1. Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
2. Center for Tsukuba Advanced Research Alliance and Institute of Applied Biochemistry, University of Tsukuba, Tsukuba 305, Japan, and Division of Molecular and Structural Biology, Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, 610 University Avenue, Toronto, Ontario, Canada M5G 2M9