Affiliations

• ¹ Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California San Francisco, 600 16th Street, San Francisco, CA 94158, USA.
• ² Department of Cell Biology and Howard Hughes Medical Institute, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA Present address: The Rockefeller University, 1230 York Avenue, New York, NY 10065, USA.