1. Departments of Biochemistry, Medical Biophysics and Molecular & Medical Genetics, and Protein Engineering NetworkCentres of Excellence, University of Toronto, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada, Division of Molecular and Structural Biology, Ontario Cancer Institute, 610 University Avenue, Toronto, Ontario M5G 2M9, Canada, and Lash Miller Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 1A1, Canada