Cyclin dependent kinases (CDKs) play a major role in many cellular processes including the mammalian cell cycle. CDKs are heterodimeric structures that contain two subunits, the catalytic kinase and the regulatory cyclin. This complex is responsible for the phosphorylation of various substrates to tightly control cell proliferation. Other proteins also regulate the cell cycle such as CDK inhibitors and tumor suppressor proteins.

The mammalian cell cycle is the process in which cells replicate and divide to form two daughter cells from a single parental cell. There are four consecutive phases of the cell cycle that must occur in order for the cell to properly divide. These phases are Gap 1 (G1), Synthesis (S), Gap 2 (G2), and Mitosis (M). Along with the CDK/cyclin complexes, there are other regulators that are required for the cell to successfully progress through the cell cycle. The regulation of cell proliferation is aided by inhibitory proteins which bind to CDKs (CDKIs). There are two distinct families of CDKIs, INK4 and Cip/Kip. The INK4 family preferentially inactivates CDK4/6 and consists of at least four members (p15INK4b, p16INK4a, p18INK4c, and p19INK4d). These proteins bind to monomeric CDK to prevent their association with the D-type cyclins1. The activities of the INK4 family results in the inhibition of the cell cycle in G1 by disrupting the ability of CDK4/6 to phosphorylate pRb. Without pRb phosphorylation, E2F-1 activity is inhibited and the cell never enters S phase. The mechanism of action in which INK4 inhibitors bind and inhibit the CDK protein is through binding near the adenosine triphosphate (ATP) binding site which alters its conformation and allosterically prevents ATP from binding.

The Cip/Kip family inactivates the CDK/cyclin complexes of CDK1, CDK2, CDK4, and CDK6. These inhibitory proteins include p21WAF1, p21Cip1, p27Cip2, and p57Kip2. Each of these proteins contains a conserved sequence known as the cyclin binding motif (CBM). These proteins interact with the cyclin through this motif via protein-protein interactions (PPI) at a shallow hydrophobic region on the cyclin known as the cyclin binding groove (CBG). Other proteins (i.e. E2F-1, pRb, etc) also contain the CBM and interact with the CBG in a similar manner prior to their phosphorylation by the CDK/cyclin complexes. The expression of p21WAF1 is under transcriptional control of p53, a tumor suppressor protein. The levels of p53 are generally kept low with in the cell however, when there is DNA damage (replication errors, external damaging agents such as X-rays or UV light) p53 levels accumulate enough for the expression of p21WAF1 to occur, resulting in cell cycle arrest.

Also controlled by CDKs are checkpoints throughout the cell cycle. These checkpoints aid in the regulation of proliferation, ensure that the cell is in its optimum state prior to the transition from one phase to the next, and are sensitive to problems within DNA replication or within the cellular environment. The first type of checkpoint is the restriction point late in G1. This checkpoint is sensitive to the physiological state of the cell and its environment and with inappropriate mitogenic signaling, the cell will not transition into S phase. This restriction point is considered the “point of no return” for the cell cycle1. It is at this point that the cell either commits to or abandons cell cycle progression. The second type of checkpoint is the DNA damage checkpoint. G1, S, and G2 phases each have this checkpoint which has the ability to block cell proliferation. If DNA damage is detected, the checkpoint will arrest the cell cycle to allow time for DNA repair1. If the damage is too advanced the cell will undergo apoptosis.

The third type of checkpoint is the DNA replication checkpoint. This checkpoint has the ability to detect unreplicated DNA or malfunctioned replication machinery. If such problems arise, this checkpoint is able to aid in the stabilization of the replication machinery to promote repair or to further DNA replication7. The last type of checkpoint is the spindle assembly checkpoint. This checkpoint ensures that all chromosomes have properly attached to the mitotic spindle prior to chromosome segregation. If improper attachment to the spindle is detected, the cell will arrest in mitosis.


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