The third extracellular loop of the beta2-adrenergic receptor can modulate receptor/G protein affinity

Diverse residues of intracellular loop 5 of the Na+/H+ exchanger modulate proton sensing, expression, activity and targeting

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is an integral membrane protein that regulates intracellular pH (pH_i) by removing a single intracellular proton in exchange for one extracellular sodium ion. It is involved in cardiac hypertrophy and ischemia reperfusion damage to the heart and elevation of its activity is a trigger for breast cancer metastasis. NHE1 has an extensive 500 amino acid N-terminal membrane domain that mediates transport and consists of 12 transmembrane segments connected by intracellular and extracellular loops. Intracellular loops are hypothesized to modulate the sensitivity to pH_i. In this study, we characterized the structure and function of intracellular loop 5 (IL5), specifically amino acids 431-443. Mutation of eleven residues to alanine caused partial or nearly complete inhibition of transport; notably, mutation of residues L432, T433, I436, N437, R440 and K443 demonstrated these residues had critical roles in NHE1 function independent of effects on targeting or expression. The nuclear magnetic resonance (NMR) solution spectra of the IL5 peptide in a membrane mimetic sodium dodecyl sulfate solution revealed that IL5 has a stable three-dimensional structure with substantial alpha helical character. NMR chemical shifts indicated that K438 was in close proximity with W434. Overall, our results show that IL5 is a critical, intracellular loop with a propensity to form an alpha helix, and many residues of this intracellular loop are critical to proton sensing and ion transport.

Recent structural advances of β1 and β2 adrenoceptors yield keys for ligand recognition and drug design

Because they represent attractive drug targets, adrenoceptors have been widely studied. Recent progress in structural data of β-adrenoceptors allows us to understand and predict key interactions in ligand recognition and receptor activation. Nevertheless, an important aspect of this process has only begun to be explored: the stabilization of a conformational state of these receptors upon contact with a ligand and the capacity of a ligand to influence receptor conformation through allosteric modulation, biased signaling, and selectivity. The aim of the present Perspective is to identify the well-defined orthosteric binding site and possible allosteric sites and to analyze the importance of the ligand-receptor interaction in the stabilization of certain receptor conformations. For this purpose, we have reviewed recent advances made through the use of X-ray data from ligand-β-adrenoceptor (including ADRB1 and ADRB2) crystal structures. Most importantly, implications in the medicinal chemistry field are explored in relation to drug design.

New insights into structural determinants for prostanoid thromboxane A2 receptor- and prostacyclin receptor-G protein coupling

G protein-coupled receptors (GPCRs) interact with heterotrimeric G proteins and initiate a wide variety of signaling pathways. The molecular nature of GPCR-G protein interactions in the clinically important thromboxane A2 (TxA(2)) receptor (TP) and prostacyclin (PGI(2)) receptor (IP) is poorly understood. The TP activates its cognate G protein (Gαq) in response to the binding of thromboxane, while the IP signals through Gαs in response to the binding of prostacyclin. Here, we utilized a combination of approaches consisting of chimeric receptors, molecular modeling, and site-directed mutagenesis to precisely study the specificity of G protein coupling. Multiple chimeric receptors were constructed by replacing the TP intracellular loops (ICLs) with the ICL regions of the IP. Our results demonstrate that both the sequences and lengths of ICL2 and ICL3 influenced G protein specificity. Importantly, we identified a precise ICL region on the prostanoid receptors TP and IP that can switch G protein specificities. The validities of the chimeric technique and the derived molecular model were confirmed by introducing clinically relevant naturally occurring mutations (R60L in the TP and R212C in the IP). Our findings provide new molecular insights into prostanoid receptor-G protein interactions, which are of general significance for understanding the structural basis of G protein activation by GPCRs in basic health and cardiovascular disease.

The third extracellular loop of the human calcitonin receptor-like receptor is crucial for the activation of adrenomedullin signalling

BACKGROUND AND PURPOSE

The extracellular loops (ECLs) in Family A GPCRs are important for ligand binding and receptor activation, but little is known about the function of Family B GPCR ECLs, especially ECL3. Calcitonin receptor-like receptor (CLR), a Family B GPCR, functions as a calcitonin gene-related peptide (CGRP) and an adrenomedullin (AM) receptor in association with three receptor activity-modifying proteins (RAMPs). Here, we examined the function of the ECL3 of human CLR within the CGRP and AM receptors.

