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J Biol Chem, Vol. 274, Issue 22, 15837-15846, May 28, 1999
From the Departments of Pediatrics and Genetics, and the Howard
Hughes Medical Institute, Stanford University School of Medicine,
Stanford, California 94305-5428
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.
Down-regulation of Melanocortin Receptor Signaling Mediated by
the Amino Terminus of Agouti Protein in Xenopus
Melanophores
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.
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