Department of Biophysics and Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, 6001 Forest Park Road, Dallas, TX, 75390-9050, USA. jing.tong@utsouthwestern.edu.|Howard Hughes Medical Institute, University of Texas Southwestern Medical Center at Dallas, 6001 Forest Park Road, Dallas, TX, 75390-9050, USA. jpei@chop.swmed.edu.|Department of Biophysics and Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, 6001 Forest Park Road, Dallas, TX, 75390-9050, USA. grishin@chop.swmed.edu.