EXPERIMENTAL APPROACH

A CLR ECL3 chimera, in which the ECL3 of CLR was substituted with that of VPAC2 (a Family B GPCR that is unable to interact with RAMPs), and CLR ECL3 point mutants were constructed and transiently transfected into HEK-293 cells along with each RAMP. Cell-surface expression of each receptor complex was then measured by flow cytometry; [(125) I]-CGRP and [(125) I]-AM binding and intracellular cAMP accumulation were also measured.

KEY RESULTS

Co-expression of the CLR ECL3 chimera with RAMP2 or RAMP3 led to significant reductions in the induction of cAMP signalling by AM, but CGRP signalling was barely affected, despite normal cell-surface expression of the receptors and normal [(125) I]-AM binding. The chimera had significantly decreased AM, but not CGRP, responses in the presence of RAMP1. Not all CLR ECL3 mutants supported these findings.

CONCLUSIONS AND IMPLICATIONS

The human CLR ECL3 is crucial for AM-induced cAMP responses via three CLR/RAMP heterodimers, and activation of these heterodimers probably relies on AM-induced conformational changes. This study provides a clue to the molecular basis of the activation of RAMP-based Family B GPCRs.

Structural and functional analysis of extracellular loop 4 of the Nhe1 isoform of the Na(+)/H(+) exchanger

The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) is a ubiquitously expressed plasma membrane protein. It regulates intracellular pH by removing a single intracellular H(+) in exchange for one extracellular Na(+). The membrane domain of NHE1 comprises the 500 N-terminal amino acids and is made of 12 transmembrane segments. The extracellular loops of the transmembrane segments are thought to be involved in cation coordination and inhibitor sensitivity. We have characterized the structure and function of amino acids 278-291 representing extracellular loop 4. When mutated to Cys, residues F277, F280, N282 and E284 of EL4 were sensitive to mutation and reaction with MTSET inhibiting NHE1 activity. In addition they were found to be accessible to extracellular applied MTSET. A peptide of the amino acids of EL4 was mostly unstructured suggesting that it does not provide a rigid structured link between TM VII and TM VIII. Our results suggest that EL4 makes an extension upward from TM VII to make up part of the mouth of the NHE1 protein and is involved in cation selectivity or coordination. EL4 provides a flexible link to TM VIII which may either allow movement of TM VII or allow TM VIII to not be adjacent to TM VII.

Enhanced hypotensive, bradycardic, and hypnotic responses to alpha2-adrenergic agonists in spinophilin-null mice are accompanied by increased G protein coupling to the alpha2A-adrenergic receptor

We previously identified spinophilin as a regulator of alpha(2) adrenergic receptor (alpha(2)AR) trafficking and signaling in vitro and in vivo (Science 304:1940-1944, 2004). To assess the generalized role of spinophilin in regulating alpha(2)AR functions in vivo, the present study examined the impact of eliminating spinophilin on alpha(2)AR-evoked cardiovascular and hypnotic responses, previously demonstrated to be mediated by the alpha(2A)AR subtype, after systemic administration of the alpha(2)-agonists 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14,304) and clonidine in spinophilin-null mice. Mice lacking spinophilin expression display dramatically enhanced and prolonged hypotensive, bradycardic, and sedative-hypnotic responses to alpha(2)AR stimulation. Whereas these changes in sensitivity to alpha(2)AR agonists occur independent of any changes in alpha(2A)AR density or intrinsic affinity for agonist in the brains of spinophilin-null mice compared with wild-type control mice, the coupling of the alpha(2A)AR to cognate G proteins is enhanced in spinophilin-null mice. Thus, brain preparations from spinophilin-null mice demonstrate enhanced guanine nucleotide regulation of UK14,304 binding and evidence of a larger fraction of alpha(2A)AR in the guanine-nucleotide-sensitive higher affinity state compared with those from wild-type mice. These findings suggest that eliminating spinophilin expression in native tissues leads to an enhanced receptor/G protein coupling efficiency that contributes to sensitization of receptor mediated responses in vivo.

