Transcription
The cell cycle is stimulated by an important group of cytosolic proteins called cyclin-dependent kinases or cdks. The cdk's protein group can be thought of as the engine driving the cell cycle.
The name of these serine/threonine kinases, cyclin-dependent kinases, indicates their mechanism of activation. Cyclins themselves have no enzymatic activity, yet they are able to activate cdks. In the absence of cyclins, cdks are completely inactive. If cdks are the motor, then cyclins are the fuel, keeping the cell engine running.
By activating the expression, the ubiquitination, and degradation of cyclins, the cell is able to vary cyclin concentrations by several magnitudes. The genes for d- and e-group cyclins are target genes with important signaling pathways, including map kinase and wnt. Thus, whenever mitotic signals reach the cell and activate one of these pathways, the expression of these cyclins is very quickly upregulated.
Aberrant cyclins are responsible for the uncontrolled proliferation of tumor cells. For example, in many breast tumors and in certain types of leukemia, the proto-oncogene encoding cyclin D1 is overexpressed or overactivated.
X-ray crystallographic studies have given interesting insights into the structural aspects of this activation. The findings are summed up in this simplified graphic. In a non-activated cdk, the atp binding site is buried in a deep cleft. The cleft is blocked off by what's called a t-loop.
The binding of cyclin to the cdk leads to the conformational change of the l12 helix, which allows the t-loop to swing out and leave the cleft open. Additionally, the p-stair helix changes position, leading to the movement of several amino acid residues within the active site. This movement facilitates further activation of the cdk by a cdk-activating kinase or cak.
Caks are able to phosphorylate several different cdk residues, leading to a wider opening of the cleft, which makes the active site more accessible for atp. The atp-binding cdk-cyclin complex is now fully active. It is able to bind and to phosphorylate substrate proteins. Most substrates bind to cdk and to cyclin simultaneously. Thus, cyclins are not only necessary for activation of the cdk but also for substrate recruitment to the cdk.
There are more than 100 known cdk substrates. Among them are transcription factors, cytoskeleton proteins, and enzymes. In their phosphorylated and active state, these proteins are responsible for replication, transcription, and cytoskeleton reorganization.
A prominent example of the role of cyclins in a complex with kinases is providing the biochemical basis for overcoming the g1/s checkpoint. Before the start of the g1 phase, the retinoblastoma tumor suppressor protein, rb1, is hypophosphorylated. The non-phosphorylated rb1 protein binds tightly to the transcription factor e2f1, locking it in an inactive state.
During the g1 phase, rb1 is partially phosphorylated by the cdks cdk4 and cdk6. In many tumors, both cdks are overexpressed due to gene amplification. Partial phosphorylation of rb1 by cdk4 and cdk6 leads to the dissociation and activation of e2f1. E2f1 activates the expression of cyclin e and of other genes. Cyclin e activates cdk2, which phosphorylates rb1. Hyperphosphorylated rb1 is able to induce genes to initiate dna replication, thereby starting the s phase.
Cdks can be inactivated by degradation but also by interaction with specific cdk inhibitors. The typical inactivation mechanism is based on the binding of a cdk inhibitor to the cdk-cyclin complex and a partial conformational rotation of the cdk. The cyclin is thereby forced to release the t-loop and dissociate from the cdk. Furthermore, the cdk inhibitor is able to insert a small helix into the cleft, blocking the cleft and also blocking the active site of the cdk.
A human cell contains many different cyclins binding to different cdks and activating specific cell cycle phases. With the exception of the d-cyclins, which are present throughout the entire cell cycle, cyclins emerge only in their own specific cell cycle phase. Each cyclin-cdk complex is specific to the cell cycle phase it activates. Distinct cdks can be identified as belonging to the g1, the m, and the s phases. Very soon after each phase, most cyclins are marked by ubiquitin for proteolysis and subsequently degraded by the proteasome.
Cdk inhibitors can be seen as the brackets of the cell cycle. To date, seven major cdk inhibitors are known. These can be divided into two different functional groups. First, p16ink4a, p15ink4a, p18ink4a, and p19ink4a are inhibitors of the ink4 group and are able to block kinases of the cdk4 family. Second, p21cip, p27kip, and p57kip are inhibitors of the cip/kip group. Both inhibitor families are able to block the cell cycle at the g1/s checkpoint. In addition, cdk inhibitors of the cip/kip family can also block the cycle in other phases.
Around 30 percent of all tumors harbor inactivating mutations in at least one cdk inhibitor. This identifies the genes encoding cdk inhibitors as important tumor suppressor genes. In addition, many tumors show mutations in cdks, which makes them insensitive to inactivation by cdk inhibitors. Thus, genes encoding cdks and cyclins can act as oncogenes.