Role of the extracellular amino terminus and first membrane-spanning helix of dopamine D1 and D5 receptors in shaping ligand selectivity and efficacy

Transmembrane (TM) helices of human D1-like dopaminergic receptors (hD1R and hD5R) harbor the same residues implicated in ligand binding and activation of catecholamine G protein-coupled receptors (GPCRs). Yet, hD1R and hD5R naturally display the distinct functional properties shared by wild type and constitutively active mutant GPCRs, respectively. Interestingly, we show in the present study that a class of synthetic phenylbenzazepine agonists containing a methyl on the azepine ring exhibited lower affinity for the more constitutively activated hD5R. These results cannot be explained by the “allosteric ternary complex model” postulating a higher agonist affinity for constitutively active GPCRs. We have also explored the functional role of distinct extracellular amino terminus (NT) and TM1 regions of hD1R and hD5R using a chimerical approach. Of these two regions, our studies suggest that TM1 predominantly shapes D1-like ligand affinity and selectivity. Additionally, NT and TM1 of hD1R and hD5R play no role in receptor constitutive activity but differentially modulate dopamine-mediated responsiveness. The TM1 exchange mediated drastic changes in intrinsic efficacy and activity of phenylbenzazepine drugs displaying partial agonism at hD1R and hD5R. Phenylbenzazepines were converted into strong partial agonists or full agonists in cells expressing hD1R-TM1^D5 chimera while being switched from full agonists to partial agonists and partial agonists to antagonists in cells harboring hD5R-TM1^D1 chimera. TM1 exchange had no effect on antipsychotic-mediated inverse agonism. In summary, our study shows that NT and TM1 of D1-like receptors control ligand binding and agonist-induced activation, poising these regions as important structural determinants for catecholamine GPCR function.     

Mechanisms of inverse agonism of antipsychotic drugs at the D(2) dopamine receptor: use of a mutant D(2) dopamine receptor that adopts the activated conformation

The antipsychotic drugs have been shown to be inverse agonists at the D(2) dopamine receptor. We have examined the mechanism of this inverse agonism by making mutations in residue T343 in the base of the sixth transmembrane spanning region of the receptor. T343R, T343S and T343K mutant D(2) dopamine receptors were made and the T343R mutant characterized in detail. The T343R mutant D(2) dopamine receptor exhibits properties of a receptor that resides more in the activated state, namely increased agonist binding affinity (independent of G-protein coupling and dependent on agonist efficacy), increased agonist potency in functional tests (adenylyl cyclase inhibition) and increased inverse agonist effects. The binding of agonists to the mutant receptor also shows sensitivity to sodium ions, unlike the native receptor, so that isomerization of the receptor to its inactive state may be driven by sodium ions. The binding of inverse agonists to the receptor is, however, unaffected by the mutation. We conclude that inverse agonism at this receptor is not achieved by the inverse agonist binding preferentially to the non-activated state of the receptor over the activated state. Rather the inverse agonist appears to bind to all forms of the receptor but then renders the receptor inactive.     

Plasticity of the ligand binding pocket in the bitter taste receptor T2R7

Bitter taste receptors (T2Rs) are a group of 25 G protein-coupled receptors (GPCRs) in humans. The cognate agonists and the mechanism of ligand binding to the majority of the T2Rs remain unknown. Here we report the first structure-function analysis of T2R7 and study the ability of this receptor to bind to different agonists by site-directed mutagenesis. Screening of ligands for T2R7 in calcium based assays lead to the identification of novel compounds that activate this receptor. Quinine, diphenidol, dextromethorphan and diphenhydramine showed substantial activation of T2R7. Interestingly, these bitter compounds showed different pharmacological characteristics. To investigate the structural features in T2R7 that might contribute to the observed differences in agonist specificities, molecular model guided ligand docking and site-directed mutagenesis was pursued. Amino acids D65, D86, W89, N167, T169, W170, S181, T255 and E271 in the ligand-binding pocket were replaced and the mutants characterized pharmacologically. Our results suggest D86, S181 and W170 present on the extracellular side of transmembrane 3 (TM3), TM5 and in extracellular loop 2 (ECL2) are essential for agonist binding in T2R7. Mutations of these amino acids lead to loss-of-function. We also identified gain-of-function residues that are agonist specific. These results suggest that agonists bind at an extracellular site rather than deep within the TM core involving residues present in both ECL2 and TM helices in T2R7. Similar to majority of the Class A GPCRs, ECL2 in T2R7 plays a significant role in agonist binding and activation.     

