1. Structural Biology Laboratory, The Salk Institute, La Jolla, California 92037, Division of Biological Sciences, University of California, San Diego, La Jolla, California 92093, Human Genome Sciences Inc., Rockville, Maryland 20850, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, and Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229