Abstract
H2 relaxin activates the relaxin family peptide receptor-1 (RXFP1), a class A G-protein coupled receptor, by a poorly understood mechanism. The ectodomain of RXFP1 comprises an N-terminal LDLa module, essential for activation, tethered to a leucine-rich repeat (LRR) domain by a 32-residue linker. H2 relaxin is hypothesized to bind with high affinity to the LRR domain enabling the LDLa module to bind and activate the transmembrane domain of RXFP1. Here we define a relaxin-binding site on the LDLa-LRR linker, essential for the high affinity of H2 relaxin for the ectodomain of RXFP1, and show that residues within the LDLa-LRR linker are critical for receptor activation. We propose H2 relaxin binds and stabilizes a helical conformation of the LDLa-LRR linker that positions residues of both the linker and the LDLa module to bind the transmembrane domain and activate RXFP1.
Highlights
His12 and Val13 (H2) relaxin activates the relaxin family peptide receptor-1 (RXFP1), a class A G-protein coupled receptor, by a poorly understood mechanism
While the low density lipoprotein class A (LDLa) module of RXFP1 is indispensable for receptor activation[10] and the leucine-rich repeat (LRR) domain is considered as the primary H2 relaxin-binding site[8,9], the role of the 32 residues linking the two domains has not been investigated
The conclusions we draw from the data presented here show that the 32-residue linker between the LDLa module and LRR domain binds with reasonable affinity to both H2 relaxin and EL2 of the transmembrane domain (TMD)
Summary
H2 relaxin activates the relaxin family peptide receptor-1 (RXFP1), a class A G-protein coupled receptor, by a poorly understood mechanism. In a recent study[14], both the LDLa module and H2 relaxin were shown to interact with exoloops-1 and -2 (EL1 and EL2) of the TMD when grafted onto an engineered scaffold protein[15] These observations support that the LDLa module is the true ligand that interacts with the TMD to cause conformational rearrangement and G-protein coupling, this mechanism requires H2 relaxin binding. This is clearly a unique mode of GPCR activation, where the ligand itself is not the activator but drives a receptor-attached module to initiate downstream signalling. We demonstrate that the LDLa-LRR linker may interact with the exoloops of the TMD supporting the critical role of this region in receptor activation
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