Charybdotoxin block of Shaker K+ channels suggests that different types of K+ channels share common structural features

Charybdotoxin (CTX), a 37 amino acid protein isolated from the venom of L. quinquestriatus, is a high-affinity blocker of various Ca2(+)-activated K+ channels. CTX also blocks Drosophila Shaker (Sh) clone H4 transient K+ currents expressed in Xenopus oocytes with similar affinity (Kd = 3.6 nM). CTX blocks both the open and the closed states of Sh channels with no apparent change in gating behavior. In addition, the block is enhanced as the ionic strength is lowered. These properties are identical to those of CTX block of Ca(+)-activated K+ channels, and these results suggest that the external pore openings of these two functionally dissimilar K+ channels may share common structural features.