1. Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, IL 60637
2. Department of Physiology, University of Wisconsin-Madison, Madison, WI 53706
3. Department of Biochemistry and Immunology, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil