Using site-directed mutagenesis in combination with whole cell recordings of transfected mammalian cells, we investigated the current through wt and mutant (H399T alone, L418C alone, H399T + L418C) Kv1.3 channels. Whole cell currents were investigated by 200 ms depolarizing voltage steps to +40 mV every 30 s. Compared to wt, the H399T mutant alone resulted in currents with a slow rate of inactivation and a fast recovery from the inactivated state while the L418C mutant alone resulted predominantly in a strongly reduced recovery from inactivation. Combining both mutations (Kv1.3 H399T/L418C) led to currents that are similar to that of Kv1.3 H399T. However, in contrast to the single mutants, a slow deactivation resulting in open channels at hyperpolarized potentials could be observed during voltage ramps from -120 to +40 mV every s. We therefore conclude that the L418C mutation might lead to a stable accumulation of Kv1.3 in the inactivated state and in addition to an impairment of open to closed state transitions, the latter being detectable only in the non-inactivating background of the double mutant (H399T + L418C). It seems that the position 418 in Kv1.3 can interfere with the activation gating machinery of the channel.

Supported by grants from the 4SC AG, Martinsried, Germany.