Acetylcholine mustard labels the binding site aspartate in muscarinic acetylcholine receptors

Novel Irreversible Agonists Acting at the A1 Adenosine Receptor

The A₁ adenosine receptor (A₁AR) is an important G protein-coupled receptor that regulates a range of physiological functions. Herein we report the discovery of novel irreversible agonists acting at the A₁AR, which have the potential to serve as useful research tools for studying receptor structure and function. A series of novel adenosine derivatives bearing electrophilic substituents was synthesized, and four compounds, 8b, 15a, 15b, and 15d, were shown to possess similar potency and efficacy to the reference high efficacy agonist, NECA, in an assay of ERK1/2 phosphorylation assay. Insensitivity to antagonist addition in a real-time, label-free, xCELLigence assay was subsequently used to identify compounds that likely mediated their agonism through an irreversible interaction with the A₁AR. Of these compounds, 15b and 15d were more directly validated as irreversible agonists of the A₁AR using membrane-based [³H]DPCPX and [³⁵S]GTPγS binding experiments.

Covalent molecular probes for class A G protein-coupled receptors: advances and applications

Covalent modification of G protein-coupled receptors (GPCRs) by employing specific molecular probes has for decades provided a successful strategy to facilitate the elucidation of the structure and function of this pharmacologically important class of membrane proteins. The ligands typically comprise a pharmacophore that generates affinity for a given GPCR and contain a reactive functionality that may form a covalent bond with a suitably positioned amino acid residue. Covalent ligands have been successfully applied to circumvent poor affinity of compounds when stable labeling of receptor populations was required, and they have been used in the isolation, purification, and pharmacological characterization of specific subtypes of GPCRs. Recently, structural studies have demonstrated that covalent molecular probes are effective at stabilizing GPCRs to obtain X-ray crystal structures, thus providing valuable insights for the development of novel therapeutics. Herein, we review covalently binding molecular probes for class A GPCRs with a focus on ligands comprising cross-linking groups that do not require photoactivation and further highlight their significant and diverse applications.

Molecular mechanisms of bitopic ligand engagement with the M1 muscarinic acetylcholine receptor

TBPB and 77-LH-28-1 are selective agonists of the M1 muscarinic acetylcholine receptor (mAChR) that may gain their selectivity through a bitopic mechanism, interacting concomitantly with the orthosteric site and part of an allosteric site. The current study combined site-directed mutagenesis, analytical pharmacology,and molecular modeling to gain further insights into the structural basis underlying binding and signaling by these agonists. Mutations within the orthosteric binding site caused similar reductions in affinity and signaling efficacy for both selective and prototypical orthosteric ligands. In contrast, the mutation of residues within transmembrane helix (TM) 2 and the second extracellular loop (ECL2) discriminated between the different classes of ligand. In particular, ECL2 appears to be involved in the selective binding of bitopic ligands and in coordinating biased agonism between intracellular calcium mobilization and ERK1/2 phosphorylation. Molecular modeling of the interaction between TBPB and the M1 mAChR revealed a binding pose predicted to extend from the orthosteric site up toward a putative allosteric site bordered by TM2, TM3, and TM7, thus consistent with a bitopic mode of binding. Overall, these findings provide valuable structural and mechanistic insights into bitopic ligand actions and receptor activation and support a role for ECL2 in dictating the active states that can be adopted by a G protein-coupled receptor. This may enable greater selective ligand design and development for mAChRs and facilitate improved identification of bitopic ligands.

Synthesis and pharmacological evaluation of analogues of benzyl quinolone carboxylic acid (BQCA) designed to bind irreversibly to an allosteric site of the M ₁ muscarinic acetylcholine receptor

Activation of the M1 muscarinic acetylcholine receptor (mAChR) is a prospective treatment for alleviating cognitive decline experienced in central nervous system (CNS) disorders. Current therapeutics indiscriminately enhance the activity of the endogenous neurotransmitter ACh, leading to side effects. BQCA is a positive allosteric modulator and allosteric agonist at the M1 mAChR that has high subtype selectivity and is a promising template from which to generate higher affinity, more pharmacokinetically viable drug candidates. However, to efficiently guide rational drug design, the binding site of BQCA needs to be conclusively elucidated. We report the synthesis and pharmacological validation of BQCA analogues designed to bind irreversibly to the M1 mAChR. One analogue in particular, 11, can serve as a useful structural probe to confirm the location of the BQCA binding site; ideally, by co-crystallization with the M1 mAChR. Furthermore, this ligand may also be used as a pharmacological tool with a range of applications.

Homology modeling of human muscarinic acetylcholine receptors

We have developed homology models of the acetylcholine muscarinic receptors M₁R-M₅R, based on the β₂-adrenergic receptor crystal as the template. This is the first report of homology modeling of all five subtypes of acetylcholine muscarinic receptors with binding sites optimized for ligand binding. The models were evaluated for their ability to discriminate between muscarinic antagonists and decoy compounds using virtual screening using enrichment factors, area under the ROC curve (AUC), and an early enrichment measure, LogAUC. The models produce rational binding modes of docked ligands as well as good enrichment capacity when tested against property-matched decoy libraries, which demonstrates their unbiased predictive ability. To test the relative effects of homology model template selection and the binding site optimization procedure, we generated and evaluated a naïve M₂R model, using the M₃R crystal structure as a template. Our results confirm previous findings that binding site optimization using ligand(s) active at a particular receptor, i.e. including functional knowledge into the model building process, has a more pronounced effect on model quality than target-template sequence similarity. The optimized M₁R-M₅R homology models are made available as part of the Supporting Information to allow researchers to use these structures, compare them to their own results, and thus advance the development of better modeling approaches.

Activation and allosteric modulation of a muscarinic acetylcholine receptor

Despite recent advances in crystallography of G protein-coupled receptors (GPCRs), little is known about the mechanism of their activation process, as only the β₂ adrenergic receptor (β₂AR) and rhodopsin have been crystallized in fully active conformations. Here, we report the structure of an agonist-bound, active state of the human M₂ muscarinic acetylcholine receptor stabilized by a G-protein mimetic camelid antibody fragment isolated by conformational selection using yeast surface display. In addition to the expected changes in the intracellular surface, the structure reveals larger conformational changes in the extracellular region and orthosteric binding site than observed in the active states of the β₂AR and rhodopsin. We also report the structure of the M₂ receptor simultaneously binding the orthosteric agonist iperoxo and the positive allosteric modulator LY2119620. This structure reveals that LY2119620 recognizes a largely pre-formed binding site in the extracellular vestibule of the iperoxo-bound receptor, inducing a slight contraction of this outer binding pocket. These structures offer important insights into activation mechanism and allosteric modulation of muscarinic receptors.

