1. Graduate Group in Biophysics, University of California, San Francisco, CA 94158, USA.
2. Howard Hughes Medical Institute and Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94158, USA.
3. Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA 94305–5174, USA.
4. Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94158, USA.