Contribution of the transmembrane domain 6 of melanocortin-4 receptor to peptide [Pro⁵, dNal (2′)⁸]-γ-MSH selectivity

The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily and each of them has a different pharmacological profile regarding the relative potency of the endogenous and synthetic melanocortin peptides. Substitution of Trp with dNal (2′) in γ-MSH resulted in the loss of binding affinity and potency at hMC4R. However, the molecular mechanism of this ligand selectivity is unclear. In this study, we utilized chimeric receptors and site-directed mutagenesis approaches to investigate the molecular basis of MC4R responsible for peptide [Pro⁵, dNal (2′)⁸]-γ-MSH selectivity. Cassette substitutions of the second, third, fourth, fifth, and sixth TM of the human MC4R (hMC4R) with the homologous regions of hMC1R were constructed and the binding affinity of peptide [Pro⁵, dNal (2′)⁸]-γ-MSH at these chimeric receptors was evaluated. Our results indicate that the cassette substitutions of TM2, TM3, TM4 and TM5 of hMC4R with homologous regions of the hMC1R did not significantly increase peptide [Pro⁵, dNal (2′)⁸]-γ-MSH binding affinity and potency but substitution of the TM6 of the hMC4R with the same region of the hMC1R significantly enhances [Pro⁵, dNal (2′)⁸]-γ-MSH binding affinity and potency. Further site-directed mutagenesis study indicates that four amino acid residues, Phe267, Tyr268, Ile269 and Ser270, in TM6 of the hMC4R may play an important role in [Pro5, dNal (2′)-γ-MSH selective activity at MC4R.