Overexpression of the cytoplasmic retention signal region of cyclin B2, but not of cyclin B1, inhibits bipolar spindle formation in Xenopus oocytes

Abstract

Cyclin B, a regulatory subunit of maturation/M-phase promoting factor (MPF), has several subtypes in many vertebrate species. However, it is not known whether the different B-type cyclins have any different functions in vertebrate cells, although their subcellular localizations seem to differ largely from each other. To examine the roles of two major B-type cyclins, B1 and B2, in spindle formation in M phase, we overexpressed their N-termini in Xenopus oocytes; the N-termini of cyclins B1 and B2 contained a cytoplasmic retention signal (CRS), and hence their overexpressions were expected to competitively inhibit the subcellular localizations of the endogenous cyclins B1 and B2, respectively. Upon entry into meiosis I, oocytes overexpressing the cyclin B1 N-terminus formed an apparently normal bipolar spindle, but those oocytes overexpressing the cyclin B2 N-terminus formed a monopolar (or monoastral) spindle. This defect in bipolar spindle formation was observed only when the cyclin B2 N-terminus contained its own CRS sequence, and was able to be rescued by overexpression of full-length cyclin B2. These results suggest, for the first time, that the correct subcellular localization of cyclin B2, but not of cyclin B1, is essential for (the initiation of) bipolar spindle formation in Xenopus oocytes.

Localization and Function of Cyclin B1 and Cyclin B2 during Porcine Oocyte Maturation

“…The cyclin B binding with tubulin has also been shown in biochemical experiments [20]. [5,23,25,26]showing cyclin B localization on the spindle did not exclude its presence in the cytoplasm during the Mphase. Weak signals were always detected in the cytoplasm on immunostaining with cyclin B-specific antibodies, indicating partiality to their localization on the spindle.…”

“…2C). Indeed, the requirement of cyclin B2, but not cyclin B1, for the first meiotic spindle formation has been reported in Xenopus and Rana japonica oocytes [5,23]. In porcine oocytes, however, cyclin B2 might not always be necessary for the first meiotic spindle formation, as described below.…”

“…In human cells, cyclin B1 can cause chromosome condensation, reorganization of the microtubules, and disassembly of the nuclear lamina and of the Golgi apparatus, whereas the role of cyclin B2 is restricted only to disassembly of the Golgi apparatus [2,3]. In maturing oocytes, differences between cyclin B1 and cyclin B2 functions have been reported in the first meiotic spindle formation and the second metaphase arrest in frog and mouse oocytes, respectively [4][5][6]. In our laboratory, we have studied cyclin B functions during porcine oocyte maturation for the past several years.…”

Differential Expression of Cyclins B1 and B2 during Medaka (Oryzias latipes) Spermatogenesis.

“…In many species, at least two different types of cyclin B are present, but their functional differences remain a mystery (Yoshitome et al, 1998). We have shown here that cyclin B1 protein is expressed in spermatogonia and spermatocytes with a transient disappearance at anaphase, whereas cyclin B2 protein is expressed in these cells even at anaphase (Fig.…”

“…Although only one type of cyclin B (cyclin B1) has been discovered in goldfish , zebrafish (Kondo et al, 1997) and the rat (Markiewicz et al, 1994) to date, many species have several subtypes of cyclin B as is the case for other cyclins; i.e., the human (Jackman et al, 1995), mouse (Chapman and Wolgemuth, 1992;Chapman and Wolgemuth, 1993) and Japanese brown frog Rana japonica (Ihara et al, 1998) have cyclins B1 and B2, the domestic fowl has cyclins B2 and B3 (Gallant and Nigg, 1992;Gallant and Nigg, 1994), and the African clawed frog Xenopus laevis has cyclins B1 to B5 (Brandeis et al, 1998;Minshull et al, 1989). Subtype-dependent differences in the expression pattern, subcellular localization and phenotype of the ectopic expression or knockout of the genes were reported for Xenopus oocytes (Kobayashi et al, 1991;Stewart et al, 1994;Yoshitome et al, 1998), mouse spermatogenic cells (Brandeis et al, 1998;Chapman and Wolgemuth, 1992;Chapman and Wolgemuth, 1993;Chapman and Wolgemuth, 1994) and human cultured cells (HeLa cells) (Jackman et al, 1995), but their functional differences are still unknown.…”