A GPCRs. This review discusses the remarkable structural conservation and distinct features of the Na(+) pocket in this most populous GPCR class, as well as the conformational collapse of the pocket upon receptor activation. New insights help to explain allosteric effects of sodium on GPCR agonist binding and activation, and sodium's role as a potential co-factor in class A GPCR function.

Structural basis for allosteric regulation of GPCRs by sodium ions

Pharmacological responses of G protein-coupled receptors (GPCRs) can be fine-tuned by allosteric modulators. Structural studies of such effects have been limited due to the medium resolution of GPCR structures. We reengineered the human A(2A) adenosine receptor by replacing its third intracellular loop with apocytochrome b(562)RIL and solved the structure at 1.8 angstrom resolution. The high-resolution structure allowed us to identify 57 ordered water molecules inside the receptor comprising three major clusters. The central cluster harbors a putative sodium ion bound to the highly conserved aspartate residue Asp(2.50). Additionally, two cholesterols stabilize the conformation of helix VI, and one of 23 ordered lipids intercalates inside the ligand-binding pocket. These high-resolution details shed light on the potential role of structured water molecules, sodium ions, and lipids/cholesterol in GPCR stabilization and function.

Crucial role of copper in detection of metal-coordinating odorants

Odorant receptors (ORs) in olfactory sensory neurons (OSNs) mediate detection of volatile odorants. Divalent sulfur compounds, such as thiols and thioethers, are extremely potent odorants. We identify a mouse OR, MOR244-3, robustly responding to (methylthio)methanethiol (MeSCH(2)SH; MTMT) in heterologous cells. Found specifically in male mouse urine, strong-smelling MTMT [human threshold 100 parts per billion (ppb)] is a semiochemical that attracts female mice. Nonadjacent thiol and thioether groups in MTMT suggest involvement of a chelated metal complex in MOR244-3 activation. Metal ion involvement in thiol-OR interactions was previously proposed, but whether these ions change thiol-mediated OR activation remained unknown. We show that copper ion among all metal ions tested is required for robust activation of MOR244-3 toward ppb levels of MTMT, structurally related sulfur compounds, and other metal-coordinating odorants (e.g., strong-smelling trans-cyclooctene) among >125 compounds tested. Copper chelator (tetraethylenepentamine, TEPA) addition abolishes the response of MOR244-3 to MTMT. Histidine 105, located in the third transmembrane domain near the extracellular side, is proposed to serve as a copper-coordinating residue mediating interaction with the MTMT-copper complex. Electrophysiological recordings of the OSNs in the septal organ, abundantly expressing MOR244-3, revealed neurons responding to MTMT. Addition of copper ion and chelator TEPA respectively enhanced and reduced the response of some MTMT-responding neurons, demonstrating the physiological relevance of copper ion in olfaction. In a behavioral context, an olfactory discrimination assay showed that mice injected with TEPA failed to discriminate MTMT. This report establishes the role of metal ions in mammalian odor detection by ORs.

Role of metal ions in ligand-receptor interaction: insights from structural studies

Experimental data indicate that metal ions such as Na(+), Ca(2+) and Mg(2+), which are present in millimolar concentrations in the extracellular environment, modulate binding of ligands to plasma membrane receptors. Here, we briefly review structural studies that demonstrate that various types of ligands, including peptide hormones and drugs, bind metal ions, in particular Ca(2+), in the lipid milieu. We propose that the metal ion-bound forms of ligands represent their bioactive conformations. With a view to understanding the mechanism of modulation of ligand-receptor interactions by metal ions, we have computed a homology model of the mu-opioid receptor, a G protein-coupled receptor (GPCR), and performed docking of specific agonist and antagonist ligands in the receptor. This resulted in the formation of a ligand-metal ion-receptor (ternary) complex which accounted for the data on the structure-activity relationships of ligands and mutation data on the receptor. Based on experimental and modeling data, we have proposed a general mechanism of activation of GPCRs by their corresponding ligands wherein metal ions play a pivotal role. Studies on overexpressed segments of mu-receptor are in progress to verify the above proposal.

