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