TY - JOUR T1 - Multiple Binding Sites Revealed by Interaction of Relaxin Family Peptides with Native and Chimeric Relaxin Family Peptide Receptors 1 and 2 (LGR7 and LGR8) JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 677 LP - 687 DO - 10.1124/jpet.104.080655 VL - 313 IS - 2 AU - Michelle L. Halls AU - Courtney P. Bond AU - Satoko Sudo AU - Jin Kumagai AU - Tania Ferraro AU - Sharon Layfield AU - Ross A. D. Bathgate AU - Roger J. Summers Y1 - 2005/05/01 UR - http://jpet.aspetjournals.org/content/313/2/677.abstract N2 - Relaxin family peptide 1 (RXFP1) receptor (LGR7) and RXFP2 receptor (LGR8) were recently identified as the receptor targets for H2 relaxin and insulin-like peptide 3 (INSL3), respectively. In this study, we define the pharmacology of these two receptors by using a number of receptor chimeras and relaxin family peptides. We have identified two binding sites on these receptors: one primary, high-affinity site within the ectodomain and a secondary, lower affinity site within the transmembrane region. The primary site was found to dictate receptor binding characteristics, although the lower affinity site also exerts some influence and modulates ligand affinity for the primary site in a manner dependent upon the peptide in question. Not all relaxin peptides were able to bind to the RXFP2 receptor, indicating that the relaxin-RXFP2 receptor interaction is species-specific. INSL3 was found to exhibit characteristics of a partial agonist at the RXFP2 and chimeric RXFP1/2 receptors, with low maximal cAMP responses but high potency in coupling to this pathway. cAMP accumulation studies also revealed that the binding sites couple to cAMP signaling pathways with differing efficiency: the high-affinity site signals with high efficiency, whereas the lower affinity site signals with little to no efficiency. Comparisons between RXFP1, RXFP2, the chimeric receptors, and the truncated receptors revealed that the interaction between receptor sites is critical for optimal ligand binding and signal transduction and that the ectodomain is essential for signaling. Evidence obtained in this study supports a two-stage binding model of receptor activation: binding to the primary site allows a conformational change and interaction with the low-affinity transmembrane site. The American Society for Pharmacology and Experimental Therapeutics ER -