Topology and charged residues in the voltage sensor domain in Kv (A) and BK (B) subunits. (A) Kv subunits comprise six TM segments (S1–S6), a reentrant selectivity filter loop, and intracellular N and C termini. (B) BK α subunits comprise seven TM segments (S0–S6), a reentrant selectivity filter loop, an extracellular N terminus, and an intracellular C terminus. Positively and negatively charged residues are shown in blue and red, respectively. Neutral residues are shown in green. The pattern of charged residues in S2, S3, and S4 is conserved between Kv and BK α subunits with the following exceptions: in BK channels, E1 in S2 is replaced by D1; E2 in S2 is replaced by Y; R1 in S4 is replaced by L. A sequence alignment of the murine BK α subunit and a representative Kv subunit, Kv1.2, is shown in Fig. 1 of Liu et al., 2008.