Chimeric melanocortin MC1 and MC3 receptors: identification of domains participating in binding of melanocyte-stimulating hormone peptides.

The melanocortin receptors MC1 and MC3 are G protein-coupled receptors that have substantial structural similarities and bind melanocyte peptides but with different affinity profiles. We constructed a series of chimeric MC1/MC3 receptors to identify the epitopes that determine their selectivities for natural melanocyte peptides and synthetic analogues. The chimeric constructs were made by a polymerase chain reaction that used identical regions in or just outside transmembranes (TM) 1, 4, and 6 and divided the receptors into four segments. Saturation and competition studies on the expressed chimeric proteins indicate that TM1, TM2, TM3, and TM7 are involved in the subtype-specific binding of melanocyte peptides to these receptors. The results support the hypothesis that TM4 and TM5 may not contribute to the ligand-binding specificity of the MC receptors. This is the first report to describe the subtype-specific hormone-binding domains of the melanocortin receptor family.