1. Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, IL 60611
2. Biomolecular Research, Structural Biology, Paul Scherrer Insititut, CH-5232 Villigen PSI, Switzerland
3. Department of Cell Biology and Department of Neuroscience, Erasmus Medical Center, 3000 CA Rotterdam, Netherlands
4. FOM Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, Netherlands