Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK. marsden@biochem.ucl.ac.uk|Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK. marsden@biochem.ucl.ac.uk|Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK. marsden@biochem.ucl.ac.uk