1. Present address: Experimental Medicine, Nuffield Department of Medicine, The Peter Medawar Building for Pathogen Research, South Parks Road, University of Oxford, Oxford, OX1 3SY, UK
2. School of Molecular and Cellular Biology, Faculty of Biological Sciences and Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
3. Present address: Tsinghua-Peking Center for Life Sciences, School of Medicine, Tsinghua University, Beijing 100084, PR China