Detection of Cyclins in Individual Cells by Flow and Laser Scanning Cytometry
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Progression of cells through successive phases and checkpoints of the cell cycle is maintained by sequential phosphorylation of different sets of nuclear and cytoplasmic proteins by cyclin-dependent kinases (CDKs) (1 –12 ). By activating their partner CDKs and targeting them to the respective protein substrates cyclins play a key regulatory role in this process. Cyclins Bl, A, E, and D are expressed discontinuously during the cycle. The synthesis and degradation of these cyclins occurs at well-defined time points of the cell cycle (Table 1 ).
Table 1 Cyclins and Their Partner CDKs During the Cell Cycle
|
Cyclin |
Primary CDK partner(s) |
Presumed role in cell cycle |
Peak of expression |
Localization |
|---|---|---|---|---|
|
D type |
CDK4 and CDK6 |
pRB phosphorylation, commitment to S phase |
Early in G1 |
Nucleus |
|
E |
CDK2 |
Initiation of S |
G1/ S transition |
Nucleus |
|
A |
CDK2 and CDC2 |
S and G2 traverse |
During G2M |
Nucleus |
|
Bl |
CDC2 |
G2 traverse entrance to M |
Late G2/M |
Cytoplasm/Nucleusa |
a Cyclin B1 is localized in cytoplasm during G2 and undergoes translocation to nucleus during prophase
