1. Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, United Kingdom, Department of New Materials and Biosystems, Max-Planck Institute for Metals Research, Heisenbergstrasse 3, 70569 Stuttgart, Germany, and Department of Biophysical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany