G-protein-coupled receptors (GPCRs) are the largest class of targets for modern drug development. But the design of new GPCR-targeting compounds has been hampered by the dearth of direct structural information on interactions between GPCRs and their ligands. Fresh insights are now to hand, following the publication in the Proceedings of the National Academy of Sciences of a study that used solid-state NMR to probe the conformation of the peptide agonist neurotensin bound to one of its GPCRs, neurotensin type-1 receptor (NTS-1).

Neurotensin is a 13-amino-acid neuropeptide that modulates vascular and endocrine functions. Both the full-length peptide and its six-amino-acid C-terminus (NT(8–13)) bind with sub-nanomolar affinity to NTS-1. Such high-affinity binding precludes the use of solution-state NMR, so Marc Baldus and colleagues optimized solid-state NMR to conduct a comparative analysis of free and bound NT(8–13) at atomic resolution. Milligram quantities of high-quality receptor preparations were provided by Reinhard Grisshammer and co-workers.

To allow the NMR signals of the bound ligand to be detected unequivocally in the presence of large background signals from buffer components, detergents and lipids, NT(8–13) uniformly labelled with the NMR-active isotopes 13C and 15N was prepared by solid-phase peptide synthesis. Analysis of spectra from optimized two-dimensional 13C NMR experiments with this labelled peptide revealed that free NT(8–13) remains essentially unstructured. However, in the presence of functional, lipid-reconstituted rat NTS-1 obtained from an Escherichia coli expression system, the NT(8–13) backbone assumes a defined β-strand conformation.

This structural model of the receptor-bound peptide could represent a viable template for three-dimensional pharmacophore-based searches of chemical libraries for non-peptide ligands, which might be therapeutically applicable. More generally, these data highlight the potential of solid-state NMR as a useful tool in unravelling the structural complexity of high-affinity GPCR–ligand interactions.