Institute of Structural and Molecular Biology, Department of Biological Sciences, Birkbeck, Malet Street, London, UK.|Institute of Structural and Molecular Biology, Department of Biological Sciences, Birkbeck, Malet Street, London, UK.|Institute of Structural and Molecular Biology, Department of Biological Sciences, Birkbeck, Malet Street, London, UK.|Institute of Structural and Molecular Biology, University College London, London, UK.|Institute of Structural and Molecular Biology, Department of Biological Sciences, Birkbeck, Malet Street, London, UK.