Changes to gonadotropin-releasing hormone (GnRH) receptor extracellular loops differentially affect GnRH analog binding and activation: evidence for distinct ligand-stabilized receptor conformations

GnRH and its structural variants bind to GnRH receptors from different species with different affinities and specificities. By investigating chimeric receptors that combine regions of mammalian and nonmammalian GnRH receptors, a greater understanding of how different domains influence ligand binding and receptor activation can be achieved. Using human-catfish and human-chicken chimeric receptors, we demonstrate the importance of extracellular loop conformation for ligand binding and agonist potency, providing further evidence for GnRH and GnRH II stabilization of distinct active receptor conformations. We demonstrate examples of GnRH receptor gain-of-function mutations that apparently improve agonist potency independently of affinity, implicating a role for extracellular loops in stabilizing the inactive receptor conformation. We also show that entire extracellular loop substitution can overcome the detrimental effects of localized mutations, thereby demonstrating the importance of considering the conformation of entire domains when drawing conclusions from point-mutation studies. Finally, we present evidence implicating the configuration of extracellular loops 2 and 3 in combination differentiating GnRH analog binding modes. Because there are two endogenous forms of GnRH ligand but only one functional form of full-length GnRH receptor in humans, understanding how GnRH and GnRH II can elicit distinct functional effects through the same receptor is likely to provide important insights into how these ligands can have differential effects in both physiological and pathological situations.

Extracellular loop 3 (ECL3) and ECL3-proximal transmembrane domains VI and VII of the mesotocin and vasotocin receptors confer differential ligand selectivity and signaling activity

Mesotocin (MT) and vasotocin (VT) are the nonmammalian orthologs of mammalian oxytocin (OT) and arginine vasopressin (AVP), respectively. The OT/AVP family of peptides has arisen from gene duplication but has evolved to possess high selectivity toward their cognate receptors. The process of molecular evolution of receptors to confer high selectivity to their cognate ligands, however, is poorly understood. We constructed a series of reciprocal chimeras using a pair of bullfrog MT receptor (MTR) and VT1 receptor (VT1R) DNA fragments. Among the MTR/VT1R chimeras, the MTR chimera containing a region from transmembrane domain (TMD) VI to the carboxyl-terminal tail (C-tail) of VT1R showed an increased sensitivity to VT, while a chimeric VT1R containing TMD VI to C-tail of MTR showed an increased sensitivity to MT. Further dissection of domains using additional chimeras demonstrated that the receptor with the fragment containing extracellular loop 3 (ECL3) and ECL3-proximal TMDs VI and VII of MTR increased MT selectivity. This fragment is also important for receptor conformation that permits the signaling ability of the receptor. Particularly, the amino acids Val/Ile(6.54) in TMD VI and Pro/Glu(7.29) in ECL3 appear to be involved in this activity, since double mutation of these amino acids completely blocked signaling activity while maintaining ligand binding activity. Mutations at these residues in human OT and AVP 1a receptors markedly decreased receptor signaling activity. This study provides clues for understanding molecular coevolution of the OT/AVP peptides and their receptors with regard to receptor-ligand binding and receptor signaling activity.

The third extracellular loop of G-protein-coupled receptors: more than just a linker between two important transmembrane helices

GPCRs (G-protein-coupled receptors) are a large family of structurally related proteins, which mediate their effects by coupling with G-proteins. Despite responding to a range of very diverse stimuli, these receptors exhibit a conserved tertiary structure comprising a bundle of seven TM (transmembrane) helices linked by alternating ECLs (extracellular loops) and ICLs (intracellular loops). The hydrophobic environment formed by the cluster of TM helices is functionally important. For example, the 11-cis retinal chromophore of rhodopsin forms a protonated Schiff base linkage to a lysine in TM7, deep within the helical bundle, and small ligands, such as amine neurotransmitters and non-peptide analogues of peptide hormones, also bind within the corresponding region of their cognate receptors. In addition, activation of GPCRs involves relative movement of TM helices to present G-protein interaction sites across the intracellular face of the receptor. Consequently, it might be assumed that the ECLs of the GPCR are inert peptide linkers that merely connect important TM helices. Focusing on ECL3 (third ECL), it is becoming increasingly apparent that this extracellular domain can fulfil a range of important roles with respect to GPCR signalling, including agonist binding, ligand selectivity and receptor activation.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

