Two tarantula peptides inhibit activation of multiple sodium channels

Two peptides, ProTx-I and ProTx-II, from the venom of the tarantula Thrixopelma pruriens, have been isolated and characterized. These peptides were purified on the basis of their ability to reversibly inhibit the tetrodotoxin-resistant Na channel, Na-V 1.8, and are shown to belong to the inhibitory cystine knot (ICK) family of peptide toxins interacting with voltage-gated ion channels. The family has several hallmarks: cystine bridge connectivity, mechanism of channel inhibition, and promiscuity across channels within and across channel families. The cystine bridge connectivity of ProTx-II is very similar to that of other members of this family, i.e., C-2 to C-16, C-9 to C-21, and C-15 to C-25. These peptides are the first high-affinity ligands for tetrodotoxin-resistant peripheral nerve Na-V channels, but also inhibit other Na-V channels (IC50's < 100 nM). ProTx-I and ProTx-II shift the voltage dependence of activation of Na-V 1.5 to more positive voltages, similar to other gating-modifier ICK family members. ProTx-I also shifts the voltage dependence of activation of Ca-V 3.1 (alpha(1G), T-type, IC50 = 50 nM) without affecting the voltage dependence of inactivation. To enable further structural and functional studies, synthetic ProTx-II was made; it adopts the same structure and has the same functional properties as the native peptide. Synthetic ProTx-I was also made and exhibits the same potency as the native peptide. Synthetic ProTx-I, but not ProTx-II, also inhibits Kv 2.1 channels with 10-fold less potency than its potency on Na-V channels. These peptides represent novel tools for exploring the gating mechanisms of several Na-V and Ca-V channels.