1. Department of Chemistry and California Institute for Quantitative Biosciences, University of California—Berkeley, Berkeley, California 94720-3220, United States
2. Department of Biochemistry, Molecular Biology, and Biophysics and The Biotechnology Institute, University of Minnesota, St. Paul, Minnesota 55108, United States
3. Department of Molecular and Cell Biology, University of California—Berkeley, Berkeley, California 94720-3220, United States