1Laboratory of Structural Biology and MOE Laboratory of Protein Science, School of Medicine, Tsinghua University, Beijing 100084, China 2College of Life Sciences and Tianjin State Laboratory of Protein Science, Nankai University, Tianjin 300071, China 3National Laboratory of Macromolecules, Institute of Biophysics, Chinese Academy of Science, Beijing 100101, China 4Beijing 107 Middle School, Beijing, China *Correspondence to:Zhiyong Lou, E-mail: louzy@xtal.tsinghua.edu.cn