1. MRC Laboratory for Molecular Cell Biology, University College London, London WClE 6BT, UK;
2. Department of Neurobiology, University of Heidelberg, Im Neuenheimer Feld 364, Heidelberg; D-69120
3. Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstr. 110 Dresden, D-01307 Germany;
4. joint first authors