Effects of asparagine mutagenesis of conserved aspartic acids in helix 2 (D2.50) and 3 (D3.32) of M1-M4 muscarinic receptors on the irreversible binding of nitrogen mustard analogs of acetylcholine and McN-A-343

We investigated how asparagine mutagenesis of conserved aspartic acids in helix 2 (D2.50) and 3 (D3.32) of M1-M4 muscarinic receptors alters the irreversible binding of acetylcholine mustard and BR384 (4-[(2-bromoethyl)methyl-amino]-2-butynyl N-(3-chlorophenyl)carbamate), a nitrogen mustard derivative of McN-A-343 ([4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyl] trimethylammonium chloride). The D2.50N mutation moderately increased the affinity of the aziridinium ions of acetylcholine mustard and BR384 for M2-M4 receptors and had little effect on the rate constant for receptor alkylation. The D3.32N mutation greatly reduced the rate constant for receptor alkylation by acetylcholine mustard but not by BR384, although the affinity of BR384 was reduced. The combination of both mutations (D2.50N/D3.32N) substantially reduced the rate constant for receptor alkylation by BR384 relative to that of wild type and mutant D2.50N and D3.32N receptors. The change in binding affinity caused by the mutations suggests that the D2.50N mutation alters the interaction of acetylcholine mustard with D3.32 of the M1 and M3 receptors but not that of the M4 receptor. BR384 exhibited the converse relationship. The simplest explanation is that acetylcholine mustard and BR384 alkylate at least two residues on M1-M4 receptors and that the D2.50N mutation alters the rate of alkylation of D3.32 relative to another residue, perhaps D2.50 itself.

Development of M1 mAChR allosteric and bitopic ligands: prospective therapeutics for the treatment of cognitive deficits

Since the cholinergic hypothesis of memory dysfunction was first reported, extensive research efforts have focused on elucidating the mechanisms by which this intricate system contributes to the regulation of processes such as learning, memory, and higher executive function. Several cholinergic therapeutic targets for the treatment of cognitive deficits, psychotic symptoms, and the underlying pathophysiology of neurodegenerative disorders, such as Alzheimer's disease and schizophrenia, have since emerged. Clinically approved drugs now exist for some of these targets; however, they all may be considered suboptimal therapeutics in that they produce undesirable off-target activity leading to side effects, fail to address the wide variety of symptoms and underlying pathophysiology that characterize these disorders, and/or afford little to no therapeutic effect in subsets of patient populations. A promising target for which there are presently no approved therapies is the M1 muscarinic acetylcholine receptor (M1 mAChR). Despite avid investigation, development of agents that selectively activate this receptor via the orthosteric site has been hampered by the high sequence homology of the binding site between the five muscarinic receptor subtypes and the wide distribution of this receptor family in both the central nervous system (CNS) and the periphery. Hence, a plethora of ligands targeting less structurally conserved allosteric sites of the M1 mAChR have been investigated. This Review aims to explain the rationale behind allosterically targeting the M1 mAChR, comprehensively summarize and critically evaluate the M1 mAChR allosteric ligand literature to date, highlight the challenges inherent in allosteric ligand investigation that are impeding their clinical advancement, and discuss potential methods for resolving these issues.

On homology modeling of the M₂ muscarinic acetylcholine receptor subtype

Twelve homology models of the human M2 muscarinic receptor using different sets of templates have been designed using the Prime program or the modeller program and compared to crystallographic structure (PDB:3UON). The best models were obtained using single template of the closest published structure, the M3 muscarinic receptor (PDB:4DAJ). Adding more (structurally distant) templates led to worse models. Data document a key role of the template in homology modeling. The models differ substantially. The quality checks built into the programs do not correlate with the RMSDs to the crystallographic structure and cannot be used to select the best model. Re-docking of the antagonists present in crystallographic structure and relative binding energy estimation by calculating MM/GBSA in Prime and the binding energy function in YASARA suggested it could be possible to evaluate the quality of the orthosteric binding site based on the prediction of relative binding energies. Although estimation of relative binding energies distinguishes between relatively good and bad models it does not indicate the best one. On the other hand, visual inspection of the models for known features and knowledge-based analysis of the intramolecular interactions allows an experimenter to select overall best models manually.

Structure-function studies of muscarinic acetylcholine receptors

There has been great interest in the structure-function relationships of the muscarinic acetylcholine receptors (mAChRs) because these prototypical Family A/class 1 G protein-coupled receptors (GPCRs) are attractive therapeutic targets for both peripheral and central nervous system disorders. A multitude of drugs that act at the mAChRs have been identified over the years, but many of these show minimal selectivity for any one of the five mAChR subtypes over the others, which has hampered their development into therapeutics due to adverse side effects. The lack of drug specificity is primarily due to high sequence similarity in this family of receptor, especially in the orthosteric binding pocket. Thus, there remains an ongoing need for a molecular understanding of how mAChRs bind their ligands, and how selectivity in binding and activation can be achieved. Unfortunately, there remains a paucity of solved high-resolution structures of GPCRs, including the mAChRs, and thus most of our knowledge of structure-function mechanisms related to this receptor family to date has been obtained indirectly through approaches such as mutagenesis. Nonetheless, such studies have revealed a wealth of information that has led to novel insights and may be used to guide future rational drug design campaigns.

