Molecular characterization of the ligand binding site of the human galanin receptor type 2, identifying subtype selective interactions.

To define the specific role of the galanin receptors when mediating the effect of galanin, effective tools for distinct activation and inhibition of the different receptor subtypes are required. Several of the physiological effects modulated by galanin are implicated to be mediated via the GalR2 subtype and have been distinguished from GalR1 effects by utilizing the Gal(2-11) peptide, recognizing only GalR2 and GalR3. In this study, we have performed a mutagenesis approach on the GalR2 subtype and present, for the first time, a molecular characterization of the interactions responsible for ligand binding and receptor activation at this receptor subtype. Our results identify four residues, His(252) and His(253) located in transmembrane domain 6 and Phe(264) and Tyr(271) in the extracellular loop 3, to be of great significance. We show evidence for the N-terminal tail of GalR2 to participate in ligand binding and that selective binding of Gal(2-11) includes interaction with the Ile(256) residue, located at the very top of TM 6. In conclusion, we present a mutagenesis study on GalR2 and confer interactions responsible for ligand binding and receptor activation as well as selective recognition of the Gal(2-11) peptide at this receptor subtype. The presented observations could be of major importance for the design and development of new and improved peptide and non-peptide ligands, selectively activating the GalR2 subtype.