1. Department of Biochemistry, Gene Center and Center for Integrated Protein Science Munich, University of Munich, 81377 Munich, Germany.
2. Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.
3. Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
4. Cryo–Electron Microscopy, Bijvoet Center for Biomolecular Research, Utrecht University, 3584 CH Utrecht, Netherlands.