Expression and spectroscopic characterization of a large fragment of the μ-opioid receptor

We report here a procedure for the production in Escherichia coli and subsequent purification and characterization of an 80-residue fragment of the human mu-opioid receptor. The fragment ('TM2-3'), which comprises the second and third transmembrane segments as well as the first extracellular loop of the receptor, was expressed as a fusion with glutathione-S-transferase. The fusion protein, which accumulated in insoluble inclusion bodies, was solubilized with N-lauroylsarcosine, and TM2-3 was obtained by thrombin cleavage of the fusion protein followed by reversed-phase HPLC purification. CD spectroscopy of TM2-3 in lysophosphatidylcholine micelles showed that TM2-3 adopts approximately 50% alpha-helical structure in this environment, with the remainder consisting of disordered and/or beta-structure. This is consistent with the assumption of an alpha-helical structure by the two membrane-spanning regions and a nonhelical structure in the loop region of TM2-3. Fluorescence spectroscopy and fluorescence quenching experiments suggested that the extracellular loop lies near the surface of the lysophosphatidylcholine micelle. Our work shows that the study of large receptor fragments is a technically accessible approach to the study of the structural properties of the mu-opioid receptor and, possibly, other G-protein-coupled receptors as well.

Intramolecular signal transduction by the bradykinin B2 receptor

To study the intramolecular signal transduction, we performed single point and cassette mutations in transmembranal and intracellular regions of the bradykinin B2 receptor. We studied the influence of the two intramembranal Cys residues at positions 304 and 348, the role of Arg at position 177 in the highly conserved tripeptide sequence Asp-Arg-Tyr, the cytosolic G-protein binding area, and attempted to verify the general hypothesis of an ion tunnel-like interface in GPCRs. Wild type receptor, His-tagged receptor, and His-tagged mutant receptors were expressed in COS-7 cells and functionally compared by bradykinin-induced formation of inositolphosphate and arachidonic acid. To investigate the expression, all mutants were modified at the N-terminus by insertion of two successive His-tags and detected with an anti-poly-His antibody. Replacement of the second and third cytosolic loop by a loop from another membrane protein as well as single replacement of Arg at position 177 by Ala leads to a fully inactive receptor mutant without any ligand binding affinity and stimulatory activity. Mutants with replacement of Cys residues 304 and 348 by Ser showed only moderate effects. Regardless of the replacement of Asp 407 by Ala, the receptor is able to increase the agonist-induced levels of inositolphosphate and of arachidonic acid, indicating that our studies can not verify the postulated ion tunnel hypothesis.

26th European Peptide Symposium. September 2000. Abstracts

The biologically relevant conformation of substance P is likely to be dictated by the lipid milieu wherein the hormone would interact with its receptor. Assuming that specific constraints to the hormone structure may be imparted by its interaction with Ca2+ ions in the low dielectric lipid medium, the interaction of substance P and its inactive analog, Ala7-substance P, has been characterized in a lipid-mimetic solvent. Circular dichroism (CD) and NMR spectral methods were employed to study the conformation of the free and Ca2+-bound forms of the peptides and the conformational changes that occur on Ca2+ binding. The results show that both peptides assume a helical structure in the non-polar solvent used, a mixture of acetonitrile and trifluoroethanol. The N-terminal region is, however, less ordered in the analog peptide compared with the native hormone. Ca2+ addition causes significant conformational changes in both the peptides. However, while substance P binds two Ca2+ ions in a cooperative manner, Ala7-substance P binds only one Ca2+ ion with a relatively weaker affinity. Computations of the minimum-energy conformations of the free and Ca2+-bound peptides were performed using interproton distances derived from nuclear Overhauser enhancement spectra of the two peptides, as well as the information provided by changes in proton chemical shifts caused by Ca2+ addition. Taken together, the results of this study suggest that differences in the interaction of substance P and Ala7-substance P with Ca2+ in the non-polar milieu, which in turn leads to differences in their Ca2+-bound conformations, may be the basis for the differences in their biological potencies.

Homology models of mu-opioid receptor with organic and inorganic cations at conserved aspartates in the second and third transmembrane domains

Metal ions affect ligand binding to G-protein-coupled receptors by as yet unknown mechanisms. In particular, Na(+) increases the affinity for antagonists but decreases it for agonists. We had modeled the mu-opioid receptor (muR) based on the low-resolution structure of rhodopsin by G. F. X. Schertler, C. Villa, and R. Henderson (1993, Nature 362, 770-772) and proposed that metal ions may be directly involved in the binding of ligands and receptor activation (B. S. Zhorov and V. S. Ananthanarayanan, 1998, J. Biomol. Struct. Dyn. 15, 631-637). Developing this concept further, we present here homology models of muR using as templates the structure of rhodopsin elaborated by I. D. Pogozheva, A. L. Lomize, and H. I. Mosberg (1997, Biophys. J. 70, 1963-1985) and J. M. Baldwin, G. F. X. Schertler, and V. M. Unger (1997, J. Mol. Biol., 272, 144-164). Using the Monte Carlo minimization (MCM) method, we docked the Na(+)-bound forms of muR ligands: naloxone, bremazocine, and carfentanyl. The resultant low-energy complexes showed that the two positive charges in the protonated metal-bound ligands interact with the two negative charges at Asp(3.32) and Asp(2.50) (for notations, see J. A. Ballesteros and H. Weinstein, 1995, Methods Neurosci. 25, 366-426). MCM computation on morphine docked inside the model of muR by I. D. Pogozheva, A. L. Lomize, and H. I. Mosberg (1998, Biophys. J. 75, 612-634) yielded two binding modes with the ligand's ammonium group salt-bridged either to Asp(3.32) (generally regarded as the ligand recognition site) or to Asp(2.50). The latter is the presumed site for Na(+) ion, which is known to modulate ligand binding. Assuming that in the low-dielectric transmembrane region of muR, organic and inorganic cations would compete for Asp(3.32) and Asp(2.50), we propose that ligand binding, as visualized in the above models, would first displace Na(+) from Asp(3.32). A subsequent progress of the ligand toward Asp(2.50) would result in either the retention of Na(+) at Asp(2.50) in the case of antagonists or the displacement of Na(+) from Asp(2.50) in the case of agonists. The displaced Na(+) would move toward the salt-bridged Asp(3.49)-Arg(3.50) and disengage the salt bridge. This, in turn, would result in conformational changes at the cytoplasmic face of the receptor that facilitate the interaction with the G-protein.

Constitutive activation of the delta opioid receptor by mutations in transmembrane domains III and VII

We have investigated whether transmembrane amino acid residues Asp128 (domain III), Tyr129 (domain III) [corrected], and Tyr308 (domain VII) in the mouse delta opioid receptor play a role in receptor activation. To do so, we have used a [35S]GTPgammaS (where GTPgammaS is guanosine 5'-3-O-(thio)triphosphate) binding assay to quantify the activation of recombinant receptors transiently expressed in COS cells and compared functional responses of D128N, D128A, Y129F, Y129A, and Y308F point-mutated receptors to that of the wild-type receptor. In the absence of ligand, [35S]GTPgammaS binding was increased for every mutant receptor under study (1.6-2.6-fold), suggesting that all mutations are able to enhance constitutive activity at the receptor. In support of this finding, the inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (where Aib represents alpha-aminobutyric acid) efficiently reduced basal [35S]GTPgammaS binding in the mutated receptor preparations. The potent agonist BW373U86 stimulated [35S]GTPgammaS binding above basal levels with similar (D128N, Y129F, and Y129A) or markedly increased (Y308F) efficacy compared with wild-type receptor. BW373U86 potency was maintained or increased. In conclusion, our results demonstrate that the mutations under study increase functional activity of the receptor. Three-dimensional modeling suggests that Asp128 (III) and Tyr308 (VII) interact with each other and that Tyr129 (III) undergoes H bonding with His278 (VI). Thus, Asp128, Tyr129, and Tyr308 may be involved in a network of interhelical bonds, which contributes to maintain the delta receptor under an inactive conformation. We suggest that the mutations weaken helix-helix interactions and generate a receptor state that favors the active conformation and/or interacts with heterotrimeric G proteins more effectively.