Modeling GPCR active state conformations: the β(2)-adrenergic receptor

The recent publication of several G protein-coupled receptor (GPCR) structures has increased the information available for homology modeling inactive class A GPCRs. Moreover, the opsin crystal structure shows some active features. We have therefore combined information from these two sources to generate an extensively validated model of the active conformation of the β(2)-adrenergic receptor. Experimental information on fully active GPCRs from zinc binding studies, site-directed spin labeling, and other spectroscopic techniques has been used in molecular dynamics simulations. The observed conformational changes reside mainly in transmembrane helix 6 (TM6), with additional small but significant changes in TM5 and TM7. The active model has been validated by manual docking and is in agreement with a large amount of experimental work, including site-directed mutagenesis information. Virtual screening experiments show that the models are selective for β-adrenergic agonists over other GPCR ligands, for (R)- over (S)-β-hydroxy agonists and for β(2)-selective agonists over β(1)-selective agonists. The virtual screens reproduce interactions similar to those generated by manual docking. The C-terminal peptide from a model of the stimulatory G protein, readily docks into the active model in a similar manner to which the C-terminal peptide from transducin, docks into opsin, as shown in a recent opsin crystal structure. This GPCR-G protein model has been used to explain site-directed mutagenesis data on activation. The agreement with experiment suggests a robust model of an active state of the β(2)-adrenergic receptor has been produced. The methodology used here should be transferable to modeling the active state of other GPCRs.

Mutagenesis of nucleophilic residues near the orthosteric binding pocket of M1 and M2 muscarinic receptors: effect on the binding of nitrogen mustard analogs of acetylcholine and McN-A-343

Investigating how a test drug alters the reaction of a site-directed electrophile with a receptor is a powerful method for determining whether the drug acts competitively or allosterically, provided that the binding site of the electrophile is known. In this study, therefore, we mutated nucleophilic residues near and within the orthosteric pockets of M(1) and M(2) muscarinic receptors to identify where acetylcholine mustard and 4-[(2-bromoethyl)methyl-amino]-2-butynyl-N-(3-chlorophenyl)carbamate (BR384) bind covalently. BR384 is the nitrogen mustard analog of [4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyl]trimethylammonium chloride (McN-A-343). Mutation of the highly conserved aspartic acid in M(1) (Asp105) and M(2) (Asp103) receptors to asparagine largely prevented receptor alkylation by acetylcholine mustard, although modest alkylation still occurred at M(2) D103N at high concentrations of the mustard. Receptor alkylation by BR384 was also greatly inhibited in the M(1) D105N mutant, but some alkylation still occurred at high concentrations of the compound. In contrast, BR384 rapidly alkylated the M(2) D103N mutant. Its affinity was reduced to one tenth, however. The alkylation of M(2) D103N by BR384 was competitively inhibited by N-methylscopolamine and allosterically inhibited by gallamine. Mutation of a variety of other nucleophilic residues, some in combination with D103N, had little effect on M(2) receptor alkylation by BR384. Our results suggest that BR384 alkylates at least one residue other than the conserved aspartic acid at the ligand-binding site of M(1) and M(2) receptors. This additional residue seems to be located within or near the orthosteric-binding pocket and is not part of the allosteric site for gallamine.

Allosteric activators of muscarinic receptors as novel approaches for treatment of CNS disorders

Muscarinic acetylcholine receptors (mAChRs) represent exciting therapeutic targets for the treatment of multiple CNS disorders. The high degree of conservation of amino acids comprising the orthosteric acetylcholine (ACh) binding site between individual mAChR subtypes has hindered the development of subtype-selective compounds that bind to this site. As a result, many academic and industry researchers are now focusing on developing allosteric activators of mAChRs including both positive allosteric modulators (PAMs) and allosteric agonists. In the past 10 years major advances have been achieved in the discovery of allosteric ligands that possess much greater selectivity for individual mAChR subtypes when compared to previously developed orthosteric agents. These novel allosteric modulators of mAChRs may provide therapeutic potential for treatment of a number of CNS disorders such as Alzheimer's disease and schizophrenia.

Investigating the interaction of McN-A-343 with the M muscarinic receptor using its nitrogen mustard derivative and ACh mustard

BACKGROUND AND PURPOSE

We investigated how McN-A-343 inhibited the alkylation of the M(1) muscarinic receptor by its nitrogen mustard derivative and that of ACh to identify whether it interacts allosterically or orthosterically.

EXPERIMENTAL APPROACH

We incubated the M(1) muscarinic receptor expressed in Chinese hamster ovary cells with ACh mustard for various periods of time in the presence of McN-A-343 or known allosteric and orthosteric ligands. After stopping the reaction and removing unreacted ligands, unalkylated receptors were measured using [(3)H]N-methylscopolamine. Analogous experiments were done using a nitrogen mustard analog of McN-A-343. Affinity constants, cooperativity values for allosteric interactions and rate constants for receptor alkylation were estimated using a mathematical model.

KEY RESULTS

The kinetics of receptor alkylation by the nitrogen mustard derivatives of ACh and McN-A-343 were consistent with a two-step model in which the aziridinium ion rapidly forms a reversible receptor complex, which converts to a covalent complex at a slower rate. The inhibition of receptor alkylation by acetycholine, N-methylscopolamine and McN-A-343 was consistent with competitive inhibition, whereas that caused by gallamine was consistent with allosterism. Affinity constants estimated from alkylation kinetics agreed with those measured by displacement of [(3)H]N-methylscopolamine binding.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that McN-A-343 and its nitrogen mustard derivative interact competitively with ACh and N-methylscopolamine at the orthosteric site on the M(1) muscarinic receptor. Measuring how drugs modulate the kinetics of receptor alkylation by an irreversible ligand is a powerful approach for distinguishing between negative allosteric modulators and competitive inhibitors.

A conserved motif in the membrane proximal C-terminal tail of human muscarinic m1 acetylcholine receptors affects plasma membrane expression

We investigated the functional role of a conserved motif, F(x)(6)LL, in the membrane proximal C-tail of the human muscarinic M(1) (hM(1)) receptor. By use of site-directed mutagenesis, several different point mutations were introduced into the C-tail sequence (423)FRDTFRLLL(431). Wild-type and mutant hM(1) receptors were transiently expressed in Chinese hamster ovary cells, and the amount of plasma membrane-expressed receptor was determined by use of intact, whole-cell [(3)H]N-methylscopolamine binding assays. The plasma membrane expression of hM(1) receptors possessing either L430A or L431A or both point mutations was significantly reduced compared with the wild type. The hM(1) receptor possessing a L430A/L431A double-point mutation was retained in the endoplasmic reticulum (ER), and atropine treatment caused the redistribution of the mutant receptor from the ER to the plasma membrane. Atropine treatment also caused an increase in the maximal response and potency of carbachol-stimulated phosphoinositide hydrolysis elicited by the L430A/L431A mutant. The effect of atropine on the L430A/L431A receptor mutant suggests that L(430) and L(431) play a role in folding hM(1) receptors, which is necessary for exit from the ER. Using site-directed mutagenesis, we also identified amino acid residues at the base of transmembrane-spanning domain 1 (TM1), V(46) and L(47), that, when mutated, reduce the plasma membrane expression of hM(1) receptors in an atropine-reversible manner. Overall, these mutagenesis data show that amino acid residues in the membrane-proximal C-tail and base of TM1 are necessary for hM(1) receptors to achieve a transport-competent state.

