1. School of Molecular and Cellular Biology, Faculty of Biological Sciences, and Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
2. Present address: Division of Virology, Department of Pathology, University of Cambridge, Cambridge, CB2 0QQ, UK
3. Faculty of Health and Medical Sciences, School of Biosciences and Medicine, University of Surrey, Guildford, GU2 7XH, UK