MiRP1 is encoded by KCNE2, which contains a single membrane-spanning domain. Inherited mutations in KCNE2 have been identified in patients with long QT syndrome (LQT6). However the mechanism by which KCNE2 mutations increase the risk of arrhythmias in patients remains unclear. Previous work has reported that MiRP1 plays an important role in regulating transient outward current (I_to) function and maintaining the ventricular electrical stability. The role of two kinds of long QT syndrome (LQT6)-associated mutations in KCNE2, I57T and V65M in regulation of Kv4.3, the major α subunit of I_to was investigated in COS-7 cells to elucidate the mechanism for pro-arrhythmia induced by these two inherited mutations. The results demonstrate that coexpression of KCNE2 with Kv4.3 slowed the rate of activation and inactivation, and caused a positive shift of voltage-dependence of inactivation of Kv4.3 channel. Moreover, KCNE2 accelerated recovery of Kv4.3 channel from inactivation. This is in agreement with previous study． In comparison with the effect of wild type KCNE2, I57T exerted no significant effect on Kv4.3 gating kinetics and recovery property. The phenotype of I57T and Kv4.3 coexpressed channel was similar to that of Kv4.3 expressed alone. In contrast, V65M enhanced all the effects of KCNE2 on Kv4.3, including slower activation and inactivation, more positive shift of voltage-dependence of inactivation and faster recovery from inactivation versus Kv4.3+KCNE2. Furthermore, V65M dramatically decreased Kv4.3 current amplitude. These results suggest that I57T causes “loss of function” while V65M results in “gain of function” of KCNE2. It is therefore proposed that KCNE2 is an important modulator of I_to. LQT6-associated mutations in KCNE2 may perturb the contribution of I_to to electrical stability in the heart through changing KCNE2 regulation of Kv4.3, predisposing the heart to arrhythmia under certain conditions.