No abstract

The agouti-related protein (AGRP) is an endogenous antagonist of the melanocortin receptors MC3R and MC4R found in the hypothalamus and exhibits potent orexigenic (appetite-stimulating) activity. The cysteine-rich C-terminal domain of this protein, corresponding to AGRP(87-132), contains five disulfide bonds and exhibits receptor binding affinity and antagonism equivalent to that of the full-length protein. The three-dimensional structure of this domain has been determined by 1 H NMR at 800 MHz. The first 34 residues of AGRP(87-132) are well-ordered and contain a three-stranded antiparallel sheet, where the last two strands form a hairpin. The relative spatial positioning of the disulfide cross-links demonstrates that the ordered region of AGRP(87-132) adopts the inhibitor cystine knot (ICK) fold previously identified for numerous invertebrate toxins. Interestingly, this may be the first example of a mammalian protein assigned to the ICK superfamily. The hairpin's turn region presents a triplet of residues (Arg-Phe-Phe) known to be essential for melanocortin receptor binding. The structure also suggests that AGRP possesses an additional melanocortin-receptor contact region within a loop formed by the first 16 residues of its C-terminal domain. This specific region shows little sequence homology to the corresponding region of the agouti protein, which is an MC1R antagonist involved in pigmentation. Consideration of these sequence differences, along with recent experiments on mutant and chimeric melanocortin receptors, allows us to postulate that this loop in the first 16 residues of its C-terminal domain confers AGRP's distinct selectivity for MC3R and MC4R.Obesity and associated disorders such as diabetes are now at epidemic levels in the United States and other developed countries (1, 2). An understanding at the molecular level of the normal processes governing energy homeostasis and weight regulation is therefore essential for developing a comprehensive understanding of these disorders and producing effective pharmaceutical treatments. Recent studies have demonstrated the key role played by brain melanocortin receptors (MCRs) in the regulation of energy homeostasis (3-6). Although MCRs are found in various tissues, these studies indicate that the specific receptors MC3R and MC4R are directly implicated in energy balance. All MCRs are G-protein-coupled receptors (GPCRs) that up-regulate the production of cAMP in the presence of small endogenous peptide agonists (7). These agonists are derived in vivo from the single pro-opiomelanocortin peptide (POMC) that is cleaved post-translationally to form the melanocortinstimulating hormones (MSHs) and the adrenocorticotropin hormone (ACTH). MCR function is also modulated by potent endogenous antagonists known as the agouti protein and the agouti-related protein (AGRP) that indeed may exert even greater control over MCR signaling than the peptide agonists (8-11). Relative to the small peptide agonists, these antagonists are larger in size and possess a five-dis...