Use of acetylcholine mustard to study allosteric interactions at the M(2) muscarinic receptor

We explored the interaction of a nitrogen mustard derivative of acetylcholine with the human M(2) muscarinic receptor expressed in Chinese hamster ovary cells using the muscarinic radioligand, [3H]N-methylscopolamine (NMS). Acetylcholine mustard caused a concentration-dependent, first-order loss of [3H]NMS binding at 37 degrees C, with the half-maximal rate constant occurring at 24 microM and a maximal rate constant of 0.16 min(-1). We examined the effects of various ligands on the rate of alkylation of M(2) receptors by acetylcholine mustard. N-methylscopolamine and 4-(trimethylamino)-2-butynyl-(3-chlorophenyl)carbamate (McN-A-343) competitively slowed the rate of alkylation, whereas the inhibition by gallamine reached a plateau at high concentrations, indicating allosteric inhibition. In contrast, 17-beta-hydroxy-17-alpha-ethynyl-5-alpha-androstano[3,2-beta]-pyrimido[1,2-alpha]benzimidazole (WIN 51708) had no effect. We also measured the inhibition of [3H]NMS binding by acetylcholine mustard at 0 degrees C, conditions under which there is little or no detectable covalent binding. In these experiments, the dissociation constant of the aziridinium ion of acetylcholine mustard was estimated to be 12.3 microM. In contrast, the parent mustard and alcoholic hydrolysis product of acetylcholine mustard were without effect. Our results show that measurement of the effects of ligands on the rate of inactivation of the orthosteric site by a small site-directed electrophile is a powerful method for discriminating competitive inhibition from allosterism.

Characterization of a CNS penetrant, selective M1 muscarinic receptor agonist, 77-LH-28-1

BACKGROUND AND PURPOSE

M1 muscarinic ACh receptors (mAChRs) represent an attractive drug target for the treatment of cognitive deficits associated with diseases such as Alzheimer's disease and schizophrenia. However, the discovery of subtype-selective mAChR agonists has been hampered by the high degree of conservation of the orthosteric ACh-binding site among mAChR subtypes. The advent of functional screening assays has enabled the identification of agonists such as AC-42 (4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine), which bind to an allosteric site and selectively activate the M(1) mAChR subtype. However, studies with this compound have been limited to recombinantly expressed mAChRs.

EXPERIMENTAL APPROACH

In this study, we have compared the pharmacological profile of AC-42 and a close structural analogue, 77-LH-28-1 (1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone) at human recombinant, and rat native, mAChRs by calcium mobilization, inositol phosphate accumulation and both in vitro and in vivo electrophysiology.

KEY RESULTS

Calcium mobilization and inositol phosphate accumulation assays revealed that both AC-42 and 77-LH-28-1 display high selectivity to activate the M1 mAChR over other mAChR subtypes. Furthermore, 77-LH-28-1, but not AC-42, acted as an agonist at rat hippocampal M1 receptors, as demonstrated by its ability to increase cell firing and initiate gamma frequency network oscillations. Finally, 77-LH-28-1 stimulated cell firing in the rat hippocampus in vivo following subcutaneous administration.

CONCLUSIONS AND IMPLICATIONS

These data suggest that 77-LH-28-1 is a potent, selective, bioavailable and brain-penetrant agonist at the M1 mAChR and therefore that it represents a better tool than AC-42, with which to study the pharmacology of the M1 mAChR.

Functional analysis of transmembrane domain 2 of the M1 muscarinic acetylcholine receptor

Ala substitution scanning mutagenesis has been used to probe the functional role of amino acids in transmembrane (TM) domain 2 of the M1 muscarinic acetylcholine receptor, and of the highly conserved Asn43 in TM1. The mutation of Asn43, Asn61, and Leu64 caused an enhanced ACh affinity phenotype. Interpreted using a rhodopsin-based homology model, these results suggest the presence of a network of specific contacts between this group of residues and Pro415 and Tyr418 in the highly conserved NPXXY motif in TM7 that exhibit a similar mutagenic phenotype. These contacts may be rearranged or broken when ACh binds. D71A, like N414A, was devoid of signaling activity. We suggest that formation of a direct hydrogen bond between the highly conserved side chains of Asp71 and Asn414 may be a critical feature stabilizing the activated state of the M1 receptor. Mutation of Leu67, Ala70, and Ile74 also reduced the signaling efficacy of the ACh-receptor complex. The side chains of these residues are modeled as an extended surface that may help to orient and insulate the proposed hydrogen bond between Asp71 and Asn414. Mutation of Leu72, Gly75, and Met79 in the outer half of TM2 primarily reduced the expression of functional receptor binding sites. These residues may mediate contacts with TM1 and TM7 that are preserved throughout the receptor activation cycle. Thermal inactivation measurements confirmed that a reduction in structural stability followed the mutation of Met79 as well as Asp71.

The predicted 3D structures of the human M1 muscarinic acetylcholine receptor with agonist or antagonist bound