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast: identification of second-site mutations that restore function to a coupling-deficient mutant M3 receptor

The M(3) muscarinic receptor is a prototypical member of the class A family of G protein-coupled receptors (GPCRs). To gain insight into the structural mechanisms governing agonist-mediated M(3) receptor activation, we recently developed a genetically modified yeast strain (Saccharomyces cerevisiae) which allows the efficient screening of large libraries of mutant M(3) receptors to identify mutant receptors with altered/novel functional properties. Class A GPCRs contain a highly conserved Asp residue located in transmembrane domain II (TM II; corresponding to Asp-113 in the rat M(3) muscarinic receptor) which is of fundamental importance for receptor activation. As observed previously with other GPCRs analyzed in mammalian expression systems, the D113N point mutation abolished agonist-induced receptor/G protein coupling in yeast. We then subjected the D113N mutant M(3) receptor to PCR-based random mutagenesis followed by a yeast genetic screen to recover point mutations that can restore G protein coupling to the D113N mutant receptor. A large scale screening effort led to the identification of three such second-site suppressor mutations, R165W, R165M, and Y250D. When expressed in the wild-type receptor background, these three point mutations did not lead to an increase in basal activity and reduced the efficiency of receptor/G protein coupling. Similar results were obtained when the various mutant receptors were expressed and analyzed in transfected mammalian cells (COS-7 cells). Interestingly, like Asp-113, Arg-165 and Tyr-250, which are located at the cytoplasmic ends of TM III and TM V, respectively, are also highly conserved among class A GPCRs. Our data suggest a conformational link between the highly conserved Asp-113, Arg-165, and Tyr-250 residues which is critical for receptor activation.     

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.     

Multiple residues in the second extracellular loop are critical for M3 muscarinic acetylcholine receptor activation

Recent studies suggest that the second extracellular loop (o2 loop) of bovine rhodopsin and other class I G protein-coupled receptors (GPCRs) targeted by biogenic amine ligands folds deeply into the transmembrane receptor core where the binding of cis-retinal and biogenic amine ligands is known to occur. In the past, the potential role of the o2 loop in agonist-dependent activation of biogenic amine GPCRs has not been studied systematically. To address this issue, we used the M(3) muscarinic acetylcholine receptor (M3R), a prototypic class I GPCR, as a model system. Specifically, we subjected the o2 loop of the M3R to random mutagenesis and subsequently applied a novel yeast genetic screen to identity single amino acid substitutions that interfered with M3R function. This screen led to the recovery of about 20 mutant M3Rs containing single amino acid changes in the o2 loop that were inactive in yeast. In contrast, application of the same strategy to the extracellular N-terminal domain of the M3R did not yield any single point mutations that disrupted M3R function. Pharmacological characterization of many of the recovered mutant M3Rs in mammalian cells, complemented by site-directed mutagenesis studies, indicated that the presence of several o2 loop residues is important for efficient agonist-induced M3R activation. Besides the highly conserved Cys(220) residue, Gln(207), Gly(211), Arg(213), Gly(218), Ile(222), Phe(224), Leu(225), and Pro(228) were found to be of particular functional importance. In general, mutational modification of these residues had little effect on agonist binding affinities. Our findings are therefore consistent with a model in which multiple o2 loop residues are involved in stabilizing the active state of the M3R. Given the high degree of structural homology found among all biogenic amine GPCRs, our findings should be of considerable general relevance.     

