1. Departments of Molecular Genetics, Biochemistry and Chemistry, The University of Toronto, Toronto, Ontario M5S 1A8, Canada
2. Department of Molecular, Microbial and Structural Biology, University of Connecticut Health Center, Farmington, Connecticut 06030, United States
3. Department of Biochemistry, University of Washington, Box 357350, 1705 NE Pacific Street, Seattle, Washington 98195-7350, United States
4. Center for Protein Engineering, Medical Research Council, Cambridge, United Kingdom