1. Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health , Bethesda , MD 20892 , USA
2. Cambridge Institute for Medical Research, University of Cambridge , Cambridge CB2 0XY , UK
3. Department of Biological Chemistry , John Innes Centre, Norwich NR4 7UH , UK
4. Department of Physics, University of California , Merced, CA 95343 , USA
5. Department of Molecular Microbiology , John Innes Centre, Norwich NR4 7UH , UK