Alexandra H. Antonioli, Jesse C. Cochrane1, Sarah V. Lipchock and Scott A. Strobel Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520-8114, USA ↵1 Present address: Department of Molecular Biology, Massachusetts General Hospital, and Department of Genetics, Harvard Medical School, 185 Cambridge Street, Boston, MA 02114, USA.