1 Cryo-Electron Microscopy, Bijvoet Center for Biomolecular Research, Utrecht University, 3584 CH Utrecht, The Netherlands; Max-Planck Institute of Biochemistry, Department of Molecular Structural Biology, 82152 Martinsried, Germany. Electronic address: f.g.forster@uu.nl.
2 Department of Molecular Cell Biology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands; Netherlands Centre for Electron Nanoscopy, Institute of Biology Leiden, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands. Electronic address: A.J.Koster@lumc.nl.