1. Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada
2. Department of Molecular Genetics, University of Toronto, Toronto, Canada
3. Department of Microbiology, Immunology and Infectious Diseases, University of Calgary, Calgary, Canada
4. Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, USA
5. Structural Biology Center, X-ray Science Division, Argonne National Laboratory, Argonne, IL, USA