[1] Univ Washington, Dept Biochem, Seattle, WA 98195 USA
[2] Univ Washington, Dept Genome Sci, Seattle, WA 98195 USA
[3] Univ Washington, Dept Biomol Struct & Design, Seattle, WA 98195 USA
[4] Lawrence Berkeley Natl Lab, Phys Biosci Div, Berkeley, CA 94720 USA