The muscarinic acetylcholine G-protein-coupled receptors are implicated in diseases ranging from cognitive dysfunctions to smooth-muscle disorders. To provide a structural basis for drug design, we used the MembStruk computational method to predict the 3D structure of the human M1 muscarinic receptor. We validated this structure by using the HierDock method to predict the binding sites for three agonists and four antagonists. The intermolecular ligand-receptor contacts at the predicted binding sites agree well with deductions from available mutagenesis experiments, and the calculated relative binding energies correlate with measured binding affinities. The predicted binding site of all four antagonists is located between transmembrane (TM) helices 3, 4, 5, 6, and 7, whereas the three agonists prefer a site involving residues from TM3, TM6, and TM7. We find that Trp 157(4) contributes directly to antagonist binding, whereas Pro 159(4) provides an indirect conformational switch to position Trp 157(4) in the binding site (the number in parentheses indicates the TM helix). This explains the large decrease in ligand binding affinity and signaling efficacy by mutations of Trp 157(4) and Pro 159(4) not previously explained by homology models. We also found that Asp 105(3) and aromatic residues Tyr 381(6), Tyr 404(7), and Tyr 408(7) are critical for binding the quaternary ammonium head group of the ligand through cation-pi interactions. For ligands with a charged tertiary amine head group, we suggest that proton transfer from the ligand to Asp 105(3) occurs upon binding. Furthermore, we found that an extensive aromatic network involving Tyr 106(3), Trp 157(4), Phe 197(5), Trp 378(6), and Tyr 381(6) is important in stabilizing antagonist binding. For antagonists with two terminal phenyl rings, this aromatic network extends to Trp 164(4), Tyr 179(extracellular loop 2), and Phe 390(6) located at the extracellular end of the TMs. We find that Asn 382(6) forms hydrogen bonds with selected antagonists. Tyr381(6) and Ser 109(3) form hydrogen bonds with the ester moiety of acetylcholine, which binds in the gauche conformation.

Structural Requirements of Transmembrane Domain 3 for Activation by the M1Muscarinic Receptor Agonists AC-42, AC-260584, Clozapine, andN-Desmethylclozapine: Evidence for Three Distinct Modes of Receptor Activation

Transmembrane domain 3 (TM3) plays a crucial role mediating muscarinic acetylcholine receptor activation by acetylcholine, carbachol, and other muscarinic agonists. We compared the effects of point mutations throughout TM3 on the interactions of carbachol, 4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine hydrogen chloride (AC-42), a potent structural analog of AC-42 called 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC-260584), N-desmethylclozapine, and clozapine with the M(1) muscarinic receptor. The binding and activation profiles of these ligands fell into three distinct patterns; one exemplified by orthosteric compounds like carbachol, another by structural analogs of AC-42, and a third by structural analogs of N-desmethylclozapine. All mutations tested severely reduced carbachol binding and activation of M(1). In contrast, the agonist actions of AC-42 and AC-260584 were greatly potentiated by the W101A mutation, slightly reduced by Y106A, and slightly increased by S109A. Clozapine and N-desmethylclozapine displayed substantially increased maximum responses at the Y106A and W101A mutants, slightly lower activity at S109A, but no substantial changes in potency. At L102A and N110A, agonist responses to AC-42, AC-260584, clozapine, and N-desmethylclozapine were all substantially reduced, but usually less than carbachol. D105A showed no functional responses to all ligands. Displacement and dissociation rate experiments demonstrated clear allosteric properties of AC-42 and AC-260584 but not for N-desmethylclozapine and clozapine, indicating that they may contact different residues than carbachol to activate M(1) but occupy substantially overlapping spaces, in contrast to AC-42 and AC-260584, which occupy separable spaces. These results show that M(1) receptors can be activated in at least three distinct ways and that there is no requirement for potent muscarinic agonists to mimic acetylcholine interactions with TM3.

Constitutive activity of muscarinic acetylcholine receptors

We review the literature describing constitutive activity of the five muscarinic acetylcholine receptors in native and recombinant systems and discuss the effect of constitutive activity on muscarinic pharmacology in the context of modern models of receptor activation. We include a summary of mutations found to cause constitutive activity and discuss the implications of these data for the structure, function, and activation mechanism of muscarinic receptors. Finally, we discuss the possible physiological significance of constitutive activity of muscarinic receptors, incorporating information provided by targeted deletion of each of the muscarinic subtypes.

Recognition of benztropine by the dopamine transporter (DAT) differs from that of the classical dopamine uptake inhibitors cocaine, methylphenidate, and mazindol as a function of a DAT transmembrane 1 aspartic acid residue

Binding of cocaine to the dopamine transporter (DAT) protein blocks synaptic dopamine clearance, triggering the psychoactive effects associated with the drug; the discrete drug-protein interactions, however, remain poorly understood. A longstanding postulate holds that cocaine inhibits DAT-mediated dopamine transport via competition with dopamine for formation of an ionic bond with the DAT transmembrane aspartic acid residue D79. In the present study, DAT mutations of this residue were generated and assayed for translocation of radiolabeled dopamine and binding of radiolabeled DAT inhibitors under identical conditions. When feasible, dopamine uptake inhibition potency and apparent binding affinity K(i) values were determined for structurally diverse DAT inhibitors. The glutamic acid substitution mutant (D79E) displayed values indistinguishable from wild-type DAT in both assays for the charge-neutral cocaine analog 8-oxa-norcocaine, a finding not supportive of the D79 "salt bridge" ligand-docking model. In addressing whether the D79 side chain contributes to the DAT binding sites of other portions of the cocaine pharmacophore, only inhibitors with modifications of the tropane ring C-3 substituent, i.e., benztropine and its analogs, displayed a substantially altered dopamine uptake inhibition potency as a function of the D79E mutation. A single conservative amino acid substitution thus differentiated structural requirements for benztropine function relative to those for all other classical DAT inhibitors. Distinguishing the precise mechanism of action of this DAT inhibitor with relatively low abuse liability from that of cocaine may be attainable using DAT mutagenesis and other structure-function studies, opening the door to rational design of therapeutic agents for cocaine abuse.

Discovery of an ectopic activation site on the M(1) muscarinic receptor

Receptors have well-conserved regions that are recognized and activated by hormones and neurotransmitters. Most drugs bind to these sites and mimic or block the action of the native ligands. Using a high-throughput functional screen, we identified a potent and selective M(1) muscarinic receptor agonist from a novel structural class. Using a series of chimeric receptors, we demonstrated that this ligand activates the receptor through a region that is not conserved among receptor subtypes, explaining its unprecedented selectivity. This region of the receptor is distinct from the conserved region that is recognized by traditional ligands. The finding that receptors for small-molecule transmitters can have multiple, structurally distinct activation sites has broad implications for the study of receptor structure/function and the potential for the discovery of novel ligands with high selectivity.

