1. Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
2. Institute for Protein Design, University of Washington, Seattle, WA 98195, USA
3. Graduate Program in Biological Physics, Structure and Design, University of Washington, Seattle, WA 98195, USA
4. Department of Biochemistry and Biophysics, Oregon State University, Corvallis, OR 97331-7305, USA