1. Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, U.K.
2. Department of Chemistry, University of Modena and Reggio Emilia, Via Campi 183, 41125 Modena, Italy
3. Department of Biological Chemistry, John Innes Centre, Norwich NR4 7UH, U.K.