1. Department of Biochemistry, University of Utah, Salt Lake City, UT 84112, USA.
2. Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO 63110, USA.
3. Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA.
4. Physical Bioscience Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. FEI Company, Hillsboro, OR 97124, USA.