Structural conservation of the pores of calcium-activated and voltage-gated potassium channels determined by a sea anemone toxin

The structurally defined sea anemone peptide toxins ShK and BgK potently bl ock the intermediate conductance, Ca2+-activated potassium channel IKCa1, a well recognized therapeutic target present in erythrocytes, human T-lympho cytes, and the colon. The well characterized voltage-gated Kv1.3 channel in human T-lymphocytes is also blocked by both peptides, although ShK has a s imilar to 1,000-fold greater affinity for Kv1.3 than IKCa1. To gain insight into the architecture of the toxin receptor in IKCa1, we used alanine-scan ning in combination with mutant cycle analyses to map the ShK-IKCa1 interfa ce, and compared it with the ShK-Kv1.3 interaction surface. ShK uses the sa me five core residues, all clustered around the critical Lys(22), to intera ct with IKCa1 and Kv1.3, although it relies on a larger number of contacts to stabilize its weaker interactions with IKCa1 than with Kv1.3. The toxin binds to IKCa1 in a region corresponding to the external vestibule of Kv1.3 , and the turret and outer pore of the structurally defined bacterial potas sium channel, KcsA. Based on the NMR structure of ShK, we deduce the toxin receptor in IKCa1 to have x-y dimensions of similar to 22 Angstrom, a diame ter of similar to 31 Angstrom, and a depth of similar to 8 Angstrom; we est imate that the ion selectivity lies similar to 13 Angstrom below the outer lip of the toxin receptor, These dimensions are in good agreement with thos e of the KcsA channel determined from its crystal structure, and the inferr ed structure of Kv1.3 based on mapping with scorpion toxins. Thus, these di stantly related channels exhibit architectural similarities in the outer po re region. This information could facilitate development of specific and po tent modulators of the therapeutically important IKCa1 channel.