The role of transmembrane helix 5 in agonist binding to the human H3 receptor

We have used alanine scanning mutagenesis to identify residues in transmembrane domain 5 of the histamine H3 receptor that are important for agonist binding. All of the mutants generated were functionally expressed as demonstrated by their ability to bind [(125)I]iodoproxyfan with comparable affinity to the wild-type receptor and their ability to inhibit forskolin-stimulated cAMP formation when activated by histamine. Many mutations produced small changes in the potency of histamine, but the most pronounced reduction in potency and affinity of the agonists, histamine, R-alpha-methylhistamine, imetit, and impentamine, was seen with mutation of glutamate 206. Our modeling suggests that this residue plays a key role in ligand binding by interacting with the imidazole ring of histamine. Interestingly, L199A greatly reduced agonist potency in functional assays but had only minor effects on agonist affinity, implicating a role for this residue in the mechanism of receptor activation. We also studied the functional effects of the mutations by linking the receptor to calcium signaling using a chimeric G protein. A comparison of the two functional assays demonstrated contrasting effects on agonist activity. Histamine, imetit, and impentamine were full agonists in the cAMP assay, but imetit exhibited only partial agonist activity through the chimeric G protein. Furthermore, impentamine, another potent agonist in the cAMP assay, was only able to activate the E206A mutant in the calcium assay despite being inactive at the wild-type receptor. These observations suggest that the agonist receptor complexes formed by these three different H3 agonists are not conformationally equivalent.

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.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Evidence for a tandem two-site model of ligand binding to muscarinic acetylcholine receptors

After short preincubations with N-[(3)H]methylscopolamine ([(3)H]NMS) or R(-)-[(3)H]quinuclidinyl benzilate ([(3)H]QNB), radioligand dissociation from muscarinic M(1) receptors in Chinese hamster ovary cell membranes was fast, monoexponential, and independent of the concentration of unlabeled NMS or QNB added to reveal dissociation. After long preincubations, the dissociation was slow, not monoexponential, and inversely related to the concentration of the unlabeled ligand. Apparently, the unlabeled ligand becomes able to associate with the receptor simultaneously with the already bound radioligand if the preincubation lasts for a long period, and to hinder radioligand dissociation. When the membranes were preincubated with [(3)H]NMS and then exposed to benzilylcholine mustard (covalently binding specific ligand), [(3)H]NMS dissociation was blocked in wild-type receptors, but not in mutated (D99N) M(1) receptors. Covalently binding [(3)H]propylbenzilylcholine mustard detected substantially more binding sites than [(3)H]NMS. The observations support a model in which the receptor binding domain has two tandemly arranged subsites for classical ligands, a peripheral one and a central one. Ligands bind to the peripheral subsite first (binding with lower affinity) and translocate to the central subsite (binding with higher affinity). The peripheral subsite of M(1) receptors may include Asp-99. Experimental data on [(3)H]NMS and [(3)H]QNB association and dissociation perfectly agree with the predictions of the tandem two-site model.

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.

Functional characterization and potential applications for enhanced green fluorescent protein- and epitope-fused human M1 muscarinic receptors

Four recombinant human M1 (hM1) muscarinic acetylcholine receptors (mAChRs) combining several modifications were designed and overexpressed in HEK293 cells. Three different fluorescent chimera were obtained through fusion of the receptor N terminus with enhanced green fluorescent protein (EGFP), potential glycosylation sites and a large part of the third intracellular (i3) loop were deleted, a hexahistidine tag sequence was introduced at the receptor C terminus, and, finally, a FLAG epitope was either fused at the receptor N terminus or inserted into its shortened i3 loop. High expression levels and ligand binding properties similar to those of the wild-type hM1 receptor together with confocal microscopy imaging demonstrated that the recombinant proteins were correctly folded and targeted to the plasma membrane, provided that a signal peptide was added to the N-terminal domain of the fusion proteins. Their functional properties were examined through McN-A-343-evoked Ca2+ release. Despite the numerous modifications introduced within the hM1 sequence, all receptors retained nearly normal abilities (EC50 values) to mediate the Ca2+ response, although reduced amplitudes (Emax values) were obtained for the i3-shortened constructs. Owing to the bright intrinsic fluorescence of the EGFP-fused receptors, their detection, quantitation, and visualization as well as the selection of cells with highest expression were straightforward. Moreover, the presence of the different epitopes was confirmed by immunocytochemistry. Altogether, this work demonstrates that these EGFP- and epitope-fused hM1 receptors are valuable tools for further functional, biochemical, and structural studies of muscarinic receptors.

ASP147 in the third transmembrane helix of the rat mu opioid receptor forms ion-pairing with morphine and naltrexone

We tested the hypotheses that the carboxylate side chain of Asp147 of the mu opioid receptor interacts with the protonated nitrogen of naltrexone and morphine and that this interaction is important for pharmacological properties of the two compounds. Mutation of Asp147 to Ala or Asn substantially reduced the affinity of naltrexone and the affinity, potency and efficacy of morphine, while the Glu mutant had similar properties as the wildtype, indicating the significant role of the carboxylate group of Asp147 in receptor binding and activation. This role could be due to its direct interaction with ligands or involvement in interhelical interactions. The unprotonated analogs of naltrexone and morphine, cyclopropylcarbonyl noroxymorphone (CPCNOM) and N-formylnormorphine (NFNM), respectively, were used to discriminate between these mechanisms. CPCNOM was much less potent as an antagonist and had substantially lower affinity for the mu receptor than naltrexone. Similarly, NFNM was unable to activate the mu receptor and had much lower affinity than morphine. These results indicate the importance of the protonated nitrogen. Notably, the D147A and D147N mutations did not appreciably affect the binding affinities of CPCNOM and NFNM. In addition, the D147E mutant had similar affinities for CPCNOM and NFNM as the D147A and D147N mu receptors. Thus, the carboxylate group of Asp147 is not important for binding of the two unprotonated compounds. These results indicate that the carboxylate group of Asp147 of the mu receptor interacts directly with the protonated nitrogen of naltrexone and morphine and this interaction is important for binding and receptor activation.

The conformational switch in 7-transmembrane receptors: the muscarinic receptor paradigm

The rhodopsin-like superfamily of 7-transmembrane receptors is the largest class of signalling molecules in the mammalian genome. Recently, a combination of mutagenesis, biophysical and modelling studies have suggested a credible model for the alpha-carbon backbone in the transmembrane region of the 7-transmembrane receptors, and have started to reveal the structural basis of the conformational switch from the inactive to the active state. A key feature may be the replacement of a network of radial constraints, centred on transmembrane helix three, which stabilise the inactive ground state of the receptor by a new set of axial interactions which help to stabilise the activated state. Transmembrane helix three may act as a rotary switch in the activation mechanism.

