1. Department of Immunology, University of Washington, Seattle, WA 98109, USA.
2. Molecular and Cellular Biology Program, University of Washington, Seattle, WA 98109, USA.
3. Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ 08854, USA.
4. Institute for Protein Design, University of Washington, Seattle, WA 98109, USA.