1. Laboratoire Mixte CNRS/INRA/Aventis, Aventis CropScience, 14-20 rue Pierre Baizet, 69263 Lyon, France, Institut de Biologie Structurale, 41 rue Jules Horowitz, 38027 Grenoble, France, Institut de Chimie des Substances Naturelles, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France, and Centre for Protein Structure, Function and Engineering, Department of Biochemistry, University of Queensland, Brisbane, QLD 4072, Australia