1. Centre de Biochimie Structurale, UMR C9955 CNRS, U414 INSERM, Université Montpellier 1, Faculté de Pharmacie, Avenue C. Flahault, F34060 Montpellier, France, and Department of Biochemistry and Molecular Biology, University College London, The Darwin Building, Gower Street, London, WC1E 6BT, U.K.