Structural insights into G-protein-coupled receptor activation

Abstract

G-protein-coupled receptors (GPCRs) are the largest family of eukaryotic plasma membrane receptors, and are responsible for the majority of cellular responses to external signals. GPCRs share a common architecture comprising seven transmembrane (TM) helices. Binding of an activating ligand enables the receptor to catalyze the exchange of GTP for GDP in a heterotrimeric G protein. GPCRs are in a conformational equilibrium between inactive and activating states. Crystallographic and spectroscopic studies of the visual pigment rhodopsin and two beta-adrenergic receptors have defined some of the conformational changes associated with activation.

Figure 1

An overlay of dark rhodopsin (purple; PDB 1gzm), low pH opsin (green; PDB 3cap), and carazolol-bound β₂AR–T4L (blue; PDB 2rh1) in the vicinity of the ionic lock. In dark rhodopsin, Arg135^3.50 in the conserved D/ERY sequence near the cytoplasmic end of TM3 forms a salt bridge with Glu247^6.30 at the cytoplasmic end of TM6. Arg^3.50 is further stabilized by a salt bridge to the preceding conserved acid at position 3.49 in both rhodopsin and β₂AR. In opsin, Arg135^3.50 interacts with Tyr223^5.58 in TM5, and Glu247^6.30 forms a salt bridge with Lys231^5.66. The homologous ionic lock residues Arg131^3.50 and Glu268^6.30 from the β₂AR are also shown. The amino acids are numbered using the Ballesteros–Weinstein system. Within each helix is a single most conserved residue among the class A GPCRs. This residue is designated x.50, where x is the number of the transmembrane helix. All other residues on that helix are numbered relative to this conserved position.

Figure 2

Comparison of rhodopsin (purple; PDB 1gzm) and carazolol-bound β₂AR–T4L (blue; PDB 2rh1). Extracellular loop 2 (ECL2) of rhodopsin forms a lid over the retinal-binding pocket, whereas the position of ECL2 of the β₂AR allows relatively free access to the carazolol-binding pocket.

Figure 3

Superimposed structures of rhodopsin (purple; PDB 1gzm) and carazolol-bound β₂AR–T4L (blue; PDB 2rh1) highlighting water molecules that form a hydrogen-bond network with conserved amino acids. Atoms of carazolol and retinal are shown as spheres. The amino acids are numbered using the Ballesteros–Weinstein system (see Figure 1).