1. Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
2. Department of Biological Chemistry and Molecular Pharmacology and Committee on Virology, Harvard Medical School, Boston, MA 02115, USA.
3. Skirball Institute, New York University School of Medicine, New York, NY 10016, USA.
4. Shanghai Institute of Organic Chemistry, Shanghai, China.
5. Department of Biological Chemistry and Howard Hughes Medical Institute, University of Michigan Medical Center, Ann Arbor, MI 48109, USA.