1. Department of Molecular and Cell Biology and California Institute for Quantitative Biosciences, University of California, Berkeley, CA 94720, USA.
2. Howard Hughes Medical Institute, Rockefeller University, New York, NY 10021, USA.
3. Department of Chemistry, University of California, Berkeley, CA 94720, USA.
4. Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA.
5. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.