1. Institute of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, LS2 9JT, United Kingdom, Astbury Centre for Structural Molecular Biology, University of Leeds, LS2 9JT, United Kingdom, Division of Virology, Department of Pathology, University of Cambridge, CB2 1QP, United Kingdom, and Department of Cellular and Molecular Medicine, School of Medical Sciences, University of Bristol, BS8 1TD, United Kingdom