Ligand-specific changes in M3 muscarinic acetylcholine receptor structure detected by a disulfide scanning strategy

G protein-coupled receptor (GPCR) function can be modulated by different classes of ligands including full and inverse agonists. At present, little is known about the conformational changes that agonist ligands induce in their target GPCRs. In this study, we employed an in situ disulfide cross-linking strategy to monitor ligand-induced structural changes in a series of cysteine (Cys)-substituted mutant M 3 muscarinic acetylcholine receptors. One of our goals was to study whether the cytoplasmic end of transmembrane domain V (TM V), a region known to be critically involved in receptor/G protein coupling, undergoes a major conformational change, similar to the adjacent region of TM VI. Another goal was to determine and compare the disulfide cross-linking patterns observed after treatment of the different mutant receptors with full versus inverse muscarinic agonists. Specifically, we generated 20 double Cys mutant M 3 receptors harboring one Cys substitution within the cytoplasmic end of TM V (L249-I253) and a second one within the cytoplasmic end of TM VI (A489-L492). These receptors were transiently expressed in COS-7 cells and subsequently characterized in pharmacological and disulfide cross-linking studies. Our cross-linking data, in conjunction with a three-dimensional model of the M 3 muscarinic receptor, indicate that M 3 receptor activation does not trigger major structural disturbances within the cytoplasmic segment of TM V, in contrast to the pronounced structural changes predicted to occur at the cytoplasmic end of TM VI. We also demonstrated that full and inverse muscarinic agonists had distinct effects on the efficiency of disulfide bond formation in specific double Cys mutant M 3 receptors. The present study provides novel information about the dynamic changes that accompany M 3 receptor activation and how the receptor conformations induced (or stabilized) by full versus inverse muscarinic agonists differ from each other at the molecular level. Because all class I GPCRs are predicted to share a similar transmembrane topology, the conclusions drawn from the present study should be of broad general relevance.     

A limited spectrum of mutations causes constitutive activation of the yeast alpha-factor receptor

Activation of G protein coupled receptors (GPCRs) by binding of ligand is the initial event in diverse cellular signaling pathways. To examine the frequency and diversity of mutations that cause constitutive activation of one particular GPCR, the yeast alpha-factor receptor, we screened libraries of random mutations for constitutive alleles. In initial screens for mutant receptor alleles that exhibit signaling in the absence of added ligand, 14 different point mutations were isolated. All of these 14 mutants could be further activated by alpha-factor. Ten of the mutants also acquired the ability to signal in response to binding of desTrp(1)?Ala(3)?lpha-factor, a peptide that acts as an antagonist toward normal alpha-factor receptors. Of these 10 mutants, at least eight alleles residing in the third, fifth, sixth, and seventh transmembrane segments exhibit bona fide constitutive signaling. The remaining alleles are hypersensitive to alpha-factor rather than constitutive. They can be activated by low concentrations of endogenous alpha-factor present in MATa cells. The strongest constitutively active receptor alleles were recovered multiple times from the mutational libraries, and extensive mutagenesis of certain regions of the alpha-factor receptor did not lead to recovery of any additional constitutive alleles. Thus, only a limited number of mutations is capable of causing constitutive activation of this receptor. Constitutive and hypersensitive signaling by the mutant receptors is partially suppressed by coexpression of normal receptors, consistent with preferential association of the G protein with unactivated receptors.     

The role of the DRY motif of human MC4R for receptor activation

We constructed several mutant human MC4R cDNAs by site directed mutagenesis and expressed these receptors in COS-1 cells. The conserved DRY motif among GPCRs was mutated to generate eight mutants. While no MC4R ligand binding was detected in any of the mutants, one mutant, D146A, resulted in higher cAMP production in cells than the wild-type receptor without ligand stimulation.     

Subunits of a yeast oligomeric G protein-coupled receptor are activated independently by agonist but function in concert to activate G protein heterotrimers

G protein-coupled receptors (GPCRs) form dimeric or oligomeric complexes in vivo. However, the function of oligomerization in receptor-mediated G protein activation is unclear. Previous studies of the yeast alpha-factor receptor (STE2 gene product) have indicated that oligomerization promotes signaling. Here we have addressed the mechanism by which oligomerization facilitates G protein signaling by examining the ability of ligand binding- and G protein coupling-defective alpha-factor receptors to form complexes in vivo and to correct their signaling defects when co-expressed (trans complementation). Newly and previously identified receptor mutants indicated that ligand binding involves the exofacial end of transmembrane domain (TM) 4, whereas G protein coupling involves ic1, ic3, the C-terminal tail, and the intracellular ends of TM2 and TM3. Mutant receptors bearing substitutions in these domains formed homo-oligomeric or hetero-oligomeric complexes in vivo, as indicated by results of fluorescence resonance energy transfer experiments. Co-expression of ligand binding- and G protein coupling-defective mutant receptors did not significantly improve signaling. In contrast, co-expression of ic1 and ic3 mutations in trans but not in cis significantly increased signaling efficiency. Therefore, we suggest that subunits of the alpha-factor receptor: 1) are activated independently rather than cooperatively by agonist, and 2) function in a concerted fashion to promote G protein activation, possibly by contacting different subunits or regions of the G protein heterotrimer.     

