G Protein-Coupled Receptors Change Seen Is an Inward Bulge of TM5 That Is Focused Around Ser2075.46

ReseaRch highlights

G PROtEIN-COuPLEd RECEPtORS change seen is an inward bulge of TM5 that is focused around Ser2075.46. In a companion publication to Crystallizing how agonists bind the study by Rasmussen et al., an irreversible agonist was designed that efficiently formed crystals Our understanding of how agonists with other subtle structural changes when bound to the low-affinity

bind to and activate members of the observed inside and outside the conformation of a human β2-AR. superfamily of G protein-coupled binding pocket, the authors propose The covalent agonist formed receptors (GPCRs) has been boosted that the degree to which an agonist hydrogen bonding contacts with by three papers published in Nature has an effect depends on whether it Ser2035.42 and Ser2075.46 on TM5 in that present ligand–receptor X-ray can fulfil three main criteria: ligand- the binding pocket, but the structure crystal structures for β-adrenergic induced conformational changes of of the cytoplasmic domains was receptors (β-ARs). Ser2155.46 and Ser2125.43 (numbers more similar to the inverse agonist-

The different conformational in superscript correspond to the bound, inactive-state β2-AR than to states that β-ARs can adopt and Ballesteros–Weinstein numbering the nanobody-stabilized active-state the inherent instability of agonist– system for conserved GPCR residues) structure. Therefore, agonist binding receptor complexes have contributed in transmembrane segment 5 alone is not enough to stabilize to the lack of crystal structures (TM5), and contraction of the the active conformation at the available to date. In these papers, binding pocket. Accordingly, the full cytoplasmic surface. different methods were used to agonists used in this paper (such as Overall, these data offer a valu- produce crystals that diffracted well carmoterol and isoprenaline) fulfil able and new understanding of how enough for the structures of various all three criteria, whereas partial GPCRs are activated; until now, the complexes to be determined. agonists (for example, salbutamol only seven-transmembrane domain Warne and colleagues used a and dobutamine) do not interact receptor to be crystallized in different 5.46 thermostabilized turkey β1-AR to with Ser215 , and the antagonist conformational states was rhodopsin, explore the initial conformational cyanopindolol does not induce any of which is activated by light rather changes induced upon binding of these changes. than ligand binding. Moreover, these full and partial agonists. Together Rasmussen and colleagues gener- insights could aid the rational design ated a camelid antibody fragment of ligands for this large and diverse (known as a nanobody) targeted to family of receptors.

the human β2-AR, which was able to Man Tsuey Tse mimic G protein-like behaviour and ORIGINAL RESEARCH PAPERS Warne, T. et al. allowed the formation of a high-affinity The structural basis for agonist and partial

agonist-bound active-state crystal agonist action on a β1-adrenergic receptor. structure. This structure revealed Nature 469, 241–244 (2011) | Rasmussen, S. G. F. et al. Structure of a nanobody-stabilized active major changes in the cytoplasmic state of the β2 adrenoceptor. Nature 469, ends of TM5 and TM6, which are 175–180 (2011) | Rosenbaum, D. M. et al. outwardly displaced, whereas TM3 Structure and function of an irreversible agonist–β2 adrenoceptor complex. and TM7 move inwards. In the Nature 469, 236–240 (2011) ligand-binding pocket, the largest