1. Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.
2. Department of Medicine, Duke University Medical Center, Durham, NC 27710, USA.
3. Howard Hughes Medical Institute, Duke University Medical Center, Durham, NC 27710, USA.
4. Department of Cell Biology, Duke University Medical Center, Durham, NC 27710, USA.
5. Department of Cell Biology, Harvard University Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.