1. Institut für Zellbiologie und Biosystemtechnik, FB Biowissenschaften, Universität Rostock, D-18051 Rostock, Germany;
2. European Molecular Biology Laboratory, Heidelberg, Germany;
3. National University Medical Institute, Clinical Research Centre, Singapore 117597, Republic of Singapore; and
4. Laboratory of Cell Biology, National Institutes of Health, Bethesda, Maryland 20892