1. UMR CNRS 6032, UFR de Pharmacie, 27 Bd Jean Moulin, 13385 Marseille Cedex 5, France, Laboratoire de Biologie et Genomique Structurales, IGBMC, rue Laurent Fries, 67404 Illkirch, France, Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, Vavilov Street 32, Moscow 119991, Russia, and Aventis Pharma, 102 route de Noisy, 93235 Romainville Cedex, France