1. School of Biological Sciences, University of Liverpool, Crown Street, Liverpool L69 7ZB, U.K., Faculty of Life Sciences, University of Manchester, Manchester M60 1QD, U.K., Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, U.K., and Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, U.K.