Graduate Group in Biophysics, University of California, Berkeley, CA 94720, USA. dpollard@berkeley.edu|Graduate Group in Biophysics, University of California, Berkeley, CA 94720, USA. dpollard@berkeley.edu|Graduate Group in Biophysics, University of California, Berkeley, CA 94720, USA. dpollard@berkeley.edu|Graduate Group in Biophysics, University of California, Berkeley, CA 94720, USA. dpollard@berkeley.edu