Control of a final gating charge transition by a hydrophobic residue in the S2 segment of a K+ channel voltage sensor

It is now well established that the voltage-sensor domains present in voltage-gated ion channels and some phosphatases operate by transferring several charged residues (gating charges), mainly arginines located in the S4 segment, across the electric field. The conserved phenylalanine F-290 located in the S2 segment of the Shaker K channel is an aromatic residue thought to interact with all the four gating arginines carried by the S4 segment and control their transfer [Tao X, et al. (2010) Science 328: 67-73]. In this paper we study the possible interaction of the gating charges with this residue by directly detecting their movement with gating current measurements in 12 F-290 mutants. Most mutations do not significantly alter the first approximately 80-90% of the gating charge transfer nor the kinetics of the gating currents during activation. The effects of the F-290 mutants are (i) the modification of a final activation transition accounting for approximately 10-20% of the total charge, similar to the effect of the ILT mutant [Ledwell JL, et al. (1999) J Gen Physiol 113: 389-414] and (ii) the modification of the kinetics of the gating charge movement during deactivation. These effects are well correlated with the hydrophobicity of the substituted residue, showing that a hydrophobic residue at position 290 controls the energy barrier of the final gating transition. Our results suggest that F-290 controls the transfer of R-371, the fourth gating charge, during gating while not affecting the movement of the other three gating arginines.