1. Cellular and Molecular Biology Laboratory, The Institute of Physical and Chemical Research (RIKEN), Wako-shi; Saitama 351-0198 Japan
2. The Graduate School of Science and Engineering, Saitama University, Urawa-shi; Saitama 338-8570 Japan
3. Faculty of Science, Ochanomizu University; Bunkyo-ku Tokyo 112-8610 Japan
4. Department of Biology, Osaka University, Toyonaka; Osaka 560 Japan