1. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.
2. Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Biophysics Graduate Group, University of California, Berkeley, Berkeley, CA 94720, USA.
4. Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, CA 94720, USA.
5. Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA.