Centre for Integrative Biology (CBI), Department of Integrated Structural Biology, IGBMC (Institute of Genetics and of Molecular and Cellular Biology), Illkirch, France, Centre National de la Recherche Scientifique (CNRS) UMR 7104, Illkirch, France, Institut National de la Santé et de la Recherche Médicale (INSERM) U964, Illkirch, France and Université de Strasbourg, Strasbourg, France.