1. Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California92093, United States
2. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland20892, United States
3. Electron Microscopy Laboratory, Cancer Research Technology Program, Frederick National Laboratory for Cancer Research Sponsored by the National Cancer Institute, Frederick, Maryland21702, United States
4. Department of Integrative Structural and Computational Biology and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California92037, United States