Pro7.33(303) of the human GnRH receptor regulates selective binding of mammalian GnRH

Mammalian gonadotropin releasing hormone (GnRH) receptors have a conserved acidic residue (Glu7.32(301) or Asp7.32(302)) in extracellular loop (ECL) three that confers selectivity for mammalian GnRH, which has Arg8. Comparison of mammalian and non-mammalian GnRH receptors suggested that the acidic residue is not the only determinant of ligand selectivity in mammalian receptors. The acidic residue is followed by a conserved Pro7.33 in mammalian GnRH receptors, but not non-mammalian receptors. Unique structural constraints imposed by Pro residues suggested that Pro7.33 determines selective binding of Arg8-containing GnRH, by stabilising the conformation of the third extracellular loop of the receptor. Substituting Pro7.33(303) or introducing Pro to position 7.31 decreased affinity for GnRH, but not analogs lacking Arg8. Substituting Pro7.33(303) changed the predicted alpha-helix content of the loop-helix interface. These results show that Pro7.33(303) of the human GnRH receptor is required for selective high affinity binding of mammalian GnRH and supports the hypothesis that Pro7.33(303) stabilises a loop conformation that is necessary for selective ligand binding.

Opioid receptor random mutagenesis reveals a mechanism for G protein-coupled receptor activation

The high resolution structure of rhodopsin has greatly enhanced current understanding of G protein-coupled receptor (GPCR) structure in the off-state, but the activation process remains to be clarified. We investigated molecular mechanisms of delta-opioid receptor activation without a preconceived structural hypothesis. Using random mutagenesis of the entire receptor, we identified 30 activating point mutations. Three-dimensional modeling revealed an activation path originating from the third extracellular loop and propagating through tightly packed helices III, VI and VII down to a VI-VII cytoplasmic switch. N- and C-terminal determinants also influence receptor activity. Findings for this therapeutically important receptor may apply to other GPCRs that respond to diffusible ligands.

Integrity of extracellular loop 1 of the human cannabinoid receptor 1 is critical for high-affinity binding of the ligand CP 55,940 but not SR 141716A

Like other G-protein coupled receptors with hydrophobic ligands, the human cannabinoid receptor 1 (CB1) is thought to bind its ligands within the transmembrane region of the receptor. However, for some of these receptors the extracellular loops (ECs) have also been shown to play a role in ligand recognition and selectivity. We have taken a mutagenesis approach to examine the role of the amino terminus, EC1, and EC3 of CB1 in ligand binding. Eight mutant receptors, each with a dipeptide insertion, were constructed, expressed, and evaluated for binding to the cannabinoid ligands (-)-cis-3[2-hydroxy-4-(1',1'-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP 55,940) and N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR 141716A). Mutants with insertions in the membrane distal region of the amino terminus or EC3 maintained affinity for both ligands. Those with insertions in the membrane proximal region of the amino terminus or EC1 exhibited a loss of affinity for CP 55,940 while retaining wild-type affinity for SR 141716A. Representative mutants were analyzed for agonist-induced inhibition of cyclic AMP accumulation, and the results indicated that G-protein coupling remained intact. Alanine substitution mutants were made to address whether it was the perturbation of the overall structure of the region or the displacement of particular side chains that was responsible for the loss of CP 55,940 binding. We conclude that a structurally intact EC1, but not the comparably short EC3, is essential for high-affinity CP 55,940 binding.

Insight into the mechanism of dopamine D1-like receptor activation. Evidence for a molecular interplay between the third extracellular loop and the cytoplasmic tail

A chimeric D1A dopaminergic receptor harboring the cytoplasmic tail (CT) of the D1B subtype (D1A-CTB) has been used previously to show that CT imparts high dopamine (DA) affinity and constitutive activity to the D1B receptors. However, the D1A-CTB chimera, unlike the D1B subtype, exhibits a significantly lower DA potency for stimulating adenylyl cyclase and a drastically lower maximal binding capacity (Bmax). Here, using a functional complementation of chimeric D1-like receptors, we have identified the human D1B receptor regions regulating the intramolecular relationships that lead to an increased DA potency and contribute to Bmax. We demonstrate that the addition of variant residues of the third extracellular loop (EL3) of the human D1B receptor into D1A-CTB chimera leads to a constitutively active mutant receptor displaying an increased DA affinity, potency, and Bmax. These results strongly suggest that constitutively active D1-like receptors can adopt multiple active conformations, notably one that confers increased DA affinity with decreased DA potency and Bmax and another that imparts increased DA affinity with a strikingly increased DA potency and Bmax. Overall, we show that a novel molecular interplay between EL3 and CT regulates multiple active conformations of D1-like receptors and may have potential implications for other G protein-coupled receptor classes.

