1. Program in Macromolecular Structure, Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, Departments of Medicine and Nephrology and of Biochemistry, Vanderbilt University Medical Center, Nashville, Tennessee 37232-2372, Syrrx, Inc., 10410 Science Center Drive, San Diego, California 92121, and The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037