1. Department of Physiology & Biophysics, University of California, Irvine, California 92697-4056, Himeji Institute of Technology, Harima Science Garden City, Kamigori, Hyogo 678-12, Japan, Department of Structural Biology, Biomolecular Engineering Research Institute, Suita, Osaka 565, Japan, and Department of Biochemistry, Wayne State University, Detroit, Michigan 48201