Conformation of the core sequence in melanocortin peptides directs selectivity for the melanocortin MC3 and MC4 receptors

Melanocortin peptides regulate a variety of physiological processes. Five m elanocortin receptors (MC-R) have been cloned and the MC3R and MC4R are the main brain MC receptors. The aim of this study was to identify structural requirements in both ligand and receptor that determine gamma-melanocyte-st imulating hormone (MSH) selectivity for the MC3R versus the MC4R, Substitut ion of Asp(10) in [Nle(4)]Lys-gamma(2)-MSH for Gly(10) from [Nle(4)]alpha-M SH, increased both activity and affinity for the MC4R while the MC3R remain ed unaffected. Analysis of chimeric MC3R/MC4Rs and mutant MC4Rs showed that Tyr(268) Of the MC4R mainly determined the low affinity for [Nle(4)]Lys-ga mma(2)-MSH. The data demonstrate that Asp(10) determines selectivity for th e MC3R, however, not through direct side chain interactions, but probably b y influencing how the melanocortin core sequence is presented to the recept or-binding pocket. This is supported by mutagenesis of Tyr(268) to lie in t he MC4R which increased affinity and activity for [Nle(4)]Lys-gamma(2)-MSH, but decreased affinity for two peptides with constrained cyclic structure of the melanocortin core sequence, MT-II and [D-Tyr(4)]MT-II, that also dis played lower affinity for the MC3R. This study provides a general concept f or peptide receptor selectivity, in which the major determinant for a selec tive receptor interaction is the conformational presentation of the core se quence in related peptides to the receptor-binding pocket.