1. Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, U.K.
2. Department of Physiology, Anatomy & Genetics, University of Oxford, South Parks Road, Oxford, OX1 3PT, U.K.
3. Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford, OX1 3RE, U.K.
4. Division of Structural Biology, Wellcome Centre for Human Genetics, University of Oxford, Roosevelt Drive, Oxford, OX3 7BN, U.K.
5. School of Life Sciences and Department of Chemistry, University of Warwick, Coventry, CV4 7AL, U.K.