基金Acknowledgments We thank Dr Emily Liman (University of Southern California, USA) for providing the pGEMHE vector for the Xenopus oocyte experiments. We also thank Dr Richer Gaber (Northwestern Uni- versity, USA) for providing the yeast mutant strain with K+ transport deficiency. We are grateful to Dr Rainer Hedrich (University of Wurzburg, Germany) for critical discussion. This work was supported by the National Natural Science Foundation of China (grant no. 30830013 to WHW), the Beijing Municipal Education Commission (grant no. YB20081001901 to WHW) and the Program of Introducing Talents of Discipline to Universities (grant no. B06003 to WHW).