1. Department of Biochemistry, University of Washington, Seattle, WA 98195, USA.
2. Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
3. Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA.
4. Division of Basic Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA.