1. Contribution from the Molecular Structural Biology Unit, National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland 20892, Protein Expression Lab, National Institute of Arthritis and Musculoskeletal and Skin Diseases, Bethesda, Maryland 20892, Structural Biochemistry Program, NCI-Frederick Cancer Research and Development Center, SAIC-Frederick, Frederick, Maryland 21702, and Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607, Japan