1. Max Planck Institute for Biophysical Chemistry, Department of Molecular Biology, Am Fassberg 11, 37077 Göttingen, Germany.
2. Department of Biochemistry, University of Wisconsin–Madison, Madison, WI 53706, USA.
3. Max Planck Institute for Biophysics, Department of Structural Biology, Max-von-Laue-Straße 3, 60438 Frankfurt am Main, Germany.