Cell Cycle

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For the separation of chromosomes that occurs as part of the cell cycle, see mitosis. For the Academic journal, see Cell Cycle.

See also: Cell division

Each turn of the cell cycle divides the chromosomes in a cell nucleus.

The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication (replicationreeling in fish). In cells without a nucleus (prokaryotic), the cell cycle occurs a process termed binary fission. In cells with a nucleus (eukaryotes), the cell cycle can be divided in two periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and the mitosis (M) phase, during which the cell splits itself into two distinct cells, often called "daughter cells" and the final phase, cytokinesis, where the new cell is completely divided. The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.

Contents

1 Phases o 1.1 Resting (G 0 phase)o 1.2 Interphase

1.2.1 G 1 phase 1.2.2 S phase 1.2.3 G 2 phase

o 1.3 Mitosis (M Phase/Mitotic phase) 2 Regulation of eukaryotic cell cycle

o 2.1 Role of cyclins and CDKs 2.1.1 General mechanism of cyclin-CDK interaction

2.1.2 Specific action of cyclin-CDK complexes o 2.2 Inhibitors o 2.3 Transcriptional Regulatory Network

3 Checkpoints 4 Role in tumor formation 5 Synchronization of cell cultures 6 See also 7 References 8 Further reading 9 External links

Phases

The cell cycle consists of four distinct phases: G1 phase, S phase (synthesis), G2 phase (collectively known as interphase) and M phase (mitosis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two sister cells, and cytokinesis, in which the cell's cytoplasm divides in half forming distinct cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.

Schematic of the cell cycle. outer ring: I = Interphase, M = Mitosis; inner ring: M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis; not in ring: G0 = Gap 0/Resting.[1]

State Description Abbreviation

quiescent/senescent

Gap 0 G0A resting phase where the cell has left the cycle and has stopped dividing.

Interphase

Gap 1 G1Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.

Synthesis S DNA replication occurs during this phase.

Gap 2 G2

During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divide.

Cell division

Mitosis M

Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells. A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.

After cell division, each of the daughter cells begin the interphase of a new cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division.

Resting (G0 phase)

The term "post-mitotic" is sometimes used to refer to both quiescent an senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical reaction; division of such a cell could, for example, become cancerous. Some cells enter the g0 phase semi-permanentally e.g., some liver and kidney cells.

Interphase

Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during the interphase. Interphase proceeds in three stages, G1, S, and G2. Cell division operates in a cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokinesis. Interphase is also known as preparatory phase, in this stage nucleus and cytosol division does not occur. The cell prepares to divide.

G1 phase

The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis is called G1 (G indicating gap). It is also called the growth phase. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by the use of 20 amino acids to form millions of proteins and later on enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same species. It is under the control of the p53 gene.

S phase

The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. During this phase, synthesis is completed as quickly as possible due to the exposed base pairs being sensitive to external factors such as any drugs taken or any mutagens (such as nicotine).[2]

G2 phase

The cell then enters the G2 phase, which lasts until the cell enters mitosis. Again, significant biosynthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.

Mitosis (M Phase/Mitotic phase)

Main article: Mitosis

The relatively brief M phase consists of nuclear division (karyokinesis). The M phase has been broken down into several distinct phases, sequentially known as:

prophase , metaphase , anaphase , telophase cytokinesis (strictly speaking, cytokinesis is not part of mitosis but is an event that directly

follows mitosis in which cytoplasm is divided into two daughter cells)

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei.[3] It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.[4] Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.[5]

Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "M phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei in a process called endoreplication. This occurs most notably among the fungi and slime moulds, but is found in various groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.[6] Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer.

Regulation of eukaryotic cell cycle

Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division. The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle.

Role of cyclins and CDKs

Nobel Laureate Paul Nurse

Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell's progress through the cell cycle.[7] Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules.[8] Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components. Many of the relevant genes were first identified by studying yeast, especially Saccharomyces cerevisiae;[9] genetic nomenclature in yeast dubs many of these genes cdc (for "cell division cycle") followed by an identifying number, e.g., cdc25 or cdc20.

Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle. Different cyclin-CDK combinations determine the downstream proteins targeted. CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals.[10]

General mechanism of cyclin-CDK interaction

This section needs additional citations for verification. (July 2010)

Upon receiving a pro-mitotic extracellular signal, G1 cyclin-CDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication. The G1 cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome.

Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G1 phase on DNA replication origins. The phosphorylation serves two purposes: to activate each already-assembled pre-replication complex, and to prevent new complexes from forming. This ensures that every portion of the cell's genome will be replicated once and only once. The reason for prevention of gaps in replication is fairly clear, because daughter cells that are missing all or part of crucial genes will die. However, for reasons related to gene copy number effects, possession of extra copies of certain genes is also deleterious to the daughter cells.

Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G2 phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly. A critical complex activated during this process is a ubiquitin ligase known as the anaphase-promoting complex (APC), which promotes degradation of structural proteins associated with the chromosomal kinetochore. APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.

Specific action of cyclin-CDK complexes

Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (e.g. growth factors). Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex. Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (Rb). The hyperphosphorylated Rb dissociates from the E2F/DP1/Rb complex (which was bound to the E2F responsive genes, effectively "blocking" them from transcription), activating E2F. Activation of E2F results in transcription of various genes like cyclin E, cyclin A, DNA polymerase, thymidine kinase, etc. Cyclin E thus produced binds to CDK2, forming the cyclin E-CDK2 complex, which pushes the cell from G1 to S phase (G1/S transition). Cyclin B along with cdc2 (cdc2 - fission yeasts (CDK1 - mammalia)) forms the cyclin B-cdc2 complex, which initiates the G2/M transition.[11] Cyclin B-cdc2 complex activation causes breakdown of nuclear envelope and initiation of prophase, and subsequently, its deactivation causes the cell to exit mitosis.[10]

Inhibitors

Overview of signal transduction pathways involved in apoptosis, also known as "programmed cell death".

Two families of genes, the cip/kip family (CDK interacting protein/Kinase inhibitory protein) and the INK4a/ARF (Inhibitor of Kinase 4/Alternative Reading Frame) prevent the progression of the cell cycle. Because these genes are instrumental in prevention of tumor formation, they are known as tumor suppressors.

The cip/kip family includes the genes p21, p27 and p57. They halt cell cycle in G1 phase, by binding to, and inactivating, cyclin-CDK complexes. p21 is activated by p53 (which, in turn, is triggered by DNA damage e.g. due to radiation). p27 is activated by Transforming Growth Factor β (TGF β), a growth inhibitor.

The INK4a/ARF family includes p16INK4a, which binds to CDK4 and arrests the cell cycle in G1 phase, and p19ARF which prevents p53 degradation.

Synthetic inhibitors of Cdc25 could also be useful for the arrest of cell cycle and therefore be useful as antineoplastic and anticancer agents.[12]

Transcriptional Regulatory Network

Evidence suggests that a semi-autonomous transcriptional network acts in concert with the CDK-cyclin machinery to regulate the cell cycle. Several gene expression studies in Saccharomyces cerevisiae have identified approximately 800 to 1200 genes that change expression over the course of the cell cycle;[9][13][14] they are transcribed at high levels at specific points in the cell cycle, and remain at lower levels throughout the rest of the cell cycle. While the set of identified genes differs between studies due to the computational methods and criterion used to identify them, each study indicates that a large portion of yeast genes are temporally regulated.[15]

Many periodically expressed genes are driven by transcription factors that are also periodically expressed. One screen of single-gene knockouts identified 48 transcription factors (about 20% of all non-essential transcription factors) that show cell cycle progression defects.[16] Genome-wide studies using high throughput technologies have identified the transcription factors that bind to the promoters of yeast genes, and correlating these findings with temporal expression patterns have allowed the identification of transcription factors that drive phase-specific gene expression.[13][17] The expression profiles of these transcription factors are driven by the transcription factors that peak in the prior phase, and computational models have shown that a CDK-autonomous network of these transcription factors is sufficient to produce steady-state oscillations in gene expression).[14][18]

Experimental evidence also suggests that gene expression can oscillate with the period seen in dividing wild-type cells independently of the CDK machinery. Orlando et al. used microarrays to measure the expression of a set of 1,271 genes that they identified as periodic in both wild type cells and cells lacking all S-phase and mitotic cyclins (clb1,2,3,4,5,6). Of the 1,271 genes assayed, 882 continued to be expressed in the cyclin-deficient cells at the same time as in the wild type cells, despite the fact that the cyclin-deficient cells arrest at the border between G1 and S phase. However, 833 of the genes assayed changed behavior between the wild type and mutant cells, indicating that these genes are likely directly or indirectly regulated by the CDK-cyclin machinery. Some genes that continued to be expressed on time in the mutant cells were also expressed at different levels in the mutant and wild type cells. These findings suggest that while the transcriptional network may oscillate independently of the CDK-cyclin oscillator, they are coupled in a manner that requires both to ensure the proper timing of cell cycle events.[14] Other work indicates that phosphorylation, a post-translational modification, of cell cycle transcription factors by Cdk1 may alter the localization or activity of the transcription factors in order to tightly control timing of target genes (Ubersax et al. 2003; Sidorova et al. 1995; White et al. 2009).[16][19][20]

While oscillatory transcription plays a key role in the progression of the yeast cell cycle, the CDK-cyclin machinery operates independently in the early embryonic cell cycle. Before the

midblastula transition, zygotic transcription does not occur and all needed proteins, such as the B-type cyclins, are translated from maternally loaded mRNA.[21]

Checkpoints

Main article: Cell cycle checkpoint

Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle.[22] Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of DNA damage. The cell cannot proceed to the next phase until checkpoint requirements have been met.

Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. Two main checkpoints exist: the G1/S checkpoint and the G2/M checkpoint. G1/S transition is a rate-limiting step in the cell cycle and is also known as restriction point.[10] An alternative model of the cell cycle response to DNA damage has also been proposed, known as the postreplication checkpoint.

p53 plays an important role in triggering the control mechanisms at both G1/S and G2/M checkpoints.

Role in tumor formation

A disregulation of the cell cycle components may lead to tumor formation. As mentioned above, some genes like the cell cycle inhibitors, RB, p53 etc., when they mutate, may cause the cell to multiply uncontrollably, forming a tumor. Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle, the proportion of cells that are in active cell division (versus quiescent cells in G0 phase) in tumors is much higher than that in normal tissue. Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same.

The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation. This fact is made use of in cancer treatment; by a process known as debulking, a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G0

to G1 phase (due to increased availability of nutrients, oxygen, growth factors etc.). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle.[10]

The fastest cycling mammalian cells in culture, crypt cells in the intestinal epithelium, have a cycle time as short as 9 to 10 hours. Stem cells in resting mouse skin may have a cycle time of more than 200 hours. Most of this difference is due to the varying length of G1, the most variable phase of the cycle. M and S do not vary much.

In general, cells are most radiosensitive in late M and G2 phases and most resistant in late S.

For cells with a longer cell cycle time and a significantly long G1 phase, there is a second peak of resistance late in G1

The pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell. Sulfhydryls are natural radioprotectors and tend to be at their highest levels in S and at their lowest near mitosis.

Synchronization of cell cultures

Several methods can be used to synchronise cell cultures by halting the cell cycle at a particular phase. For example, serum starvation [23] and treatment with thymidine or aphidicolin [24] halt the cell in the G1 phase, mitotic shake-off, treatment with colchicine [25] and treatment with nocodazole [26] halt the cell in M phase and treatment with 5-fluorodeoxyuridine halts the cell in S phase. In addition, partial cell division cycle synchrony can be achieved in budding yeast as a consequence of metabolic synchrony.[27] This metabolic synchrony at the level of synchronized cultures is an emergent, self-organized phenomenon based on a single-cell autonomous cell growth cycle taking place in the G1 phase of the cell division cycle.[27][28]

Cyclin-dependent kinases (CDKs) are a family of protein kinases first discovered for their role in regulating the cell cycle. They are also involved in regulating transcription, mRNA processing, and the differentiation of nerve cells.[1] They are present in all known eukaryotes, and their regulatory function in the cell cycle has been evolutionarily conserved. In fact, yeast cells can proliferate normally when their CDK gene has been replaced with the homologous human gene.[1][2] CDKs are relatively small proteins, with molecular weights ranging from 34 to 40 kDa, and contain little more than the kinase domain.[1] By definition, a CDK binds a regulatory protein called a cyclin. Without cyclin, CDK has little kinase activity; only the cyclin-CDK complex is an active kinase. CDKs phosphorylate their substrates on serines and threonines, so they are serine-threonine kinases.[1] The consensus sequence for the phosphorylation site in the amino acid sequence of a CDK substrate is [S/T*]PX[K/R], where S/T* is the phosphorylated serine or threonine, P is proline, X is any amino acid, K is lysine, and R is arginine [1]

Contents

1 Types 2 CDKs and Cyclins in the Cell Cycle 3 Regulation of CDK activity

o 3.1 Cyclin bindingo 3.2 Phosphorylationo 3.3 CDK Inhibitorso 3.4 Suk1 or Ckso 3.5 Non-cyclin CDK Activators

3.5.1 Viral Cyclins 3.5.2 CDK5 Activators 3.5.3 RINGO/Speedy

4 History 5 Medical significance 6 References 7 External links

Types

Table 1: Known CDKs, their cyclin partners, and their functions in the human [3] and consequences of deletion in mice.[4]

CDKCyclin partner

Function Deletion Phenotype in Mice

Cdk1 Cyclin B M phase None. ~E2.5.

Cdk2 Cyclin E G1/S transitionReduced size, imparted neural progenitor cell proliferation. Viable, but both males & females sterile.

Cdk2 Cyclin A S phase, G2 phaseCdk3 Cyclin C G1 phase ? No defects. Viable, fertile.

Cdk4 Cyclin D G1 phaseReduced size, insulin deficient diabetes. Viable, but both male & female infertile.

Cdk5 p35 TranscriptionSevere neurological defects. Died immediately after birth.

