1. Department of Biochemistry, University of Toronto; Toronto Ontario M5S 1A8 Canada
2. Department of Biochemistry, Faculty of Medicine and Dentistry, University of Western Ontario; London Ontario N6A 5C1 Canada
3. Division of Immunology, Beth Israel Deaconess Medical Center, Harvard Medical School; Boston MA 02215 USA
4. Department of Molecular and Medical Genetics, University of Toronto; Toronto Ontario M5S 1A8 Canada
5. Department of Biology, Massachusetts Institute of Technology; 77 Massachusetts Avenue Cambridge MA 02139 USA
6. Department of Chemistry, University of Toronto; Toronto Ontario M5S 1A8 Canada
7. Samuel Lunenfeld Research Institute, Mt Sinai Hospital; Toronto Ontario M5G 1X5 Canada
8. Program in Structural Biology and Biochemistry, The Hospital for Sick Children; Toronto Ontario M5G 1X8 Canada