1. Max-Planck-Institut für Zellbiologie, Rosenhof, Ladenburg, Germany.
2. A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, Russia.
3. Present address: Max-Planck-Institut für Medizinische Forschung, Heidelberg, Germany.
4. Present address: Heinrich-Pette Institute for Experimental Virology and Immunology, Hamburg University, Hamburg, Germany.
5. Present address: Institute for Cellular and Molecular Botany, Bonn University, Bonn, Germany.