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Fig. 3. The interactions between PCNA and cell cycle regulatory networks. The
eukaryotic cell cycle can be divided into the four phases: G1 (green), where
cells grow in size, assess their metabolic status and get ready to divide; S
(yellow), where the actual genome duplication takes place; G2 (green), where
cells check for completion of DNA replication and prepare to divide and M
(red), where mitosis and cytodieresis take place. Each phase is under the
general control of specific CDK-cyclin complexes: the CDK4,6–cyclin-D
complex regulates progression through G1, CDK2–cyclin-E is involved in
regulating the transition from G1 to S phase (also known as the restriction
point), CDK2–cyclin-A and CDK1–cyclin-A act throughout the S
phase, whereas CDK1–Cyclin-B regulates mitosis. Moreover, specific
checkpoint mechanisms can be activated halting the progression through the
cell cycle in case problems arise. PCNA forms complexes with all these
CDK–cyclin complexes as well as with critical checkpoint proteins,
transducing both positive (indicated as arrows) and negative signals
(indicated as T-bars).
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-helices at the N-terminus (pink);
and (3) the C-terminal tail (blue). For clarity, only one region per each
monomer is highlighted. The PCNA structure was generated with the program
Swiss-PDBViewer from the PDB file 1AXC, rendered with QuickTime 3D and
composed in Adobe PhotoShop.
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holoenzyme (pol