Ginseng Gintonin Activates the Human Cardiac Delayed Rectifier K+ Channel: Involvement of Ca2+/Calmodulin Binding Sites

Abstract

Gintonin, a novel, ginseng-derived G protein-coupled lysophosphatidic acid (LPA) receptor ligand, elicits [Ca2+]i transients in neuronal and non-neuronal cells via pertussis toxin-sensitive and pertussis toxin-insensitive G proteins. The slowly activating delayed rectifier K+ (I-Ks) channel is a cardiac K+ channel composed of KCNQ1 and KCNE1 subunits. The C terminus of the KCNQ1 channel protein has two calmodulin-binding sites that are involved in regulating I-Ks channels. In this study, we investigated the molecular mechanisms of gintonin-mediated activation of human I-Ks channel activity by expressing human I-Ks channels in Xenopus oocytes. We found that gintonin enhances I-Ks channel currents in concentration- and voltage-dependent manners. The EC50 for the I-Ks channel was 0.05 +/- 0.01 mu g/ml. Gintonin-mediated activation 1 of the I-Ks channels was blocked by an LPA1/3 receptor antagonist, an active phospholipase C inhibitor, an IP3 receptor antagonist, and the calcium chelator BAPTA. Gintonin-mediated activation of both the I-Ks channel was also blocked by the calmodulin (CaM) blocker calmidazolium. Mutations in the KCNQ1 [Ca2+]i/CaM-binding IQ motif sites (S373P, W392R, or R539W) blocked the action of gintonin on I-Ks channel. However, gintonin had no effect on hERG K+ channel activity. These results show that gintonin-mediated enhancement of I-Ks channel currents is achieved through binding of the [Ca2+](i)/CaM complex to the C terminus of KCNQ1 subunit.