1. Structural Biology and Biochemistry, Research Institute, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada, Department of Biochemistry, Faculty of Medicine, Medical Sciences Building, University of Toronto, Toronto, Ontario M5S 1A8, Canada, and Department of Molecular Genetics and Microbiology, Faculty of Medicine, Medical Sciences Building, University of Toronto, Toronto, Ontario M5S 1A8, Canada