1. Department of Structural Biology, Stanford University , Stanford, CA 94305-5126, USA
2. Program in Biophysics, Stanford University , Stanford, CA 94305-5126, USA
3. Department of Molecular Biophysics and Biochemistry, Yale University , New Haven, CT 06511, USA
4. Department of Biochemistry and Molecular Biophysics, Columbia University , New York, NY 10032, USA
5. Department of Cell and Molecular Biology, Uppsala University , Uppsala 751 24, Sweden
6. Department of Chemistry, Yale University , New Haven, CT 06511, USA