Evidence for multiple, biochemically distinguishable states in the G protein-coupled receptor, rhodopsin

Sinorhizobium meliloti nifA mutant induces different gene expression profile from wild type in Alfalfa nodules

Several studies have demonstrated that the Rhizobium nifA gene is an activator of nitrogen fixation acting in nodule bacteria. To understand the effects of the Sinorhizobium meliloti nifA gene on Alfalfa, the cDNA-AFLP technique was employed to study the changes in gene expression in nifA mutant nodules. Among the approximately 3,000 transcript-derived fragments, 37 had differential expression levels. These expression levels were subsequently confirmed by reverse Northern blot and RT-polymerase chain reaction. Sequence analyses revealed that 21 cDNA fragments corresponded to genes involved in signal communication, protein degradation, nutrient metabolism, cell growth and development.

Molecular Characterization of a Purified 5-HT4 Receptor

Serotonin 5-HT(4(a)) receptor, a G-protein-coupled receptor (GPCR), was produced as a functional isolated protein using Escherichia coli as an expression system. The isolated receptor was characterized at the molecular level by circular dichroism (CD) and steady-state fluorescence. A specific change in the near-UV CD band associated with the GPCR disulfide bond connecting the third transmembrane domain to the second extracellular loop (e2) was observed upon agonist binding to the purified receptor. This is a direct experimental evidence for a change in the conformation of the e2 loop upon receptor activation. Different variations were obtained depending whether the ligand was an agonist (partial or full) or an inverse agonist. In contrast, antagonist binding did not induce any variation. These observations provide a first direct evidence for the fact that free (or antagonist-occupied), active (partial- or full agonist-occupied) and silent (inverse agonist-occupied) states of the receptor involve different arrangements of the e2 loop. Finally, ligand-induced changes in the fluorescence emission profile of the purified receptor confirmed that the partial agonist stabilized a single, well-defined, conformational state and not a mixture of different states. This result is of particular interest in a pharmacological perspective since it directly demonstrates that the efficacy of a drug is likely due to the stabilization of a ligand-specific state rather than selection of a mixture of different conformational states of the receptor.

“Network Leaning” as a Mechanism of Insurmountable Antagonism of the Angiotensin II Type 1 Receptor by Non-peptide Antagonists

A mechanistic understanding of the insurmountable antagonism of the angiotensin II type 1 (AT(1)) receptor could be fundamental in the quest for discovery and improvement of drugs. Candesartan and EXP3174 are competitive, reversible insurmountable antagonists of the AT(1) receptor. They contain di-acidic substitutions, whereas the surmountable antagonist, losartan, contains only one acidic group. We tested the hypothesis that these two classes of ligands interact with the AT(1) receptor through similar but not identical bonds and that the differences in the acid-base group contacts are critical for insurmountable antagonism. By pharmacological characterization of site-directed AT(1) receptor mutants expressed in COS1 cells we show that specific interactions with Gln(257) in transmembrane 6 distinguishes insurmountable antagonists and that abolishing these interactions transforms insurmountable to surmountable antagonism. In the Q257A mutant, the dissociation rate of [(3)H]candesartan is 2.8-fold more than the rate observed with wild type, and the association rate was reduced 4-fold lower than the wild type. The pattern of antagonism of angiotensin II concentration-response in the Q257A mutant pretreated with EXP3174 and candesartan is surmountable. We propose that leaning ability of insurmountable antagonists on Gln(257) in the wild-type receptor is the basis of an antagonist-mediated conformational transition, which is responsible for both slow dissociation and inhibition of maximal IP response.

A 5-HT4 receptor transmembrane network implicated in the activity of inverse agonists but not agonists

Activation of G protein-coupled receptors is thought to involve disruption of intramolecular interactions that stabilize their inactive conformation. Such disruptions are induced by agonists or by constitutively active mutations. In the present study, novel potent inverse agonists are described to inhibit the constitutive activity of 5-HT(4) receptors. Using these compounds and specific receptor mutations, we investigated the mechanisms by which inverse agonists may reverse the disruption of intramolecular interactions that causes constitutive activation. Two mutations (D100(3.32)A in transmembrane domain (TMD)-III and F275(6.51)A in TMD-VI) were found to completely block inverse agonist effects without impairing their binding properties nor the molecular activation switches induced by agonists. Based on the rhodopsin model, we propose that these mutated receptors are in equilibrium between two states R and R* but are unable to reach a third "silent" state stabilized by inverse agonists. We also found another mutation in TMD-VI (W272(6.48)A) that stabilized this silent state. This mutant remained fully activated by agonists. Molecular modeling indicated that Asp-100, Phe-275, and Trp-272 might constitute a network required for stabilization of the silent state by the described inverse agonists. However, this network is not necessary for agonist activity.

