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 60-74 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). 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. 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.