1. School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K.
2. Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States
3. Department of Structural and Molecular Biology, University College London, London WC1E 6BT, U.K.,
4. Department of Biochemistry and Molecular Biology, The University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73104, United States