title: Novel structural and functional insights into M3 muscarinic receptor dimer/oligomer formation.
abstract: Class A G protein-coupled receptors (GPCRs) are able to form homodimers and/or oligomeric arrays. We recently proposed, based on bioluminescence resonance energy transfer studies with the M3 muscarinic receptor (M3R), a prototypic class A GPCR, that the M3R is able to form multiple, structurally distinct dimers that are probably transient in nature (McMillin, S. M., Heusel, M., Liu, T., Costanzi, S., and Wess, J. (2011) J. Biol. Chem. 286, 28584-28598). To provide more direct experimental support for this concept, we employed a disulfide cross-linking strategy to trap various M3R dimeric species present in a native lipid environment (transfected COS-7 cells). Disulfide cross-linking studies were carried out with many mutant M3Rs containing single cysteine (Cys) substitutions within two distinct cytoplasmic M3R regions, the C-terminal portion of the second intracellular loop (i2) and helix H8 (H8). The pattern of cross-links that we obtained, in combination with molecular modeling studies, was consistent with the existence of two structurally distinct M3R dimer interfaces, one involving i2/i2 contacts (TM4-TM5-i2 interface) and the other one characterized by H8-H8 interactions (TM1-TM2-H8 interface). Specific H8-H8 disulfide cross-links led to significant impairments in M3R-mediated G protein activation, suggesting that changes in the structural orientation or mobility of H8 are critical for efficient receptor-G protein coupling. Our findings provide novel structural and functional insights into the mechanisms involved in M3R dimerization (oligomerization). Because the M3R shows a high degree of sequence similarity with many other class A GPCRs, our findings should be of considerable general interest.
authors.affiliation: From the Molecular Signaling Section, Laboratory of Bioorganic Chemistry, NIDDK, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: Jianxin
authors.initials: J
authors.lastname: Hu
authors.forename: Kelly
authors.initials: K
authors.lastname: Hu
authors.forename: Tong
authors.initials: T
authors.lastname: Liu
authors.forename: Matthew K
authors.initials: MK
authors.lastname: Stern
authors.forename: Rajendra
authors.initials: R
authors.lastname: Mistry
authors.forename: R A John
authors.initials: RA
authors.lastname: Challiss
authors.forename: Stefano
authors.initials: S
authors.lastname: Costanzi
authors.forename: Jürgen
authors.initials: J
authors.lastname: Wess
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002522:Chlorocebus aethiops; D003545:Cysteine / Q000737:chemistry; D016297:Mutagenesis, Site-Directed; D010716:Phosphatidylinositols / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D043587:Receptor, Muscarinic M3 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D010716:Phosphatidylinositols; D043587:Receptor, Muscarinic M3; D003545:Cysteine
keywords: Cholinergic Receptor; G Protein-coupled Receptors (GPCR); Neurotransmitter Receptors; Receptor Structure-function; Site-directed Mutagenesis