1. Department of Pathology, Harvard Medical School and Brigham and Women's Hospital, 75 Francis Street, Boston, MA 02115, USA
2. Harvard Cryo-Electron Microscopy Center for Structural Biology, Harvard Medical School, 250 Longwood Avenue, Boston, MA 02115, USA
3. Department of Biological Chemistry and Molecular Pharmacology, Blavatnik Institute, Harvard Medical School, 250 Longwood Avenue, Boston, MA 02115, USA