Intramolecular interactions that control voltage sensitivity in the jShak1 potassium channel from Polyorchis...

Voltage-gated potassium ion (Kv) channel proteins respond to changes in membrane potential by changing the probability of K⁺ flux through an ion-selective pore. Kv channels from different paralogous and orthologous families have widely varying V₅₀ values. The voltage-sensing transmembrane helices (S4) of different channels contain four to seven basic residues that are responsible for transducing changes in transmembrane potential into the energy required to shift the equilibrium between the open- and closed-channel conformations. These residues also form electrostatic interaction networks with acidic residues in the S2 and S3 helices that stabilize the open and the closed states to different extents. The length and composition of the extracellular loop connecting the S3 and S4 helices (S3-S4 loop) also shape the voltage response. We describe mutagenesis experiments on the jellyfish (Polyorchis penicillatus) Kv1 family jShak1 channel to evaluate how variants of the S3-S4 loop affect the voltage sensitivity of this channel. In combination with changes in the length and composition of the S3-S4 linker, we mutated a residue on the S2 helix (N227) that in most Kv1 family channels is glutamate (E226 in mouse Kv1.2, E283 in D. melanogaster Shaker). Some individual loop replacement mutants cause major changes in voltage sensitivity, depending on a combination of length and composition. Pairwise combinations of the loop mutations and the S2 mutations interact to yield quantitatively distinct, non-additive changes in voltage sensitivity. We conclude that the S3-S4 loop interacts energetically with the residue at position N227 during the transitions between open and closed states of the channel.