Comparison of H5, S6, and H5-S6 exchanges on pore properties of voltage-dependent K+ channels.

Evidence is now substantial that membrane segments besides H5 contribute to the pore of K+ channels. We found that substitution of the H5 region of Shaker B (ShB) with the corresponding sequence of NGK2 expressed channels which retained the single channel K+ conductance (gK+) of the host ShB channel. A reverse chimera with ShB H5 region transplanted into NGK2 also retained the gK+ of the host NGK2. Point mutations V443L+T449Y in ShB H5 converted internal tetraethylammonium (TEA) affinity to NGK2 values, and T449Y converted external TEA affinity and Rb+ conductance (gRb+) to NGK2 values. In ShB, exchanging a short stretch of 9 amino acids located just past the transmembrane segment referred to as S6, post-S6+ produced a large increase in gK+ with no effect on internal or external TEA blockade. Within S6, 3 important residues for internal TEA blockade were identified. Thus, H5 determines external TEA blockade and both H5 and S6 may determine internal TEA blockade, but neither H5 nor S6 alone restored the donor gK+. However, chimeric channels in which H5, S6, and post-S6 were exchanged transferred gK+ of NGK2 to ShB or the gK+ of ShB to NGK2. Thus, contributions from H5, S6, and its cytoplasmic extension post-S6 make the pore of voltage-dependent Shaker K+ channels a polysegmental mosaic structure.