Agonist-bound structures of G protein-coupled receptors Guillaume Lebon 1 , Tony Warne 2 and Christopher G Tate 2 G protein-coupled receptors (GPCRs) play a major role in intercellular communication by binding small diffusible ligands (agonists) at the extracellular surface. Agonist-binding induces a conformational change in the receptor, which results in the binding and activation of heterotrimeric G proteins within the cell. Ten agonist-bound structures of non-rhodopsin GPCRs published last year defined for the first time the molecular details of receptor activated states and how inverse agonists, partial agonists and full agonists bind to produce different effects on the receptor. In addition, the structure of the b 2 - adrenoceptor coupled to a heterotrimeric G protein showed how the opening of a cleft in the cytoplasmic face of the receptor as a consequence of agonist binding results in G protein coupling and activation of the G protein. Addresses 1 Institut de Ge ´ nomique Fonctionnelle, UMR 5203 CNRS - U 661 INSERM - Univ. Montpellier I & II, 141, rue de la cardonille, 34094 Montpellier Cedex 05, France 2 MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 0QH, UK Corresponding author: Tate, Christopher G (cgt@mrc-lmb.cam.ac.uk) Current Opinion in Structural Biology 2012, 22:482–490 This review comes from a themed issue on Membranes Edited by Tamir Gonen and Gabriel Waksman For a complete overview see the Issue and the Editorial Available online 3rd April 2012 0959-440X/$ – see front matter, Crown Copyright # 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sbi.2012.03.007 Introduction The year 2011 resulted in a quantum leap in our under- standing of how G protein-coupled receptors (GPCRs) are activated by agonists, due to the publication of six papers describing ten structures of agonist-bound GPCRs in var- ious different conformations (Table 1). The first three papers appeared together in the same issue of Nature and discussed structures of the b 2 -adrenoceptor (b 2 AR) bound to an agonist either in the presence [1  ] or absence [2  ] of a single-domain camelid antibody fragment acting as a G protein mimetic, and the structure of a thermo- stabilised avian b 1 AR bound to different agonists and partial agonists [3  ]. These papers were closely followed by the structures of the human adenosine A 2A receptor (A 2A R) bound to three different agonists [4  ,5  ], in- cluding the endogenous agonist adenosine. Finally, the structure of agonist-bound b 2 AR in complex with the heterotrimeric G protein Gs gave the first picture of how a G protein binds to a GPCR [6  ]. In addition to these structures of GPCRs that are activated by diffusible ligands, structures of rhodopsin in the light-activated Meta II state [7  ] and of a constitutively active mutant [8  ] were also determined. This review will focus pre- dominantly on how structures of the bARs and A 2A R have enhanced our understanding of ligand-induced confor- mation changes, with only a brief comparison to light- activated states of rhodopsin. For recent reviews on the photocycle of rhodopsin and the associated structural re- arrangements in the receptor, interested readers should refer to Refs. [9,10]. The intention of this review is to compare the agonist- bound GPCR structures within the framework of confor- mational states that have been described pharmacologi- cally. It has long been appreciated that GPCRs exist in equilibrium between two distinct groups of confor- mations, R and R*, even in the absence of ligands. In the extended ternary complex model [11], the R state is considered to be an inactive ground state that does not activate G proteins, whereas R* is the activated state of the receptor that can couple to G proteins (Figure 1). The equilibrium between R and R* is altered upon agonist binding so that the probability of R* formation is increased. In contrast, binding of an inverse agonist will bias the equilibrium towards the R state. The crucial difference in the conformation of the ligand binding pocket of a GPCR between the R and R* states, is that agonists bind to the R* state with high affinity and to the R state with low affinity. For example, the agonist NECA binds to the adenosine A 2A receptor (A 2A R) 42 times more strongly in the presence of a G protein [12] and, similarly, the agonist isoprenaline binds 95–100 times more strongly to the b 2 -adrenoceptor (b 2 AR) in the presence of either a G protein [6  ] or a G protein mimetic [1  ]. With the determination of many structures of GPCRs bound to many different ligands, the obvious question is ‘What conformation of the receptor is represented by each structure?’ Initially, this has been straightforward to deduce. The first structure of a GPCR that was determined, dark-state rhodopsin [13] is clearly in the inactive R state because of the characteristic absorbance spectrum of the receptor within the crystal. In addition, rhodopsin will remain indefinitely in the inactive state if kept in the dark due to the extremely large activation energy barrier that has to be overcome to reach the active state [14]. Subsequent biochemical and structural data identified a 6 A ˚ movement of transmembrane helix 6 (TM6) that is characteristic of the light-activated R* state Available online at www.sciencedirect.com Current Opinion in Structural Biology 2012, 22:482–490 www.sciencedirect.com