Expression, Localisation and Stability of Mitotic Cyclins in Tobacco BY-2 Cells
Cyclin-dependent kinases (CDKs) control the cell cycle transitions in all eukaryotes (Nigg 1995, 2001). In yeast, different cyclins act with a single catalytic subunit (Cdc28 in budding yeast and Cdc2 in fission yeast), whereas in multicellular eukaryotes, different cyclins act with different CDKs. It is the sequential waves of different cyclin-CDK activities that regulate the progress through the cell cycle (Pines and Rieder 2001). The activity of the CDKs is dependent on cyclin binding, which is also involved in the subcellular localisation and substrate specificity of the CDK complexes. Since cyclin protein levels oscillate during the cell cycle, these proteins directly determine the timing of CDK activation. Cyclins that specifically activate CDKs have been grouped into different families: the G1 cyclins (called CLNs in budding yeast and D- and E-type cyclins in animals) and the mitotic cyclins (called CLBs in budding yeast and A- and B-type cyclins in animals). In animals, mitotic cyclins have been further subdivided in two different A-type cyclins (A1 and A2) and three different B-type cyclins (B1, B2 and B3). The G1 cyclins function during G1 phase to control cell cycle commitment in response to growth and mitogenic signals (Sherr 1994). Whereas vertebrate A-type cyclin binds to both CDK1 and CDK2 and is required for S-phase and early mitotic events, cyclin B1 bound to CDK1 is assumed to fulfil only mitotic functions. Both CDK1 and CDK2 are closely related to the yeast Cdc28/Cdc2 CDKs, which share the conserved PSTAIRE motif in the cyclin binding domain.
KeywordsSpindle Checkpoint Mitotic Cyclins Plant Cyclin Cyclin Degradation Cyclin Binding Domain
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