1. Laboratory of Structural Biology, School of Life Science, University of Science and Technology of China, Hefei, Anhui, PRC, Shanghai Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences, Yue-Yang Road 320, Shanghai 200031, PRC, Institute of Human Virology, University of Maryland, 725 West Lombard Street, Baltimore, Maryland 21201, and Institute of Protein Research, Tong-Ji University, Shanghai 200092, PRC