1. Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
2. Department of Theoretical Biophysics, Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany.
3. Faculty of Arts and Sciences, Center for Systems Biology, Harvard University, Cambridge, MA 02138, USA.
4. Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
5. Present address: Department of Pharmacology, Yale University School of Medicine, New Haven, CT 06520, USA.
6. Howard Hughes Medical Institute and Department of Biochemistry and Molecular Pharmacology, New York University School of Medicine, New York, NY 10016, USA.
7. Institute of Biophysics, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany.