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α-conotoxin IMI (alpha-conotoxin IMI) is a conopeptide that has been isolated from the venom of the cone snail Conus imperialis. α-conotoxin IMI acts on postsynaptic membranes, and binds onto and inhibits nicotinic acetylcholine receptors (nAChR). α-conotoxin IMI is a selective blocker of α7 homomeric nicotinic acetylcholine receptors (IC50 ~ 200 nM). It has no effect on nAChRs composed of α2/ß2, α3/ß2, α4/ß2, α2/ß4, α3/ß4, or α4/ß4 subunits. α-conotoxins IMI acts independently of voltage value. This toxin is highly active against the neuromuscular receptor in frog.
AA sequence: Gly-Cys2-Cys3-Ser-Asp-Pro-Arg-Cys8-Ala-Trp-Arg-Cys12-NH2
Disulfide bonds: Cys2-Cys8 and Cys3-Cys12
Length (aa): 12
Formula: C52H78N20O15S4
Molecular Weight: 1352.92 Da
Appearance: White lyophilized solid
Solubility: water or saline buffer
CAS number: [156467-85-5]
Source: Synthetic
Purity rate: > 95 %
A novel conotoxin, alpha-conotoxin ImII (alpha-CTx ImII), identified from Conus imperialis venom ducts, was chemically synthesized. A previously characterized C. imperialis conotoxin, alpha-conotoxin ImI (alpha-CTx ImI), is closely related; 9 of 12 amino acids are identical. Both alpha-CTx ImII and alpha-CTx ImI functionally inhibit heterologously expressed rat alpha7 nAChRs with similar IC(50) values. Furthermore, the biological activities of intracranially applied alpha-CTx ImI and alpha-CTx ImII are similar over the same dosage range, and are consistent with alpha7 nAChR inhibition. However, unlike alpha-CTx ImI, alpha-CTx ImII was not able to block the binding of alpha-bungarotoxin to alpha7 nAChRs. alpha-Conotoxin ImI and alpha-bungarotoxin-binding sites have been well characterized as overlapping and located at the cleft between adjacent nAChR subunits. Because alpha-CTx ImI and alpha-CTx ImII share extensive sequence homology, the inability of alpha-CTx ImII to compete with alpha-BgTx is surprising. Furthermore, functional studies in oocytes indicate that there is no overlap between functional binding sites of alpha-CTx ImI and alpha-CTx ImII. Like alpha-CTx ImI, the block by alpha-CTx ImII is voltage-independent. Thus, alpha-CTx ImII represents a probe for a novel antagonist binding site, or microsite, on the alpha7 nAChR.
Ellison, M., et al. (2003) Alpha-conotoxins ImI and ImII. Similar alpha 7 nicotinic receptor antagonists act at different sites, J Biol Chem. PMID: 12384509
Alpha-Conotoxins are small disulfide-constrained peptide toxins which act as antagonists at specific subtypes of nicotinic acetylcholine receptors (nACh receptors). In this study, we analyzed the structures and activities of three mutants of alpha-conotoxin ImI, a 12 amino acid peptide active at alpha7 nACh receptors, in order to gain insight into the primary and tertiary structural requirements of neuronal alpha-conotoxin specificity. NMR solution structures were determined for mutants R11E, R7L, and D5N, resulting in representative ensembles of 20 conformers with average pairwise RMSD values of 0.46, 0.52, and 0.62 A from their mean structures, respectively, for the backbone atoms N, C(alpha), and C’ of residues 2-11. The R11E mutant was found to have activity near that of wild-type ImI, while R7L and D5N demonstrated activities reduced by at least two orders of magnitude. Comparison of the structures reveals a common two-loop architecture, with variations observed in backbone and side-chain dihedral angles as well as surface electrostatic potentials upon mutation. Correlation of these structures and activities with those from previously published studies emphasizes that existing hypotheses regarding the molecular determinants of alpha-conotoxin specificity are not adequate for explaining peptide activity, and suggests that more subtle features, visualized here at the atomic level, are important for receptor binding. These data, in conjunction with reported characterizations of the acetylcholine binding site, support a model of toxin activity in which a single solvent-accessible toxin side-chain anchors the complex, with supporting weak interactions determining both the efficacy and the subtype specificity of the inhibitory activity.
Rogers, J. P., et al. (2000) Structure-activity relationships in a peptidic alpha7 nicotinic acetylcholine receptor antagonist, J Mol Biol. PMID: 11124036
Through a study of cloned nicotinic receptors expressed in Xenopus oocytes, we provide evidence that alpha-conotoxin ImI, a peptide marine snail toxin that induces seizures in rodents, selectively blocks subtypes of nicotinic acetylcholine receptors. alpha-Conotoxin ImI blocks homomeric alpha 7nicotinic receptors with the highest apparent affinity and homomeric alpha 9 receptors with 8-fold lower affinity. This toxin has no effect on receptors composed of alpha 2 beta 2, alpha 3 beta 2, alpha 4 beta 2, alpha 2 beta 4, alpha 3 beta 4, or alpha 4 beta 4 subunit combinations. In contrast toalpha-bungarotoxin, which has high affinity for alpha 7, alpha 9, and alpha 1 beta 1 gamma delta receptors, alpha-conotoxin ImI has low affinity for the muscle nAChR. Related Conus peptides, alpha-conotoxins MI and GI, exhibit a distinct specificity, strictly targeting the muscle subtype receptor but notalpha 7 or alpha 9 receptors. alpha-Conotoxins thus represent selective tools for the study of neuronal nicotinic acetylcholine receptors.
Johnson, D. S., et al. (1995) Alpha-Conotoxin ImI exhibits subtype-specific nicotinic acetylcholine receptor blockade: preferential inhibition of homomeric alpha 7 and alpha 9 receptors, Mol Pharmacol. PMID: 7651351
We report the isolation, characterization, and total synthesis of a small peptide ligand for nicotinic acetylcholine receptors (nAChRs). It is highly active against the neuromuscular receptor in frog but not in mice. In contrast, it induces seizures when injected centrally in mice and rats, suggesting that it may target neuronal nAChRs in mammals. Although such receptors may be important in both normal cognition and the pathophysiology of several neuropsychiatric disorders, there are few ligands to discriminate between the multiple receptor subtypes. The new peptide is a highly divergent alpha-conotoxin from the snail Conus imperialis, which preys on polychaete worms. In this article, the purification, structural analysis, synthesis, and preliminary physiological characterization of alpha-conotoxin ImI (alpha-CTx-ImI) are reported. The sequence of the peptide is: Gly-Cys-Cys-Ser-Asp-Pro-Arg-Cys-Ala-Trp-Arg-Cys-NH2. The peptide shows striking sequence differences from all alpha-conotoxins of fish-hunting Conus, but its disulfide-bridging is similar: [2-8; 3-12]. We suggest that cone venoms may provide an array of ligands with selectivity for various neuronal nAChR subtypes.
McIntosh, J. M., et al. (1994) A nicotinic acetylcholine receptor ligand of unique specificity, alpha-conotoxin ImI, J Biol Chem. PMID: 8206995
