Molecular basis for differential sensitivity of KCNQ and I(Ks) channels to the cognitive enhancer XE991

Channels formed by coassembly of the KCNQ1 (KvLQT1) subunit and the minK subunit underlie slowly activating cardiac delayed rectifier (I(Ks)) in the heart, whereas two other members of the KCNQ channel family, KCNQ2 and KCNQ3, coassemble to underlie the M current in the nervous system. Because of their important physiological function, KCNQ channels have potential as drug targets, and an understanding of possible mechanisms that would enable tissue-specific targeting of these channels will be of significant value to drug development. In this study, we examined the role of the minK subunit in determining the response of KCNQ1 channels to blockade by the cognitive enhancer XE991. Coexpression with minK markedly decreased the sensitivity of KCNQ1 to blockade by XE991. When measured at the end of a 500-ms step, XE991 blockade of the KCNQ1+minK current had a K(D) value of 11.1 +/- 1.8 microM, approximately 14-fold less sensitive than the block of the KCNQ1 current (K(D) = 0.78 +/- 0.05 microM). In addition, XE991 reduced activation and deactivation time constants and caused a rightward shift in the activation curve of KCNQ1+minK, but affected none of these parameters for KCNQ1 alone. Also, XE991 block of KCNQ1+minK, but not of KCNQ1, was time- and voltage-dependent. We conclude that the presence of minK in the I(Ks) channel complex gives rise to differential sensitivity of KCNQ and I(Ks) channels to blockade by XE991. Our results have implications for drug development by demonstrating the important potential role of accessory subunits in determining the pharmacological properties of KCNQ channels.