Tea⁺-Sensitive Kcnq1 Constructs Reveal Pore-Independent Access to Kcne1 in Assembled I_Ks Channels

I_Ks, a slowly activating delayed rectifier K⁺ current through channels formed by the assembly of two subunits KCNQ1 (KvLQT1) and KCNE1 (minK), contributes to the control of the cardiac action potential duration. Coassembly of the two subunits is essential in producing the characteristic and physiologically critical kinetics of assembled channels, but it is not yet clear where or how these subunits interact. Previous investigations of external access to the KCNE1 protein in assembled I_Ks channels relied on occlusion of the pore by extracellular application of TEA⁺, despite the very low TEA⁺ sensitivity (estimated EC₅₀ > 100 mM) of channels encoded by coassembly of wild-type KCNQ1 with the wild type (WT) or a series of cysteine-mutated KCNE1 constructs. We have engineered a high affinity TEA⁺ binding site into the h-KCNQ1 channel by either a single (V319Y) or double (K318I, V319Y) mutation, and retested it for pore-delimited access to specific sites on coassembled KCNE1 subunits. Coexpression of either KCNQ1 construct with WT KCNE1 in Chinese hamster ovary cells does not alter the TEA⁺ sensitivity of the homomeric channels (IC₅₀ ≈ 0.4 mM [TEA⁺]_out), providing evidence that KCNE1 coassembly does not markedly alter the structure of the outer pore of the KCNQ1 channel. Coexpression of a cysteine-substituted KCNE1 (F54C) with V319Y significantly increases the sensitivity of channels to external Cd²⁺, but neither the extent of nor the kinetics of the onset of (or the recovery from) Cd²⁺ block was affected by [TEA⁺]_o at 10× the IC₅₀ for channel block. These data strongly suggest that access of Cd²⁺ to the cysteine-mutated site on KCNE1 is independent of pore occlusion caused by TEA⁺ binding to the outer region of the KCNE1/V319Y pore, and that KCNE1 does not reside within the pore region of the assembled channels.