1. Department of Chemistry, Umeå University, SE-90187 Umeå, Sweden
2. Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110
3. Institute of Structural and Molecular Biology, University College London/Birkbeck, Malet Street, London WC1E 7HX, U.K.
4. Department of Cellular and Molecular Interactions, Laboratory for Structural and Molecular Microbiology, Structural Biology Brussels, Vrije Universiteit Brussel/VIB, 1050 Brussels, Belgium