1. Department of Experimental Pathology, Lund University; S-221 85 Lund Sweden
2. Swiss Federal Institute of Technology, Department of Cell Biology; CH-8093 Zürich Switzerland
3. Department of Histology, University of Göttingen; Göttingen Germany
4. Cell and Molecular Biology, CIEMAT; 28040 Madrid Spain
5. Max Planck Institute for Biochemistry; D-82152 Martinsried Germany