Molecular evolution of the mammalian alpha 2B adrenergic receptor

The alpha 2B adrenergic receptor (A2AB) is a heptahelical G protein-coupled receptor for catecholamines. We compared the almost complete coding region (about 1,175 bp) of the A2AB gene from 48 mammalian species, including eight newly determined sequences, representing all the 18 eutherian and two marsupial orders. Comparison of the encoded proteins reveals that residues thought to be involved in agonist binding are highly conserved, as are the regions playing a role in G protein-coupling. The three extracellular loops are generally more variable than the transmembrane domains and two of the intracellular loops, indicating a lower functional constraint. However, the greatest variation is observed in the very long, third intracellular loop, where only a few residues and a polyglutamyl tract are preserved. Although this polyglutamyl domain displays a great variation in length, its presence in all described A2ABs confirms its proposed role in agonist-dependent phosphorylation of the third intracellular loop. Phylogenetic analyses of the A2AB data set, including Bayesian methods, recognized the superordinal clades Afrotheria, Laurasiatheria, and Euarchontoglires, in agreement with recent molecular evidence, albeit with lower support. Within Afrotheria, A2AB strongly supports the paenungulate clade and the association of the continental African otter shrew with Malagasy tenrecs. Among Laurasiatheria, A2AB confirms the nesting of whales within the artiodactyls, as a sister group to hippopotamus. Within the Euarchontoglires, there is constant support for rodent monophyly.

The binding site of aminergic G protein-coupled receptors: the transmembrane segments and second extracellular loop

In the current chapter, we review approaches to the identification of the residues forming the binding sites for agonists, antagonists, and allosteric modulators in the family of aminergic G protein-coupled receptors (GPCRs). We then review the structural bases for ligand binding and pharmacological specificity based on the application of these methods to muscarinic cholinergic, adrenergic, dopaminergic, serotonergic, and histaminergic receptors, using the high resolution rhodopsin structure as a template. Furthermore, we propose a critical role of the second extracellular loop in forming the binding site for small molecular weight aminergic ligands, much as this loop dives down into the binding-site crevice and contacts retinal in rhodopsin.

Role of aspartate7.32(302) of the human gonadotropin-releasing hormone receptor in stabilizing a high-affinity ligand conformation

Mammalian gonadotropin-releasing hormone (GnRH) receptors preferentially bind mammalian GnRH, which has Arg in position eight. The Glu(7.32(301)) residue, which determines selectivity of the mouse GnRH receptor for Arg(8)-containing GnRH, is Asp(7.32(302)) in the human GnRH receptor. We have confirmed that Asp(7.32(302)) confers selectivity of the human GnRH receptor for Arg(8) of GnRH and investigated the mechanism of this specificity using site-directed mutagenesis and ligand modification. We find that although Arg(8) and Asp(7.32(302)) are required for high-affinity binding of GnRH, conformationally constrained peptides, with D-amino acid substitutions in position six or with a 6,7 gamma-lactam, bind the human GnRH receptor with high affinity, which is independent of the presence of Asp(7.32(302)) in the receptor or Arg(8) in the ligand. The ability of the ligand constraints to compensate for the absence of both Arg(8) and Asp(7.32(302)) indicates that these residues both have roles in stabilizing a high affinity ligand conformation and that their roles are complementary. This suggests that the Arg(8) and Asp(7.32(302)) side chains interact to induce a high affinity conformation of native GnRH. Thus, Asp(7.32(302)) of the human GnRH receptor determines selectivity for mammalian GnRH by its ability to induce a high affinity conformation of its native ligand. However, this initial interaction seems not to contribute to the final ligand-receptor complex. We propose that Arg(8) interacts transiently with Asp(7.32(302)) to induce a high-affinity ligand conformation of GnRH, which then interacts with a binding pocket that is common for both constrained and unconstrained analogs of GnRH.

