1. Department of Virology II
2. Department of Biochemistry and Cell Biology, National Institute of Infectious Diseases, Tokyo 162-8640
3. Department of Molecular Virology, Research Institute for Microbial Diseases, Osaka University, Osaka 565-0871, Japan
4. Department of Molecular Microbiology and Immunology, University of Southern California, Los Angeles, California 90033-1054