1. M.E. Müller Institute for Microscopy, Biozentrum, University of Basel Klingelbergstrasse 70 CH‐4056 Basel Switzerland
2. Department of Structural Biology, Biozentrum, University of Basel Klingelbergstrasse 70 CH‐4056 Basel Switzerland
3. Department of Biological Chemistry, Johns Hopkins University School of Medicine Baltimore MD 21205 USA
4. Max‐Planck‐Institute for Biophysics Kennedyallee 70 D‐60596 Frankfurt Germany