1. Howard Hughes Medical Institute Laboratories, Departments of Cell Biology1, Duke University Medical Center, Durham, North Carolina 27710;,
2. Howard Hughes Medical Institute Laboratories, Departments of Medicine3, Duke University Medical Center, Durham, North Carolina 27710;
3. Present Address: Department of Pharmacology, The Ohio State University College of Medicine and Public Health, Columbus, Ohio 43210;
4. Howard Hughes Medical Institute Laboratories, Departments of Biochemistry2, Duke University Medical Center, Durham, North Carolina 27710;