1. Structural Biology of Molecular Machines Group, Protein Structure & Function Programme, Novo Nordisk Foundation Center for Protein Research, Faculty of Health and Medical Sciences; University of Copenhagen, Blegdamsvej 3B; Copenhagen 2200 Denmark
2. Departamento de Estructura de Macromoleculas; Centro Nacional de Biotecnologia (CSIC), Calle Darwin 3; E-28049 Madrid Spain
3. Department of Biochemistry and Molecular Biology, 301 University Blvd; University of Texas Medical Branch; Galveston TX 77555-0647 USA