American Society for Biochemistry and Molecular Biology
Journal of Biological Chemistry vol:284 issue:36 pages:24568-24582
Magi 4, now renamed delta-hexatoxin-Mg1a, is a 43-residue neurotoxic peptide from the venom of the hexathelid Japanese funnel-web spider (Macrothele gigas) with homology to delta-hexatoxins from Australian funnel-web spiders. It binds with high affinity to receptor site 3 on insect voltage-gated sodium (Nav) channels but, unlike delta-hexatoxins, does not compete for the related site 3 in rat brain despite being previously shown to be lethal by intracranial injection. To elucidate differences in Nav channel selectivity, we have undertaken the first characterization of a peptide toxin on a broad range of mammalian and insect Nav channel subtypes showing that delta-hexatoxin-Mg1a selectively slows channel inactivation of mammalian Nav1.1, Nav1.3, and Nav1.6 but more importantly shows higher affinity for insect Nav1 (para) channels. Consequently, delta-hexatoxin-Mg1a induces tonic repetitive firing of nerve impulses in insect neurons accompanied by plateau potentials. In addition, we have chemically synthesized and folded delta-hexatoxin-Mg1a, ascertained the bonding pattern of the four disulfides, and determined its three-dimensional solution structure using NMR spectroscopy. Despite modest sequence homology, we show that key residues important for the activity of scorpion alpha-toxins and delta-hexatoxins are distributed in a topologically similar manner in delta-hexatoxin-Mg1a. However, subtle differences in the toxin surfaces are important for the novel selectivity of delta-hexatoxin-Mg1a for certain mammalian and insect Nav channel subtypes. As such, delta-hexatoxin-Mg1a provides us with a specific tool with which to study channel structure and function and determinants for phylum- and tissue-specific activity.