[1] Univ Calif San Francisco, Dept Biopharmaceut Sci, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Dept Pharmaceut Chem, San Francisco, CA 94143 USA
[3] Univ Calif San Francisco, Calif Inst Quantitat Biomed Res, San Francisco, CA 94143 USA
[4] Univ Calif Berkeley, Dept Mol & Cell Biol, Stanley Donner ASU, Berkeley, CA 94720 USA
[5] Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley Ctr Struct Biol, Berkeley, CA 94720 USA
[6] Max Planck Inst Biochem, Dept Biol Struct, D-82152 Martinsried, Germany