Acknowledgements
We thank Thomas Sorenson and Liz Duke for assistance with data collection at I03 at the Diamond Light Source. We thank the staff at the European Synchrotron Radiation Facility protein crystallography beamlines and Martin Noble, Ed Lowe, and other members of the Laboratory of Molecular Biophysics at the University of Oxford for assistance with data collection. We thank Stephen Graham for helpful suggestions during data collection and refinement. We are grateful to Tony Willis for N-terminal sequencing and Joanne Nettleship for collection of mass spectrometry data. JED was funded by a Wellcome Trust Grant (No. 077082) to SML. PR and SJ were funded by grants to SML from the Medical Research Council (MRC) of the United Kingdom (grants G0400775 and G0400389). CK was funded by a MRC studentship.