G protein-coupled receptors--recent advances

Abstract

The years 2000 and 2007 witnessed milestones in current understanding of G protein-coupled receptor (GPCR) structural biology. In 2000 the first GPCR, bovine rhodopsin, was crystallized and the structure was solved, while in 2007 the structure of β(2)-adrenergic receptor, the first GPCR with diffusible ligands, was determined owing to advances in microcrystallization and an insertion of the fast-folding lysozyme into the receptor. In parallel with those crystallographic studies, the biological and biochemical characterization of GPCRs has advanced considerably because those receptors are molecular targets for many of currently used drugs. Therefore, the mechanisms of activation and signal transduction to the cell interior deduced from known GPCRs structures are of the highest importance for drug discovery. These proteins are the most diversified membrane receptors encoded by hundreds of genes in our genome. They participate in processes responsible for vision, smell, taste and neuronal transmission in response to photons or binding of ions, hormones, peptides, chemokines and other factors. Although the GPCRs share a common seven-transmembrane α-helical bundle structure their binding sites can accommodate thousands of different ligands. The ligands, including agonists, antagonists or inverse agonists change the structure of the receptor. With bound agonists they can form a complex with a suitable G protein, be phosphorylated by kinases or bind arrestin. The discovered signaling cascades invoked by arrestin independently of G proteins makes the GPCR activating scheme more complex such that a ligand acting as an antagonist for G protein signaling can also act as an agonist in arrestin-dependent signaling. Additionally, the existence of multiple ligand-dependent partial activation states as well as dimerization of GPCRs result in a 'microprocessor-like' action of these receptors rather than an 'on-off' switch as was commonly believed only a decade ago.

Figure 1. A scheme of shapes and tilts of transmembrane helices of GPCRs based on the representative crystal structure of β₂-adrenergic receptor (PDB id: 2RH1)

Location of a ligand is marked by a red sphere whereas the location of protein G by a blue sphere.

Figure 2

Histograms of sequence identity between members of four branches of rhodopsin-like family of GPCRs.

Figure 3. Crystal structures of rhodopsin (PDB id: 1F88) and β₂-adrenergic receptor (PDB id: 2RH1)

Top, view along the membrane plane, bottom, from the extracellular side.

Figure 4

Crystal structure of β₂-adrenergic receptor in complex with heterotrimeric Gaβ protein (PDB id: 3SN6). View along the membrane plane.

Figure 5. Crystal structure of arrestin (PDB id: 1CF1) with characteristic elements indicated

Orange balls indicate regions that change upon GPCR binding but are not directly involved in the interaction with receptor. Colored residues are important for arrestin stability (a salt bridge in blue and red in polar core region) or initial recognition of receptor (two Lys residues in green).

Figure 6. GPCR signaling

In response to ligand binding a stimulation signaling can occur via G-protein-mediated pathway terminated by subsequent GRK/arrestin binding, or/and via β-arrestin-mediated pathway.

Figure 7. A schematic representation of arrestin-biased signaling

Binding of standard agonist to receptor induces an active conformation (R*) whereas binding of arrestin-biased agonist induces a different active conformation (R**). Distinct active conformations of receptor are coupled to different active conformation of arrestin which govern different functional outcomes.

Figure 8. Crystal structures of chemokine receptors

(A) Crystal structure of CXCR4 (PDB id: 3ODU) with small molecule antagonist IT1t. (B) NMR structure of CXCR1 (PDB id: 2LNL). Top, view along the membrane plane, bottom, from the extracellular side.