Synthesis, Solution Structure, and Phylum Selectivity of a Spider δ-Toxin That Slows Inactivation of Specific Voltage-gated Sodium Channel Subtypes.

Magi 4, now renamed δ-hexatoxin-Mg1a, is a 43-residue neurotoxic peptide from the venom of the hexathelid Japanese funnel-web spider (Macrothele gigas) with homology to δ-hexatoxins from Australian funnel-web spiders. It binds with high affinity to receptor site 3 on insect voltage-gated sodium (Nay) channels but, unlike δ-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 5-hexatoxin-Mgla 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, δ-hexatoxin-Mg1a induces tonic repetitive firing of nerve impulses in insect neurons accompanied by plateau potentials. In addition, we have chemically synthesized and folded δ-hexatoxin-Mg1a, ascertained the bonding pattern of the four disuffides, 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 α-toxins and δ-hexatoxins are distributed in a topologically similar manner in δ-hexatoxin-Mg1a. However, subtle differences in the toxin surfaces are important for the novel selectivity of δ-hexatoxin-Mg1a for certain mammalian and insect Nav channel subtypes. As such, δ-hexatoxin-Mg1a provides us with a specific tool with which to study channel structure and function and determinants for phylumand tissue-specific activity. [ABSTRACT FROM AUTHOR]