Location and Orientation of minK within the I_Ks Potassium Channel Complex

The slowly activating cardiac potassium current (I_Ks) is generated by a heteromultimeric potassium channel complex consisting of pore-forming (KvLQT1) and accessory (minK) subunits belonging to the KCNQ and KCNE gene families, respectively. Evidence indicating that minK residues line the I_Ks pore originates from the observation that two minK cysteine mutants (G55C and F54C) render I_Ks Cd²⁺-sensitive. We have identified a single cysteine residue in the KvLQT1 S6 segment (Cys-331) that contributes to Cd²⁺ coordination in conjunction with cysteine residues engineered into the minK transmembrane domain. This observation indicates that minK resides in close proximity to S6 in the I_Ks channel complex. On the basis of homology modeling that compares the KvLQT1 S6 segment with the structure of the bacterial potassium channel KcsA, we predict that the sulfhydryl side chain of Cys-331 projects away from the central axis of the KvLQT1 pore and suggest that minK resides outside of the permeation pathway. A preliminary model illustrating the orientation of minK with S6 was validated by successful prediction of a novel Cd²⁺ binding site created within the I_Ks channel complex by engineering additional cysteine residues into both subunits. Our results indicate the location and orientation of minK within the I_Ks channel complex and further suggest that Cd²⁺ exerts its effect on I_Ks through an allosteric mechanism rather than direct pore blockade.