1. Department
of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
2. Jack
Skirball Center for Chemical Biology and Proteomics, The Salk Institute, 10010 N. Torrey Pines Road, La Jolla, California 92037, United States
3. Howard Hughes Medical Institute, 10010 N. Torrey Pines Road, La Jolla, California 92037, United States