Mutations in the minK gene encoding the protein KCNE1 have been associated with the long QT syndrome 5 (LQT5). KCNE1 is an auxiliary subunit that associates with the potassium channel KCNQ1 to form a complex in cardiac myocytes that produces the slow delayed rectifier current IKs. Recent experiments have shown that abnormal trafficking of KCNQ1 mutants can influence disease pathogenesis in LQT1 (Wilson et al., 2005), but the role of abnormal trafficking of KCNE1 in LQT5 remains to be firmly established. To investigate whether abnormal trafficking of KCNE1 contributes to LQT5 three KCNE1 mutants were generated, G52R, T58P/L59P and R98W, and their effects on trafficking and channel function examined. To examine defects in trafficking the level of colocalisation of green fluorescent protein tagged KCNQ1 (KCNQ1-GFP) with an endoplasmic reticulum marker when overexpressed in conjunction with the KCNE1 mutants, in CHO-K1 cells, was investigated. For G52R, T58P/L59P and R98W a modest, but significant (P<0.05), trafficking defect was observed that was similar to the retention level seen when KCNQ1-GFP was expressed without KCNE1. To assess the effects of the KCNE1 mutants on channel function the mutants were coexpressed with KCNQ1-GFP, in CHO-K1 cells, and the currents recorded by whole-cell patch clamping. G52R and T58P/L59P produced currents that lacked the slow activating IKs current and were similar to currents produced by KCNQ1-GFP expression alone. R98W produced currents that had the slow activating IKs profile but with reduced amplitude and steady state activation kinetics that were shifted to the right. To determine whether the lack of IKs current produced by G52R and T58P/L59P was due to a lack of expression, these mutants were cotransfected with KCNQ1, in CHO-K1 cells, and subjected to western blotting. Both G52R and T58P/L59P were found to be expressed at a similar level to wild type KCNE1, which suggests that the inability of G52R and T58P/L59P to form a pore complex that produces a slow activating IKs current is not due to a lack of expression. These results highlight two possible mechanisms for a role of abnormal trafficking in LQT5. Firstly, that the modest trafficking defect and a functional defect combine to increase dysfunction or secondly that certain KCNE1 mutants fail to enter the pore complex which results in a severe loss of function. We are currently developing techniques with the aim to distinguish between these two potential mechanisms.