1. Howard Hughes Medical Institute, University of California Los Angeles, Los Angeles, California 90095, United States
2. Department of Biological Chemistry, University of California Los Angeles, 615 Charles E.Young Drive South, Los Angeles, California 90095, United States
3. Department of Chemistry − BMC, Uppsala University, Husargatan 3, 75237 Uppsala, Sweden
4. Department of Physiology, University of California Los Angeles, 615 Charles E. Young Drive South, Los Angeles, California 90095, United States