Inhibits tetrodotoxin-sensitive voltage-gated sodium channels (Nav) by binding to site 3. Slows the inactivation, and causes a prolongation of action potential duration resulting in repetitive firing in autonomic and motor nerve fibers. Does not depolarize the resting potential. Does not affect tetrodotoxin-resistant sodium channels. This lethal neurotoxin is active on both insect and mammalian voltage-gated sodium channels. Pan-neuronal expression in Drosophila is lethal but flies engineered to express the toxin only in pacemaker neurons have profound defects in circadian rhythm but a normal lifespan. Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. LD(50) is 0.06 ug/kg by intracerebroventricular injection into mice. LD(50) is 0.2 mg/kg by subcutaneous injection into mice. LD(50) is 770 pmol/g when injected into crickets. PD(50) is 200 +-36 pmol/g when injected into crickets. PD(50) is 54 +- 14 pmol/g when injected into blowfly larvae. Is the sole lethal toxin in male and female A.versutus venom. Belongs to the neurotoxin 06 (delta-actx) family.