1. Departments of Molecular Genetics, Biochemistry and Chemistry, The University of Toronto, Toronto, Ontario M5S1A8, Canada
2. Department of Molecular, Microbial, and Structural Biology, University of Connecticut Health Center, 263 Farmington Ave, Farmington, Connecticut 06030-3305, United States
3. Hospital for Sick Children, Program in Molecular Structure and Function, 555 University Avenue, Toronto, Ontario M5G1X8, Canada