No abstract

No abstract

An important and exciting challenge in the postgenomic era is to understand the functions of newly discovered proteins based on their structures. The main thrust is to find the common structural motifs that contribute to specific functions. Using this premise, here we report the purification, solution NMR, and functional characterization of a novel class of weak potassium channel toxins from the venom of the scorpion Heterometrus fulvipes. These toxins, -hefutoxin1 and -hefutoxin2, exhibit no homology to any known toxins. NMR studies indicate that -hefutoxin1 adopts a unique three-dimensional fold of two parallel helices linked by two disulfide bridges without any ␤؊sheets. Based on the presence of the functional diad (Tyr 5 /Lys 19 ) at a distance (6.0 ؎ 1.0 Å) comparable with other potassium channel toxins, we hypothesized its function as a potassium channel toxin. -Hefutoxin 1 not only blocks the voltage-gated K ؉ -channels, Kv1.3 and Kv1.2, but also slows the activation kinetics of Kv1.3 currents, a novel feature of -hefutoxin 1, unlike other scorpion toxins, which are considered solely pore blockers. Alanine mutants (Y5A, K19A, and Y5A/K19A) failed to block the channels, indicating the importance of the functional diad.

Venoms have evolved over millions of years into potent cocktails of bioactive peptides and proteins. These compounds can be of great value to the pharmaceutical industry for numerous clinical applications. In this study, a novel proteomic - bioinformatic approach was utilised, where chromatography followed by gel electrophoresis was utilised to separate the venom peptides/proteins of Heterometrus longimanus (Asian black scorpion). Purified peptides were analysed by tandem mass spectrometry, de novo sequenced and then homology matched against known peptides in the Swiss-Prot protein database. Numerous potentially biologically active peptide matches were discovered, and a simple scoring system applied to putatively assign functions to the peptides. As a validation of this approach, the functional composition of the experimentally derived proteome is similar to that of other scorpions, and contains a potent mix of toxins, antimicrobials and ionic channel inhibitors.

In contrast to most short and long chain curaremimetic neurotoxins that produce virtually irreversible neuromuscular blockade in isolated nerve-muscle preparations, candoxin, a novel three-finger toxin from the Malayan krait Bungarus candidus, produced postjunctional neuromuscular blockade that was readily and completely reversible. Nanomolar concentrations of candoxin (IC 50 ‫؍‬ ϳ10 nM) also blocked acetylcholineevoked currents in oocyte-expressed rat muscle (␣␤␥␦) nicotinic acetylcholine receptors in a reversible manner. In contrast, it produced a poorly reversible block (IC 50 ‫؍‬ ϳ50 nM) of rat neuronal ␣7 receptors, clearly showing diverse functional profiles for the two nicotinic receptor subsets. Interestingly, candoxin lacks the helix-like segment cyclized by the fifth disulfide bridge at the tip of the middle loop of long chain neurotoxins, reported to be critical for binding to ␣7 receptors. However, its solution NMR structure showed the presence of some functionally invariant residues involved in the interaction of both short and long chain neurotoxins to muscle (␣␤␥␦) and long chain neurotoxins to ␣7 receptors. Candoxin is therefore a novel toxin that shares a common scaffold with long chain ␣-neurotoxins but possibly utilizes additional functional determinants that assist in recognizing neuronal ␣7 receptors.Curaremimetic or ␣-neurotoxins from snake venoms are well known to bind with high affinity and selectivity and in most instances, almost irreversibly to Torpedo and muscle (␣␤␥␦) nicotinic acetylcholine receptors (nAChR), 1 thereby affecting synaptic neurotransmission and producing flaccid paralysis (1, 2). They belong to a family of proteins called "three-finger toxins," which adopt a flat, leaf-like shape formed by three adjacent loops that emerge from a small globular core, which is the location of the four conserved disulfide bridges (3-9). Other members of this family include -bungarotoxins, which recognize neuronal nicotinic receptors (10), muscarinic toxins with selectivity toward distinct types of muscarinic receptors (11), fasciculins that inhibit acetylcholinesterase (12), calciseptins that block the L-type calcium channels (13,14), cardiotoxins (cytotoxins) that exert their toxicity by forming pores in cell membranes (15), and dendroaspins, which are antagonists of various cell adhesion processes (16). Despite their common structural fold and comparable affinity for the Torpedo and muscle (␣␤␥␦) nAChRs, ␣-neurotoxins are classified as short chain neurotoxins (e.g. erabutoxin-b (Laticauda semifasciata)) that have 60 -62 residues and four conserved disulfide bonds and long chain neurotoxins (e.g. ␣-bungarotoxin (Bungarus multicinctus); ␣-cobratoxin (Naja kaouthia)) with 66 -75 residues and five disulfide bonds (3). The additional disulfide bridge in long chain ␣-neurotoxins, as well in the neuronal -bungarotoxin (B. multicinctus) is located in the middle loop (loop II) (3,8,9). This fifth bridge, which cyclizes a helix-like conformation at the tip of loop II, has been reported to be cru...

No abstract

No abstract

5. Our studies confirmed that the rat anococcygeus muscle is an excellent nerve-smooth muscle preparation for investigating the effects of bioactive agents on noradrenergic and nitrergic transmission, as well as the direct agonist actions of these agents on post-synaptic ␣-adrenoceptors and M3 muscarinic cholinoceptors. Although many studies, including our own, have documented that scorpion venoms and toxins mediate their primary effects via a prejunctional mechanism that leads to the marked release of various autonomic neurotransmitters, our studies have shown that there are exceptions to this generally accepted phenomenon. In particular, we have provided firm evidence to show that the venoms from H. longimanus and H. spinifer do not have such a prejunctional site of action but, instead, the venoms mediate their autonomic effects through direct agonist actions on post-junctional muscarinic M3 cholinoceptors and ␣-adrenoceptors.