The Role of the Aspartate-Arginine-Tyrosine Triad in the m1 Muscarinic Receptor: Mutations of Aspartate 122 and Tyrosine 124 Decrease Receptor Expression but Do Not Abolish Signaling
- 1Division of Physical Biochemistry, Medical Research Council National Institute for Medical Research, London NW7 1AA, UK (Z.-L.L., C.A.C., E.C.H.), 2Wyeth-Ayerst Research, CN8000, Princeton, New Jersey 08543-8000 (P.G.J.), and 3Department of Pharmacology, Malaga University School of Medicine, Campus Teatinos, 29080 Malaga, Spain (J.P.)
Abstract
An Asp-Arg-Tyr triad occurs in a majority of rhodopsin-like G protein-coupled receptors. The fully conserved Arg is critical for G protein activation, but the function of the flanking residues is not well understood. We expressed in COS-7 cells m1 muscarinic receptors that were mutated at Asp122 and Tyr124. Most mutations at either position strongly attenuated or prevented the expression of binding sites for the antagonist [3H]N-methylscopolamine. However, sites that were expressed displayed unaltered affinity for the antagonist. Receptor protein, visualized with a carboxyl-terminally directed antibody, was reduced but never completely abolished. The effects of these mutations were partially reversed by the deletion of 129 amino acids from the third intracellular loop of the receptor. In several cases, comparison of immunocytochemistry with binding measurements suggested the presence of substantial amounts of inactive, presumably misfolded, receptor protein. Some of the variants that bound [3H]N-methylscopolamine underwent small changes in their affinities for acetylcholine. All retained nearly normal abilities to mediate an acetylcholine-induced phosphoinositide response. We propose that Asp122 and Tyr124 make intramolecular contacts whose integrity is important for efficient receptor folding but that they do not participate directly in signaling. The role of these residues is completely distinct from that of Arg123, whose mutation abolishes signaling without diminishing receptor expression.
Footnotes
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Send reprint requests to: Dr. E. C. Hulme, Division of Physical Biochemistry, National Institute for Medical Research, Mill Hill, London NW7 1AA, U.K. E-mail:e-hulme{at}nimr.mrc.ac.uk
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↵1 Z.-L. Lu, unpublished observations.
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↵2 E. C. Hulme and C. A. Curtis, unpublished observations.
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This work was supported by the Medical Research Council (UK) and by a Wellcome Travelling Fellowship (Z.L.).
- Abbreviations:
- ACh
- acetylcholine
- mAChR
- muscarinic acetylcholine receptor
- GPCR
- G protein-coupled receptor
- NMS
- N-methylscopolamine
- PrBCM
- propylbenzilylcholine mustard
- PI
- phosphoinositide
- TM
- transmembrane
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
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- Received July 15, 1996.
- Accepted October 28, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
