1. Banting and Best Department of Medical Research, Ontario Centre for Structural Proteomics, University of Toronto, Toronto, Ontario M5G 1L6, Canada, Midwest Center for Structural Genomics and Structural Biology Center, Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Microbiology and Cell Science, University of Florida, Gainesville, Florida 32611-0700