1. Department of Physics & Astronomy, University College London, London WC1E 6BT, United Kingdom;
2. Institute for the Physics of Living Systems, University College London, London WC1E 6BT, United Kingdom;
3. Medical Research Council Laboratory for Molecular Cell Biology, University College London, London WC1E 6BT, United Kingdom;
4. Medical Research Council Laboratory of Molecular Biology, University of Cambridge, Cambridge CB2 0QH, United Kingdom