1. Center for Membrane and Cell Physiology, University of Virginia, Charlottesville, VA 22908, USA.
2. Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA.
3. Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
4. Department of Cell Biology, University of Virginia, Charlottesville, VA 22908, USA.