1. Department of Immunology, Harvard Medical School, Boston, MA 02115, USA.
2. The Ragon Institute of MGH, MIT, and Harvard, Cambridge, MA 02139, USA.
3. Cell Biology Microscopy Facility, Harvard Medical School, Boston, MA 02115, USA.
4. Departments of Chemistry, Molecular & Cell Biology, and Helen Wills Neuroscience Institute, University of California, Berkeley, CA 94720, USA.