1. Institute of Molecular and Cell Biology A*STAR (Agency for Science, Technology and Research) Biopolis; Singapore 138673 Singapore
2. Department of Biochemistry Yong Loo Lin School of Medicine National University of Singapore; Singapore 117597 Singapore
3. Nagoya University Graduate School of Science Structural Biology Research Center and Division of Biological Sciences; Furo-cho, Chikusa-ku Nagoya 464-8601 Japan
4. Max Planck Institute for Medical Research; D69120 Heidelberg Germany
5. Research Institute for Interdisciplinary Science Okayama University; Okayama 700-8530 Japan