title: Down-regulation of melanocortin receptor signaling mediated by the amino terminus of Agouti protein in Xenopus melanophores.
abstract: Agouti protein and Agouti-related protein (Agrp) regulate pigmentation and body weight, respectively, by antagonizing melanocortin receptor signaling. A carboxyl-terminal fragment of Agouti protein, Ser73-Cys131, is sufficient for melanocortin receptor antagonism, but Western blot analysis of skin extracts reveals that the electrophoretic mobility of native Agouti protein corresponds to the mature full-length form, His23-Cys131. To investigate the potential role of the amino-terminal residues, we compared the function of full-length and carboxyl-terminal fragments of Agrp and Agouti protein in a sensitive bioassay based on pigment dispersion in Xenopus melanophores. We find that carboxyl-terminal Agouti protein, and all forms of Agrp tested, act solely by competitive antagonism of melanocortin action. However, full-length Agouti protein acts by an additional mechanism that is time- and temperature-dependent, depresses maximal levels of pigment dispersion, and is therefore likely to be mediated by receptor down-regulation. Apparent down-regulation is not observed for a mixture of amino-terminal and carboxyl-terminal fragments. We propose that the phenotypic effects of Agouti in vivo represent a bipartite mechanism: competitive antagonism of agonist binding by the carboxyl-terminal portion of Agouti protein and down-regulation of melanocortin receptor signaling by an unknown mechanism that requires residues in the amino terminus of the Agouti protein.
authors.affiliation: Departments of Pediatrics and Genetics, and the Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305-5428, USA.
authors.forename: M M
authors.initials: MM
authors.lastname: Ollmann
authors.forename: G S
authors.initials: GS
authors.lastname: Barsh
mesh_terms: D054366:Agouti Signaling Protein; D054369:Agouti-Related Protein; D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002478:Cells, Cultured; D015536:Down-Regulation / Q000187:drug effects; D036341:Intercellular Signaling Peptides and Proteins; D007700:Kinetics; D008547:Melanophores / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000494:pharmacology; D010858:Pigmentation / Q000187:drug effects; D011506:Proteins / Q000378:metabolism*; D018043:Receptors, Corticotropin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D044101:Receptors, Melanocortin; D011994:Recombinant Proteins / Q000378:metabolism; D016415:Sequence Alignment; D015398:Signal Transduction; D012867:Skin / Q000378:metabolism; D014981:Xenopus / Q000378:metabolism*; D000521:alpha-MSH / Q000037:antagonists & inhibitors
chemical_list: D054366:Agouti Signaling Protein; D054369:Agouti-Related Protein; C515199:Agrp protein, mouse; D036341:Intercellular Signaling Peptides and Proteins; D010446:Peptide Fragments; D011506:Proteins; D018043:Receptors, Corticotropin; D044101:Receptors, Melanocortin; D011994:Recombinant Proteins; C515183:a protein, mouse; D000521:alpha-MSH