Detection of Cyclins in Individual Cells by Flow and Laser Scanning Cytometry

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 1Cyclins 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