A novel heterodimeric three-finger neurotoxin, irditoxin, was isolated from venom of the brown treesnake Boiga irregularis (Colubridae). Irditoxin subunit amino acid sequences were determined by Edman degradation and cDNA sequencing. The crystal structure revealed two subunits with a three-finger protein fold, typical for "nonconventional" toxins such as denmotoxin, bucandin, and candoxin. This is the first colubrid three-finger toxin dimer, covalently connected via an interchain disulfide bond. Irditoxin showed taxon-specific lethality toward birds and lizards and was nontoxic toward mice. It produced a potent neuromuscular blockade at the avian neuromuscular junction (IC(50)=10 nM), comparable to alpha-bungarotoxin, but was three orders of magnitude less effective at the mammalian neuromuscular junction. Covalently linked heterodimeric three-finger toxins found in colubrid venoms constitute a new class of venom peptides, which may be a useful source of new neurobiology probes and therapeutic leads.

Yong QC, Lee SW, Foo CS, Neo KL, Chen X, Bian JS. Endogenous hydrogen sulphide mediates the cardioprotection induced by ischemic postconditioning.

Snake venoms are a mixture of pharmacologically active proteins and polypeptides that have led to the development of molecular probes and therapeutic agents. Here, we describe the structural and functional characterization of a novel neurotoxin, haditoxin, from the venom of Ophiophagus hannah (King cobra). Haditoxin exhibited novel pharmacology with antagonism toward muscle (␣␤␥␦) and neuronal (␣ 7 , ␣ 3 ␤ 2 , and ␣ 4 ␤ 2 ) nicotinic acetylcholine receptors (nAChRs) with highest affinity for ␣ 7 -nAChRs. The high resolution (1.5 Å ) crystal structure revealed haditoxin to be a homodimer, like -neurotoxins, which target neuronal ␣ 3 ␤ 2 -and ␣ 4 ␤ 2 -nAChRs. Interestingly however, the monomeric subunits of haditoxin were composed of a three-finger protein fold typical of curaremimetic shortchain ␣-neurotoxins. Biochemical studies confirmed that it existed as a non-covalent dimer species in solution. Its structural similarity to short-chain ␣-neurotoxins and -neurotoxins notwithstanding, haditoxin exhibited unique blockade of ␣ 7 -nAChRs (IC 50 180 nM), which is recognized by neither shortchain ␣-neurotoxins nor -neurotoxins. This is the first report of a dimeric short-chain ␣-neurotoxin interacting with neuronal ␣ 7 -nAChRs as well as the first homodimeric three-finger toxin to interact with muscle nAChRs.Snake venoms are a rich source of pharmacologically active proteins and polypeptides targeting a variety of receptors with high affinity and specificity (1). Because of their high specificity, some of these molecules have contributed significantly (a) to the isolation and characterization of different receptors and their subtypes in the field of molecular pharmacology and (b) as lead compounds in the development of therapeutic agents (2, 3). For example, the discovery of ␣-bungarotoxin, a postsynaptic neurotoxin from the venom of Bungarus multicinctus, led to the identification of the nicotinic acetylcholine receptor (nAChR), 3 the first isolated receptor protein (4) as well as the first one to be characterized electrophysiologically (5) and biochemically (6, 7). Subsequently, it was also used to characterize several other nAChRs (8 -10).Snake venom proteins can be broadly classified as enzymatic and non-enzymatic proteins. Three-finger toxins (3FTxs) are the largest group of non-enzymatic snake venom proteins (1, 11). They are most commonly found in the venoms of elapid and hydrophiid snakes. Recently, our laboratory has also demonstrated the presence of 3FTxs from colubrid venoms (12, 13), and 3FTx transcripts have been found in the venom gland transcriptome of viperid snakes (14, 15). The proteins in this family of toxins share a common structural scaffold of three ␤-sheeted loops emerging from a central core (11,16). Despite the overall similarity in structure, these proteins have diverse functional properties. Members of this family include neurotoxins targeting the cholinergic system (7, 11, 16), cytotoxins/cardiotoxins interacting with the cell membranes (17), calciseptine and related toxins that block th...

Background and Purpose Animal toxins have contributed significantly to our understanding of the neurobiology of receptors and ion channels. We studied the venom of the coral snake Micrurus fulvius fulvius and identified and characterized the structure and pharmacology of a new homodimeric neurotoxin, fulditoxin, that exhibited novel pharmacology at nicotinic ACh receptors (nAChRs). Experimental Approach Fulditoxin was isolated by chromatography, chemically synthesized, its structure determined by X‐ray crystallography, and its pharmacological actions on nAChRs characterized by organ bath assays and two‐electrode voltage clamp electrophysiology. Key Results Fulditoxin's distinct 1.95‐Å quaternary structure revealed two short‐chain three‐finger α‐neurotoxins (α‐3FNTxs) non‐covalently bound by hydrophobic interactions and an ability to bind metal and form tetrameric complexes, not reported previously for three‐finger proteins. Although fulditoxin lacked all conserved amino acids canonically important for inhibiting nAChRs, it produced postsynaptic neuromuscular blockade of chick muscle at nanomolar concentrations, comparable to the prototypical α‐bungarotoxin. This neuromuscular blockade was completely reversible, which is unusual for snake α‐3FNTxs. Fulditoxin, therefore, interacts with nAChRs by utilizing a different pharmacophore. Unlike short‐chain α‐3FNTxs that bind only to muscle nAChRs, fulditoxin utilizes dimerization to expand its pharmacological targets to include human neuronal α4β2, α7, and α3β2 nAChRs which it blocked with IC50 values of 1.8, 7, and 12 μM respectively. Conclusions and Implications Based on its distinct quaternary structure and unusual pharmacology, we named this new class of dimeric Micrurus neurotoxins represented by fulditoxin as Σ‐neurotoxins, which offers greater insight into understanding the interactions between nAChRs and peptide antagonists.

Anticoagulant therapy is used for the prevention and treatment of thromboembolic disorders. Blood coagulation is initiated by the interaction of factor VIIa (FVIIa) with membrane-bound tissue factor (TF) to form the extrinsic tenase complex which activates FX to FXa. Thus, it is an important target for the development of novel anticoagulants. Here, we report the isolation and characterization of a novel anticoagulant ringhalexin from the venom of Hemachatus haemachatus (African Ringhals Cobra). Amino acid sequence of the protein indicates that it belongs to the three-finger toxin family and exhibits 94% identity to an uncharacterized Neurotoxin-like protein NTL2 from Naja atra. Ringhalexin inhibited FX activation by extrinsic tenase complex with an IC50 of 123.8 ± 9.54 nM. It is a mixed-type inhibitor with the kinetic constants, Ki and Ki’ of 84.25 ± 3.53 nM and 152.5 ± 11.32 nM, respectively. Ringhalexin also exhibits a weak, irreversible neurotoxicity on chick biventer cervicis muscle preparations. Subsequently, the three-dimensional structure of ringhalexin was determined at 2.95 Å resolution. This study for the first time reports the structure of an anticoagulant three-finger toxin. Thus, ringhalexin is a potent inhibitor of the FX activation by extrinsic tenase complex and a weak, irreversible neurotoxin.

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