1. Department of Microbiology and Immunology, University of Illinois at Chicago, Chicago, IL 60612–7344, USA.
2. Laboratoire de Cristallographie et de Cristallogénèse des Protéins, IBS, Institut de Biologie Structurale Jean-Pierre Ebel; CEA; CNRS; Université Joseph Fourier, 41 rue Jules Horowitz, F-38207 Grenoble, France.
3. Children's Hospital Oakland Research Institute, Oakland, CA 94609–1673, and Department of Nutritional Science and Molecular Toxicology, University of California, Berkeley, Berkeley, CA 94720–3104, USA.