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...