Determinants of Apamin and d-Tubocurarine Block in SK Potassium Channels

TLDR

SK channels exhibit distinct pharmacology, with selective block by apamin and d‑tubocurarine that is reproduced in cloned channels. Introducing a tyrosine at the position analogous to the tetraethylammonium‑sensitivity determinant of voltage‑gated K⁺ channels confers equivalent TEP sensitivity on SK channels, indicating similar outer vestibules. Two residues flanking the deep pore determine apamin and d‑tubocurarine sensitivity, linking SK channel pharmacology to outer‑pore structure and suggesting SK1 and SK2 subunits can form heteromeric channels.

Abstract

Small conductance calcium-activated potassium channels show a distinct pharmacology. Some, but not all, are blocked by the peptide toxin apamin, and apamin-sensitive channels are also blocked by <i>d</i>-tubocurarine. Cloned SK channels (small conductance calcium-activated potassium channel) recapitulate these properties. We have investigated the structural basis for these differences and found that two amino acid residues on either side of the deep pore are the primary determinants of sensitivity to apamin and differential block by <i>d</i>-tubocurarine. Therefore, the pharmacology of SK channels compared with other potassium channels correlates with structural differences in the outer pore region. However, introduction of a tyrosine residue in the position analogous to that which determines sensitivity to external tetraethylammonium for voltage-gated potassium channels endows SK channels with an equivalent tetraethylammonium sensitivity, indicating that the outer vestibules of the pores are similar. The pharmacology of channels formed in oocytes coinjected with SK1 and SK2 mRNAs, or with SK1-SK2 dimer mRNA, show that SK subunits may form heteromeric channels.