1. Department of Biochemistry, University of Leicester, Lancaster Road, Leicester LE1 9HN, U.K.
2. Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
3. Department of Structural and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
4. ISMB Biophysics Centre, Department of Biological Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, U.K.
5. Department of Biochemistry and Molecular Biology and Center for Biomolecular Structure and Function, University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77005, United States