Center for Structural Biology, Vanderbilt University, Nashville, Tennessee, United States of America.|Department of Biochemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Vanderbilt Vaccine Center, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America.|Center for Structural Biology, Vanderbilt University, Nashville, Tennessee, United States of America, Department of Chemistry, Vanderbilt University, Nashville, Tennessee, United States of America.