Cdk6 Cyclin D G1 phase

Cdk7 Cyclin HCDK-activating kinase, transcription

Cdk8 Cyclin C TranscriptionCdk9 Cyclin T Transcription Embryonic lethalCdk11 Cyclin L  ? Mitotic defects. E3.5. ? Cyclin F  ? Defects in extraembryonic tissues. E10.5. ? Cyclin G  ?

CDKs and Cyclins in the Cell Cycle

Most of the known cyclin-CDK complexes regulate the progression through the cell cycle. Animal cells contain at least nine CDKs, four of which, Cdk1, 2, 3, and 4, are directly involved in cell cycle regulation.[1] In mammalian cells, CDK1, with its partners cyclin A2 and B1, alone can drive the cell cycle.[4] Another one, Cdk5, is involved indirectly as the CDK-activating kinase.[1] Cyclin-CDK complexes phosphorylate substrates appropriate for the particular cell cycle phase.[3] Cyclin-CDK complexes in earlier cell-cycle phase help activate cyclin-CDK complexes in later phases.[1]

Table 2: Cyclins and CDKs by Cell-Cycle Phase

Phase Cyclin CDKG0 C Cdk3G1 D, E Cdk4, Cdk2, Cdk6S A, E Cdk2G2 A Cdk2, Cdk1M B Cdk1

Table 3: Cyclin-dependent kinases that control the cell cycle in model organisms.[1]

Species Name Original name Size (amino acids) FunctionSaccharomyces cerevisiae Cdk1 Cdc28 298 All cell-cycle stagesSchizosaccharomyces pombe Cdk1 Cdc2 297 All cell-cycle stagesDrosophila melanogaster Cdk1 Cdc2 297 M

Cdk2 Cdc2c 314 G1/S, S, possibly MCdk4 Cdk4/6 317 G1, promotes growth

Xenopus laevis Cdk1 Cdc2 301 MCdk2 297 S, possibly M

Homo sapiens Cdk1 Cdc2 297 MCdk2 298 G1, S, possibly MCdk4 301 G1Cdk6 326 G1

A list of CDKs with their regulator protein, cyclin or other.

CDK1 ; cyclin A, cyclin B CDK2 ; cyclin A, cyclin E CDK3 ; cyclin C CDK4 ; cyclin D1, cyclin D2, cyclin D3 CDK5 ; CDK5R1, CDK5R2. See also CDKL5. CDK6 ; cyclin D1, cyclin D2, cyclin D3 CDK7 ; cyclin H CDK8 ; cyclin C

CDK9 ; cyclin T1, cyclin T2a, cyclin T2b, cyclin K CDK10 CDK11 (CDC2L2) ; cyclin L CDK12 (CRKRS) ; cyclin L CDK13 (CDC2L5) ; cyclin L

Regulation of CDK activity

CDK levels remains relatively constant throughout the cell cycle and most regulation is post-translational. Most knowledge of CDK structure and function is based on CDKs of S. pombe (Cdc2), S. cerevisiae (CDC28), and vertebrates (CDC2 and CDK2). The four major mechanisms of CDK regulation are cyclin binding, CAK phosphorylation, regulatory inhibitory phosphorylation, and binding of CDK inhibitory subunits (CKIs).[5]

Cyclin binding

The active site, or ATP-binding site, of all kinases is a cleft between a small amino-terminal lobe and a larger carboxy-terminal lobe.[1] The structure of human Cdk2 revealed that CDKs have a modified ATP-binding site that can be regulated by cyclin binding.[1] Phosphorylation by CDK-activating kinase (CAK) at Thr 161 on the T-loop increases the complex activity. Without cyclin, a flexible loop called the activation loop or T-loop blocks the cleft, and the position of several key amino acid residues is not optimal for ATP-binding.[1] With cyclin, two alpha helices change position to permit ATP binding. One of them, the L12 helix that comes just before the T-loop in the primary sequence, becomes a beta strand and helps rearrange the T-loop, so it no longer blocks the active site.[1] The other alpha helix called the PSTAIRE helix rearranges and helps changes the position of the key amino acid residues in the active site.[1]

There is considerable specificity in which cyclin binds with CDK.[3] Furthermore, cyclin binding determines the specificity of the cyclin-CDK complex for particular substrates.[3] Cyclins can directly bind the substrate or localize the Cdk to a subcellular area where the substrate is found. Substrate specificity of S cyclins is imparted by the hydrophobic batch (centered on the MRAIL sequence), which has affinity for substrate proteins that contain a hydrophobic RXL (or Cy) motif. Cyclin B1 and B2 can localize Cdk1 to the nucleus and the Golgi, respectively, through a localization sequence outside the Cdk-binding region.[1]

