Non-conventional toxins from Elapid venoms

Non-conventional toxins constitute a poorly characterized class of three-finger toxins isolated exclusively from Elapidae venoms. These toxins are monomers of 62–68 amino acid residues and contain five disulfide bridges. However, unlike α/κ-neurotoxins and κ-neurotoxins which have the fifth disulfide bridge in their middle loop (loop II), the fifth disulfide bridge in non-conventional toxins is located in loop I (N-terminus loop). Overall, non-conventional toxins share ∼28–42% identity with other three-finger toxins including α-neurotoxins, α/κ-neurotoxins and κ-neurotoxins. Recent structural studies have revealed that non-conventional toxins also display the typical three-finger motif. Non-conventional toxins are typically characterized by a lower order of toxicity (LD 50∼5–80 mg/kg) in contrast to prototype α-neurotoxins (LD 50∼0.04–0.3 mg/kg) and hence they are also referred to as ‘weak toxins’. Further, it is generally assumed that non-conventional toxins target muscle (α 2βγδ) receptors with low affinities several orders of magnitude lower than α-neurotoxins and α/κ-neurotoxins. However, it is now known that some non-conventional toxins also antagonize neuronal α7 nicotinic acetylcholine receptors. Hence, non-conventional toxins are not a functionally homogeneous group and other, yet unknown, molecular targets for this class of snake venom toxins may exist. Non-conventional toxins may therefore be a useful source of ligands with novel biological activity targeting the plethora of neuronal nicotinic receptors as well as other physiological processes.