Phe-308 and Phe-312 in transmembrane domain 7 are major sites of alpha 1-adrenergic receptor antagonist binding. Imidazoline agonists bind like antagonists

Although agonist binding in adrenergic receptors is fairly well understood and involves residues located in transmembrane domains 3 through 6, there are few residues reported that are involved in antagonist binding. In fact, a major docking site for antagonists has never been reported in any G-protein coupled receptor. It has been speculated that antagonist binding is quite diverse depending upon the chemical structure of the antagonist, which can be quite different from agonists. We now report the identification of two phenylalanine residues in transmembrane domain 7 of the alpha(1a)-adrenergic receptor (Phe-312 and Phe-308) that are a major site of antagonist affinity. Mutation of either Phe-308 or Phe-312 resulted in significant losses of affinity (4-1200-fold) for the antagonists prazosin, WB4101, BMY7378, (+) niguldipine, and 5-methylurapidil, with no changes in affinity for phenethylamine-type agonists such as epinephrine, methoxamine, or phenylephrine. Interestingly, both residues are involved in the binding of all imidazoline-type agonists such as oxymetazoline, cirazoline, and clonidine, confirming previous evidence that this class of ligand binds differently than phenethylamine-type agonists and may be more antagonist-like, which may explain their partial agonist properties. In modeling these interactions with previous mutagenesis studies and using the current backbone structure of rhodopsin, we conclude that antagonist binding is docked higher in the pocket closer to the extracellular surface than agonist binding and appears skewed toward transmembrane domain 7.

Epinephrine correction of impaired platelet thromboxane receptor signaling

This study evaluated the mechanism of epinephrine potentiation of platelet secretion induced by thromboxane A(2) (TXA(2)). Dog platelets that do not secrete in response to TXA(2) alone (TXA(2)-) were compared with dog platelets that do secrete (TXA(2)+) and with human platelets. TXA(2)- platelets had impaired TXA(2) receptor (TP receptor)-G protein coupling, indicated by 1) impaired stimulated GTPase activity, 2) elevated basal guanosine 5'-O-(3-thiotriphosphate) binding, and 3) elevated Galpha(q) palmitate turnover that was corrected by preexposure to epinephrine. Kinetic agonist binding studies revealed biphasic dog and human platelet TP receptor association and dissociation. TXA(2)- and TP receptor-desensitized TXA(2)+ dog and human platelets had altered ligand binding parameters compared with untreated TXA(2)+ or human platelets. These parameters were reversed, along with impaired secretion, by epinephrine. Basal phosphorylation of TXA(2)- platelet TP receptors was elevated 60% and was normalized by epinephrine. Epinephrine potentiates platelet secretion stimulated by TXA(2) by reducing basal TP receptor phosphorylation and facilitating TP receptor-G protein coupling in TXA(2)- platelets and, probably, in normal platelets as well.

Novel aromatic residues in transmembrane domains IV and V involved in agonist binding at alpha(1a)-adrenergic receptors

We examined the role that aromatic residues located in the transmembrane helices of the alpha(1a)-adrenergic receptor play in promoting antagonist binding. Since alpha(1)-antagonists display low affinity binding at beta(2)-adrenergic receptors, two phenylalanine residues, Phe-163 and Phe-187, of the alpha(1a)-AR were mutated to the corresponding beta(2)-residue. Neither F163Q nor F187A mutations of the alpha(1a) had any effect on the affinity of the alpha(1)-antagonists. However, the affinity of the endogenous agonist epinephrine was reduced 12.5- and 8-fold by the F163Q and F187A mutations, respectively. An additive loss in affinity (150-fold) for epinephrine was observed at an alpha(1a) containing both mutations. The loss of agonist affinity scenario could be reversed by a gain of affinity with mutation of the corresponding residues in the beta(2) to the phenylalanine residues in the alpha(1a). We propose that both Phe-163 and Phe-187 are involved in independent aromatic interactions with the catechol ring of agonists. The potency but not the efficacy of epinephrine in stimulating phosphatidylinositol hydrolysis was reduced 35-fold at the F163Q/F187A alpha(1a) relative to the wild type receptor. Therefore, Phe-163 and Phe-187 represent novel binding contacts in the agonist binding pocket of the alpha(1a)-AR, but are not involved directly in receptor activation.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Defect of receptor-G protein coupling in human gallbladder with cholesterol stones