Rapid identification of functionally critical amino acids in a G protein-coupled receptor

G protein-coupled receptors (GPCRs) comprise one of the largest protein families found in nature. Here we describe a new experimental strategy that allows rapid identification of functionally critical amino acids in the rat M(3) muscarinic acetylcholine receptor (M3R), a prototypic class I GPCR. This approach involves low-frequency random mutagenesis of the entire M3R coding sequence, followed by the application of a new yeast genetic screen that allows the recovery of inactivating M3R single point mutations. The vast majority of recovered mutant M3Rs also showed substantial functional impairments in transfected mammalian (COS-7) cells. A subset of mutant receptors, however, behaved differently in yeast and mammalian cells, probably because of the specific features of the yeast expression system used. The screening strategy described here should be applicable to all GPCRs that can be expressed functionally in yeast.     

Role of receptor-attached phosphates in binding of visual and non-visual arrestins to G protein-coupled receptors

Arrestins are a small family of proteins that regulate G protein-coupled receptors (GPCRs). Arrestins specifically bind to phosphorylated active receptors, terminating G protein coupling, targeting receptors to endocytic vesicles, and initiating G protein-independent signaling. The interaction of rhodopsin-attached phosphates with Lys-14 and Lys-15 in β-strand I was shown to disrupt the interaction of α-helix I, β-strand I, and the C-tail of visual arrestin-1, facilitating its transition into an active receptor-binding state. Here we tested the role of conserved lysines in homologous positions of non-visual arrestins by generating K2A mutants in which both lysines were replaced with alanines. K2A mutations in arrestin-1, -2, and -3 significantly reduced their binding to active phosphorhodopsin in vitro. The interaction of arrestins with several GPCRs in intact cells was monitored by a bioluminescence resonance energy transfer (BRET)-based assay. BRET data confirmed the role of Lys-14 and Lys-15 in arrestin-1 binding to non-cognate receptors. However, this was not the case for non-visual arrestins in which the K2A mutations had little effect on net BRET(max) values for the M2 muscarinic acetylcholine (M2R), β(2)-adrenergic (β(2)AR), or D2 dopamine receptors. Moreover, a phosphorylation-deficient mutant of M2R interacted with wild type non-visual arrestins normally, whereas phosphorylation-deficient β(2)AR mutants bound arrestins at 20-50% of the level of wild type β(2)AR. Thus, the contribution of receptor-attached phosphates to arrestin binding varies depending on the receptor-arrestin pair. Although arrestin-1 always depends on receptor phosphorylation, its role in the recruitment of arrestin-2 and -3 is much greater in the case of β(2)AR than M2R and D2 dopamine receptor.     

Identification of two serine residues involved in agonist activation of the beta-adrenergic receptor

Pharmacophore mapping of the ligand binding domain of the beta-adrenergic receptor has revealed specific molecular interactions which are important for agonist and antagonist binding to the receptor. Previous site-directed mutagenesis experiments have demonstrated that the binding of amine agonists and antagonists to the receptor involves an interaction between the amine group of the ligand and the carboxylate side chain of Asp113 in the third hydrophobic domain of the receptor (Strader, C. D., Sigal, I. S., Candelore, M. R., Rands, E., Hill, W. S., and Dixon, R. A. F. (1988) J. Biol. Chem. 263, 10267-10271). We have now identified 2 serine residues, at positions 204 and 207 in the fifth hydrophobic domain of the beta-adrenergic receptor, which are critical for agonist binding and activation of the receptor. These serine residues are conserved with G-protein-coupled receptors which bind catecholamine agonists, but not with receptors whose endogenous ligands do not have the catechol moiety. Removal of the hydroxyl side chain from either Ser204 or Ser207 by substitution of the serine residue with an alanine attenuates the activity of catecholamine agonists at the receptor. The effects of these mutations on agonist activity are mimicked selectively by the removal of the catechol hydroxyl moieties from the aromatic ring of the agonist. The data suggest that the interaction of catecholamine agonists with the beta-adrenergic receptor involves two hydrogen bonds, one between the hydroxyl side chain of Ser204 and the meta-hydroxyl group of the ligand and a second between the hydroxyl side chain of Ser207 and the para-hydroxyl group of the ligand.     

