1. Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-01, Japan, Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel, Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201, and Department of Physiology and Biophysics, University of California, Irvine, California 92717