Lifei Li1,2, Lijuan Yan1,2,3, Zhucheng Chen4,5,6. 1. MOE Key Laboratory of Protein Science, Tsinghua University, Beijing, P. R. China. 2. School of Life Science, Tsinghua University, Beijing, P. R. China. 3. Boston Children's Hospital, Boston, MA, USA. 4. MOE Key Laboratory of Protein Science, Tsinghua University, Beijing, P. R. China. zhucheng_chen@tsinghua.edu.cn. 5. School of Life Science, Tsinghua University, Beijing, P. R. China. zhucheng_chen@tsinghua.edu.cn. 6. Tsinghua-Peking Joint Center for Life Sciences, Beijing Advanced Innovation Center for Structural Biology, Beijing, P. R. China. zhucheng_chen@tsinghua.edu.cn.