1. Department of Molecular and Cell Biology, and Leicester Institute of Chemical Biology, University of Leicester, Leicester LE1 7RH, U.K.
2. Centre for Enzyme Innovation, School of Biological Sciences, University of Portsmouth, Portsmouth, Hampshire PO1 2DY, U.K.
3. Core Biotechnology Services Electron Microscopy Facility, University of Leicester, Leicester LE1 7RH, U.K.
4. School of Medicine, Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth PO1 2DT, U.K.