D242N, a K_V7.1 LQTS mutation uncovers a key residue for I_Ks voltage dependence

K_V7.1 and KCNE1 co-assemble to give rise to the I_Ks current, one of the most important repolarizing currents of the cardiac action potential. Its relevance is underscored by the identification of >500 mutations in K_V7.1 and, at least, 36 in KCNE1, that cause Long QT Syndrome (LQTS). The aim of this study was to characterize the biophysical and cellular consequences of the D242N K_V7.1 mutation associated with the LQTS. The mutation is located in the S4 transmembrane segment, within the voltage sensor of the K_V7.1 channel, disrupting the conserved charge balance of this region. Perforated patch-clamp experiments show that, unexpectedly, the mutation did not disrupt the voltage-dependent activation but it removed the inactivation and slowed the activation kinetics of D242N K_V7.1 channels. Biotinylation of cell-surface protein and co-immunoprecipitation experiments revealed that neither plasma membrane targeting nor co-assembly between K_V7.1 and KCNE1 was altered by the mutation. However, the association of D242N K_V7.1 with KCNE1 strongly shifted the voltage dependence of activation to more depolarized potentials (+50mV), hindering I_Ks current at physiologically relevant membrane potentials. Both functional and computational analysis suggest that the clinical phenotype of the LQTS patients carrying the D242N mutation is due to impaired action potential adaptation to exercise and, in particular, to increase in heart rate. Moreover, our data identify D242 aminoacidic position as a potential residue involved in the KCNE1-mediated regulation of the voltage dependence of activation of the K_V7.1 channel.