1. Department of Biochemistry, University of Washington, Seattle, WA 98195, USA.
2. Institute for Protein Design, University of Washington, Seattle, WA 98195, USA.
3. Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California, Los Angeles (UCLA), CA 90095, USA.
4. Department of Bioengineering and Biophysics Group, University of California, Berkeley, Berkeley, CA 94720, USA.
5. Department of Biological Structure, University of Washington, Seattle, WA 98195, USA.
6. Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA.