1. Department of Biochemistry and Molecular Biology, University College London, London WCIE 6BT, UK, Department of Biochemistry and Molecular Biology, The University of Leeds, Leeds LS2 9JT, UK, CLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK, Department of Physical and Metabolic Sciences, Astra Charnwood, Bakewell Road, Loughborough, Leicestershire LE11 5RH, UK, and Department of Discovery Biology, Pfizer Central Research, Sandwich, Kent CT13 9NJ, UK