1. Department of Biochemistry, McGill University, Montreal, QC, H3G 1Y6 Canada, York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5YW, U.K., Biotechnology Research Institute, National Research Council of Canada, Montreal, QC, H4P 2R2 Canada, and Institute for Biological Sciences, National Research Council of Canada, Ottawa, ON, K1A 0R6 Canada