1. Department of Biochemistry and Molecular Biophysics, California Institute of Technology, MS 128-95, Pasadena, California 91125, Department of Bioengineering, Stanford University and Howard Hughes Medical Institute, James H. Clark Center E300, 318 Campus Drive, Stanford, California 94305, Howard Hughes Medical Institute and Departments of Molecular and Cellular Physiology, Neurology and Neurological Sciences, Structural Biology and Stanford Synchrotron Radiation Laboratory, Stanford University, James H....