1. Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.
2. Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, NC 27710, USA.
3. Department of Cell Biology, Duke University Medical Center, Durham, NC 27710, USA.
4. Department of Neurobiology, Duke University Medical Center, Durham, NC 27710, USA.
5. Howard Hughes Medical Institute, Durham, NC 27710, USA.