1. Department of Pathology & Immunology, Washington University School of Medicine, St. Louis, MO 63110, USA.
2. Department of Biochemistry, Biophysics & Molecular Biology, Iowa State University, Ames, IA 50011, USA.
3. Department of Biochemistry, University of Iowa, Iowa City, IA 52242, USA.
4. Department of Pathology, Geisel School of Medicine at Dartmouth, Lebanon, NH 03756, USA.
5. Department of Microbiology and Immunology, Geisel School of Medicine at Dartmouth, Lebanon, NH 03756, USA.