Background: GHS-R1a activates multiple signaling pathways mediating feeding and addictive behaviors. Results: Some GHS-R1a ligands activate G q but not G i/o and fail to recruit ␤-arrestin2; others act as selective inverse agonists at G q compared with G 13 . Conclusion: Synthetic ligands can selectively activate or reverse G q -dependent signaling at GHS-R1a. Significance: Ligand-biased signaling can be exploited for the development of selective drugs to treat GHS-R1a-mediated disorders.

How G protein-coupled receptor conformational dynamics control G protein coupling to trigger signaling is a key but still open question. We addressed this question with a model system composed of the purified ghrelin receptor assembled into lipid discs. Combining receptor labeling through genetic incorporation of unnatural amino acids, lanthanide resonance energy transfer, and normal mode analyses, we directly demonstrate the occurrence of two distinct receptor:Gq assemblies with different geometries whose relative populations parallel the activation state of the receptor. The first of these assemblies is a preassembled complex with the receptor in its basal conformation. This complex is specific of Gq and is not observed with Gi. The second one is an active assembly in which the receptor in its active conformation triggers G protein activation. The active complex is present even in the absence of agonist, in a direct relationship with the high constitutive activity of the ghrelin receptor. These data provide direct evidence of a mechanism for ghrelin receptor-mediated Gq signaling in which transition of the receptor from an inactive to an active conformation is accompanied by a rearrangement of a preassembled receptor:G protein complex, ultimately leading to G protein activation and signaling.GPCR | G protein | preassembly | conformation dynamics | signaling G protein-coupled receptors (GPCRs), one of the largest cell surface receptor families, are involved in many cellular signaling processes (1). Based on this property, as well as their importance as drug targets, the molecular aspects of GPCR functioning have been extensively investigated. In particular, coupling to heterotrimeric G proteins has been the focus of numerous studies. Indeed, delineating the molecular mechanisms responsible for receptor:G protein interaction is absolutely required to better understand how signaling is controlled. Recent years have seen spectacular advances that have culminated in elucidation of the 3D structure of the β 2 -adrenergic receptor:Gs complex (2). Nevertheless, the need for further progress remains, in particular to fully understand the dynamics of this interaction. This is a crucial question, given that how the receptor interacts with its G protein partner governs signaling, and thus biological and pathophysiological responses.To date, two different models for GPCR:G protein interaction have been proposed: collision coupling and preassembly. Originally, it was proposed that receptors and G proteins couple by collision (3, 4). One of the main features of this model is that only activated receptors interact with G proteins. Since then, alternative models of signaling have been developed. One of these, the preassembly model, proposes that the receptor and the G protein make a complex even in the absence of agonist (5-8).

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