1. Department of Biochemistry; University of Washington; Seattle Washington 98195
2. Institut Pasteur, Département de Virologie, Unité de Virologie Structurale; Paris France
3. CNRS UMR 3569 Virologie; Paris France
4. Virology Division, Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine; Utrecht University; 3584 CL Utrecht The Netherlands