School of Biological Sciences, Stopford Building, The University of Manchester, Oxford Road, Manchester M13 6PT, UK. wright@biochemistry.ucl.ac.uk|School of Biological Sciences, Stopford Building, The University of Manchester, Oxford Road, Manchester M13 6PT, UK. wright@biochemistry.ucl.ac.uk|School of Biological Sciences, Stopford Building, The University of Manchester, Oxford Road, Manchester M13 6PT, UK. wright@biochemistry.ucl.ac.uk