Cyclin dependent kinases, also known as cdks, are a family of kinases that are important for a variety of eukaryotic cellular processes, most notably the cell cycle. They are responsible for moving the cell from G1 phase to S phase to G2 phase and finally into mitosis where the cell divides into two daughter cells. Cdks are made up of two subunits; a kinase subunit also called cdk and a cyclin subunit responsible for activating the cdk. Cdks were originally discovered in yeast and were called cdcs, or cell-division-cycle genes. The cdc genes were also found in humans and other higher eukaryotes, where they were relabeled as cdks.
Mechanism and Regulation
Cdks work in tandem with another group of proteins called cyclins. The amount of the various cyclins oscillates characteristically throughout the cell cycle. For example, cyclin D protein is present at its highest amount during G1 phase. When there is enough cyclin D protein they are able to bind to and activate a particular cdk, such as cdk4. Cdk4 is then able to phosphorylate a variety of proteins to help the cell move from G1 phase into S phase. At the end of G1 phase the cyclin D proteins are broken down and cdk4 is turned off. The cdks have a variety of protein targets that they phosphorylate to move the cell through the cell cycle. For example, cdks4 and cdk6 phosphorylate a protein called retinoblastoma during G1 phase, which releases its control on a transcription factor that then induces the expression of proteins that will be needed in S phase. Cdk2 phosphorylates Histone H1, which is thought to help condense the chromosomes during S phase. Cdk1 phosphorylates lamins and microtubules that help dissociate the nuclear membrane and assemble the mitotic spindle for mitosis.
Two events must occur in order to activate the cdks. First, they must be bound to their respective cyclins. This only occurs when there is enough of the cyclin present, which only occurs during a certain phase of the cell cycle. Additionally, once the cyclins are degraded at the end of the cell cycle phase the cdks are inactivated. Second, they must be phosphorylated at certain residues by a kinase called CAK. This phosphorylation strengthens the binding of the cdk to its cyclin and allows the cdk to bind ATP so it can phosphorylate its substrates.
Cdks are also negatively controlled in two ways. First, the cdks often have residues that are phosphorylated that keep the cdk from becoming active. These residues are different from the residues that must be phosphorylated to activate the cdk as described above. Specific phosphatases are responsible for removing the phosphorylation. Second, a group of proteins called cdk inhibitors, or ckis, are responsible for inhibiting cdk activity. These proteins bind either directly to the cdk or to the cdk-cyclin complex and prevent them from phosphorylating their substrates. Ckis are divided into two groups, the INK4 group and the cip/kip group. The INK 4 members directly inhibit cdk4 and cdk6, while the cip/kip members inhibit any cdk-cyclin complex. The ckis are inactivated by the cdks themselves. When there is enough cdk activity the cdks can phosphorylate and inactivate their respective ckis, removing this negative regulation.
Structure and Types
Cdks are basically made up of two main domains, an "amino-terminal" (N-terminal) domain and a "carboxyl-terminal" (C-terminal) domain. The N-terminal domain is largely made up of alpha-helices while the C-terminal domain is largely made up of beta-sheets. The crystal structures of several cdks have been determined to find the three dimensional structure of the proteins. These structures show that the two domains form two lobes on the protein. The ATP binding site of the cdks is located between these two lobes.
There are nine cdks that have been discovered in humans, labeled cdk1 through cdk9. A brief description of these cdks follows:
- cdk1 - This was the first cdk discovered in humans and is also known as cdc2. It binds first to cyclin A in G2 phase and later to cyclin B to promote the entry into mitosis.
- cdk2 - cdk2 binds to many different cyclins. It first binds to cyclin D1, D2, and D3 in early G1 phase, then to cyclin E to promote the transition from G1 phase into S phase, and finally to cyclin A to move the cell through S phase.
- cdk3 - cdk3 is involved in G1 phase, but its cyclin partner and functions have not yet been identified.
- cdk4 - cdk4 binds to cyclin D1, D2, and D3 during G1 phase.
- cdk5 - cdk5 is present mainly in neuronal cells. It binds to cyclin D1, D2, and D3 during G1 phase and may play a role in the progression of Alzheimer's Disease.
- cdk6 - cdk6 binds to cyclin D1, D2, and D3 during G1 phase.
- cdk7 - cdk7 combines with cyclin H to form a kinase called cyclin-activating-kinase or CAK. This kinase is responsible for phosphorylating and activating a variety of other cdks involved in the cell cycle. cdk7 may also be involved in transcription.
- cdk8 - Details on this cdk are not well known, although it appears to be involved in transcription.
- cdk9 - cdk9 combines with cyclin T to form a kinase called p-TEFb or positive transcription elongation factor b that is important for transcription. p-TEFb is responsible for phosphorylating and activating RNA polymerase II. This activation allows the polymerase to elongate RNA transcripts.
Drugs that Target Cdks
Because cdks are so important in regulating the cell cycle they make excellent targets for anticancer drugs. By targeting cdks the drugs could arrest the cancer cells at various stages of the cell cycle and could cause the cells to undergo apoptosis, or die. Roughly fifty cdk inhibitors have been discovered so far. All of the inhibitors work by binding to the ATP binding site of the cdk, preventing ATP from binding which prevents the cdk from phosphorylating its substrates. This causes the cell to stop moving through the cell cycle. The best known cdk inhibitors include the drugs roscovitine, olomoucine, and flavopiridol. Various cdk inhibitors are currently being tested in the lab and a few are in the stages of human clinical trials. None are yet licensed for public use.
Current Protocols in Cell Biology, Volume 1, Section 8, 2003
Albert's "Molecular Biology of the Cell", Third Edition, 1994
Rang's "Pharmacology", Fourth Edition, 1999