Functional differences in Kv1.5 currents expressed in mammalian cell lines are due to the presence of endogenous Kvβ2.1 subunits

The voltage-sensitive currents observed following hKv1.5 α subunit expression in HEK 293 and mouse L-cells differ in the kinetics and voltage dependence of activation and slow inactivation. Molecular cloning, immunopurification, and Western blot analysis demonstrated that an endogenous L-cell Kvβ2.1 subunit assembled with transfected hKv1.5 protein. In contrast, both mRNA and protein analysis failed to detect a β subunit in the HEK 293 cells, suggesting that functional differences observed between these two systems are due to endogenous L-cell Kvβ2.1 expression. In the absence of Kvβ2.1, midpoints for activation and inactivation of hKv1.5 in HEK 299 cells were -0.2 ± 2.0 and -9.6 ± 1.8 mV, respectively. In the presence of Kvβ2.1 these values were -14.1 ± 1.8 and -22.1 ± 3.7 mV, respectively. The β subunit also caused a 1.5-fold increase in the extent of slow inactivation at 50 mV, thus completely reconstituting the L-cell current phenotype in the HEK 293 cells. These results indicate that 1) the Kvβ2.1 subunit can alter Kv1.5 α subunit function, 2) β subunits are not required for α subunit expression, and 3) endogenous β subunits are expressed in heterologous expression systems used to study K ⁺ channel function.