[1] Stanford Univ, Howard Hughes Med Inst, Stanford, CA 94305 USA
[2] Stanford Univ, Dept Mol & Cellular Physiol, Stanford, CA 94305 USA
[3] Stanford Univ, Dept Neurol & Neurol Sci, Stanford, CA 94305 USA
[4] Stanford Univ, Stanford Synchrotron Radiat Lab, Stanford, CA 94305 USA
[5] Univ Cambridge, Dept Biochem, Cambridge CB2 1GA, England