1. Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143, USA.
2. Department of Molecular Biology and Genetics, University of Aarhus, 8000 Aarhus, Denmark.
3. Department of Physiology, University of California, San Francisco, CA 94143, USA.
4. Howard Hughes Medical Institute, University of California, San Francisco, CA 94143, USA.