[1] Stanford Univ, Sch Med, Dept Cellular & Mol Physiol, Stanford, CA 94305 USA
[2] Scripps Res Inst, Dept Mol Biol, La Jolla, CA 92037 USA
[3] Stanford Univ, Sch Med, Dept Biol Struct, Stanford, CA 94305 USA
[4] Scripps Res Inst, Dept Cell Biol, La Jolla, CA 92037 USA