1. Biological Chemistry and Crop Protection Department, Rothamsted Research, Harpenden AL5 2JQ, U.K.
2. Department of Cell Biology, University Medical Center Groningen, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands
3. Department of Infectious Diseases, St. Jude Children's Research Hospital, 262 Danny Thomas Place, Memphis, TN 38105-3678, U.S.A.
4. Institute of Structural and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.