1. Max-Planck-Institut für Zellbiologie, Rosenhof, D-68526 Ladenburg, Germany.
2. Present address: Heinrich-Pette Institute for Experimental Virology and Immunology, Hamburg University, Hamburg, Germany.
3. Present address: Robert Wood Johnson Medical School, Department of Biochemistry, University of Medicine and Dentistry of New Jersey, Piscataway, New Jersey.
4. Present address: Max-Planck-Institut für Medizinische Forschung, Heidelberg, Germany.
5. Present address: Institute for Cellular and Molecular Botany, Bonn University, Bonn, Germany.