1. Department of Biochemistry and Molecular Biology, Department of Chemistry, The Michael Smith Laboratories, and The Protein Engineering Network of Centres of Excellence, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3, Department of Microbiology and Immunology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E5, and The Plant Biotechnology Institute, National Research Council of Canada, 110 Gymnasium Place, Saskatoon, Saskatchewan, Canada S7N 0W9