Phosphorylation

Full kinase activity requires an activating phosphorylation on a threonine adjacent to the active site.[1] The identity of the CDK-activating kinase (CAK) that performs this phosphorylation varies across the model organisms.[1] The timing of this phosphorylation varies as well. In mammalian cells, the activating phosphorylation occurs after cyclin binding.[1] In yeast cells, it occurs before cyclin binding.[1] CAK activity is not regulated by known cell-cycle pathways and cyclin binding is the limiting step for CDK activation.[1]

Unlike activating phosphorylation, CDK inhibitory phosphorylation is vital for regulation of the cell cycle. Various kinases and phosphatases regulate their phosphorylation state. One of the

kinases that place the tyrosine phosphate is Wee1, a kinase conserved in all eukaryotes.[1] Fission yeast also contains a second kinase Mik1 that can phosphorylate the tyrosine.[1] Vertebrates contain a different second kinase called Myt1 that is related to Wee1 but can phosphorylate both the threonine and the tyrosine.[1] Phosphatases from the Cdc25 family dephosphorylate both the threonine and the tyrosine.[1]

CDK Inhibitors

A cyclin-dependent kinase inhibitor (CKI) is a protein that interacts with a cyclin-CDK complex to block kinase activity, usually during G1 or in response to signals from the environment or from damaged DNA.[1] In animal cells, there are two major CKI families: the INK4 family and the CIP/KIP family.[1] The INK4 family proteins are strictly inhibitory and bind CDK monomers. Crystal structures of CDK6-INK4 complexes show that INK4 binding twists the CDK to distort cyclin binding and kinase activity. The CIP/KIP family proteins bind both the cyclin and the CDK of a complex and can be inhibitory or activating. CIP/KIP family proteins activate cyclin D and CDK4 or CDK6 complexes by enhancing complex formation.[1]

In yeast and Drosophila, CKIs are strong inhibitors of S- and M-CDK, but do not inhibit G1/S-CDKs. During G1, high levels of CKIs prevent cell cycle events from occurring out of order, but do not prevent transition through the Start checkpoint, which is initiated through G1/S-CDKs. Once the cell cycle is initiated, phosphorylation by early G1/S-CDKs leads to destruction of CKIs, relieving inhibition on later cell cycle transitions. In mammalian cells, the CKI regulation works differently. Mammalian protein p27 (Dacapo in Drosophila) inhibits G1/S- and S-CDKs, but does not inhibits S- and M-CDKs.[1]

Suk1 or Cks

The CDKs directly involved in the regulation of the cell cycle associate with small, 9- to 13-kiloDalton proteins called Suk1 or Cks.[3] These proteins are required for CDK function, but their precise role is unknown.[3] Cks1 binds the carboxy lobe of the Cdk, and recognizes phosphorylated residues. It may help the cyclin-CDK complex with substrates that have multiple phosphorylation sites by increasing affinity for the substrate.[3]

Non-cyclin CDK Activators

Viral Cyclins

Viruses can encode proteins with sequence homology to cyclins. One much-studied example is K-cyclin (or v-cyclin) from Kaposi sarcoma herpes virus (see Kaposi’s sarcoma), which activates CDK6. Viral cyclin-CDK complexes have different substrate specificities and regulation sensitivities.[6]

CDK5 Activators

The proteins p35 and p39 activate CDK5. Although they lack cyclin sequence homology, crystals structures show that p35 folds in a similar way as the cyclins. However, activation of CDK5 does not require activation loop phosphorylation.[6]

RINGO/Speedy

Proteins with no homology to the cyclin family can be direct activators of CDKs.[7] One family of such activators is the RINGO/Speedy family,[7] which was originally discovered in Xenopus. All five members discovered so far directly activate Cdk1 and Cdk2, but the RINGO/Speedy-CDK2 complex recognizes different substrates than cyclin A-CDK2 complex.[6]

History

Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse received the 2001 Nobel Prize in Physiology or Medicine for their complete description of cyclin and cyclin-dependent kinase mechanisms, which are central to the regulation of the cell cycle.

Medical significance

CDKs are considered a potential target for anti-cancer medication. If it is possible to selectively interrupt the cell cycle regulation in cancer cells by interfering with CDK action, the cell will die. At present, some CDK inhibitors such as Seliciclib are undergoing clinical trials. Although it was originally developed as a potential anti-cancer drug, in recent laboratory tests Seliciclib has also proven to induce apoptosis in neutrophil granulocytes, which mediate inflammation.[8] This means that novel drugs for treatment of chronic inflammation diseases such as arthritis and cystic fibrosis could be developed.

Flavopiridol (Alvocidib) is the first CDK inhibitor to be tested in clinical trials after being identified in an anti-cancer agent screen in 1992. It competes for the ATP site of the CDKs.[9]

More research is required, however, because disruption of the CDK-mediated pathway has potentially serious consequences; while CDK inhibitors seem promising, it has to be determined how side-effects can be limited so that only target cells are affected. As such diseases are currently treated with glucocorticoids, which have often serious side-effects, even a minor success would mean an improvement.

