1. Department of Molecular Biology, Cancer Research Institute; Sapporo Medical University School of Medicine; Sapporo Hokkaido 060-8556 Japan
2. Division of Cell Biology, Cancer Institute; Hokkaido University School of Medicine; Sapporo Hokkaido 060-8638 Japan
3. Department of Microbiology; Keio University School of Medicine; Shinjuku-ku Tokyo 160-8582 Japan