RT Journal Article SR Electronic T1 High-speed AFM reveals accelerated binding of agitoxin-2 to a K+ channel by induced fit JF Science Advances JO Sci Adv FD American Association for the Advancement of Science SP eaax0495 DO 10.1126/sciadv.aax0495 VO 5 IS 7 A1 Sumino, A. A1 Sumikama, T. A1 Uchihashi, T. A1 Oiki, S. YR 2019 UL http://advances.sciencemag.org/content/5/7/eaax0495.abstract AB Agitoxin-2 (AgTx2) from scorpion venom is a potent blocker of K+ channels. The docking model has been elucidated, but it remains unclear whether binding dynamics are described by a two-state model (AgTx2-bound and AgTx2-unbound) or a more complicated mechanism, such as induced fit or conformational selection. Here, we observed the binding dynamics of AgTx2 to the KcsA channel using high-speed atomic force microscopy. From images of repeated binding and dissociation of AgTx2 to the channel, single-molecule kinetic analyses revealed that the affinity of the channel for AgTx2 increased during persistent binding and decreased during persistent dissociation. We propose a four-state model, including high- and low-affinity states of the channel, with relevant rate constants. An induced-fit pathway was dominant and accelerated binding by 400 times. This is the first analytical imaging of scorpion toxin binding in real time, which is applicable to various biological dynamics including channel ligands, DNA-modifier proteins, and antigen-antibody complexes.