Human gallbladders with cholesterol stones (ChS) exhibit an impaired muscle contraction and relaxation and a lower CCK receptor-binding capacity compared with those with pigment stones (PS). This study was designed to determine whether there is an abnormal receptor-G protein coupling in human gallbladders with ChS using (35)S-labeled guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding, (125)I-labeled CCK-8 autoradiography, immunoblotting, and G protein quantitation. CCK and vasoactive intestinal peptide caused significant increases in [(35)S]GTPgammaS binding to Galpha(i-3) and G(s)alpha, respectively. The binding was lower in ChS than in PS (P < 0.01). The reduced [(35)S]GTPgammaS binding in ChS was normalized after the muscles were treated with cholesterol-free liposomes (P < 0.01). Autoradiography and immunoblots showed a decreased optical density (OD) for CCK receptors, an even lower OD value for receptor-G protein coupling, and a higher OD for uncoupled receptors or Galpha(i-3) protein in ChS compared with PS (P < 0.001). G protein quantitation also showed that there were no significant differences in the Galpha(i-3) and G(s)alpha content in ChS and PS. We conclude that, in addition to an impaired CCK receptor-binding capacity, there is a defect in receptor-G protein coupling in muscle cells from gallbladder with ChS. These changes may be normalized after removal of excess cholesterol from the plasma membrane.

Delineation of the structural basis for the activation properties of the dopamine D1 receptor subtypes

To delineate the structural determinants involved in the constitutive activation of the D1 receptor subtypes, we have constructed chimeras between the D1A and D1B receptors. These chimeras harbored a cognate domain corresponding to transmembrane regions 6 and 7 as well as the third extracellular loop (EL3) and cytoplasmic tail, a domain referred herein to as the terminal receptor locus (TRL). A chimeric D1A receptor harboring the D1B-TRL (chimera 1) displays an increased affinity for dopamine that is indistinguishable from the wild-type D1B receptor. Likewise, a chimeric D1B receptor containing the D1A-TRL cassette (chimera 2) binds dopamine with a reduced affinity that is highly reminiscent of the dopamine affinity for the wild-type D1A receptor. Furthermore, we show that the agonist independent activity of chimera 1 is identical to the wild-type D1B receptor whereas the chimera 2 displays a low agonist independent activity that is indistinguishable from the wild-type D1A receptor. Dopamine potencies for the wild-type D1A and D1B receptor were recapitulated in cells expressing the chimera 2 or chimera 1, respectively. However, the differences observed in agonist-mediated maximal activation of adenylyl cyclase elicited by the D1A and D1B receptors remain unchanged in cells expressing the chimeric receptors. To gain further mechanistic insights into the structural determinants of the TRL involved in the activation properties of the D1 receptor subtypes, we have engineered two additional chimeric D1 receptors that contain the EL3 region of their respective cognate wild-type counterparts (hD1A-EL3B and hD1B-EL3A). In marked contrast to chimera 1 and 2, dopamine affinity and constitutive activation were partially modulated by the exchange of the EL3. Meanwhile, hD1A-EL3B and hD1B-EL3A mutant receptors display a full switch in the agonist-mediated maximal activation, which is reminiscent of their cognate wild-type counterparts. Overall, our studies suggest a fundamental role for the TRL in shaping the intramolecular interactions implicated in the constitutive activation and coupling properties of the dopamine D1 receptor subtypes.

