Isolation and Structural and Pharmacological Characterization of α-Elapitoxin-Dpp2d, an Amidated Three Finger Toxin from Black Mamba Venom

We isolated a novel, atypical long-chain three-finger toxin (TFT), alpha-elapitoxin-Dpp2d (alpha-EPTX-Dpp2d), from black mamba (Dendroaspis polylepis polylepis) venom. Proteolytic digestion with trypsin and V8 protease, together with MS/MS de novo sequencing, indicated that the mature toxin has an amidated C-terminal arginine, a posttranslational modification rarely observed for snake TFTs. alpha-EPTX-Dpp2d was found to potently inhibit alpha 7 neuronal nicotinic acetylcholine receptors (nAChR; IC50, 58 +/- 24 nM) and muscle-type nAChR (IC50, 114 +/- 37 nM) but did not affect alpha 3 beta 2 and alpha 3 beta 4 nAChR isoforms at 1 mu M concentrations. Competitive radioligand binding assays demonstrated that alpha-EPTX-Dpp2d competes with epibatidine binding to the Lymnea stagnalis acetylcholine-binding protein (Ls-AChBP; IC50, 4.9 +/- 2.3 nM). The activity profile and binding data are reminiscent of classical long-chain TFTs with a free carboxyl termini, suggesting that amidation does not significantly affect toxin selectivity. The crystal structure of alpha-EPTX-Dpp2d was determined at 1.7 angstrom resolution and displayed a dimeric toxin assembly with each monomer positioned in an antiparallel orientation. The dimeric structure is stabilized by extensive intermolecular hydrogen bonds and electrostatic interactions, which raised the possibility that the toxin may exist as a noncovalent homodimer in solution. However, chemical cross-linking and size-exclusion chromatography coupled with multiangle laser light scattering (MALLS) data indicated that the toxin is predominantly monomeric under physiological conditions. Because of its high potency and selectivity, we expect this toxin to be a valuable pharmacological tool for studying the structure and function of nAChRs.