Relaxin family peptide receptors (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database
Relaxin family peptide receptors (RXFP, nomenclature as agreed by the NC-IUPHAR Subcommittee on Relaxin family peptide receptors [18, 75]) may be divided into two pairs, RXFP1/2 and RXFP3/4. Endogenous agonists at these receptors are heterodimeric peptide hormones structurally related to insulin: relaxin-1, relaxin, relaxin-3 (also known as INSL7), insulin-like peptide 3 (INSL3) and INSL5. Species homologues of relaxin have distinct pharmacology and relaxin interacts with RXFP1, RXFP2 and RXFP3, whereas mouse and rat relaxin selectively bind to and activate RXFP1 . relaxin-3 is the ligand for RXFP3 but it also binds to RXFP1 and RXFP4 and has differential affinity for RXFP2 between species . INSL5 is the ligand for RXFP4 but is a weak antagonist of RXFP3. relaxin and INSL3 have multiple complex binding interactions with RXFP1  and RXFP2  which direct the N-terminal LDLa modules of the receptors together with a linker domain to act as a tethered ligand to direct receptor signaling . INSL5 and relaxin-3 interact with their receptors using distinct residues in their B-chains for binding, and activation, respectively [211, 97].
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