The Helix Ring Peptide U11 from the Venom of the Ant, Tetramorium bicarinatum, Acts as a Putative Pore-Forming Toxin

Abstract

An insect neuroactive helix ring peptide called U11-MYRTX-Tb1a (abbreviated as U11) from the venom of the ant, Tetramorium bicarinatum. U11 is a 34-amino-acid peptide that is claimed to be one of the most paralytic peptides ever reported from ant venoms acting against blowflies and honeybees. The peptide features a compact triangular ring helix structure stabilized by a single disulfide bond, which is a unique three-dimensional scaffold among animal venoms. Pharmacological assays using Drosophila S2 cells have demonstrated that U11 is not cytotoxic but instead suggest that it may modulate potassium channels via the presence of a functional dyad. In our work described here, we have tested this hypothesis by investigating the action of synthetically made U11 on a wide array of voltage-gated K and Na channels since it is well known that these channels play a crucial role in the phenomenon of paralysis. Using the Xenopus laevis oocyte heterologous expression system and voltage clamp, our results have not shown any modulatory effect of 1 μM U11 on the activity of Kv1.1, Kv1.3, Kv1.4, Kv1.5, Shaker IR, Kv4.2, Kv7.1, Kv10.1, Kv11.1 and KQT1, nor on DmNav and BgNav. Instead, 10 μM U11 caused a quick and irreversible cytolytic effect, identical to the cytotoxic effect caused by Apis mellifera venom, which indicates that U11 can act as a pore-forming peptide. Interestingly, the paralytic dose (PD50) on blowflies and honeybees corresponds with the concentration at which U11 displays clear pore-forming activity. In conclusion, our results indicate that the insecticidal and paralytic effects caused by U11 may be explained by the putative pore formation of the peptide.