1. Department of Pharmacology, University of California, San Diego, La Jolla, California 92093, United States
2. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States
3. Novartis Institutes for Biomedical Research, Cambridge, Massachusetts 02139, United States
4. Department of Bioengineering, University of California, San Diego, La Jolla, California 92093, United States