1. CIC bioGUNE, Structural Biology Unit, Bldg 800, Biscay Technology Park Derio 48160 Basque Country Spain
2. MRC Laboratory of Molecular Biology Cambridge CB2 2QH UK
3. Department of Biochemistry and Molecular Biology, Faculty of Science and Technology, University of the Basque Country PO Box 644 E‐48080 Bilbao Spain