title: Role of third intracellular loop of the melanocortin 4 receptor in the regulation of constitutive activity.
abstract: The melanocortin 4 receptor (MC4R) has been reported to display constitutive activity, which is probably relevant to the maintenance of a normal energy balance. Among the clinically reported mutants of MC4R in human obesity patients, we investigated the functional characteristics of seven mutants characterized by mutations in the third intracellular (i3) loop of MC4R. Via a CRE (cAMP responsive element)-mediated luciferase reporter gene assay, we show that most of these mutants displayed significantly reduced basal activity with reduced reporter gene activity, whereas the P230L mutant manifested significantly increased basal activity. When the dominant negative G(s) mutant was co-expressed, the majority of the mutants, including the P230L mutant, showed reduced basal activity. These results suggest that the i3 loop of MC4R is essential not only for the functional activity but also for the regulation and maintenance of an optimal constitutive activity of MC4R in association with G protein coupling, in the control of energy homeostasis.
authors.affiliation: Division of Life Sciences, School of Life Sciences and Biotechnology, Korea University, Seoul 136-701, South Korea.
authors.forename: Do-Hun
authors.initials: DH
authors.lastname: Kim
authors.forename: Seung Woo
authors.initials: SW
authors.lastname: Shin
authors.forename: Ja-Hyun
authors.initials: JH
authors.lastname: Baik
mesh_terms: D054369:Agouti-Related Protein / Q000494:pharmacology; D020816:Amino Acid Motifs; D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology; D004734:Energy Metabolism / Q000235:genetics; D005786:Gene Expression Regulation; D017930:Genes, Reporter; D006706:Homeostasis / Q000235:genetics; D006801:Humans; D008156:Luciferases / Q000032:analysis / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D009765:Obesity / Q000235:genetics*; D010446:Peptide Fragments / Q000494:pharmacology; D044105:Receptor, Melanocortin, Type 4 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020218:Response Elements; D014158:Transcription, Genetic / Q000187:drug effects
chemical_list: D054369:Agouti-Related Protein; D010446:Peptide Fragments; D044105:Receptor, Melanocortin, Type 4; C430443:agouti-related protein (86-132), human; D000242:Cyclic AMP; D008156:Luciferases