The slow I_KS K⁺ channel plays a major role in repolarizing the cardiac action potential and consists of the assembly of KCNQ1 and KCNE1 subunits. Mutations in either KCNQ1 or KCNE1 genes produce the long-QT syndrome, a life-threatening ventricular arrhythmia. Here, we show that long-QT mutations located in the KCNQ1 C terminus impair calmodulin (CaM) binding, which affects both channel gating and assembly. The mutations produce a voltage-dependent macroscopic inactivation and dramatically alter channel assembly. KCNE1 forms a ternary complex with wild-type KCNQ1 and Ca²⁺-CaM that prevents inactivation, facilitates channel assembly, and mediates a Ca²⁺-sensitive increase of I_KS-current, with a considerable Ca²⁺-dependent left-shift of the voltage-dependence of activation. Coexpression of KCNQ1 or I_KS channels with a Ca²⁺-insensitive CaM mutant markedly suppresses the currents and produces a right shift in the voltage-dependence of channel activation. KCNE1 association to KCNQ1 long-QT mutants significantly improves mutant channel expression and prevents macroscopic inactivation. However, the marked right shift in channel activation and the subsequent decrease in current amplitude cannot restore normal levels of I_KS channel activity. Our data indicate that in healthy individuals, CaM binding to KCNQ1 is essential for correct channel folding and assembly and for conferring Ca²⁺-sensitive I_KS-current stimulation, which increases the cardiac repolarization reserve and hence prevents the risk of ventricular arrhythmias.