Positioning and Guidance of the Voltage Sensor S4 Within the Omega/Gating-Pore in the Shaker K-Channel
Inwardly leaking omega-currents in resting Shaker K-channels were first found when the long charged residue R362 on S4 was made short, creating a pore in the voltage-sensing domain (Tombola et al., 2005, 2007). We recently demonstrated that in fact a pair of adjacent charged residues on S4 must be short to build a double-gap (Gamal El-Din et al., 2010). These residues located at every third position A359, R362, R365, R368, R371, K374 can be seen as a rail, shorthand notation “aRRRRK”. Omega-currents were obtained for the double-gap constructs “asRRRK”, “RssRRK” and “aRssRK” (short residues lower case). In a mechanistic view, the long residues slide like bolts in a guidance groove which becomes the leaking omega-pore when two occluding bolts are shortened.Presently, we study the wall and length of this guidance groove. Especially, we checked in the closed state of S4 whether at the outer end of the groove, E283 on S2 opposes A359 on S4, and whether E293, F290 form the inner end and oppose R362 on S4. Firstly, we kept the inner part open (R362S) and studied different mutant pairs at positions 283(S2) and 359(S4). The size of the resulting omega-current clearly corresponded to the cleft width obtained from molecular modeling. Secondly, leaving the outer part open with A359, we demonstrated that the omega-currents now depended on the cleft width between residues at 362(S4) and 293, 290 on S2.In conclusion, the omega-pore represents a guidance groove for the gating charges of S4. E283 and E293 located at the outer and inner end of the groove determine the length of the membrane voltage drop. This length also guarantees that during gating always at least one residue senses the field in the pore.