1. Department of Physiology, University of California, San Francisco, CA 94158, USA.
2. Institute of Biology 1, University of Freiburg, 79104 Freiburg, Germany.
3. Department of Biochemistry and Biophysics and Cardiovascular Research Institute, University of California, San Francisco, CA 94158, USA.
4. Max Planck Institute of Molecular Cell Biology and Genetics, 01307 Dresden, Germany.