The role of charge interactions in muscarinic agonist binding, and receptor-response coupling

Site-directed mutagenesis has been used to evaluate the roles of the key aspartate and arginine residues in transmembrane domain three of the muscarinic receptors. The results suggest that the formation of an ionic bond between the Asp carboxylate group and the onium headgroup is essential to anchor acetylcholine in its active, bound conformation in both binary agonist-receptor and ternary agonist-receptor-G-protein complexes, but that secondary, non-productive binding modes, promoted by non-polar forces, may contribute to binary complex formation by other ligands. The positive charge of the arginyl side-chain is central to the recognition, and subsequent activation of G-proteins by the agonist-M1 mAChR complex.

Muscarinic acetylcholine receptors: structural basis of ligand binding and G protein coupling

Muscarinic acetylcholine receptors (m1-m5) were studied by a combined molecular genetic/pharmacologic approach to elucidate the molecular characteristics of the ligand binding site and of the receptor domains involved in G protein coupling. Site-directed mutagenesis studies of the rat m3 muscarinic receptor suggest that the acetylcholine binding domain is formed by a series of hydrophilic amino acids located in the "upper" half of transmembrane domains (TM) III, V, VI, and VII. Moreover, we showed that mutational modification of a TM VI Asn residue (Asn507 in the rat m3 receptor sequence) which is characteristic for the muscarinic receptor family has little effect on high-affinity acetylcholine binding and receptor activation, but results in dramatic reductions in binding affinities for certain subclasses of muscarinic antagonists. The N-terminal portion of the third intracellular loop (i3) of muscarinic and other G protein-coupled receptors has been shown to play a central role in determining the G protein coupling profile of a given receptor subtype. Insertion mutagenesis studies with the rat m3 muscarinic receptor suggest that this region forms an amphiphilic alpha-helix and that the hydrophobic side of this helix represents an important G protein recognition surface. Further mutational analysis of this receptor segment showed that Tyr254 located at the N-terminus of the i3 loop of the m3 muscarinic receptor plays a key role in muscarinic receptor-induced Gq activation. The studies described here, complemented by biochemical and biophysical approaches, should eventually lead to a detailed structural model of the ligand-receptor-G protein complex.

The functional topography of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor, revealed by scanning mutagenesis

Alanine-scanning mutagenesis has been applied to residues 100-121 in transmembrane domain 3 of the M1 muscarinic acetylcholine receptor. This study complements a previous investigation of the triad Asp122-Arg123-Tyr124 (Lu, Z-L., Curtis, C. A., Jones, P. G., Pavia, J., and Hulme, E. C. (1997) Mol. Pharmacol. 51, 234-241). The results demonstrate the alpha-helical secondary structure of the domain and suggest its orientation with respect to the other transmembrane domains. The C-terminal part of the helix appears to be largely buried within the receptor structure. On its surface, there is a patch of three residues, Val113, Leu116, and Ser120, which may form intramolecular contacts that help to stabilize the inactive ground state of the receptor. Mutagenic disruption of these increased agonist affinity and signaling efficacy. In two cases (L116A and S120A), this led to constitutive activation of the receptor. Parallel to the helix axis and spanning the whole transmembrane region, a distinct strip of residues on one face of transmembrane domain 3 forms intermolecular (acetylcholine-receptor, receptor-G protein) or intrareceptor bonds that contribute to the activated state. The binding of acetylcholine may destabilize the first set of contacts while favoring the formation of the second.

Double mutant cycle analysis of aspartate 69, 97, and 103 to asparagine mutants in the m2 muscarinic acetylcholine receptor

Double mutant cycles provide a method for analyzing the effects of a mutation at a defined position in the protein structure on the properties of an amino acid at a second site. This approach was used to map potential interactions between aspartates 69, 97, and 103 in the m2 muscarinic acetylcholine receptor transmembrane helices 2 and 3. Receptors containing single and double aspartate to asparagine mutants were expressed in Chinese hamster ovary cells and their effects on ligand binding, signal transduction, and thermal stability determined. Analysis of the double mutant cycles showed that the mutations had approximately additive effects on ligand binding, signal transduction, and thermal stability. Ligand binding and thermal inactivation results support the conclusion that aspartate-103 is the ligand amine counterion. Effector coupling properties of the mutant receptors showed that aspartate-103 was also required for signal transduction activity. The mutation of aspartate-69 to asparagine completely eliminated signal transduction by the agonists acetylcholine, carbachol, and pilocarpine but not oxotremorine M, which caused reduced but significant inhibition of adenylyl cyclase and stimulation of phospholipase C. In contrast, adenylyl cyclase stimulation by the asparagine-69 mutant was elicited only by acetylcholine and carbachol but not by oxotremorine M. The variation in agonist-dependent effector coupling properties provides evidence that the asparagine-69 mutant can exist in activated receptor states that are different from the wild-type m2 muscarinic receptor.

Fluorescent muscarinic EGFP-hM1 chimeric receptors: design, ligand binding and functional properties

We describe the construction, expression and characterization of recombinant proteins comprising the enhanced green fluorescent protein (EGFP) fused to the amino-terminal part of the muscarinic hM1 receptor together or not with an additional hexahistidine tag placed at the C-terminal end of the receptor. Expression of the fluorescent proteins reaches levels identical to those of the wt hM1 receptor, provided that fusion takes place at the very N-terminal end of the receptor. Also correct protein folding and targeting to plasma membrane is obtained upon addition of a signal peptide promoting amino-terminal domain translocation through the membrane. Ligand binding properties of--and activation of the calcium release response by--the fusion proteins are almost identical to those of the wild-type muscarinic receptor, indicating that such fluorescently-labelled receptors are valuable model systems for further functional, biochemical and structural studies.

The second intracellular loop of the m5 muscarinic receptor is the switch which enables G-protein coupling

We have completed a systematic search of the intracellular loops of a muscarinic acetylcholine receptor for domains that govern G-protein coupling. A unique feature of the second intracellular (i2) loop was an ordered cluster of residues where diverse substitutions cause constitutive activation. A second group of residues in i2 was identified where mutations compromised receptor/G-protein coupling. The residues of each group alternate and are spaced three to four positions apart, suggesting an alpha-helical structure where these groups form opposing faces of the helix. We propose that the constitutively activating face normally constrains the receptor in the "off-state," while the other face couples G-proteins in the "on-state." Therefore, the i2 loop functions as the switch enabling G-protein activation.

