1. Department of Biological Sciences, Institute of Structural and Molecular Biology, Birkbeck College, London WC1E 7HX, U.K.
2. Department of Chemistry, Christopher Ingold Building, University College London, London WC1H 0AJ, U.K.
3. Department of Biochemical Engineering, Bernard Katz Building, University College London, London WC1E 6BT, U.K.