1. Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, U.K.
2. Grenoble Outstation, European Molecular Biology Laboratory, 6 rue Jules Horowitz, BP181, 38042 Grenoble Cedex 9, France
3. Department of Biological Chemistry, John Innes Centre, Norwich NR4 7UH, U.K.