1. Institute of Molecular, Cell and Systems Biology, Glasgow University, Glasgow G12 8QQ, UK.
2. MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
3. Materials and Structural Analysis, Thermo Fisher Scientific, Achtseweg Noord 5, 5651 GG Eindhoven, Netherlands.
4. Department of Molecular Biology, The University of Sheffield, Sheffield S10 2TN, UK.