Cooperative Binding of Insulin-Like Peptide 3 to a Dimeric Relaxin Family Peptide Receptor 2

Insulin-like peptide 3 (INSL3) binds to a G protein-coupled receptor (GPCR) called relaxin family peptide receptor 2 (RXFP2). RXFP2 belongs to the leucine-rich repeat-containing subgroup (LGR) of class A GPCRs. Negative cooperativity has recently been demonstrated in other members of the LGR subgroup. In this work, the kinetics of INSL3 binding to HEK293 cells stably transfected with RXFP2 (HEK293-RXFP2) have been investigated in detail to study whether negative cooperativity occurs and whether this receptor functions as a dimer. Our results show that negative cooperativity is present and that INSL3-RXFP2 binding shows both similarities and differences with insulin binding to the insulin receptor. A dose-response curve for the negative cooperativity of INSL3 binding had a reverse bell shape reminiscent of that seen for the negative cooperativity of insulin binding to its receptor. This suggests that binding of INSL3 may happen in a trans rather than in a cis way in a receptor dimer. Bioluminescence resonance energy transfer (BRET(2)) experiments confirmed that RXFP2 forms constitutive homodimers. Heterodimerization between RXFP2 and RXFP1 was also observed.