1. Cardiovascular Research Institute, University of California, San Francisco, San Francisco, CA 94158, USA.
2. Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
3. Departments of Biochemistry and Biophysics, and Cellular and Molecular Pharmacology, University of California, San Francisco, San Francisco, CA 94158, USA.
4. California Institute for Quantitative Biomedical Research, San Francisco, CA 94158, USA.
5. Kavli Institute for Fundamental Neuroscience, University of California, San Francisco, San Francisco, CA 94158, USA.
6. Molecular Biophysics and Integrated Bio-imaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.