Irditoxin
Irditoxin is a three-finger toxin protein found in the venom of the brown [tree snake] and likely in other members of the genus Boiga. It is a heterodimer composed of two distinct protein chains, each of the three-finger protein fold, linked by an intermolecular disulfide bond. This structure is unusual for 3FTx proteins, which are most commonly monomeric.
Structure
Three-finger toxin proteins canonically consist of approximately 60-80 amino acid residues that assume a structure with three "finger"-like beta strand-containing loops projecting from a core stabilized by four intramolecular disulfide bonds. Irditoxin is a covalent heterodimer in which two subunits are linked by an intermolecular disulfide bond. Each subunit is of the three-finger toxin protein superfamily and is most closely related to the "non-conventional" 3FTx subclass, characterized by the presence of an additional disulfide bond in the first of the canonical three "finger" loops. Each subunit thus contains 11 cysteine residues: the eight canonical residues that form the core disulfide bonds, the two in the first loop forming the non-conventional disulfide, and the one that forms the dimeric linkage. Irditoxin subunits A and B are 75 and 77 amino acid residues long, respectively, and each possess a seven-residue extension with a pyroglutamic acid post-translational modification at the N-terminus.Irditoxin's structure is highly unusual within the 3FTx superfamily. Most 3FTx proteins are monomers. The best-studied exception is kappa-bungarotoxin, a non-covalent homodimer with a very different protein-protein interaction surface; the recently described alpha-cobratoxin also forms both covalent homodimers and low-abundance covalent heterodimers with other 3FTx proteins found in monocled cobra venom. It is as yet unclear how irditoxin's two subunits contribute to its biological activities.