Charybdotoxin block of potassium channels. (A) Cartoon illustrating charybdotoxin binding to the outer pore of the BK channel and the position of a basic residue near to where potassium ions bind within the outer pore of the channel. The cartoon is reproduced from MacKinnon and Miller (1988). (B) X-ray structure of the Kv1.2/2.1 paddle chimera in complex with charybdotoxin (PDB accession no. 4JTA). Only S5-S6 helices are shown for clarity, with neighboring subunits colored white or wheat. The toxin is colored blue, with K27 shown in stick representation (carbon colored yellow and nitrogen blue).