Double-knot toxin-K2 (DkTx-K2) is one of the two inhibitory cysteine-knot (ICK) lobes that are the receptor-binding subunits of the full-length DkTx toxin ^(1,2). DkTx, a peptide toxin originally isolated from the venom of the Chinese bird spider Cyriopagopus schmidti (formerly known as Ornithoctonus huwena), selectively and irreversibly activates the transient receptor potential vanilloid 1 (TRPV1) channel by targeting the outer pore domain ⁽¹⁾. The two ICK motifs that comprise DkTx (K1 and K2) are highly homologous yet differ in their potency, affinity, and binding orientation to the TRPV1 channel.

Full-length DkTx partitions into the cellular membrane and binds bivalently to TRPV1, triggering long-lasting channel activation. In contrast, its monovalent single knots partition into the membrane poorly and activate TRPV1 in a rapidly reversible manner ^(3-6). DkTx-K2 is a more effective activator of TRPV1 than K1, since K2 binds to the outer channel as part of a protein-protein interface, while K1 initiates membrane interaction. DkTx-K2 therefore binds TRPV1 with higher affinity than K1 ^(1-3), which makes it a more flexible and specific research tool than native DkTx or capsaicin.

One of the most versatile pain receptors, TRPV1, is expressed in nociceptors and plays important roles in the transduction of noxious stimuli and thermosensation. In addition, TRPV1 is widely expressed in non-neuronal cells and has been shown to play an important role in the immune system. As a receptor for multiple injurious stimuli, TRPV1 has emerged as a new and promising target for developing analgesic and anti-inflammatory drugs ^(7,8).