1. Institute of Botany, Department of Plant Cell Biology, University of Bonn, Kirschallee 1, D–53115 Bonn, Germany (F.B., D.V.);
2. Department of Plant Physiology, Faculty of Sciences, Charles University, Viničná 5, CZ–128 44 Prague, Czech Republic (F.C.); and
3. Structural Studies Division, Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, United Kingdom (J.K.-J.)