Structure of a beta1-adrenergic G-protein-coupled receptor

Abstract

G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.

Figure 1

Schematic representations of the turkey β₁AR structure. (A) Diagram of the turkey β₁AR sequence in relation to secondary structure elements. Amino sequence in white circles indicates regions that are well ordered, but sequences in grey circles were not resolved in the structure. Sequences on an orange background were deleted to make the β₁AR construct for expression. Thermostabilising mutations are in red and two other mutations C116L (increases functional expression) and C358A (eliminates palmitoylation site) are in blue. The Na⁺ ion is in purple. Numbers refer to the first and last amino acid residues in each helix (blue boxes), with the Ballesteros-Weinstein numbering in superscript. Helices were defined using the Kabasch & Sander algorithm, with helix distortions being defined as residues that have main chain torsion angles that differ by more than 40° from standard α-helix values (−60°,−40°). (B) Ribbon representation of the β₁AR structure in rainbow colouration (N-terminus blue, C-terminus red), with the Na⁺ ion in pink, the two disulphide bonds in yellow and cyanopindolol as a space-filling model. Extracellular loop 2 (EL2) and cytoplasmic loops 1 and 2 (CL1, CL2) are labelled.

Figure 2

Comparison of the CL2 loop regions in four GPCR structures. (A) β₁AR, (B) β₂AR:T4 lysozyme fusion, (C) β₂:Fab complex and (D) rhodopsin. Residues DR from the highly conserved conserved D^3.48R^3.49Y^3.50 motif are shown. Residue E^6.30, which is half of the putative ionic lock, is also shown as E247 in rhodopsin, and E285 and E268 in β₁AR and β₂AR respectively: E247^6.30 was thought to form a salt bridge with R135^3.49 in rhodopsin, but the evidence is weak. Finally, Y149 in β₁ forms a hydrogen bond with D138^3.49 in β₁AR.

Figure 3

Structure of the ligand binding pocket. (A) 2F_o-F_c map prior to inclusion of cyanopindolol (CYP) in the model showing the interaction of CYP with Thr203 and Phe201 in EL2. (B) Amino acid residues that interact with the ligand cyanopindolol (yellow) by polar interactions (aquamarine) or non-polar interactions (grey).

Figure 4

Comparisons between β receptor ligand binding pockets and the binding of different ligands. (A) Superposition of β₁AR molecule B with β₂AR (PDB code 2RH110) in the region surrounding the ligand binding site. Shown are side chains that have different rotamer conformations (N310^6.55 and S211^5.42) along with two residues that are conserved yet consistently different between β₁ and β₂ receptors (F325/Y308^7.35 and V172/T164^4.56). Cyanopindolol (CYP) is in the ligand binding pocket of the β₁ receptor and carazolol (CAR) is in the β₂ receptor. The biggest backbone deviation is seen at the V172/T164^4.56 position. (B) Superposition of a model of the agonist, adrenaline (yellow), with the structure of the antagonist, cyanopindolol (pink), as it binds to β₁AR, showing the distances (Å, red) to the nearest side chains known to interact with the hydoxyl groups on the catechol ring of the agonist. It is clear that a 2-3 Å tightening of the pocket around the ligand must occur on agonist binding.