Localization and Dynamics of Cdc2-Cyclin B during Meiotic Reinitiation in Starfish Oocytes

Abstract

The Cdc2-cyclin B kinase has a central role in regulating the onset of M phase. In starfish oocytes, Cdc2-cyclin B begins to be activated ϳ10 min after application of maturation hormone, followed by accumulation in the nucleus then nuclear envelope breakdown. By immunofluorescence and by expressing a green fluorescent (GFP) chimera of cyclin B, we find that cyclin B is present in aggregates in the cytoplasm of immature oocytes. The aggregates disperse at ϳ10 min, suggesting that the dispersal is closely related to the activation of the kinase. Using cyclin B-GFP, the dispersion begins from the region containing the centrosomes. Extractability of Cdc2-cyclin B changes with similar kinetics during maturation. Active Cdc25 phosphatase released Cdc2-cyclin B from the detergent-insoluble fraction independently of its phosphatase activity. Live cell imaging also showed that Cdc2-cyclin B begins to accumulate in the nucleus before changes in nuclear pore permeability, consistent with Cdc2-cyclin B-induced disassembly of the pores. INTRODUCTIONThe Cdc2/cdk1 kinase triggers the onset of M phase (Nurse, 1990). Activation of Cdc2 requires association with cyclin B and dephosphorylation of Thr 14 and Tyr 15 on Cdc2; the phosphorylation state is governed by the relative activities of Wee1 family kinases and Cdc25 phosphatase (reviewed in Lew and Kornbluth, 1996). Inactive Cdc2-cyclin B is present in the cytoplasm during interphase and after it is activated, accumulates in the nucleus where it phosphorylates multiple targets to cause nuclear envelope breakdown (Pines and Hunter, 1991;Ookata et al., 1992).There are still many aspects of this system that are not well understood. The pathway leading to activation is different in different organisms and has not been established except in starfish (Okumura et al., 2002). Phosphorylation of cyclin is probably involved in the regulation of nuclear entry (Li et al., 1997;Hagting et al., 1998;Toyoshima et al., 1998;Yang et al., 1998) but may have other roles as well (Peter et al., 2002).Another subject of uncertainty regards Cdc2-cyclin B localization in the cytoplasm and its possible associations with other molecules in the cytoplasm. There are several indications that inactive Cdc2-cyclin B is not free to diffuse in the cytosol but instead, is associated with other molecules or intracellular structures and undergoes changes in associations when it becomes activated. Among various reports, Picard and Peaucellier (1998) observed cyclin B in vesiclelike structures in starfish oocytes, Westendorf et al. (1989) found evidence for cyclin B aggregates in immature clam oocytes, and Lee and Kirschner (1996) found evidence for a binding inhibitor of Cdc2-cyclin B that is associated with membranes. More recently, inactive Cdc2-cyclin B was found to be associated with annulate lamellae in immature Xenopus oocytes (Beckhelling et al., 2003).Current techniques for localization are subject to limitations of various kinds. The use of green fluorescent protein (GFP) chimeras is promising because loc...

Spatial regulation of APCCdh1-induced cyclin B1 degradation maintains G2 arrest in mouse oocytes

“…2H). This cortical distribution was reminiscent of that observed for cyclin B in Xenopus laevis (Beckhelling et al, 2003); and in keeping with the cytoplasmic accumulation observed in oocytes of other species (Terasaki et al, 2003;Westendorf et al, 1989) and in adult cells (Clute and Pines, 1999). Such cytoplasmic accumulation of cyclin B1 may be due to its ability to anchor to cytoplasmic proteins (Charrasse et al, 2000;Jackman et al, 1995;Ookata et al, 1995).…”

The role of the actin cytoskeleton in calcium signaling in starfish oocytes

“…Studies in a number of cell types have indicated that the perinuclear/centrosomal region is the site of MPF accumulation (Bechelling et al, 2000). Recently, using cyclin B-GFP, it was shown that MPF enters the starfish nucleus starting from the animal pole side, which is the location of the centrosomes (Terasaki et al, 2003). The timing of its accumulation in the nucleus is consistent with its proposed role in disassembling the nuclear envelope, as the event takes place after the phosphorylation of the nuclear pore complexes (Macaulay et al, 1995).…”

A Novel Class of Cyclin-dependent Kinase Inhibitors Identified by Molecular Docking Act through a Unique Mechanism

“…Another potential explanation for the induced aggregation is that the Cdks may be inherently prone to aggregation themselves. Cdk1-Cyclin B complexes exist in aggregates in Xenopus oocytes before resolubilizing and becoming active for the initiation of mitosis (30,31). Moreover, smaller and less frequent Cdk aggregates appear to be naturally present in some cell types, as they can be observed in vehicle-treated cells.…”