1. Department of Structural Biology, St. Jude Children’s Research Hospital, 262 Danny Thomas Place, Memphis, Tennessee 38105, Hartwell Center for Bioinformatics and Biotechnology, St. Jude Children’s Research Hospital, 262 Danny Thomas Place, Memphis, Tennessee 38105, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032, and Department of Molecular Sciences, University of Tennessee Health Sciences Center, Memphis, Tennessee 38163