1. University College London Department of Structural and Molecular Biology Gower Street London WC1E 6BT UK
2. Current address: Imperial College London Exhibition Road London SW7 2AZ UK
3. Rega Institute for Medical Research KU Leuven Herestraat, 49 box 1041 3000 Leuven Belgium
4. Current address: Touchlight Genetics Ltd. Morelands & Riverdale Buildings Lower Sunbury Road Hampton TW12 2ER UK
5. Institute of Structural and Molecular Biology Birkbeck College University of London Malet Street London WC1E 7HX UK