1. TaiGen Biotechnology Co., Taipei 114, Taiwan, ROC, Institute of Biochemical Sciences, National Taiwan University, Taipei 106, Taiwan, ROC, Institute of Biological Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC, Core Facility for Protein X-ray Crystallography, Academia Sinica, Taipei 115, Taiwan, ROC, and Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, Texas 77555