Down-regulation of melanocortin receptor signaling mediated by the amino terminus of Agouti protein in Xenopus melanophores

J Biol Chem. 1999 May 28;274(22):15837-46. doi: 10.1074/jbc.274.22.15837.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Agouti Signaling Protein
  • Agouti-Related Protein
  • Amino Acid Sequence
  • Animals
  • Binding, Competitive
  • Cells, Cultured
  • Down-Regulation / drug effects
  • Intercellular Signaling Peptides and Proteins*
  • Kinetics
  • Melanophores / metabolism*
  • Mice
  • Molecular Sequence Data
  • Peptide Fragments / pharmacology
  • Pigmentation / drug effects
  • Proteins / metabolism*
  • Receptors, Corticotropin / antagonists & inhibitors
  • Receptors, Corticotropin / metabolism*
  • Receptors, Melanocortin
  • Recombinant Proteins / metabolism
  • Sequence Alignment
  • Signal Transduction
  • Skin / metabolism
  • Xenopus / metabolism*
  • alpha-MSH / antagonists & inhibitors

Substances

  • Agouti Signaling Protein
  • Agouti-Related Protein
  • Agrp protein, mouse
  • Intercellular Signaling Peptides and Proteins
  • Peptide Fragments
  • Proteins
  • Receptors, Corticotropin
  • Receptors, Melanocortin
  • Recombinant Proteins
  • a protein, mouse
  • alpha-MSH