Conformational changes in G-protein-coupled receptors-the quest for functionally selective conformations is open

Abstract

The G-protein-coupled receptors (GPCRs) represent one the largest families of drug targets. Upon agonist binding a receptor undergoes conformational rearrangements that lead to a novel protein conformation which in turn can interact with effector proteins. During the last decade significant progress has been made to prove that different conformational changes occur. Today it is mostly accepted that individual ligands can induce different receptor conformations. However, the nature or molecular identity of the different conformations is still ill-known. Knowledge of the potential functionally selective conformations will help to develop drugs that select specific conformations of a given GPCR which couple to specific signalling pathways and may, ultimately, lead to reduced side effects. In this review we will summarize recent progress in biophysical approaches that have led to the current understanding of conformational changes that occur during GPCR activation.

Figure 1

Schematic representation of the helix movements as delineated from experimental data described by the different approaches. The crystal structure data from bovine rhodopsin (PDB access code: 1U19, Okada et al., 2004) were used to generate the figure. Colour code: TMIII, blue; TMVI, green; TMVII, dark orange. Left: view from the cytoplasmic side; right: view from the extracellular side. Helix movements are indicated by arrows and are meant to indicate the general movements as mentioned in the text. Blue arrows: data delineated from the rhodopsin system; green arrows: data delineated from the β₂-adrenoceptor system; red arrows: data delineated from the M3 muscarinic ACh receptor system; yellow arrows: data delineated from the metal-ion chelator approach. PDB, Protein Data Bank; TM, transmembrane domain.

Figure 2

(a) Schematic representation of a GPCR modified with the cyan and yellow fluorescent protein. Crystal structure data from bovine rhodopsin and GFP were used to generate the figure. (b) Size comparison of GFP (left) and FlAsH (right). A phenylalanine side chain of GFP is shown to indicate that FlAsH was matched to size. Side and top view of both fluorophores are shown. (c) Changes in the relative fluorescence of CFP or FlAsH in response to 100 μM adenosine from a single HEK-293 cell expressing the A_2A-FlAsH3-CFP construct. (d) Comparison of FRET signals in FlAsH/CFP- and CFP/YFP-labelled receptors. Normalized FRET ratios in response to 1 mM adenosine from single HEK-293 cells expressing A_2A-FlAsH3-CFP (red) or A_2A-CFP-YFP (black) are shown. (e) Same data, as in panel d, with the amplitude (response to 1 mM adenosine) set to 100% for both traces. Figure is reproduced with permission from Hoffmann et al. (2005). FlAsH, fluorescein arsenical hairpin binder; FRET, fluorescence resonance energy transfer; GFP, green fluorescent protein; GPCR, G-protein-coupled receptor.