1. Institute of Cell Biology, University of Bern, Baltzerstrasse 4, CH-3012 Bern, Switzerland.
2. Mercachem, Kerkenbos 1013, PO Box 6747, 6503 Ge Nijmegen, The Netherlands
3. Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599-7260, USA