Importance of the extracellular domain for prostaglandin EP(2) receptor function

The ligand binding pocket of biogenic amine G protein-coupled receptors is embedded in the membrane-spanning regions of these receptors, whereas the extracellular domains of the peptidergic receptors play a key role in the structure and function of this class of receptors. To examine the role of the extracellular sequences in prostaglandin receptor-ligand interaction, chimeras were constructed with the two G(s)-coupled E-prostanoid (EP) receptors, replacing each of the extracellular sequences of the human EP(2) receptor with the corresponding human EP(4) receptor residues. Replacement of the third extracellular loop (ECIII) yielded a receptor that binds [(3)H]prostaglandin E(2) (PGE(2); K(d) = 6.3 nM) with similar affinity as the EP(2) wild-type receptor (K(d) = 12.9 nM). Similarly, replacement of the nonconserved carboxyl-terminal portion of ECII resulted in a receptor that maintains [(3)H]PGE(2) binding (K(d) = 8.8 nM). In contrast, replacement of the amino terminus, ECI, the entire ECII region, or the residues within the highly conserved motif of the amino-terminal half of ECII yielded chimeras that displayed neither detectable [(3)H]PGE(2) binding nor receptor-evoked cAMP generation. Immunoprecipitation demonstrated that each chimera is expressed at levels near that of wild-type receptors; however, enzyme-linked immunosorbent assay revealed that inactive chimeras have reduced cell surface expression. Similarly, chimeras that exchange the multiple extracellular loop sequences N/ECI, ECII/ECIII, or all four sequences lacked detectable binding and signal transduction, and although expressed, were not detected on the cell surface. These data suggest that the extracellular sequences of the EP(2) receptor are critical determinants of receptor structure and/or function, unlike other G protein-coupled receptors that bind small molecules.

Activation mechanism of human oxytocin receptor: a combined study of experimental and computer-simulated mutagenesis

The aim of this study was to investigate the molecular changes associated with the transition of the human oxytocin receptor from its inactive to its active states. Mutation of the conserved arginine of the glutamate/aspartate-arginine-tyrosine motif located in the second intracellular domain gave rise to the first known constitutively active oxytocin receptor (R137A), whereas mutation of the aspartic acid located in the second transmembrane domain led to an inactive receptor (D85A). The structural features of the constitutively active and inactive receptor mutants were compared with those of the wild type in its free and agonist-bound states. The results suggest that, although differently triggered, the activation process induced by the agonist and the activating mutation are characterized by the opening of a solvent exposed site formed by the 2nd intracellular loop, the cytosolic extension of helix 5, and the 3rd intracellular loop; on the contrary, the D85A mutation prevents oxytocin from triggering the opening of a cytosolic site. On the basis of these findings, we hypothesize that this cytosolic crevice plays an important role in G protein recognition. Finally, comparative analysis of the free- and agonist-bound forms of the wild-type oxytocin receptor and alpha1B adrenergic receptor suggests that the highly conserved polar amino acids and the seven helices play similar mechanistic roles in the different G protein-coupled receptors.

Chimeric receptor analysis of the ketanserin binding site in the human 5-Hydroxytryptamine1D receptor: importance of the second extracellular loop and fifth transmembrane domain in antagonist binding

The 5-hydroxytryptamine (5-HT)1B/1D receptor subtypes are involved in the regulation of 5-HT release and have gained particular interest because of their apparent role in migraine. Although selective antagonists for both receptor subtypes recently have been developed, the receptor domains involved in the pharmacological specificity of these antagonists are defined poorly. This was investigated with a chimeric 5-HT1B/1D receptor analysis and using ketanserin as a selective antagonist of h5-HT1D (h5-HT1D) Ki = 24-27 nM) as opposed to h5-HT1B (Ki = 2193-2902 nM) receptors. A domain of the h5-HT1D receptor encompassing the second extracellular loop and the fifth transmembrane domain is necessary and sufficient to promote higher affinity binding (Ki = 65-115 nM) for ketanserin to the h5-HT1B receptor. The same domain of the h5-HT1B receptor, when exchanged in the h5-HT1D receptor, abolished high affinity binding of ketanserin (Ki = 364-1265 nM). A similar observation was made with the antagonist ritanserin and seems specific because besides the unmodified binding affinities for 5-HT and zolmitriptan, only minor modifications (2-4-fold) were observed for the agonists L 694247 and sumatriptan and the antagonists GR 127935 and SB 224289. Generating point mutations of divergent amino acids compared with the h5-HT1B receptor did not demonstrate a smaller peptide region related to a significant modification of ketanserin binding. The antagonists ketanserin and ritanserin are likely to bind the h5-HT1D receptor by its second extracellular loop, near the exofacial surface of the fifth transmembrane domain, or both.