Chimeras of the agouti-related protein: insights into agonist and antagonist selectivity of melanocortin receptors.

The specific melanocortin receptors, MC3R and MC4R, are directly linked to metabolism and body weight control. These receptors are activated by the peptide hormone alpha-MSH and antagonized by the agouti-related protein (AGRP). Whereas alpha-MSH acts broadly on most members of the MCR family (with the exception of MC2R), AGRP is highly specific for only MC3R and MC4R. AGRP is a complex ligand of approximately 100 amino acids. Within AGRP, MCR recognition and antagonism is localized to a 34 residue, cysteine-rich domain that adopts an inhibitor cystine knot (ICK) fold. An oxidatively folded peptide corresponding to this domain, referred to as mini-AGRP, exhibits full antagonist function and selectivity for MC3R and MC4R. Here we investigate a series of chimera proteins based on the mini-AGRP scaffold. Amino acid sequences derived from peptide agonists are grafted into the mini-AGRP active loop, implicated in receptor recognition, with the goal of producing ICK based agonists specific for MC3R and MC4R. Several constructs indeed exhibited potent agonist activity; however, with all chimeras, receptor selectivity is significantly altered. Pharmacologic data indicate that the chimeras do not interact with MC receptors through native AGRP like contacts. A model to explain the data suggest that there is only partial overlap of the agonist versus antagonist binding surfaces within MC receptors. Moreover, accessibility to the binding pocket is highly receptor specific with MC3R being the least tolerant of ligand alterations.