Whicher, J. | Search bioRxiv

bioRxiv2025

Mechanism of SK2 channel gating and its modulation by the bee toxin apamin and small molecules

Small-conductance calcium-activated potassium channel 2 (SK2) serves a variety of biological functions by coupling intracellular calcium dynamics with membrane potential. SK2 modulators are in development for the treatment of neurological and cardiovascular diseases, though the mechanisms of pharmacological modulation remain incompletely understood. We determined structures of an SK2-4 chimeric channel in Ca2+-bound and Ca2+-free conformations and in complex with the bee toxin apamin, a small molecule inhibitor, and a small molecule activator. The structures revealed that the S3-S4 linker forms a hydrophobic constriction at the extracellular opening of the pore. Apamin binds to this extracellular constriction and blocks the exit of potassium ions. Furthermore, we identified a structurally related SK2 inhibitor and activator that bind to the transmembrane domains. The compounds exert opposing effects on gating by differentially modulating the conformation of the S6 helices. These results provide important mechanistic insights to facilitate the development of targeted SK2 channel therapeutics.

pharmacology and toxicology↗

bioRxiv2024

Scorpion α-toxin LqhαIT specifically interacts with a glycan at the pore domain of voltage-gated sodium channels

Voltage-gated sodium (Nav) channels sense membrane potential and drive cellular electrical activity. Numerous protein toxins have been identified that modulate Nav gating, and structures of Nav channels in complex with these toxins helped elucidate the molecular mechanisms of voltage-dependent channel gating. The deathstalker scorpion -toxin LqhIT exerts a strong action potential prolonging effect on Nav channels. Biochemical studies show that LqhIT features a functionally essential epitope at its C-terminus that is not shared with related scorpion -toxins. To elucidate the mechanism of action of LqhIT, we determined a 3.9 [A] cryo-electron microscopy (cryo-EM) structure of LqhIT in complex with the Nav channel from Periplaneta americana (NavPas). We found that LqhIT binds to voltage sensor domain 4 and traps it in a "S4 down" conformation to stabilize the open state. To promote binding, the functionally essential C-terminal epitope of LqhIT forms an extensive interface with the glycan scaffold linked to Asn330 of NavPas that augments a small protein-protein interface between NavPas and LqhIT. A combination of molecular dynamics simulations, structural comparisons, and prior mutagenesis experiments demonstrate the functional importance of this toxin-glycan interaction. These findings help establish a structural basis for the specificity achieved by scorpion -toxins and provide crucial insights for the development and optimization of new Nav channel modulators.

biophysics↗