1. Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093-0340, Department of Chemistry, Stanford University, Stanford, California 94305-5080, Department of Pharmacology and Howard Hughes Medical Institute, University of California San Diego, La Jolla, California 92093-0647, and Cell Function & Dynamics, Brain Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan