Three-finger toxin

Three-finger toxins (abbreviated 3FTx) are a protein superfamily of small toxin proteins found in the venom of snakes. Three-finger toxins are in turn members of a larger superfamily of three-finger protein domains which includes non-toxic proteins that share a similar protein fold. The group is named for its common structure consisting of three beta strand loops connected to a central core containing four conserved disulfide bonds. The 3FP protein domain has no enzymatic activity and is typically between 60 and 74 amino acid residues long. Despite their conserved structure, three-finger toxin proteins have a wide range of pharmacological effects. Most members of the family are neurotoxins that act on cholinergic intercellular signaling; the alpha-neurotoxin family interacts with muscle nicotinic acetylcholine receptors (nAChRs), the kappa-bungarotoxin family with neuronal nAChRs, and muscarinic toxins with muscarinic acetylcholine receptors (mAChRs).

Structure The three-finger toxin superfamily is defined by a common tertiary structure consisting of three beta strand-containing loops (designated loops I, II, and III) projecting from a small hydrophobic core containing four conserved disulfide bonds. This structure is thought to resemble a hand with three fingers, giving rise to the name. The proteins are typically 60-74 amino acid residues long, though some have additional N- or C-terminal extensions. An additional disulfide bond may be present in either loop I or loop II. The superfamily can be broadly divided into three classes:

short-chain toxins have under 66 residues and four core disulfide bonds. long-chain toxins have at least 66 residues, a disulfide bond in loop II, and possibly a C-terminal extension. non-conventional toxins have a disulfide bond in loop I and possibly terminal extensions.

Oligomerization

Most 3FTx proteins are monomers. However, some 3FTx subgroups form functional non-covalent homodimers. The kappa-bungarotoxin group is the best characterized dimeric 3FTx, and interacts through an antiparallel dimer interface composed of the outer strand of loop III. Haditoxin is another example of a dimeric 3FTx; it is a member of the short-chain group and has a similar dimer interface but distinct pharmacology compared to the long-chain-like kappa-bungarotoxins. A few examples of covalently linked dimers have also been described. These proteins, from the non-conventional group, are linked through intermolecular disulfide bonds. Some, such as irditoxin, are heterodimers linked by cysteines in loops I and II. Others, such as α-cobratoxin, can form both homodimers and heterodimers which have distinct pharmacological activities in vitro, though their functional significance is unclear due to their very low concentration in venom.

Function Despite their conserved shared structure, 3FTx proteins have a wide range of pharmacological effects mediating their toxicity. Many members of the family are neurotoxins that bind to receptor proteins in the cell membrane, particularly nicotinic acetylcholine receptors. Others, including the second-largest 3FTx subgroup, are cardiotoxins.

Cellular targets

Nicotinic acetylcholine receptors Many of the most well-characterized 3FTx proteins exert their toxic effects through binding to nicotinic acetylcholine receptors (nAChRs), a family of ligand-gated ion channels. 3FTx binding interferes with cholinergic intercellular signaling particularly at neuromuscular junctions and causes paralysis. The alpha-neurotoxin family is a group of 3FTx proteins that bind muscle nAChRs, preventing the binding of the neurotransmitter acetylcholine. Alpha-bungarotoxin, the alpha-neurotoxin from the many-banded krait (Bungarus multicinctus), has a long history of use in molecular biology research; it was through the study of this toxin that nAChRs were isolated and characterized, which facilitated the study of the subunit composition of tissue-specific nAChRs and the detailed pharmacological understanding of the neuromuscular junction. In general, short-chain 3FTx members of this group bind muscle nAChRs only, and long-chain members bind both muscle and neuronal receptors. This 3FTx group is sometimes referred to as the "curaremimetic" toxins due to the similarity of their effects with the plant alkaloid curare. Other groups of 3FTx proteins also bind to different nAChR subtypes; for example, kappa-neurotoxins, which are long-chain dimers, bind neuronal nAChRs, and haditoxin, which is a short-chain dimer, binds both muscle and neuronal subtypes. Non-conventional 3FTx proteins also often bind nAChRs; these were thought to be weaker toxins when first discovered, but the class has been found to possess a range of binding affinities. Recently, a new class of nAChR antagonist 3FTx proteins called omega-neurotoxins has been described.

Muscarinic acetylcholine receptors A smaller class of 3FTx proteins binds instead to muscarinic acetylcholine receptors, a family of G-protein-coupled receptors. Muscarinic toxins can be either receptor agonists or receptor antagonists, and in some cases the same 3FTx protein is an agonist at one receptor subtype and an antagonist at another. Muscarinic toxins are generally of the short-chain type.

Acetylcholinesterase A class of 3FTx proteins called fasciculins bind the enzyme acetylcholinesterase and inhibit its activity by blocking access of acetylcholine to the enzyme's active site, thereby preventing acetylcholine breakdown. This class derives its name from its clinical effect, causing muscle fasciculations.

Cardiac targets The second-largest class of 3FTx proteins causes toxicity in cardiac myocytes and can cause increased heart rate and eventually cardiac arrest. These cardiotoxins also often have generalized cytotoxic effects and are sometimes known as cytolysins. The protein targets in myocytes are not generally known for this class, though some members may cause physical damage to the cell by establishing pores in the cell membrane. Another class, called the beta-cardiotoxins, causes decreased heart rate and are thought to function as beta blockers, antagonists for the beta-1 and beta-2 adrenergic receptors.

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