Role of the S3-S4 Linker in Shaker Potassium Channel Activation

Structural models of voltage-gated channels suggest that flexibility of the S3-S4 linker region may beimportant in allowing the S4 region to undergo large conformational changes in its putative voltage-sensing function. We report here the initial characterization of 18 mutations in the S3-S4 linker of the Shaker channel, including deletions, insertions, charge changes, substitution of prolines, and chimeras replacing the 25-residue Shakerlinker with 7- or 9-residue sequences from Shab, Shaw, or Shal. As measured in Xenopus oocytes with a two-microelectrode voltage clamp, each mutant construct yielded robust currents. Changes in the voltage dependence of activation were small, with activation voltage shifts of 13 mV or less. Substitution of linkers from the slowly activatingShab and Shaw channels resulted in a three- to fourfold slowing of activation and deactivation. It is concluded thatthe S3-S4 linker is unlikely to participate in a large conformational change during channel activation. The linker,which in some channel subfamilies has highly conserved sequences, may however be a determinant of activationkinetics in potassium channels, as previously has been suggested in the case of calcium channels.