Identification of a ligand-dependent switch within a muscarinic receptor

G-protein-coupled receptors spontaneously switch between active and inactive conformations. Agonists stabilize the active conformation, whereas antagonists stabilize the inactive conformation. In a systematic search for residues that participate in receptor function, several regions of the m5 muscarinic receptor were randomly mutated and tested for their functional properties. Mutations spanning one face of transmembrane 6 (TM6) were found to induce high levels of receptor activity in the absence of agonists (constitutive activity). The same face of TM6 contained several residues crucial for receptor activation by agonists and one residue identified as a contact site for both agonists and antagonists. In addition, one mutation induced agonist-like responses from the receptor when exposed to classical antagonists. These results suggest that TM6 is a switch that defines the activation state of the receptor, and that ligand interactions with TM6 stabilize the receptor in either an active or an inactive conformation.

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor has revealed a highly-differentiated alpha-helical structure. Lipid-facing residues are distinguished from a patch of residues which selectively stabilise the ground state of the receptor, and from a band of amino acids extending the full length of the helix, which contribute to the active agonist-receptor-G protein complex. The most important residues are strongly conserved in the GPCR superfamily.

Direct identification of a distinct site of interaction between the carboxyl-terminal residue of cholecystokinin and the type A cholecystokinin receptor using photoaffinity labeling

Mechanisms of ligand binding and activation of G protein-coupled receptors are particularly important, due to their ubiquitous expression and potential as drug targets. Molecular interactions between ligands and these receptors are best defined for small molecule ligands that bind within the transmembrane helices. Extracellular domains seem to be more important for peptide ligands, based largely on effects of receptor mutagenesis, where interference with binding or activity can reflect allosteric as well as direct effects. We now take the more direct approach of photoaffinity labeling the active site of the cholecystokinin (CCK) receptor, using a photolabile analogue of CCK having a blocked amino terminus. This probe, 125I-desaminotyrosyl-Gly-[Nle28,31, pNO2-Phe33]CCK-(26-33), binds specifically, saturably, and with high affinity (Ki = 3.3 nM) and has full agonist activity. This makes likely its being sited in a natural position within the receptor. As substrate, we used CHO-CCK receptor cells overexpressing functional recombinant rat type A CCK receptor. Covalent labeling of the appropriate Mr = 85,000-95,000 plasma membrane glycoprotein with core of Mr = 42,000 was established by SDS-polyacrylamide gel electrophoresis and autoradiography. A single domain adjacent to transmembrane 1 was labeled, as established by cyanogen bromide cleavage and separation by gel and/or high pressure liquid chromatography. The site of interaction was further defined by additional proteolysis with trypsin, with purification of the labeled fragment, followed by manual Edman degradation and radiochemical sequencing. This demonstrated that Trp39 was specifically labeled and likely resides proximate to the carboxyl-terminal pNO2-Phe33 residue of the probe. A model of this ligand-bound receptor has been constructed and will be used to plan future experiments to refine our understanding of this interaction.

Constitutive activation of the m5 muscarinic receptor by a series of mutations at the extracellular end of transmembrane 6

The m5 muscarinic acetylcholine receptor was constitutively activated by a wide range of amino acid substitutions at a residue (serine 465) that is positioned at the junction of the sixth transmembrane domain and the extracellular loop. Of 13 substitutions tested, 11 produced significant increases in constitutive activity. Replacement of serine 465 with large (phenylalanine and valine) or basic residues (arginine and lysine) increased the constitutive activity of the receptor to between 55 and 110% of the maximum response of the wild-type receptor to the agonist carbachol. Other substitutions (e.g., cysteine and leucine) increased the constitutive activity to an intermediate level (30%), while small and acidic residues (glycine, aspartate, and glutamate) caused small or insignificant increases. The increase in the constitutive activity of each mutant receptor correlated with an increase in the potency of carbachol in both binding and functional assays, with the most constitutively activated receptors showing a 40-fold decrease in the EC50 of carbachol. The negative antagonist atropine bound to and reversed the constitutive activity of all mutant receptors with equal potency. These data were fitted to a two-state model of receptor function. The data are consistent with the primary effect of substitutions to serine 465 being to selectively destabilize the inactive state of the receptor, thus favoring formation of the active state in the absence of agonists. Our data strongly support this two-state model of receptor function and identify a critical role of this domain in the activation of muscarinic receptors.

Covalent labeling of muscarinic acetylcholine receptors by tritiated aryldiazonium photoprobes

p-dimethylamino (A) and p-dibutylamino (B) benzenediazonium salts, previously characterized as efficient labels of membrane-bound and solubilized muscarinic receptor sites, are endowed with overall interesting photochemical and alkylating properties that allow their use as structural probes of the muscarinic ligand binding domain to be considered. Under reversible binding conditions, these antagonists display no binding selectivity towards the 5 muscarinic acetylcholine receptor (mAChR) subtypes. They were used here, in a tritiated form, as photoaffinity labels of purified muscarinic receptors from porcine striatum, and their irreversible binding was assessed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. When irradiated under energy transfer conditions, [3H]A and [3H]B were both found to covalently label purified muscarinic receptor sites in a light-dependent and atropine-protectable manner. The electrophoretic migration properties of the alkylated sites were similar to those of [3H]propylbenzilylcholine mustard (PrBCM)-labeled mAChRs. Specific radioactive incorporation showed a clear dependency on probe concentration. Labeling efficiency was rather high, with up to 30% and even 60% of the receptor population being photolabeled by [3H]A and [3H]B, respectively. These two photoactivatable ligands have proven to be powerful tools for the structural analysis of other cholinergic targets (acetylcholinesterase and the nicotinic acetylcholine receptor) by allowing the characterization of a number of different residues belonging to their acetylcholine-binding domain. Altogether, these results reinforce the interest of our site-directed labeling approach because [3H]A- and [3H]B-alkylated mAChRs may now be considered as suitable materials to investigate the muscarinic receptor-binding pocket through peptide mapping, sequence analyses, and identification of radiolabeled amino acid residues.