1. School of Biological Sciences, University of Southampton, SouthamptonSO16 1BJ, U.K.
2. Institute of Structural and Molecular Biology, Division of Bioscience, University College London, LondonWC1E 6BT, U.K.
3. Institute of Structural and Molecular Biology, Birkbeck College, University of London, LondonWC1E 7HX, U.K.
4. Department of Chemistry, Stony Brook University, 100 Nicolls Rd., Stony Brook, New York11794, United States