The structural changes involved in ligand-dependent activation of G protein-coupled receptors are not well understood at present. To address this issue, we developed an in situ disulfide cross-linking strategy using the rat M 3 muscarinic receptor, a prototypical G q -coupled receptor, as a model system. It is known that a tyrosine residue (Tyr 254 ) located at the C terminus of transmembrane domain (TM) V and several primarily hydrophobic amino acids present within the cytoplasmic portion of TM VI play key roles in determining the G protein coupling selectivity of the M 3 receptor subtype. To examine whether M3 receptor activation involves changes in the relative orientations of these functionally critical residues, pairs of cysteine residues were substituted into a modified version of the M 3 receptor that contained a factor Xa cleavage site within the third intracellular loop and lacked most endogenous cysteine residues. All analyzed mutant receptors contained a Y254C point mutation and a second cysteine substitution within the segment Lys 484-Ser 493 at the intracellular end of TM VI. Following their transient expression in COS-7 cells, mutant receptors present in their native membrane environment (in situ) were subjected to mild oxidizing conditions, either in the absence or in the presence of the muscarinic agonist, carbachol. The successful formation of disulfide cross-links was monitored by studying changes in the electrophoretic mobility of oxidized, factor Xa-treated receptors on SDS gels. The observed cross-linking patterns indicated that M 3 receptor activation leads to structural changes that allow the cytoplasmic ends of TM V and TM VI to move closer to each other and that also appear to involve a major change in secondary structure at the cytoplasmic end of TM VI. This is the first study employing aninsitudisulfidecross-linkingstrategytoexamineagonistdependent dynamic structural changes in a G proteincoupled receptor. G protein-coupled receptors (GPCRs)1 constitute the largest class of signaling molecules in the mammalian genome (1-3).All GPCRs are predicted to share a conserved molecular architecture consisting of a bundle of seven ␣-helically arranged transmembrane domains (TM I-VII) linked by alternating intracellular and extracellular loops (Fig. 1). Despite the remarkable structural diversity of the ligands that exert their physiological functions via interaction with specific classes of GPCRs, all GPCRs are thought to share a conserved mechanism of activation. Several lines of evidence indicate that the binding of ligands to the extracellular side of the receptor leads to changes in the arrangement of distinct TM helices, which are then propagated to the intracellular surface of the receptor, thus enabling the receptor to recognize and activate specific classes of heterotrimeric G proteins (4 -7).Accumulating evidence suggests that GPCR activation may involve a change in the relative disposition of TM III and VI (4 -11). Elegant site-directed spin labeling studies (9) carried out with t...

The conformational changes that convert G proteincoupled receptors (GPCRs) activated by diffusible ligands from their resting into their active states are not well understood at present. To address this issue, we used the M 3 muscarinic acetylcholine receptor, a prototypical class A GPCR, as a model system, employing a recently developed disulfide cross-linking strategy that allows the formation of disulfide bonds using Cys-substituted mutant M 3 muscarinic receptors present in their native membrane environment. In the present study, we generated and analyzed 30 double Cys mutant M 3 receptors, all of which contained one Cys substitution within the C-terminal portion of transmembrane domain (TM) VII (Val-541 to Ser-546) and another one within the C-terminal segment of TM I (Val-88 to Phe-92). Following their transient expression in COS-7 cells, all mutant receptors were initially characterized in radioligand binding and second messenger assays (carbachol-induced stimulation of phosphatidylinositol hydrolysis). This analysis showed that all 30 double Cys mutant M 3 receptors were able to bind muscarinic ligands with high affinity and retained the ability to stimulate G proteins with high efficacy. In situ disulfide cross-linking experiments revealed that the muscarinic agonist, carbachol, promoted the formation of crosslinks between specific Cys pairs. The observed pattern of disulfide cross-links, together with receptor modeling studies, strongly suggested that M 3 receptor activation induces a major rotational movement of the C-terminal portion of TM VII and increases the proximity of the cytoplasmic ends of TM I and VII. These findings should be of relevance for other family A GPCRs.

The activity of G protein-coupled receptors can be modulated by different classes of ligands, including agonists that promote receptor signaling and inverse agonists that reduce basal receptor activity. The conformational changes in receptor structure induced by different agonist ligands are not well understood at present. In this study, we employed an in situ disulfide crosslinking strategy to monitor ligand-induced conformational changes in a series of cysteine-substituted mutant M 3 muscarinic acetylcholine receptors. The observed disulfide cross-linking patterns indicated that muscarinic agonists trigger a separation of the N-terminal segment of the cytoplasmic tail (helix 8) from the cytoplasmic end of transmembrane domain I. In contrast, inverse muscarinic agonists were found to increase the proximity between these two receptor regions. These findings provide a structural basis for the opposing biological effects of muscarinic agonists and inverse agonists. This study also provides the first piece of direct structural information as to how the conformations induced by these two functionally different classes of ligands differ at the molecular level. Given the high degree of structural homology found among most G protein-coupled receptors, our findings should be of broad general relevance.

