1. Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
2. The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
3. Department of Chemistry, The Scripps Research Institute, La Jolla, CA 92037, USA.
4. Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037, USA.
5. Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.