alpha -Conotoxins ImI and ImII Target Distinct Regions of the Human alpha 7 Nicotinic Acetylcholine Receptor and Distinguish Human Nicotinic Receptor Subtypes

The Conus peptides alpha -conotoxin ImI ( alpha -ImI) and ImII ( alpha -ImII) differ by only three of 11 residues in their primary sequences and yet are shown to inhibit the human alpha 7 nicotinic acetylcholine receptor (nAChR) by targeting different sites. Mutations at both faces of the classical ligand binding site of the alpha 7 nAChR strongly affect antagonism by alpha -ImI but not alpha -ImII. The effects of the mutations on alpha -ImI binding and functional antagonism are explained by computational docking of the NMR structure of alpha -ImI to a homology model of the ligand binding domain of the alpha 7 nAChR. A distinct binding site for alpha -ImII is further demonstrated by its weakened antagonism for a chimeric receptor in which the membrane-spanning domains and intervening linkers of the alpha 7 nAChR are replaced with the corresponding sequence from the serotonin type-3 receptor (5HT sub(3)). The two toxins also discriminate between different subtypes of human nicotinic receptors; alpha -ImII most strongly blocks the human alpha 7 and alpha 1 beta 1 delta epsilon receptor subtypes, while alpha -ImI most potently blocks the human alpha 3 beta 2 subtype. Collectively, the data show that while alpha -ImI targets the classical competitive ligand binding site in a subtype selective manner, alpha -ImII is a probe of a novel inhibitory site in homomeric alpha 7 nAChRs.