Institute of Structural and Molecular Biology, School of Biological Sciences, The University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.|Institute of Structural and Molecular Biology, School of Biological Sciences, The University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.|Institute of Structural and Molecular Biology, School of Biological Sciences, The University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.|Institute of Structural and Molecular Biology, School of Biological Sciences, The University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.