Bitter taste receptors: Novel insights into the biochemistry and pharmacology

Bitter taste receptors (T2Rs) belong to the super family of G protein-coupled receptors (GPCRs). There are 25 T2Rs expressed in humans, and these interact with a large and diverse group of bitter ligands. T2Rs are expressed in many extra-oral tissues and can perform diverse physiological roles. Structure-function studies led to the identification of similarities and dissimilarities between T2Rs and Class A GPCRs including amino acid conservation and novel motifs. However, the efficacy of most of the T2R ligands is not yet elucidated and the biochemical pharmacology of T2Rs is poorly understood. Recent studies on T2Rs characterized novel ligands including blockers for these receptors that include inverse agonist and antagonists. In this review we discuss the techniques used for elucidating bitter blockers, concept of ligand bias, generic amino acid numbering, the role of cholesterol, and conserved water molecules in the biochemistry and pharmacology of T2Rs.     

Long Range Effect of Mutations on Specific Conformational Changes in the Extracellular Loop 2 of Angiotensin II Type 1 Receptor

The topology of the second extracellular loop (ECL2) and its interaction with ligands is unique in each G protein-coupled receptor. When the orthosteric ligand pocket located in the transmembrane (TM) domain is occupied, ligand-specific conformational changes occur in the ECL2. In more than 90% of G protein-coupled receptors, ECL2 is tethered to the third TM helix via a disulfide bond. Therefore, understanding the extent to which the TM domain and ECL2 conformations are coupled is useful. To investigate this, we examined conformational changes in ECL2 of the angiotensin II type 1 receptor (AT1R) by introducing mutations in distant sites that alter the activation state equilibrium of the AT1R. Differential accessibility of reporter cysteines introduced at four conformation-sensitive sites in ECL2 of these mutants was measured. Binding of the agonist angiotensin II (AngII) and inverse agonist losartan in wild-type AT1R changed the accessibility of reporter cysteines, and the pattern was consistent with ligand-specific “lid” conformations of ECL2. Without agonist stimulation, the ECL2 in the gain of function mutant N111G assumed a lid conformation similar to AngII-bound wild-type AT1R. In the presence of inverse agonists, the conformation of ECL2 in the N111G mutant was similar to the inactive state of wild-type AT1R. In contrast, AngII did not induce a lid conformation in ECL2 in the loss of function D281A mutant, which is consistent with the reduced AngII binding affinity in this mutant. However, a lid conformation was induced by [Sar¹,Gln²,Ile⁸] AngII, a specific analog that binds to the D281A mutant with better affinity than AngII. These results provide evidence for the emerging paradigm of domain coupling facilitated by long range interactions at distant sites on the same receptor.     

Inhibition of sequestration of human B2 bradykinin receptor by phenylarsine oxide or sucrose allows determination of a receptor affinity shift and ligand dissociation in intact cells

Depending on their interaction with intracellular proteins, G protein-coupled receptors (GPCR) often display different affinities for agonists at 37 degrees C. Determining the affinity at that temperature is often difficult in intact cells as most GPCRs are internalized after activation. When sequestration of the B2 bradykinin receptor (B2R) was inhibited by either 0.5 M sucrose or phenylarsine oxide (PAO), a shift in the affinity was detected when the incubation temperature was raised from 4 degrees C to 37 degrees C or lowered from 37 degrees C to 4 degrees C. In contrast, binding of the antagonist [3H]NPC 17731 was temperature-independent. B2R mutants displayed different affinity shifts allowing conclusions on the role of the involved amino acids. By inhibiting receptor sequestration it was possible to determine also dissociation of [3H]BK and of [3H]NPC 17731 from intact cells at 37 degrees C. Surprisingly, both dissociation rates were markedly enhanced by the addition of unlabeled ligand, most likely via prevention of reassociation of dissociated [3H]ligand. This suggests that dissociated [3H]ligand cannot move freely away from the receptor. In summary, our data demonstrate that inhibition of receptor internalization either by PAO or sucrose provides an excellent method to study receptor function and the effects of mutations in intact cells.     

