Double gaps along Shaker S4 demonstrate omega currents at three different closed states

The aim of the present study was to investigate in detail how the voltage sensor in the Shaker potassium channel moves during the gating process. After the publication of the open channel structure from the crystallized K(V)Ap channel in 2003, an alternative so-called paddle model was put forward in contrast to the existing helical screw model. The voltage sensor S4 contains 4 arginine residues relevant for gating, R1(362), R2(365), R3(368) and R4(371), each separated by 2 neutral residues. These charged residues coil as one of three threads on the S4-alpha-helix. Based on a previous finding that the mutation RIS leads to the so-called omega leak current through a gating-pore in the closed state, we introduced gaps systematically along the arginine thread substituting long arginines by short serines. Mutations R2S or R3S did neither create transient nor steady leaks. The fact that the native residue A359, which is located three amino acids in front of R1, is a short one, motivated us to check its role. Mutation of A359 to arginine blocked the omega current in the R1S mutant indicating that the omega pore is occupied by A359 and R1. Introducing further double gaps (RR to ss) at sequential positions (0 + 1, 1 + 2, 2 + 3), produced clear leak currents which were remarkably stable over a wide voltage range. These leaks contradict that S4 would swing together with S3 in lipid according to the paddle hypothesis. Rather, our results show that during gating the S4 segment moves in 3 helical steps through a fixed pore formed by the channel protein.