1. York Structural Biology Laboratory, Department of Chemistry, University of York , York YO10 5DD, UK
2. Structural and Computational Biology Unit, European Molecular Biology Laboratory , 69117 Heidelberg , Germany
3. MRC Mitochondrial Biology Unit, University of Cambridge, The Keith Peters Building, Cambridge Biomedical Campus , Hills Road , Cambridge CB2 0XY, UK
4. Department of Structural Biology, School of Medicine, University of Pittsburgh , Pittsburgh , PA 15260, USA