Division of Cell Biology, German Research Centre for Biotechnology (GBF) Mascheroder Weg 1, D-38124 Braunschweig, Germany. gdi@gbf.de|Division of Cell Biology, German Research Centre for Biotechnology (GBF) Mascheroder Weg 1, D-38124 Braunschweig, Germany. gdi@gbf.de|Division of Cell Biology, German Research Centre for Biotechnology (GBF) Mascheroder Weg 1, D-38124 Braunschweig, Germany. gdi@gbf.de|Division of Cell Biology, German Research Centre for Biotechnology (GBF) Mascheroder Weg 1, D-38124 Braunschweig, Germany. gdi@gbf.de