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Cell Death, Cell Cycle and Senescence

The C-Terminal Seven Amino Acids in the Cytoplasmic Retention Signal Region of Cyclin B2 are Required for Normal Bipolar Spindle Formation in Xenopus Oocytes and Embryos¹

¹ Department of Biomedical Sciences, School of Life Sciences, Tottori University, Yonago, Japan;
² Laboratory for Amphibian Biology, Graduate School of Science, Hiroshima University, Higashihiroshima, Japan; and
³ Department of Biology, Graduate School of Sciences, Kyushu University, Fukuoka, Japan

Requests for reprints: Satoshi Yoshitome, Department of Biomedical Sciences, School of Life Sciences, Tottori University, 86 Nishi-machi, Yonago 683-8503, Japan. Phone: 81-859-34-8044; Fax: 81-859-34-8208. E-mail: yoshitom{at}grape.med.tottori-u.ac.jp; or Nobuaki Furuno, Laboratory for Amphibian Biology, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima 739-8526, Japan. Phone & Fax: 81-824-24-7483. E-mail: nfuruno{at}hiroshima-u.ac.jp

In many vertebrates, cyclin B has several subtypes, but the functional differences among them are largely unclear. Previously, we have shown that Xenopus cyclin B2, not cyclin B1, is involved in bipolar spindle formation through its cytoplasmic retention signal (CRS) region. However, identification of a nuclear export signal (NES) in the CRS region of cyclin B1 raised the possibility that an NES-like sequence (NELS) present in the CRS region of cyclin B2 might be involved in bipolar spindle formation. We show here that cyclin B2 is actually exported from the nucleus via its NELS, but that overexpression of the cyclin B2 CRS region, having a mutated NELS, still inhibits bipolar spindle formation in oocytes. In contrast, overexpression of the cyclin B2 CRS region lacking its C-terminal seven amino acids no longer inhibits bipolar spindle formation in oocytes or embryos. These results suggest strongly that the CRS region, especially its C-terminal seven acidic residues, of cyclin B2 is required for bipolar spindle formation in both the meiotic and mitotic cell divisions.

Key Words: spindle apparatus • cyclin B • nuclear export • Xenopus oocytes