1. Departments of Molecular Cell Research and
2. Cell Physiology, Max-Planck-Institute for Medical Research, D-69120 Heidelberg, Germany;
3. Laboratoire de Biochimie et Biophysique des Systèmes Intégrés, UMR5092-CNRS, CEA, 38054 Grenoble, France;
4. Institute of Cell Biology, Ludwig-Maximilians University, D-80336 Munich, Germany; and
5. Department of Biological Sciences, Alexander Fleming Building, Imperial College of Science Technology and Medicine, London SW7 2AZ, UK