1. Max-Planck-Institute for Metals Research, Department New Materials & Biosystems, Heisenbergstrasse 3, 70569 Stuttgart, Germany, University of Heidelberg, Department of Biophysical Chemistry, INF 253, 69120 Heidelberg, Germany, and European Molecular Biology Laboratory, Cell Biology and Biophysics Unit, Meyerhofstrasse 1, 69117 Heidelberg, Germany