1. Department of Pediatrics, University of Iowa Carver College of Medicine, 240 EMRB, Iowa City, IA 52242, U.S.A.
2. Department of Biochemistry and Protein Crystallography Facility, University of Iowa Carver College of Medicine, 240 EMRB, Iowa City, IA 52242, U.S.A.
3. Department of Microbiology, University of Washington, Seattle, WA 98195, U.S.A.
4. Department of Medicine, University of Washington, Seattle, WA 98195, U.S.A.
5. Department of Medicine, University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.
6. Department of Biological Sciences, California State University, Los Angeles, CA 90032, U.S.A.