1. Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
2. Jack H. Skirball Center for Chemical Biology and Proteomics, Salk Institute for Biological Studies, La Jolla, California 92037, United States
3. Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, California 92037, United States