1. Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, (EMBL), Meyerhofstraße 1, Heidelberg D-69117, Germany
2. Department of Medicine, Surgery and Health Sciences, University of Trieste, Trieste 34149, Italy
3. Electron Microscopy Core Facility, Europen Molecular Biology Laboratory (EMBL), Meyerhofstraße 1, Heidelberg D-69117, Germany