Identification of the specific muscarinic acetylcholine receptor (mAChR) subtypes mediating stimulation of salivary secretion is of considerable clinical interest. Recent pharmacological and molecular genetic studies have yielded somewhat confusing and partially contradictory results regarding the involvement of individual mAChRs in this activity. In the present study, we re-examined the roles of M 1 and M 3 mAChRs in muscarinic agonist-mediated stimulation of salivary secretion by using M 1 and M 3 receptor single-knockout (KO) mice and newly generated M 1 /M 3 receptor double-KO mice. When applied at a low dose (1 mg/kg, s.c.), the muscarinic agonist pilocarpine showed significantly reduced secretory activity in both M 1 and M 3 receptor single-KO mice. However, when applied at higher doses, pilocarpine induced only modestly reduced (5 mg/kg, s.c.) or unchanged (15 mg/kg, s.c.) salivation responses, respectively, in M 1 and M 3 receptor single-KO mice, indicating that the presence of either M 1 or M 3 receptors is sufficient to mediate robust salivary output. Quantitative reverse transcriptase-polymerase chain reaction studies with salivary gland tissue showed that the inactivation of the M 1 or M 3 mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Strikingly, the sialagogue activity of pilocarpine was abolished in M 1 /M 3 receptor double-KO mice. However, salivary glands from M 1 /M 3 receptor double-KO mice remained responsive to stimulation by the ␤-adrenergic receptor agonist, (S)-isoproterenol. Taken together these studies support the concept that a mixture of M 1 and M 3 receptors mediates cholinergic stimulation of salivary flow.

In this study, we employed an in situ disulfide cross-linking strategy to gain insight into the structure of the inactive and active state of the M(3) muscarinic acetylcholine receptor. Specifically, this study was designed to identify residues in TM I that are located in close to Cys532 (position 7.42), an endogenous cysteine residue present in the central portion of TM VII. Cysteine residues were substituted, one at a time, into 10 consecutive positions of TM I (Ala71-Val80) of a modified version of the M(3) muscarinic receptor that lacked most endogenous cysteine residues and contained a factor Xa cleavage site within the third intracellular loop. Following their expression in COS-7 cells, the 10 resulting cysteine mutant receptors were oxidized in their native membrane environment, either in the absence or in the presence of muscarinic ligands. Disulfide cross-link formation was monitored by examining changes in the electrophoretic mobility of oxidized and factor Xa-digested receptors on SDS gels. When molecular iodine was used as the oxidizing agent, the L77C receptor (position 1.42) was the only mutant receptor that displayed significant disulfide cross-linking, either in the absence or in the presence of muscarinic agonists or antagonists. On the other hand, when the Cu(II)-(1,10-phenanthroline)(3) complex served as the redox catalyst, muscarinic ligands inhibited disulfide cross-linking of the L77C receptor, probably because of impaired access of this relatively bulky oxidizing agent to the ligand binding crevice. The iodine cross-linking data suggest that M(3) muscarinic receptor activation is not associated with significant changes in the relative orientations of the outer and/or central segments of TM I and VII. In bovine rhodopsin, the residues present at the positions corresponding to Cys532 and Leu77 in the rat M(3) muscarinic receptor are not located directly adjacent to each other, raising the possibility that the relative orientations of TM I and VII are not identical among different class I GPCRs. Alternatively, dynamic protein backbone fluctuation may occur, enabling Cys532 to move within cross-linking distance of Leu77 (Cys77).

To study the conformational changes that convert G protein-coupled receptors (GPCRs) from their resting to their active state, we used the M 3 muscarinic acetylcholine receptor, a prototypical class A GPCR, as a model system. Specifically, we employed a recently developed in situ disulfide cross-linking strategy that allows the formation of disulfide bonds in Cys-substituted mutant M 3 muscarinic receptors present in their native membrane environment. At present, little is known about the conformational changes that GPCR ligands induce in the immediate vicinity of the ligand-binding pocket. To address this issue, we generated 11 Cys-substituted mutant M 3 muscarinic receptors and characterized these receptors in transfected COS-7 cells. All analyzed mutant receptors contained an endogenous Cys residue (Cys-532 7.42 ) located within the exofacial segment of transmembrane domain (TM) VII, close to the agonist-binding site. In addition, all mutant receptors harbored a second Cys residue that was introduced into the exofacial segment of TM III, within the sequence Leu-142 3.27 -Asn-152 3.37 . Disulfide cross-linking studies showed that muscarinic agonists, but not antagonists, promoted the formation of a disulfide bond between S151 3.36 C and Cys-532. A three-dimensional model of the inactive state of the M 3 muscarinic receptor indicated that Cys-532 and Ser-151 face each other in the center of the TM receptor core. Our cross-linking data therefore support the concept that agonist activation pulls the exofacial segments of TMs VII and III closer to each other. This structural change may represent one of the early conformational events triggering the more pronounced structural reorganization of the intracellular receptor surface. To the best of our knowledge, this is the first direct demonstration of a conformational change occurring in the immediate vicinity of the binding site of a GPCR activated by a diffusible ligand.

Muscarinic acetylcholine receptors (mAChRs) expressed by pancreatic acinar cells play an important role in mediating acetylcholine-dependent stimulation of digestive enzyme secretion. To examine the potential roles of M 1 and M 3 mAChRs in this activity, we used M 1 and M 3 receptor single knockout (KO) and M 1 /M 3 receptor double KO mice as novel experimental tools. Specifically, we examined the ability of the muscarinic agonist carbachol to stimulate amylase secretion in vitro, using dispersed pancreatic acini prepared from wild-type and mAChR mutant mice. Quantitative reverse transcription-polymerase chain reaction studies using RNA prepared from mouse pancreatic acini showed that deletion of the M 1 or M 3 mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Moreover, immunoprecipitation studies with M 1 and M 3 mAChR-selective antisera demonstrated that both mAChR subtypes are expressed by mouse pancreatic acini. Strikingly, carbachol-induced stimulation of amylase secretion was significantly impaired in acinar preparations from both M 1 and M 3 receptor single KO mice and abolished in acinar preparations from M 1 /M 3 receptor double KO mice. However, another pancreatic secretagogue, bombesin, retained its ability to fully stimulate amylase secretion in acinar preparations from M 1 /M 3 receptor double KO mice. Together, these studies support the concept that cholinergic stimulation of pancreatic amylase secretion is mediated by a mixture of M 1 and M 3 mAChRs and that other mAChR subtypes do not make a significant contribution to this activity. These findings clarify the long-standing question regarding the molecular nature of the mAChR subtypes mediating the secretion of digestive enzymes from the exocrine pancreas.

The ligand-induced activation of G protein-coupled receptors (GPCRs) is predicted to involve pronounced conformational changes on the intracellular surface or the receptor proteins. A reorientation of the cytoplasmic end of transmembrane domain VI (TM VI) is thought to play a key role in GPCR activation and productive receptor/G protein coupling. Disulfide cross-linking studies with solubilized, Cys-substituted mutant versions of bovine rhodopsin and the M3 muscarinic acetylcholine receptor suggested that the cytoplasmic end of TM VI is conformationally highly flexible, even in the absence of activating ligands (Farrens, D. L., et al. (1996) Science 274, 768-770; Zeng, F. Y., et al. (1999) J. Biol. Chem. 274, 16629-16640). To test the hypothesis that the promiscuous disulfide cross-linking pattern observed in these studies was caused by the use of solubilized receptor proteins endowed with increased conformational flexibility, we employed a recently developed in situ disulfide cross-linking strategy that allows the detection of disulfide bonds in Cys-substituted mutant M3 muscarinic receptors present in their native membrane environment. Specifically, we used membranes prepared from transfected COS-7 cells to analyze a series of double Cys mutant M3 receptors containing one Cys residue within the sequence K484(6.29) to S493(6.38) at the cytoplasmic end of TM VI and a second Cys residue at the cytoplasmic end of TM III (I169C(3.54)). This analysis revealed a disulfide cross-linking pattern that was strikingly more restricted than that observed previously with solubilized receptor proteins, both in the absence and in the presence of the muscarinic agonist, carbachol. Carbachol stimulated the formation of disulfide bonds in only two of the 10 analyzed mutant muscarinic receptors, I169C(3.54)/K484C(6.29) and I169C(3.54)/A488C(6.33), consistent with an agonist-induced rotation of the cytoplasmic end of TM VI. These findings underline the usefulness of analyzing the structural and dynamic properties of GPCRs in their native lipid environment.