1. Structural Biology Laboratory, London Research Institute, Cancer Research UK, London WC2A 3LY, 2Cancer Research Technology Ltd, Wolfson Institute for Biomedical Research, The Cruciform Building, London, WC1E 6BT, UK and 3Institute of Structural and Molecular Biology, Department of Biological Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK