Molecular modeling of human MT2 melatonin receptor: the role of Val204, Leu272 and Tyr298 in ligand binding

A model of the helical part of the human MT2 melatonin (hMT2) receptor, a member of the G protein-coupled receptors superfamily has been generated, based on the structure of bovine rhodopsin. Modeling has been combined with site-directed mutagenesis to investigate the role of the specific amino acid residues within the transmembrane domains (TM) numbers V, VI and VII of hMT2 receptor in the interaction with 2-iodomelatonin. Saturation binding assays with 2-iodomelatonin demonstrated that the substitution V204A (TMV) resulted in total loss of binding while the mutation V205A had no effect. The replacement of F209 with alanine led to a significant decrease in the Bmax value of receptor binding while mutations V205A and F209A also within TM V did not significantly change binding properties of the hMT2 receptor. In the case of TM VI, the substitution G271T caused substantial decrease in 2-iodomelatonin binding to the hMT2 receptor. The change L272A (TM VI) as well as mutation Y298A within TM VII completely abolished ligand binding to the receptor. These data suggest that several new amino acid residues within TM V, VI and VII are involved in ligand-MT2 receptor interaction.