Complications of developing a CDK drug include the fact that many CDKs are not involved in the cell cycle such as transcription, viral infection, neural physiology, and glucose homeostatsis.[10]

Table 4: Cyclin-dependent kinase inhibitor drugs [10]

Drug CDKs InhibitedFlavopiridol (Alvocidib) 1, 2, 4, 6, 7, 9Olomoucine 1, 2, 5

Roscovitine 1, 2, 5Purvalanol 1, 2, 5Paullones 1, 2, 5Butryolactone 1, 2, 5Thio/oxoflavopiridols 1Oxindoles 2Aminothiazoles 4Benzocarbazoles 4Pyrimidines 4Seliciclib ?

Cyclin-dependent kinase 1From Wikipedia, the free encyclopedia

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Cyclin-dependent kinase 1

Available structures

PDBOrtholog search: PDBe, RCSB

[show]List of PDB id codes

Identifiers

Symbols CDK1; CDC2; CDC28A; P34CDC2

External

IDs

OMIM: 116940 MGI: 88351 HomoloGene: 68203

ChEMBL: 308 GeneCards: CDK1 Gene

EC

number2.7.11.23 2.7.11.22, 2.7.11.23

[show]Gene Ontology

RNA expression pattern

More reference expression data

Orthologs

Species Human Mouse

Entrez 983 12534

Ensembl ENSG00000170312 ENSMUSG00000019942

UniProt P06493 P11440

RefSeq

(mRNA)NM_001130829.1 NM_007659.3

RefSeq

(protein)NP_001163877.1 NP_031685.2

Location

(UCSC)

Chr 10:

62.54 – 62.55 Mb

Chr 10:

69.34 – 69.35 Mb

PubMed

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Cyclin-dependent kinase 1 also known as CDK1 or cell division control protein 2 homolog is a highly conserved protein that functions as a serine/threonine kinase, and is a key player in cell cycle regulation.[1] It has been highly studied in the budding yeast S. cerevisiae, and the fission yeast S. pombe, where it is encoded by genes cdc28 and cdc2, respectively.[2] In humans, Cdk1 is encoded by the CDC2 gene.[3] With its cyclin partners, Cdk1 forms complexes that phosphorylate a variety of target substrates (over 75 have been identified in budding yeast); phosphorylation of these proteins leads to cell cycle progression.[4]

Contents

1 Structure 2 Function 3 Regulation 4 Interactions 5 References 6 Further reading 7 See also

Structure

Crystal Structure of the human Cdk1 homolog, Cdk2

Cdk1 is a small protein (approximately 34 kilodaltons), and is highly conserved. The human homolog of Cdk1, CDC2, shares approximately 60-65% amino-acid similarity with its yeast homolog. Furthermore, human CDC2 is capable of rescuing budding yeast carrying a cdc28 mutation.[5][6] Cdk1 is comprised mostly by the bare protein kinase motif, which other protein kinases share. Cdk1, like other kinases, contains a cleft in which ATP fits. Substrates of Cdk1 bind near the mouth of the cleft, and Cdk1 residues catalyze the covalent bonding of the γ-phosphate to the oxygen of the hydroxyl serine/threonine of the substrate.

In addition to this catalytic core, Cdk1, like other cyclin-dependent kinases, contains a T-loop, which, in the absence of an interacting cyclin, prevents substrate binding to the Cdk1 active site. Cdk1 also contains a PSTAIRE helix, which, upon cyclin binding, moves and rearranges the actives site, facilitating Cdk1 kinase activities.[7]

Function

Fig. 1 The diagram shows the role of Cdk1 in progression through the S. cerevisiae cell cycle. Cln3-Cdk1 leads to Cln1,2-Cdk1 activity, eventually resulting in Clb5,6-Cdk1 activity and then Clb1-4-Cdk1 activity.[1]

When bound to its cyclin partners, Cdk1 phosphorylation leads to cell cycle progression. Cdk1 activity is best understood in S. cerevisaie, so Cdk1 S. cerevisiae activity is described here. In the budding yeast, initial cell cycle entry is controlled by two regulatory complexes, SBF (SCB-binding factor) and MBF (MCB-binding factor). These two complexes control G1/S gene transcription, however, they are normally inactive. SBF is inhibited by the protein Whi5, however, when phosphorylated by Cln3-Cdk1, Whi5 is ejected from the nucleus, allowing for transcription of the G1/S regulon, which includes the G1/S cyclins Cln1,2.[8] G1/S cyclin-Cdk1 activity leads to preparation for S phase entry (e.g., duplication of centromeres or the spindle pole body), and a rise in the S cyclins (Clb5,6 in S. cerevisiae). Clb5,6-Cdk1 complexes directly lead to replication origin initiation;[9] however, they are inhibited by Sic1, preventing premature S phase initiation.

