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

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 verte brate cells, although their subcellular localizations seem to differ largel y 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 X enopus oocytes; the N-termini of cyclins B1 and B2 contained a cytoplasmic retention signal (CRS), and hence their overexpressions were expected to co mpetitively 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 (o r monoastral) spindle. This defect in bipolar spindle formation was observe d only when the cyclin B2 N-terminus contained its own CRS sequence, and wa s able to be rescued by overexpression of full-length cyclin B2. These resu lts suggest, for the first time, that the correct subcellular localization of cyclin B2, but not of cyclin B1, is essential for (the initiation of) bi polar spindle formation in Xenopus oocytes. ((C) Elsevier, Paris).