ACKNOWLEDGMENTS. We thank the Edinburgh Protein Production (Biophysical Characterisation) Facility for use of circular dichroism spectroscopy and Dr. Colin Hammond for assistance with the size exclusion chromatography–multiangle laser light scattering experiment. This work was supported by Engineering and Physical Sciences Research Council Grant EP/J007404/1 and Biotechnology and Biological Sciences Research Council Grants BB/L006979/1, BB/I019464/1, and BB/L006804/1. Mass-spectrometry analysis of purified proteins was performed in the College of Life Sciences and is supported by the Wellcome Trust 097945/B/11/Z.