Switching agonist/antagonist properties of opiate alkaloids at the delta opioid receptor using mutations based on the structure of the orphanin FQ receptor

In an earlier study, we have demonstrated that by mutating five amino acid residues to those conserved in the opioid receptors, the OFQ receptor could be converted to a functional receptor that bound many opioid alkaloids with nanomolar affinities. Surprisingly, when the reciprocal mutations, Lys-214 --> Ala (TM5), Ile-277 --> Val/His-278 --> Gln/Ile-279 --> Val (TM6), and Ile-304 --> Thr (TM7), are introduced in the delta receptor, neither the individual mutations nor their various combinations significantly reduce the binding affinities of opioid alkaloids tested. However, these mutations cause profound alterations in the functional characteristics of the mutant receptors as measured in guanosine 5'-3-O-(thio)triphosphate binding assays. Some agonists become antagonists at some constructs as they lose their ability to activate them. Some alkaloid antagonists are transformed into agonists at other constructs, but their agonistic effects can still be blocked by the peptide antagonist TIPP. Even the delta inverse agonist 7-benzylidenenaltrexone becomes an agonist at the mutant containing both the Ile-277 --> Val/His-278 --> Gln/Ile-279 --> Val and Ile-304 --> Thr mutations. Thus, although the mutated residues are thought to be part of the binding pocket, they are critically involved in the control of the delta receptor activation process. These findings shed light on some of the structural bases of ligand efficacy. They are also compatible with the hypothesis that a ligand may achieve high affinity binding in several different ways, each having different effects on receptor activation.     

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast. Identification of point mutations that "silence" a constitutively active mutant M3 receptor and greatly impair receptor/G protein coupling

The M3 muscarinic receptor is a prototypical member of the class I family of G protein-coupled receptors (GPCRs). To facilitate studies on the structural mechanisms governing M3 receptor activation, we generated an M3 receptor-expressing yeast strain (Saccharomyces cerevisiae) that requires agonist-dependent M3 receptor activation for cell growth. By using receptor random mutagenesis followed by a genetic screen in yeast, we initially identified a point mutation at the cytoplasmic end of transmembrane domain (TM) VI (Q490L) that led to robust agonist-independent M3 receptor signaling in both yeast and mammalian cells. To explore further the molecular mechanisms by which point mutations can render GPCRs constitutively active, we subjected a region of the Q490L mutant M3 receptor that included TM V-VII to random mutagenesis. We then applied a yeast genetic screen to identify second-site mutations that could suppress the activating effects of the Q490L mutation and restore wild-type receptor-like function to the Q490L mutant receptor. This analysis led to the identification of 12 point mutations that allowed the Q490L mutant receptor to function in a fashion similar to the wild-type receptor. These amino acid substitutions mapped to two distinct regions of the M3 receptor, the exofacial segments of TM V and VI and the cytoplasmic ends of TM V-VII. Strikingly, in the absence of the activating Q490L mutation, all recovered point mutations severely reduced the efficiency of receptor/G protein coupling, indicating that the targeted residues play important roles in receptor activation and/or receptor/G protein coupling. This strategy should be generally applicable to identify sites in GPCRs that are critically involved in receptor function.     

Decreased agonist affinity and chloride conductance of mutant glycine receptors associated with human hereditary hyperekplexia.

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor alpha 1 subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1R271L or alpha 1R271Q) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [3H]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1R271L, 64 pS; alpha 1R271Q, 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyperexcitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor.     

Agonist-bound structures of G protein-coupled receptors

G protein-coupled receptors (GPCRs) play a major role in intercellular communication by binding small diffusible ligands (agonists) at the extracellular surface. Agonist-binding induces a conformational change in the receptor, which results in the binding and activation of heterotrimeric G proteins within the cell. Ten agonist-bound structures of non-rhodopsin GPCRs published last year defined for the first time the molecular details of receptor activated states and how inverse agonists, partial agonists and full agonists bind to produce different effects on the receptor. In addition, the structure of the β(2)-adrenoceptor coupled to a heterotrimeric G protein showed how the opening of a cleft in the cytoplasmic face of the receptor as a consequence of agonist binding results in G protein coupling and activation of the G protein.