1. Contribution from the Institut de Biologie Structurale “Jean-Pierre Ebel” (CEA-CNRS), 41 Avenue des Martyrs, 38027 Grenoble Cedex, France, DRFMC-SCIB-SCPM, 85X, CEN-Grenoble, 38041 Grenoble Cedex, France, and Department of Biochemistry, University of Arizona, Tucson, Arizona 85721