EVIDENCE THAT THE S6 SEGMENT OF THE SHAKER VOLTAGE-GATED K+ CHANNEL COMPRISES PART OF THE PORE

POTASSIUM channels are highly selective and allow the rapid flux of po tassium ions through their pore1. Several studies have implicated the H5 (P or SS1-SS2) segment2,3 as part of the pore in voltage-gated ion channels4-10. The proposal that H5 spans at least 80% of the electric potential drop across the K+ channel pore5,11 is based on altered inte rnal tetraethylammonium sensitivity arising from mutations of H5 resid ues that are 100% conserved among K+ channels having differing sensiti vity to tetraethylammonium5-7,12,13. Here we report that the S6 segmen t is also involved in K+ ion permeation and in governing the sensitivi ty to internal tetraethylammonium and barium. Transplanting the S6 seg ment of NGK2 into Shaker causes this S6 chimaera to adopt the single-c hannel conductance and sensitivity to internal tetraethylammonium and barium ions from the NGK2 channel. The differences between NGK2 and Sh aker in external tetraethylammonium sensitivity, but not single-channe l conductance, can be attributed to the differences in their H5 sequen ces. Three non-conserved S6 residues have been found to affect either single-channel conductance or internal tetraethylammonium sensitivity.