Loops and links: structural insights into the remarkable function of the agouti-related protein.

The agouti-related protein (AGRP) is an endogenous antagonist of the melanocortin receptors MC3R and MC4R found in the hypothalamus and exhibits potent orexigenic activity. The cysteine-rich C-terminal domain of this protein, corresponding to AGRP(87-132), exhibits receptor binding affinity and antagonism equivalent to that of the full-length protein. We recently determined the NMR structure of AGRP(87-132) and demonstrated that a portion of the domain adopts the inhibitor cystine-knot fold. Remarkably, this is the first identification of a mammalian protein with this specific architecture. Further analysis of the structure suggests that melanocortin receptor contacts are made primarily by two loops presented within the cystine knot. (10) To test this hypothesis we designed a 34-residue AGRP analogue corresponding to only the cystine knot. We found that this designed miniprotein folds to a homogeneous product, retains the desired cystine-knot architecture, functions as a potent antagonist, and maintains the melanocortin receptor pharmacological profile of AGRP(87-132). (26) The AGRP-like activity of this molecule supports the hypothesis that indeed the cystine-knot region possesses the melanocortin receptor contacts. Based on these design and structure studies, we propose that the N-terminal loop of AGRP(87-132) makes contact with a receptor exoloop and helps confer AGRP's selectivity for the central MCRs.