The Effects of Ginsenoside Rg3 on Human Kv1.4 Channel Currents without the N-Terminal Rapid Inactivation Domain

Kv1.4 channel belongs to the family of voltage-gated potassium channels that mediate transient and rapidly inactivating A-type currents and N-type inactivation. This N-type inactivation can be removed by the deletion of N-terminal domains, which exhibit non-inactivating currents and C-type inactivation. In our previous report, we demonstrated that 20(S)-ginsenoside Rg3 (Rg3), one of the active ingredients of ginseng saponins, inhibits human Kv1.4 (hKv1.4) channel currents through the interaction with amino acids, including Lys (K) residue, which is known as K+ activation and the extracellular tetraethylammonium (TEA) binding site. In the present study, we examined the effects of Rg3 on hKv1.4 channel currents without the N-terminal rapid inactivation domain. We constructed hKv1.4Δ2-61 channels by N-terminal deletion of 2-61 amino acid residues. We investigated the effect of Rg3 on hKv1.4Δ2-61 channel currents. We found that Rg3 preferentially inhibited non-inactivating outward currents rather than peak outward currents of hKv1.4Δ2-61 channels. The mutation of K531 hKv1.4Δ2-61 to K531Y hKv1.4Δ2-61 and raising of extracellular [K+]o abolished Rg3 inhibitions on non-inactivating outward currents. Rg3 treatment increased the C-type inactivation rate, but raising the extracellular [K+]o reversed Rg3 action. These results provide additional evidence that K531 residue also plays an important role in the Rg3-mediated non-inactivating current blockages and in Rg3-mediated increase of the C-type inactivation rate in hKv1.4Δ2-61 channels.