G-Protein-Coupled Receptors: from Structural Insights to Functional Mechanisms

The design and development of selective ligands for the human OT (oxytocin) and AVP (arginine vasopressin) receptors is a big challenge since the different receptor subtypes and their native peptide ligands display great similarity. Detailed understanding of the mechanism of OT’s interaction with its receptor is important and may assist in the ligandor structure-based design of selective and potent ligands. In the present article, we compared 69 OTand OT-like receptor sequences with regards to their molecular evolution and diversity, utilized an in silico approach to map the common ligand interaction sites of recently published G-proteincoupled receptor structures to a model of the human OTR (OT receptor) and compared these interacting residues within a selection of different OTR sequences. Our analysis suggests the existence of a binding site for OT peptides within the common transmembrane core region of the receptor, but it appears extremely difficult to identify receptor or ligand residues that could explain the selectivity of OT to its receptors. We remain confident that the presented evolutionary overview and modelling approach will aid interpretation of forthcoming OTR crystal structures. Introduction The neurohypophyseal nonapeptides OT (oxytocin) and AVP (arginine vasopressin) play an important role in many physiological functions through GPCR (G-proteincoupled receptor) signal transduction. In humans and other mammalian species, OT and AVP target the OTR (oxytocin receptor) and the three vasopressin receptors V1aR, V1bR and V2R [1]. Peripherally, OT acts as a hormone that triggers uterine smooth muscle contraction during childbirth, facilitates milk ejection for breastfeeding and is involved in male ejaculation. Centrally, it functions as a neurotransmitter where it is involved in complex social behaviour, maternal care, stress and anxiety [2]. AVP regulates fluid balance and blood pressure and is mainly involved in memory, learning, stress regulation and aggressive behaviour (summarized in [1]). Although OT and AVP elicit distinct physiological functions, they display a certain degree of cross-reactivity, which is caused by the high ligand similarity (OT and AVP only differ by two residues) and the high extracellular homology (∼80%) between the OT and AVP receptors. Contrary to many other signalling systems, selectivity is not achieved via the ligands, but via interplay of factors including receptor upor down-regulation, release of specific liganddegrading enzymes, local ligand production and receptor clustering. In terms of drug development, the extracellular