1. Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, UK.
2. INSERM U869, Université Victor Segalen Bordeaux 2, 33076 Bordeaux, France.
3. Department of Chemistry and Biochemistry, University of Berne, CH-3012 Berne, Switzerland.
4. CNRS UMR5097, Université Victor Segalen Bordeaux 2, 33076 Bordeaux, France.