1. Department of Cell Biology, University of Geneva, CH-1211 Geneva 4, Switzerland.
2. Department of Biochemistry, University of Geneva, CH-1211 Geneva 4, Switzerland.
3. Faculty of Land and Food Resource, University of Melbourne, Parkville Victoria 3010, Australia.
4. Institute of Molecular Biology, University of Zurich, Winterthurerstrasse, CH-8057 Zurich, Switzerland.
5. Department of Zoology and Animal Biology, University of Geneva, CH-1211 Geneva 4, Switzerland.