1. Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
2. Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, U.K.
3. MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.
4. Department of Pediatrics, School of Medicine, University of California San Diego, La Jolla, California 92093, United States