1. Amgen Discovery Research, Amgen, Thousand Oaks, California 91320, United States
2. Department of Biological Chemistry, University of California-Los Angeles, Los Angeles, California 90095, United States
3. Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States
4. Department of Chemistry and Biochemistry, University of California-Los Angeles, Los Angeles, California 90095, United States