1. Department of Biochemistry and Institute for Protein Design, University of Washington, Seattle, WA 98195, USA.
2. Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
3. Department of Structural Biology, Stanford University School of Medicine, Stanford, CA 94305, USA.
4. Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
5. Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA.