1. Institute de Biologie Structurale Jean-Pierre Ebel, CNRS-CEA-UJF, 41 rue Jules Horowitz, 38027-Grenoble Cedex, France, Department of NMR-Based Structural Biology and Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany