Second half of the transmembrane region from hKCNQ1 is necessary for the modulation by KCNE1. (A) The half-maximal potential (V_1/2) without (light gray bars) and with (dark gray bars) KCNE1 for human KCNQ1 (hKCNQ1; left end), Ci-KCNQ1 (right end), and their chimeras are plotted. Asterisks indicate the significant reduction in the level of V_1/2 with KCNE1 compared with that of hKCNQ1 with KCNE1 (Dunnett’s test). The design for each chimera is depicted at the bottom of the bar graphs; red regions are from human KCNQ1, and blue regions are from Ci-KCNQ1. (B) Representative current traces of chimera ciQ1hQ1(S4–S6) in the absence and presence of KCNE1. The membrane potential was stepped from −100 to +60 mV in 20-mV increments and subsequently stepped to −30 mV for tail currents. (C) G-V relationships for ciQ1hQ1(S4–S6) in the absence and presence of KCNE1. Plots are fitted with the Boltzmann equation (red curves).