Molecular modelling and site-directed mutagenesis of human GALR1 galanin receptor defines determinants of receptor subtype specificity

Human galanin is a 30 amino acid neuropeptide that elicits a range of biological activities by interaction with G protein-coupled receptors. We have generated a model of the human GALR1 galanin receptor subtype (hGALR1) based on the alpha carbon maps of frog rhodopsin and investigated the significance of potential contact residues suggested by the model using site-directed mutagenesis. Mutation of Phe186 within the second extracellular loop to Ala resulted in a 6-fold decrease in affinity for galanin, representing a change in free energy consistent with hydrophobic interaction. Our model suggests interaction between Phe186 of hGALR1 and Ala7 or Leu11 of galanin. Receptor subtype specificity was investigated by replacement of residues in hGALR1 with the corresponding residues in hGALR2 and use of the hGALR2-specific ligands hGalanin(2-30) and [D-Trp2]hGalanin(1-30). The His267Ile mutant receptor exhibited a pharmacological profile corresponding to that of hGALR1, suggesting that His267 is not involved in a receptor-ligand interaction. The mutation Phe115Ala resulted in a decreased binding affinity for hGalanin and for hGALR2-specific analogues, indicating Phe115 to be of structural importance to the ligand binding pocket of hGALR1 but not involved in direct ligand interaction. Analysis of Glu271Trp suggested that Glu271 of hGALR1 interacts with the N-terminus of galanin and that the Trp residue in the corresponding position in hGALR2 is involved in receptor subtype specificity of binding. Our model supports previous reports of Phe282 of hGALR1 interacting with Trp2 of galanin and His264 of hGALR1 interacting with Tyr9 of galanin.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Agonist-induced phosphorylation of the angiotensin II (AT(1A)) receptor requires generation of a conformation that is distinct from the inositol phosphate-signaling state

G protein-coupled receptors are thought to isomerize between distinct inactive and active conformations, an idea supported by receptor mutations that induce constitutive (agonist-independent) activation. The agonist-promoted active state initiates signaling and, presumably, is then phosphorylated and internalized to terminate the signal. In this study, we examined the phosphorylation and internalization of wild type and constitutively active mutants (N111A and N111G) of the type 1 (AT(1A)) angiotensin II receptor. Cells expressing these receptors were stimulated with angiotensin II (AngII) and [Sar(1),Ile(4),Ile(8)]AngII, an analog that only activates signaling through the constitutive receptors. Wild type AT(1A) receptors displayed a basal level of phosphorylation, which was stimulated by AngII. Unexpectedly, the constitutively active AT(1A) receptors did not exhibit an increase in basal phosphorylation nor was phosphorylation enhanced by AngII stimulation. Phosphorylation of the constitutively active receptors was unaffected by pretreatment with the non-peptide AT(1) receptor inverse agonist, EXP3174, and was not stimulated by the selective ligand, [Sar(1),Ile(4),Ile(8)]AngII. Paradoxically, [Sar(1),Ile(4), Ile(8)]AngII produced a robust ( approximately 85% of AngII), dose-dependent phosphorylation of the wild type AT(1A) receptor at sites in the carboxyl terminus similar to those phosphorylated by AngII. Moreover, internalization of both wild type and constitutive receptors was induced by AngII, but not [Sar(1),Ile(4),Ile(8)]AngII, providing a differentiation between the phosphorylated and internalized states. These data suggest that the AT(1A) receptor can attain a conformation for phosphorylation without going through the conformation required for inositol phosphate signaling and provide evidence for a transition of the receptor through multiple states, each associated with separate stages of receptor activation and regulation. Separate transition states may be a common paradigm for G protein-coupled receptors.

Role of the extracellular loops of G protein-coupled receptors in ligand recognition: a molecular modeling study of the human P2Y1 receptor

The P2Y1 receptor is a G protein-coupled receptor (GPCR) and is stimulated by extracellular ADP and ATP. Site-directed mutagenesis of the three extracellular loops (ELs) of the human P2Y1 receptor indicates the existence of two essential disulfide bridges (Cys124 in EL1 and Cys202 in EL2; Cys42 in the N-terminal segment and Cys296 in EL3) and several specific ionic and H-bonding interactions (involving Glu209 and Arg287). Through molecular modeling and molecular dynamics simulations, an energetically sound conformational hypothesis for the receptor has been calculated that includes transmembrane (TM) domains (using the electron density map of rhodopsin as a template), extracellular loops, and a truncated N-terminal region. ATP may be docked in the receptor, both within the previously defined TM cleft and within two other regions of the receptor, termed meta-binding sites, defined by the extracellular loops. The first meta-binding site is located outside of the TM bundle, between EL2 and EL3, and the second higher energy site is positioned immediately underneath EL2. Binding at both the principal TM binding site and the lower energy meta-binding sites potentially affects the observed ligand potency. In meta-binding site I, the side chain of Glu209 (EL2) is within hydrogen-bonding distance (2.8 A) of the ribose O3', and Arg287 (EL3) coordinates both alpha- and beta-phosphates of the triphosphate chain, consistent with the insensitivity in potency of the 5'-monophosphate agonist, HT-AMP, to mutation of Arg287 to Lys. Moreover, the selective reduction in potency of 3'NH2-ATP in activating the E209R mutant receptor is consistent with the hypothesis of direct contact between EL2 and nucleotide ligands. Our findings support ATP binding to at least two distinct domains of the P2Y1 receptor, both outside and within the TM core. The two disulfide bridges present in the human P2Y1 receptor play a major role in the structure and stability of the receptor, to constrain the loops within the receptor, specifically stretching the EL2 over the opening of the TM cleft and thus defining the path of access to the binding site.

Agonist-independent regulation of constitutively active G-protein-coupled receptors

G-protein-coupled receptors constitute one of the largest protein super-families in mammals. Since the cloning of the encoding genes, these important drug targets have been subjected to thorough biochemical and pharmacological studies. It has become clear that G-protein-coupled receptors not only transmit signals after stimulation by agonists but can also spontaneously couple to signal-transduction pathways. Recent findings show that constitutively active G-protein-coupled receptors can also be regulated in an agonist-independent manner, which has important implications for the interpretation of the actions of (inverse) agonists and the results of site-directed-mutagenesis studies.