Mutations in the minK gene KCNE1 have been linked to the LQT5 variant of human long QT syndrome. MinK assembles with KvLQT1 to produce the slow delayed rectifier K+ current IKs and may assemble with HERG to modulate the rapid delayed rectifier IKr. We used electrophysiological and immunocytochemical methods to compare the cellular phenotypes of wild-type minK and four LQT5 mutants co-expressed with KvLQT1 in Xenopus oocytes and HERG in HEK293 cells. We found that three mutants, V47F, W87R and D76N, were expressed at the cell surface, while one mutant, L51H, was not. Co-expression of V47F and W87R with KvLQT1 produced IKs currents having altered gating and reduced amplitudes compared with WT-minK, co-expression with L51H produced KvLQT1 current rather than IKs and co-expression with D76N suppressed KvLQT1 current. V47F increased HERG current but to a lesser extent than WT-minK, while L51H and W87R had no effect and D76N suppressed HERG current markedly. Thus, V47F interacts with both KvLQT1 and HERG, W87R interacts functionally with KvLQT1 but not with HERG, D76N suppresses both KvLQT1 and HERG, and L51H is processed improperly and interacts with neither channel. We conclude that minK is a co-factor in the expression of both IKs and IKr and propose that clinical manifestations of LQT5 may be complicated by differing effects of minK mutations on KvLQT1 and HERG.