Evidence indicating that the TM4, EL2, and TM5 of the melanocortin 3 receptor do not participate in ligand binding

The TM4, EL2 and TM5 show low amino acid homology within the MC receptor family. Three mutants of the human MC3 receptor were created in order to investigate the participation of these regions in ligand binding. The TM4, EL2 and TM5 were separately changed by multiple mutagenesis so that their amino acid sequences became identical with the human MC1 receptor. The mutants were expressed in COS cells and they bound peptide ligands in the same fashion as the wild type MC3 receptor clone. Our results indicate that the amino acids that were mutated in the MC3 receptor do nor affect the binding of MSH peptides. The data provide further evidence, that the mutated regions may not participate at all in ligand binding, as indicated by modelling experiments and homology comparison. (C) 1996 Academic Press