[1] Univ Calif Berkeley, Dept Mol & Cell Biol, 229 Stanley Hall, Berkeley, CA 94720 USA
[2] Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA
[3] Univ Calif Berkeley, Dept Bioengn, Berkeley, CA 94720 USA
[4] European Mol Biol Lab, Cell Biol & Biophys Unit, Heidelberg, Germany