α-RgIA, a novel conotoxin that blocks the α9α10 nAChR:: Structure and identification of key receptor-binding residues

alpha-Conotoxins are small disulfide-constrained peptides from cone snails that act as antagonists at specific subtypes of nicotinic acetylcholine receptors (nAChRs). The 13-residue peptide alpha-conotoxin RgIA (alpha-RgIA) is a member of the alpha-4,3 family of alpha-conotoxins and selectively blocks the alpha 9 alpha 10 nAChR subtype, in contrast to another well-characterized member of this family, alpha-Conotoxin ImI (alpha-ImI), which is a potent inhibitor of the alpha 7 and alpha 3 beta 2 nAChR subtypes. In this study, we have altered side chains in both the four-residue and the three-residue loops of alpha-RgIA, and have modified its C-terminus. The effects of these changes on activity against alpha 9 alpha 10 and alpha 7 nAChRs were measured; the solution structures of alpha-RgIA and its Y10W, D5E, and P6V analogues were determined from NMR data; and resonance assignments were made for alpha-RgIA [R9A]. The structures for alpha-RglA and its three analogues were well defined, except at the chain termini. Comparison of these structures with reported structures of alpha-ImI reveals a common two-loop backbone architecture within the alpha-4,3 family, but with variations in side-chain solvent accessibility and orientation. Asp5, Pro6, and Arg7 in loop 1 are critical for blockade of both the alpha 9 alpha 10 and the alpha 7 subtypes. In loop 2, alpha-RgIA [Y10W] had activity near that of wild-type alpha-RgIA, with high potency for 000 and low potency for alpha 7, and had a structure similar to that of wild type. By contrast, Arg9 in loop 2 is critical for specific binding to the alpha 9 alpha 10 subtype, probably because it is larger and more solvent accessible than Ala9 in alpha-IMI. Our findings contribute to a better understanding of the molecular basis for antagonism of the alpha 9 alpha 10 nAChR subtype, which is a target for the development of analgesics for the treatment of chronic neuropathic pain. (C) 2008 Elsevier Ltd. All rights reserved.