1. Bijvoet Center for Biomolecular Research, Department of Bio-Organic Chemistry, Utrecht University, P.O. Box 80.075, NL-3508 TB Utrecht, The Netherlands, Department of Molecular Biophysics, German Cancer Research Center, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany, and Max Planck Institute for Medical Research, Jahnstrasse 29, D-69120 Heidelberg, Germany