1. Department of Structural and Computational Biology, Max F. Perutz Laboratories, University of Vienna, Campus-Vienna-Biocenter 5, A-1030 Vienna, Austria, Departments of Molecular Genetics, Biochemistry, and Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A8, and Molecular Biotechnology/IFM, Linköping University, SE-581 83 Linköping, Sweden