1. Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
2. Department of Biophysics and Biochemistry, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.
3. Institute of Biological Chemistry, Academia Sinica, Taipei 115, Taiwan.
4. Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84604, USA.