In a recent issue of the Proceedings of the National Academy of Sciences, researchers describe a technique for 'trapping' small molecular fragments at the proposed binding site of a G-protein-coupled receptor (GPCR). This could reveal how the receptor is activated and also facilitate the development of small-molecule drugs that target it.

GPCRs comprise the largest single class of cell-surface receptors. They transmit signals from the outside to the inside of the cell and are involved in many important physiological processes, including the detection of light, smell, sound and taste. GPCRs are also implicated in many diseases, and almost two-thirds of currently marketed drugs are thought to interact with these transmembrane receptors. But despite their importance in biology and medicine, our understanding of how ligands bind and activate GPCRs is still relatively limited.

Now, however, Jim Wells and colleagues have used an ingenious disulphide trapping technique to characterize ligand binding and activation of the chemokine C5a receptor (C5aR), and suggest that the approach might be suitable for analysing other GPCRs.

The main requirement for disulphide trapping is that the ligands have an exchangeable thiol group (−SH), which can form reversible disulphide bonds with cysteine (Cys) residues engineered into the receptor in the vicinity of the ligand-binding/activation site. Ligands that bind the receptor are therefore brought into close proximity to the engineered Cys residues and become 'trapped' at the ligand-binding site through disulphide-bond formation, enabling their functional effects to be assessed.

Previous work has shown that thiol-containing peptides are able to bind Cys residues engineered into the transmembrane region of C5aR and modulate its activity, indicating that the receptor's binding/activation site is in the vicinity of these residues. To more thoroughly analyse the structural basis of receptor activation, the current study screened a library of 10,000 small molecules for disulphide trapping at the same Cys sites. Compounds were selected by their ability to inhibit binding of the natural C5a ligand, and a number of agonists and antagonists were identified. Analysis of these compounds showed that small changes in the structure of the trapped molecule could either greatly reduce binding, or switch a compound from being an agonist to an antagonist. Furthermore, a key amino-acid residue was identified in the receptor's binding site that has a crucial role in determining whether a ligand behaves as an agonist or an antagonist. So, structural changes to either the ligand or its receptor can modulate agonism or antagonism.

The researchers suggest that this technique could be used to trap small-molecule mimics of natural ligands for drug discovery.