Cln1,2 and/or Clb5,6-Cdk1 complex activity leads to a sudden drop in Sic1 levels, allowing for coherent S phase entry. Finally, phosphorylation by M cyclins (e.g., Clb1, 2, 3 and 4) in complex with Cdk1 leads to spindle assembly and sister chromatid alignment. Cdk1 phosphorylation also leads to the activation of the ubiquitin-protein ligase APCCdc20, an activation which allows for chromatid segregation and, furthermore, degradation of M-phase cyclins. This destruction of M cyclins leads to the final events of mitosis (e.g., spindle disassembly, mitotic exit).

Regulation

Given its essential role in cell cycle progression, Cdk1 is highly regulated. Most obviously, Cdk1 is regulated by its binding with its cyclin partners. Cyclin binding alters access to the active site of Cdk1, allowing for Cdk1 activity; furthermore, cyclins impart specificity to Cdk1 activity. At least some cyclins contain a hydrophobic patch which may directly interact with substrates, conferring target specificity.[10] Furthermore, cyclins can target Cdk1 to particular subcellular locations.

In addition to regulation by cyclins, Cdk1 is regulated by phosphorylation. A conserved tyrosine (Tyr15 in humans) leads to inhibition of Cdk1; this phosphorylation is thought to alter ATP orientation, preventing efficient kinase activity. In S. pombe, for example, incomplete DNA synthesis may lead to stabilization of this phosphorylation, preventing mitotic progression.[11] Wee1, conserved among all eukaryotes phosphorylates Tyr15, whereas members of the Cdc25 family are phosphatases, counteracting this activity. The balance between the two is thought to help govern cell cycle progression. Wee1 is controlled upstream by Cdr1, Cdr2, and Pom1.

Cdk1-cyclin complexes are also governed by direct binding of Cdk inhibitor proteins (CKIs). One such protein, already discussed, is Sic1. Sic1 is a stoichiometric inhibitor that binds directly to Clb5,6-Cdk1 complexes. Multisite phosphorylation, by Cdk1-Cln1/2, of Sic1 is thought to time Sic1 ubiquitination and destruction, and by extension, the timing of S-phase entry. Only until Sic1 inhibition is overcome can Clb5,6 activity occur and S phase initiation may begin.

Cyclin-dependent kinase 2From Wikipedia, the free encyclopedia

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"CDK2" redirects here. For the airport with the Transport Canada identifier CDK2, see Diavik Airport.

Cyclin-dependent kinase 2

PDB rendering based on 1aq1 [1] .

Available structures

PDBOrtholog search: PDBe, RCSB

[show]List of PDB id codes

Identifiers

Symbols CDK2; p33(CDK2)

External

IDs

OMIM: 116953 MGI: 104772 HomoloGene: 74409

ChEMBL: 301 GeneCards: CDK2 Gene

EC

number2.7.11.22

[show]Gene Ontology

RNA expression pattern

More reference expression data

Orthologs

Species Human Mouse

Entrez 1017 12566

Ensembl ENSG00000123374 ENSMUSG00000025358

UniProt P24941 P97377

RefSeq

(mRNA)NM_001798.3 NM_016756.4

RefSeq

(protein)NP_001789.2 NP_058036.1

Location

(UCSC)

Chr 12:

56.36 – 56.37 Mb

Chr 10:

128.7 – 128.71 Mb

PubMed

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Cyclin-dependent kinase 2 also known as cell division protein kinase 2 is an enzyme that in humans is encoded by the CDK2 gene.[2][3]

Contents

1 Function 2 Inhibitors 3 Gene regulation 4 Interactions 5 References 6 Further reading 7 External links

Function

The protein encoded by this gene is a member of the cyclin-dependent kinase family of Ser/Thr protein kinases. This protein kinase is highly similar to the gene products of S. cerevisiae cdc28, and S. pombe cdc2, also known as Cdk1 in humans. It is a catalytic subunit of the cyclin-dependent kinase complex, whose activity is restricted to the G1-S phase of the cell cycle, and is essential for the G1/S transition. This protein associates with and is regulated by the regulatory subunits of the complex including cyclin E or A. Cyclin E binds G1 phase Cdk2, which is required for the transition from G1 to S phase while binding with Cyclin A is required to progress through the S phase. Its activity is also regulated by phosphorylation. Two alternatively spliced variants and multiple transcription initiation sites of this gene have been reported.[3]

The role of this protein in G1-S transition has been recently questioned as cells lacking Cdk2 are reported to have no problem during this transition.[4]

Inhibitors

Known CDK inhibitors are p21Cip1 (CDKN1A) and p27Kip1 (CDKN1B).[5] Drugs that inhibit Cdk2 and arrest the cell cycle, such as GW8510, may reduce the sensitivity of the epithelium to many cell cycle-active antitumor agents and, therefore, represent a strategy for prevention of chemotherapy-induced alopecia.[6]

Gene regulation

In melanocytic cell types, expression of the CDK2 gene is regulated by the Microphthalmia-associated transcription factor.[7][8]

Cyclin-dependent kinase 4From Wikipedia, the free encyclopedia

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Cyclin-dependent kinase 4

Rendering based on PDB 1LD2.

