Department of Structural and Molecular Biology, UCL, London WC1E 6BT, UK. yeats@biochem.ucl.ac.uk|Department of Structural and Molecular Biology, UCL, London WC1E 6BT, UK. yeats@biochem.ucl.ac.uk|Department of Structural and Molecular Biology, UCL, London WC1E 6BT, UK. yeats@biochem.ucl.ac.uk