1. School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.
2. Protein Structure Laboratory, Zeneca Pharmaceuticals, Mereside, Alderley Park, Macclesfield SK10 4TG, U.K.
3. School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.
4. Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.