Mechanisms of Inward-Rectifier K+ Channel Inhibition by Tertiapin-Q

Abstract

Tertiapin-Q (TPN_Q) is a derivative of honey bee toxin tertiapin (TPN) whose methionine residue is replaced with a glutamine residue. TPN_Q inhibits the ROMK1 and GIRK1/4 inward-rectifier K⁺ channels with affinities very similar to TPN. However, unlike native TPN, TPN_Q is nonoxidizable by air. The stability of TPN_Q allows us to investigate how it interacts with the targeted channels. We found that the interaction between TPN_Q and the ROMK1 channel is a bimolecular reaction, i.e., one TPN_Q molecule binds to one channel. The interaction surface in TPN_Q is primarily formed by its α helix rather than the β sheets with which scorpion toxins form their interaction surface. The mutagenesis studies on both the channel and TPN_Q together strongly suggest that to block the K⁺ pore TPN_Q plugs its α helix into the vestibule of the K⁺ pore, while leaving the extended structural portion sticking out of the vestibule into the extracellular media.