MELANOCORTIN RECEPTOR-BINDING DETERMINANTS IN THE AGOUTI PROTEIN

The agouti protein plays an important role in the development of diabe tes and obesity in rodents and has been shown to be a potent antagonis t of melanocortin receptors, For this reason alanine-scanning mutagene sis was performed on the agouti protein carboxyl terminus to locate re sidues important for melanocortin receptor binding inhibition. When ag outi residues Arg116 and Phe118 are changed to alanine, very large dec reases in agouti affinity for melanocortin receptor 1, 3, and 4 result , Mutation of Phe117 to alanine causes a similar increase in agouti K- I app at melanocortin receptor 4. Substitution of agouti residue Asp10 8 with alanine results in large increases in K-I app for all three mel anocortin receptors examined. All of these residues are conserved in t he agouti-related transcript, ART, whose expression is up-regulated in animal models of obesity. The three-dimensional structure of the agou ti carboxyl terminus was modeled, and residues which decrease receptor binding by a factor of greater than or equal to 15 when mutated to al anine localize to one side of the structure, These agouti variants wit h altered receptor selectivity may be useful in determining the role o f melanocortin receptors in diabetes and obesity.