1. Department of Structural Biology and Biochemistry, Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8, Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada M5G 1L5, and Department of Molecular and Cellular Biology and Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1