Dendrotoxin-like effects of noxiustoxin

On the convergent evolution of animal toxins. Conservation of a diad of functional residues in potassium channel-blocking toxins with unrelated structures

BgK is a K+ channel-blocking toxin from the sea anemone Bunodosoma granulifera. It is a 37-residue protein that adopts a novel fold, as determined by NMR and modeling. An alanine-scanning-based analysis revealed the functional importance of five residues, which include a critical lysine and an aromatic residue separated by 6.6 +/- 1.0 A. The same diad is found in the three known homologous toxins from sea anemones. More strikingly, a similar functional diad is present in all K+ channel-blocking toxins from scorpions, although these toxins adopt a distinct scaffold. Moreover, the functional diads of potassium channel-blocking toxins from sea anemone and scorpions superimpose in the three-dimensional structures. Therefore, toxins that have unrelated structures but similar functions possess conserved key functional residues, organized in an identical topology, suggesting a convergent functional evolution for these small proteins.

Modulation of acetylcholine release at mouse neuromuscular junctions by interaction of three homologous scorpion toxins with K+ channels

1. The effects of three scorpion toxins, charybdotoxin (CTX), iberiotoxin (IbTX), and noxiustoxin (NTX) have been studied on acetylcholine release and on K+ channels by means of twitch tension and electrophysiological recording techniques using isolated skeletal muscle preparations and by a radioligand binding assay using 125I-labelled dendrotoxin I (DpI) and rat brain synaptosomal membranes. 2. On chick biventer cervicis preparations, CTX and IbTX (125 nM) augmented the twitch responses to indirect muscle stimulation. Further, the increase (about 70-80% of control twitch height) was fast in onset, reaching a maximum within 25-30 min. NTX at 125 nM produced a slower augmentation of the twitch responses to indirect muscle stimulation, with the maximum response being seen after 40-50 min. 3. On mouse triangularis sterni preparations, CTX (300 nM after 35-40 min) and IbTX (100 nM after 15 min) increased quantal content of the evoked endplate potentials (e.p.p.) by about two fold. However, NTX (300 nM) caused only a small increase in e.p.p. amplitude, which was followed by repetitive e.p.ps in response to single shock nerve stimulation after 40-50 min. 4. Extracellular recording of nerve terminal current waveforms in triangularis sterni preparations revealed that CTX and IbTX (3-100 nM), but not NTX (100 nM), blocked the Ca(2+)-activated K+ current, IK-Ca. However, there was no major change in the portion of the nerve terminal waveform associated with voltage-dependent K+ currents, IKv. 5. In the radioligand binding assay, NTX potently displaced labelled [125I]-DpI, whereas CTX produced only partial displacement. However, IbTX did not displace [125I]-DpI from its binding sites on rat brain synaptosomal membranes.6. We conclude that these three structurally homologous scorpion toxins act on different K+ channels and that this leads to different patterns of facilitation of acetylcholine release. IbTX acts selectively on high conductance Ca2+-activated K+ channels, leading to an increase in the amplitude of e.p.ps without any other changes. NTX acts on voltage-dependent K+ channels that are sensitive to dendrotoxin and causes repetitive e.p.ps. CTX shares amino acid residues that exist in the structures of IbTX and NTX;CTX acts on both Ca2+- and voltage-dependent K+ channels.

Neuromuscular effects of some potassium channel blocking toxins from the venom of the scorpion Leiurus quinquestriatus hebreus

The scorpion venom Leiurus quinquestriatus hebreus was fractionated by chromatography in order to isolate toxins that affected binding of radiolabelled dendrotoxin to K+ channel proteins on synaptosomal membranes and that facilitated acetylcholine release in chick biventer cervicis nerve-muscle preparations. In addition to the previously characterized charybdotoxin, three toxins were isolated: 14-2, 15-1 and 18-2. Toxin 14-2 has a blocked N-terminus and because of low quantities, it has not been sequenced; 15-1 is a newly sequenced toxin of 36 residues with some overall homology to charybdotoxin and noxiustoxin; 18-2 is identical to charybdotoxin-2. The apparent Ki against dendrotoxin binding were: charybdotoxin, 3.8 nM; 14-2, 150 nM; 15-1, 50 nM; and 18-2, 0.25 nM. Toxin 14-2 (75 nM-1.5 microM) had a presynaptic facilitatory effect on neuromuscular preparations. Toxin 15-1 augmented responses to direct muscle stimulation, probably because it blocked Ca(2+)-activated K+ currents in muscle fibres. Toxin 18-2 (charybdotoxin-2) had a potent presynaptic facilitatory action, with less effect on direct muscle stimulation. This contrasts with the relatively weak neuromuscular effects of the highly homologous charybdotoxin. On a Ca(2+)-activated K+ current in mouse motor nerve endings, charybdotoxin and toxin 18-2 produced maximal block at around 100 nM, whereas 15-1 was inactive at 300 nM. Charybdotoxin can increase quantal content, but this is more likely to result from block of voltage-dependent K+ channels than Ca(2+)-activated channels: the increase in transmitter release occurred in conditions in which little IKCa would be present; higher concentration of charybdotoxin and longer exposure times were required to increase transmitter release than those needed to block IKCa, and the facilitatory effects of charybdotoxin and toxin 18-2 correlated more with their effects on dendrotoxin binding than on block of IKCa.

Tityustoxin K alpha blocks voltage-gated noninactivating K+ channels and unblocks inactivating K+ channels blocked by alpha-dendrotoxin in synaptosomes

Two nonhomologous polypeptide toxins, tityustoxin K alpha (TsTX-K alpha) and tityustoxin K beta (TsTX-K beta), purified from the venom of the Brazilian scorpion Tityus serrulatus, selectively block voltage-gated noninactivating K+ channels in synaptosomes (IC50 values of 8 nM and 30 nM, respectively). In contrast, alpha-dendrotoxin (alpha-DTX) and charybdotoxin (ChTX) block voltage-gated inactivating K+ channels in synaptosomes (IC50 values of 90 nM and 40 nM, respectively). We studied interactions among these toxins in 125I-alpha-DTX binding and 86Rb efflux experiments. Both TsTX-K alpha and ChTX completely displaced specifically bound 125I-alpha-DTX from synaptic membranes, but TsTX-K beta had no effect on bound alpha-DTX. TsTX-K alpha and TsTX-K beta blocked the same noninactivating component of 100 mM K(+)-stimulated 86Rb efflux in synaptosomes. Both alpha-DTX and ChTX blocked the same inactivating component of the K(+)-stimulated 86Rb efflux in synaptosomes. Both the inactivating and the noninactivating components of the 100 mM K(+)-stimulated 86Rb efflux were completely blocked when 200 nM TsTX-K beta and either 600 nM alpha-DTX or 200 nM ChTX were present. The effects of TsTX-K alpha and ChTX on 86Rb efflux were also additive. When TsTX-K alpha was added in the presence of alpha-DTX, however, only the noninactivating component of the K(+)-stimulated efflux was blocked. The inactivating component could then be blocked by ChTX, which is structurally homologous to TsTX-K alpha. We conclude that TsTX-K alpha unblocks the voltage-gated inactivating K+ channels in synaptosomes when they are blocked by alpha-DTX, but not when they are blocked by ChTX. TsTX-K alpha binds to a site on the inactivating K+ channel that does not occlude the pore; its binding apparently prevents alpha-DTX (7054 Da), but not ChTX (4300 Da), from blocking the pore. The effects of TsTX-K alpha on 125I-alpha-DTX binding and 86Rb efflux are mimicked by noxiustoxin, which is homologous to TsTX-K alpha and ChTX.