1. Sealy Center for Structural Biology and Molecular Biophysics, Galveston, Texas 77555-0364, USA.
2. Department of Biochemistry and Molecular Biology, Galveston, Texas 77555-0364, USA.
3. Sealy Center for Vaccine Development, Galveston, Texas 77555-0364, USA.
4. Department of Microbiology and Immunology, Galveston, Texas 77555-0364, USA.
5. Child Health Research Center, Department of Pediatrics, University of Texas Medical Branch, 310 University Boulevard, Galveston, Texas 77555-0364, USA.