FUNCTIONAL DIFFERENCES IN KV1.5 CURRENTS EXPRESSED IN MAMMALIAN-CELL LINES ARE DUE TO THE PRESENCE OF ENDOGENOUS KV-BETA-2.1 SUBUNITS

The voltage-sensitive currents observed following hKv1.5 alpha subunit expression in HEK 293 and mouse L-cells differ in the kinetics and vo ltage dependence of activation and slow inactivation. Molecular clonin g, immunopurification, and Western blot analysis demonstrated that an endogenous L-cell Kv beta 2.1 subunit assembled with transfected hKv1. 5 protein. In contrast, both mRNA and protein analysis failed to detec t a beta subunit in the HEK 293 cells, suggesting that functional diff erences observed between these two systems are due to endogenous L-cel l Kv beta 2.1 expression. In the absence of Kv beta 2.1, midpoints for activation and inactivation of hKv1.5 in HEK 293 cells were -0.2 +/- 2.0 and -9.6 +/- 1.8 mV, respectively. In the presence of Kv beta 2.1 these values were -14.1 +/- 1.8 and -22.1 +/- 3.7 mV, respectively. Th e beta subunit also caused a 1.5-fold increase in the extent of slow i nactivation at 50 mV, thus completely reconstituting the L-cell curren t phenotype in the HEK 293 cells. These results indicate that 1) the K v beta 2.1 subunit can alter Kv1.5 alpha subunit function, 2) beta sub units are not required for alpha subunit expression, and 3) endogenous beta subunits are expressed in heterologous expression systems used t o study K+ channel function.