Available structures

PDBOrtholog search: PDBe, RCSB

[show]List of PDB id codes

Identifiers

Symbols CDK4; CMM3; PSK-J3

External

IDs

OMIM: 123829 MGI: 88357 HomoloGene: 55429

ChEMBL: 331 GeneCards: CDK4 Gene

EC

number2.7.11.22

[show]Gene Ontology

RNA expression pattern

More reference expression data

Orthologs

Species Human Mouse

Entrez 1019 12567

Ensembl ENSG00000135446 ENSMUSG00000006728

UniProt P11802 P30285

RefSeq

(mRNA)NM_000075.3 NM_009870.3

RefSeq

(protein)NP_000066.1 NP_034000.1

Location

(UCSC)

Chr 12:

58.14 – 58.15 Mb

Chr 10:

127.06 – 127.07 Mb

PubMed

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Cyclin-dependent kinase 4 is part of the cyclin-dependent kinase family.

The protein encoded by this gene is a member of the Ser/Thr protein kinase family. This protein is highly similar to the gene products of S. cerevisiae cdc28 and S. pombe cdc2. It is a catalytic subunit of the protein kinase complex that is important for cell cycle G1 phase progression. The activity of this kinase is restricted to the G1-S phase, which is controlled by the regulatory subunits D-type cyclins and CDK inhibitor p16(INK4a). This kinase was shown to be responsible for the phosphorylation of retinoblastoma gene product (Rb). Mutations in this gene as well as in its related proteins including D-type cyclins, p16(INK4a) and Rb were all found to be associated with tumorigenesis of a variety of cancers. Multiple polyadenylation sites of this gene have been reported.[1]

It is regulated by Cyclin D.

Contents

1 Interactions 2 References 3 Further reading 4 External links

Interactions

Cyclin-dependent kinase 4 has been shown to interact with SERTAD1,[2][3] CDC37,[4][5][6][7] CEBPA,[8] PCNA,[9][10] Cyclin D3,[11][12][13][14] Cyclin D1,[3][9][13][15][16][17] CDKN2C,[4][18] MyoD,[19][20] P16,[2][3][4][9][17][21] CDKN2B,[11][22] Drebrin-like [4] and CDKN1B.[13][16]

Overview of signal transduction pathways involved in apoptosis.

Cyclin-dependent kinase 6From Wikipedia, the free encyclopedia

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Cyclin-dependent kinase 6

PDB rendering based on 1bi7.

Available structures

PDBOrtholog search: PDBe, RCSB

[show]List of PDB id codes

Identifiers

Symbols CDK6; PLSTIRE

External

IDs

OMIM: 603368 MGI: 1277162 HomoloGene: 963

ChEMBL: 2508 GeneCards: CDK6 Gene

EC

number2.7.11.22

[show]Gene Ontology

RNA expression pattern

More reference expression data

Orthologs

Species Human Mouse

Entrez 1021 12571

Ensembl ENSG00000105810 ENSMUSG00000040274

UniProt Q00534 Q64261

RefSeq

(mRNA)NM_001145306.1 NM_009873.2

RefSeq

(protein)NP_001138778.1 NP_034003.1

Location

(UCSC)

Chr 7:

92.23 – 92.47 Mb

Chr 5:

3.34 – 3.52 Mb

PubMed

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Cell division protein kinase 6 is an enzyme that in humans is encoded by the CDK6 gene.[1][2]

It is regulated by Cyclin D.

The protein encoded by this gene is a member of the cyclin-dependent protein kinase (CDK) family. CDK family members are highly similar to the gene products of Saccharomyces cerevisiae cdc28, and Schizosaccharomyces pombe cdc2, and are known to be important regulators of cell cycle progression. This kinase is a catalytic subunit of the protein kinase complex that is important for cell cycle G1 phase progression and G1/S transition. The activity of this kinase first appears in mid-G1 phase, which is controlled by the regulatory subunits including D-type cyclins and members of INK4 family of CDK inhibitors. This kinase, as well as CDK4, has been shown to phosphorylate, and thus regulate the activity of, tumor suppressor protein Rb.[2]

Contents

1 Interactions 2 References 3 Further reading 4 External links

Interactions

Cyclin-dependent kinase 6 was shown to interact with CDKN2C,[3][4][5] P16,[6][7][8] PPM1B,[9] Cyclin D3,[10][11] Cyclin D1 [10] [12] and PPP2CA.[9]