1. Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 32377
2. Department of Pathology & Laboratory Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 32377
3. Institute for Research in Immunology and Cancer, University of Montréal, Montréal, QC H3C 3T5, Canada
4. Cell Biology and Biophysics Unit, EMBL, 69117 Heidelberg, Germany