title: Molecular characterization of human melanocortin-3 receptor ligand-receptor interaction.
abstract: Melanocortin-3 receptor (MC3R), primarily expressed in the hypothalamus, plays an important role in the regulation of energy homeostasis. MC3R-deficient (MC3R(-)(/)(-)) mice demonstrate increased fat mass, higher feeding efficiency, hyperleptinaemia, and mild hyperinsulinism. At least one specific mutation of MC3R has been identified to be associated with human obesity. Functional analysis of this altered MC3R (I183N) has indicated that the mutation completely abolishes agonist-mediated receptor activation. However, the specific molecular determinants of MC3R responsible for ligand binding and receptor signaling are currently unknown. The present study is to determine the structural aspects of MC3R responsible for ligand binding and receptor signaling. On the basis of our theoretical model for MC1R, using mutagenesis, we have examined 19 transmembrane domain amino acids selected for these potential roles in ligand binding and receptor signaling. Our results indicate that (i) substitutions of charged amino acid residues E131 in transmembrane domain 2 (TM2), D154 and D158 in TM3, and H298 in TM6 with alanine dramatically reduced NDP-MSH binding affinity and receptor signaling, (ii) substitutions of aromatic amino acids F295 and F296 in TM6 with alanine also significantly decreased NDP-MSH binding and receptor activity, (iii) substitutions of D121in TM2 and D332 in TM7 with alanine resulted in the complete loss of ligand binding, ligand induced receptor activation, and cell surface protein expression, and (iv) interestingly, substitution of L165 in TM3 with methionine or alanine switched antagonist SHU9119 into a receptor agonist.
  
  
  CONCLUSIONS
  Our results suggest that TM3 and TM6 are important for NDP-MSH binding, while D121 in TM2 and D332 in TM7 are crucial for receptor activity and signaling. Importantly, L165 in TM3 is critical for agonist or antagonist selectivity. These results provide important information about the molecular determinants of hMC3R responsible for ligand binding and receptor signaling.
authors.affiliation: Department of Surgery, University of Alabama at Birmingham, Birmingham, Alabama 35233, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Chen
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Aprahamian
authors.forename: Ahmet
authors.initials: A
authors.lastname: Celik
authors.forename: Keith E
authors.initials: KE
authors.lastname: Georgeson
authors.forename: W Timothy
authors.initials: WT
authors.lastname: Garvey
authors.forename: Carroll M
authors.initials: CM
authors.lastname: Harmon
authors.forename: Yingkui
authors.initials: Y
authors.lastname: Yang
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics / Q000502:physiology; D024322:Amino Acids, Aromatic / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008024:Ligands; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding / Q000235:genetics / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000235:genetics; D044104:Receptor, Melanocortin, Type 3 / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018000:Receptors, Peptide / Q000378:metabolism; D015398:Signal Transduction; D014162:Transfection; D000521:alpha-MSH / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism
chemical_list: D024322:Amino Acids, Aromatic; D008024:Ligands; D008565:Membrane Proteins; D044104:Receptor, Melanocortin, Type 3; D018000:Receptors, Peptide; C103272:SHU 9119; D000521:alpha-MSH; C027756:MSH, 4-Nle-7-Phe-alpha-; D009074:Melanocyte-Stimulating Hormones