title: The DRY motif and the four corners of the cubic ternary complex model.
abstract: Recent structural data on GPCRs using a variety of spectroscopic approaches suggest that GPCRs adopt a dynamic conformational landscape, with ligands stabilizing subsets of these states to activate one or more downstream signaling effectors. A key outstanding question posed by this emerging dynamic structural model of GPCRs is what states, active, inactive, or intermediate are captured by the numerous crystal structures of GPCRs complexed with a variety of agonists, partial agonists, and antagonists. In the early nineties the discovery of inverse agonists and constitutive activity led to the idea that the active receptor state (R⁎) is an intrinsic property of the receptor itself rather than of the RG complex, eventually leading to the formulation of the cubic ternary complex model (CTC). Here, by a careful analysis of a series of data obtained with a number of mutants of the highly conserved E/DRY motif, we show evidences for the existence of all the receptor states theorized by the CTC, four 'uncoupled (R, R⁎and HR and HR⁎), and, consequently four 'coupled' (RG, R⁎G, HRG and HR⁎G). The E/DRY motif located at the cytosolic end of transmembrane helix III of Class A GPCRs has been widely studied and analyzed because it forms a network of interactions believed to lock receptors in the inactive conformation (R), and, thus, to play a key role in receptor activation. Our conclusions are supported by recent crystal and NMR spectra, as well as by results obtained with two prototypical GPCRs using a new FRET technology that de-couples G protein binding to the receptor from signal transduction. Thus, despite its complexity and limitations, we propose that the CTC is a useful framework to reconcile pharmacological, biochemical and structural data.
authors.affiliation: Department of Pharmacological and Biomolecular Sciences, University of Milan, Milano, Italy. Electronic address: Genrico.Rovati@unimi.it.
authors.forename: G Enrico
authors.initials: GE
authors.lastname: Rovati
authors.affiliation: Department of Pharmacological and Biomolecular Sciences, University of Milan, Milano, Italy; Department of Health Science, University of Milan, Milano, Italy. Electronic address: Valerie.Capra@unimi.it.
authors.forename: Valérie
authors.initials: V
authors.lastname: Capra
authors.affiliation: Department of Pharmacology & Toxicology, Michigan State University, East Lansing, MI, USA. Electronic address: shawvinc@msu.edu.
authors.forename: Vincent S
authors.initials: VS
authors.lastname: Shaw
authors.affiliation: Department of Genetics, Cell Biology & Development, College of Biological Sciences, University of Minnesota Twin Cities, Minneapolis, MN, USA. Electronic address: rumalik@umich.edu.
authors.forename: Rabia U
authors.initials: RU
authors.lastname: Malik
authors.affiliation: Department of Genetics, Cell Biology & Development, College of Biological Sciences, University of Minnesota Twin Cities, Minneapolis, MN, USA. Electronic address: sivaraj@umn.edu.
authors.forename: Sivaraj
authors.initials: S
authors.lastname: Sivaramakrishnan
authors.affiliation: Department of Pharmacology & Toxicology, Michigan State University, East Lansing, MI, USA. Electronic address: Richard.Neubig@hc.msu.edu.
authors.forename: Richard R
authors.initials: RR
authors.lastname: Neubig
mesh_terms: D018360:Crystallography, X-Ray; D019204:GTP-Binding Proteins / Q000737:chemistry* / Q000235:genetics; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D043562:Receptors, G-Protein-Coupled / Q000737:chemistry* / Q000235:genetics; D015398:Signal Transduction / Q000235:genetics*
chemical_list: D008024:Ligands; D043562:Receptors, G-Protein-Coupled; D019204:GTP-Binding Proteins
keywords: Crystal structure; Cubic ternary complex model; E/DRY motif; Fluorescence resonance energy transfer; G protein coupled receptor; Receptor state