1. Department of Microbiology and Immunology, The University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190, Department of Medical Biochemistry & Genetics, Department of Chemistry, and Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843-1114, and The Ian Potter Foundation Protein Crystallography Laboratory, St. Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia