Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, USA.|Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX, USA.|Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX, USA.|Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX, USA.|Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, USA.