1. Department of Biochemistry and Molecular Biophysics, Columbia University, 2-221 Blackwell, 165 West 168th Street, New York, NY 10032, USA.
2. Department of Cell and Molecular Biology, Uppsala University, Box-596, BMC, 75124 Uppsala, Sweden.
3. Howard Hughes Medical Institute, Columbia University, New York, NY 10032, USA.
4. Department of Biological Sciences, Columbia University, New York, NY 10027, USA.