ProTx-I

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 (IC(50)’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, IC(50) = 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 K(V) 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.

The tarantula venom peptides ProTx-I and ProTx-II inhibit voltage-gated sodium channels by shifting their voltage dependence of activation to a more positive potential, thus acting by a mechanism similar to that of potassium channel gating modifiers such as hanatoxin and VSTX1. ProTx-I and ProTx-II inhibit all sodium channel (Nav1) subtypes tested with similar potency and represent the first potent peptidyl inhibitors of TTX-resistant sodium channels. Like gating modifiers of potassium channels, ProTx-I and ProTx-II conform to the inhibitory cystine knot motif, and ProTx-II was demonstrated to bind to sodium channels in the closed state. Both toxins have been synthesized chemically, and ProTx-II, produced by recombinant means, has been used to map the interaction surface of the peptide with the Nav1.5 channel. In comparison, beta-scorpion toxins activate sodium channels by shifting the voltage dependence of activation to more negative potentials, and together these peptides represent valuable tools for exploring the gating mechanism of sodium channels.

ProTx-I peptide, a venom toxin of the tarantula Thrixopelma pruriens, has been reported to interact with voltage-gated ion channels. ProTx-I reduced Ba(2+) currents through recombinant human T-type voltage-gated Ca(2+) channels, Ca(v)3.1 (hCa(v)3.1), with roughly 160-fold more potency than through hCa(v)3.2 channels. Chimeric channel proteins (hCa(v)3.1/S3S4 and hCa(v)3.2/S3S4) were produced by exchanging fourteen amino acids in the hCa(v)3.1 domain IV S3-S4 linker region and the corresponding region of hCa(v)3.2 between each other. The ProTx-I sensitivity was markedly reduced in the hCa(v)3.1/S3S4 chimera as compared to the original hCa(v)3.1 channel, while the hCa(v)3.2/S3S4 chimera exhibited greater ProTx-I sensitivity than the original hCa(v)3.2 channel. These results suggest that the domain IV S3-S4 linker in the hCa(v)3.1 channel may contain residues involved in the interaction of ProTx-I with T-type Ca(2+) channels.