1. Institute of Biophysics and Physical Biochemistry, University of Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany
2. Institute of Biochemistry, University of Cologne, Otto-Fischer-Strasse 12-14, D-50674 Cologne, Germany
3. Division of Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland
4. Institute of Molecular Biophysics, University of Mainz, Jakob-Welder-Weg 26, D-55128 Mainz, Germany
5. School of Biological Sciences, University of Liverpool, Crown Street, Liverpool L69 7ZB, U.K.