1. Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
2. York Structural Biology Laboratory, Department of Chemistry, The University of York, York YO10 5DD, UK
3. Architecture et Fonction des Macromolécules Biologiques (AFMB), CNRS, Aix-Marseille University, Marseille, France
4. Department of Cell and Chemical Biology, Leiden University Medical Centre, 2333 ZC Leiden, The Netherlands