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

Abstract

The structurally defined sea anemone peptide toxins ShK and BgK potently block the intermediate conductance, Ca2+-activated potassium channel IKCal, a well recognized therapeutic target present in erythrocytes, human T- lymphocytes, and the colon. The well characterized voltage-gated Kvl.3 channel in human T-lymphocytes is also blocked by both peptides, although ShK a ~ 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-scanning in combination with mutant cycle analyses to map the ShK-IKCal interface, and compared it with the ShK-Kvl.3 interaction surface. ShK uses the same five core residues, all clustered around the critical Lys22, to interact with IKCal and Kvl.3, although it relies on a larger number of contacts to stabilize its weaker interactions with IKCal than with Kvl.3. The toxin binds to IKCal in a region corresponding to the external vestibule of Kvl.3, and the turret and outer pore of the structurally defined bacterial potassium channel, -KcsA. Based on the NMR structure of ShK, we deduce the toxin receptor in IKCal to have x-y dimensions of ~22 Å, a diameter of ~31 Å, and a depth of ~8 Å; we estimate that the ion selectivity lies ~13 below the outer lip of the toxin receptor. These dimensions are in good agreement with those of the KcsA channel determined-from its crystal structure, and the inferred structure of Kvl.3 based on mapping with scorpion toxins. Thus, these distantly related channels exhibit architectural similarities in the outer pore region. This information could facilitate development of specific and potent modulators of the therapeutically important IKCal channel.