Identification of amino acid residues involved in dendrotoxin block of rat voltage-dependent potassium channels.

alpha-Dendrotoxin (DTX) is a 60-amino acid peptide belonging to the family of mamba snake neurotoxins; it is a potent blocker of some but not all voltage-gated potassium currents. Potassium currents recorded from oocytes injected with cloned potassium channel RNAs also vary in sensitivity to DTX. Expression of channels that were chimeras of the DTX-sensitive channel RBK2 and the DTX-insensitive channel RGK5 showed that the putative extracellular loop between transmembrane domains S5 and S6 contributes strongly to DTX sensitivity. Mutation of two residues (Ala352Glu353) in this region of RBK1 to conform to those at equivalent positions in RGK5 (Pro374Ser375) reduced the potency of DTX about 70-fold, and the substitution of Tyr379 in RBK1 by its counterpart in RGK5 (His401) caused an additional 2.5-fold decrease in sensitivity. Converse substitutions in RGK5 significantly increased sensitivity to DTX. The results suggest that these residues contribute significantly to the channel-toxin interaction, providing further evidence that the S5-S6 loop lies at or near the external mouth of the channel, where DTX binding leads to channel occlusion. They offer a molecular explanation for the differences in DTX sensitivity observed among native potassium channels.