CELL CYCLEMrs. OFELIA SOLANO SALUDAR

Department of Natural SciencesUniversity of St. La Salle

An overview of the cell cycle control system. The core of the cell cycle control system consists of a series of cyclin-cdk

complexes. The activity of each complex is influenced by various inhibitory mechanisms, which provide information

about the extracellular environment, cell damage, and incomplete cell cycle events. Many are missing in early

embryonic cell cycles.

http://highered.mcgraw-hill.com/olc/dl/120082/bio34a.swf

The mitosis promoting factor (MPF) contains 2 subunits: cyclin which regulates the other subunit, the cyclin-dependent kinase (Cdk).

The cell cycle is governed by the synthesis and degradation of cyclin.

Cyclin regulators found in the cytoplasm control MPF synthesis.

Components of the cell-

cycle control system

When different cyclin forms a complex with Cdk, the protein kinase is activated to trigger the different events of the cycle.

Cdk activates mitosis by phosphorylating several target proteins, including histones, the nuclear envelope lamin proteins, and the regulatory subunit of cytoplasmic myosin.

This brings about chromatin condensation, nuclear envelope depolymerization, and the organization of the mitotic spindle.

Cdk activity is terminated by cyclin degradation.

There are four classes of cyclins, each defined by the stage of the cell cycle at which they bind cdks and function. All eucaryotic cells require three of these

classes :1. G1/S-cyclins activate Cdks in late G1 and thereby

help trigger progression through Start, resulting in a commitment to cell-cycle entry. Their levels fall in S phase.

2. S-cyclins bind Cdks soon after progression through Start and help stimulate chromosome duplication. S-cyclin levels remain elevated until mitosis, and these cyclins also contribute to the control of some early mitotic events.

3. M-cyclins activate Cdks that stimulate entry into mitosis at the G2/M checkpoint. Various mechanisms later destroy M-cyclins in mid-mitosis.

4. G1-cyclins help govern the activities of the G1/S cyclins.

Cyclin-Cdk complexes of the cell-cycle control system. The concentrations of the major cyclin types oscillate during the cell cycle,

while the concentrations of Cdk do not change and exceed the amounts of cyclins. In late G1, rising G1/S-cyclin levels lead to the formation of G1/S-Cdk complexes that trigger progression through the Start checkpoint. S-

Cdk complexes form at the start of S phase and trigger DNA replication. M-Cdk complexes form during G2 but are held in an inactive state. These

complexes are activated at the end of G2 and trigger the early events of mitosis. A separate regulatory protein, the APC/C, initiates the metaphase-

to anaphase transition.

http://www.cellsalive.com/cell_cycle.htm

The mechanisms that control the activities of cyclin-Cdk complexes include phosphorylation of the Cdk subunit, binding of Cdk inhibitor proteins (CKIs), proteolysis of cyclins, and changes in the transcription of genes encoding Cdk regulators.

The cell-cycle control system also depends crucially on two additional enzyme complexes, the APC/C and SCF ubiquitin ligases, which catalyze the ubiquitylation and consequent destruction of specific regulatory proteins that control critical events in the cycle.

The structural basis of Cdk activation. The location of the bound ATP is indicated. The Enzyme is shown in three states.( A) In the

inactive state, without cyclin bound, the active site is blocked by a region of the protein called the T-loop (red). (B) The binding of cyclin causes the T-loop to move out of the active site, resulting in partial activation of the Cdk2. (C) Phosphorylation of Cdk2 ( by CAK) at a threonine residue in the T –loop further activates the enzyme by

changing the shape of the T-loop, improving the ability of the enzyme to bind its protein substrates.

The regulation of Cdk activity by inhibitory phosphorylation. The active cyclin-Cdk complex is turned off when the kinase Wee 1 phosphorylates two closely spaced sites

above the active site. Removal of these phosphates by the phosphatase Cd c25 activates the cyclin-Cdk complex (only one

inhibitory phosphate is shown). CAK adds the activating phosphate.

The inhibition of a cyclin-Cdk complex by a CK1. p27 binds to both cyclin and Cdk in the

complex, distorting the active site of the Cdk. It also inserts into the ATP-binding site, further inhibiting

the enzyme activity.

(A) The control of proteolysis by APC/C during the cell cycle. The APC/C is activated in mitosis by association with

the activating subunit Cdc 20, which recognizes specific amino acid sequences on M cyclin and other target proteins.

With the help of 2 additional proteins E1 and

E2, the APC/C transfers multiple ubiquitin

molecules onto the target protein.

The poly-ubiquitylated target is then recognized

and degraded in

the proteasome.

(B) ) The control of proteolysis by SCF during the cell cycle. the activity of the ubiquitin ligase SCF depends on different types of substrate binding subunits called F-box proteins.

The phosphorylation of a target protein (e.g. CKI), allows the target to be

recognized by a specific F-box subunit.

Control of chromosome

duplication. At G1, pre-replicative

complexes (pre-RCs) assemble at

replication origins. S-Cdk activation leads to

the formation of multiprotein preinitiation

complexes that unwind the DNA at

origins and begin the process of DNA replication. The

activation of replication origins in S

phase causes disassembly of the

pre-RCs, which does not reform at the origin until the

following G1-thereby ensuring that each

origin is activated only once in each cell

cycle.

Control of the initiation of DNA replication. The ORC remains

associated with a replication origin throughout the cell cycle. In early G1,

Cdc6 and Cdt1 associate with the ORC. The resulting protein complex then

assembles Mcm ring complexes on the adjacent DNA, resulting in the formation of the pre-RC. S-Cdk then stimulates the

assembly of severalproteins at the origin to form the pre-

initiation complex. DNA Polymerase and other replication proteins are recruited to

the origin, the Mcm protein rings are activated as DNA helicases, and DNA unwinding allows DNA replication to

begin. S –Cdk also blocks rereplication by triggering the destruction of Cdc6 and

the inactivation of the ORC. Cdt1 is inactivated by the protein geminin, which is an APC/C target. Its levels increase in S

and M phases, when APC/C is inactive. Thus, the components of the pre-RC

(Cdc6C, dt1, Mcm) cannot form a new pre-RC at the origins until M –Cdk1 is

Inactivated and the APC/C is activated at the end of mitosis.

Sister-chromatid cohesion depends on a large protein complex calledcohesin, which is deposited at many locations along the length of

each sister chromatid as the DNA is replicated in S phase. Two of the subunits of cohesin are members of a large family of proteins called SMC proteins (for Structural Maintenance of chromosomes). Cohesin forms giant ring-like structures, and it has been proposed that these

might form rings that surround the two sister chromatids

At the end of S phase, sister-chromatid cohesion facilitates the attachment of the two sister chromatids in a pair to opposite

poles of the mitotic spindle.

The activation of M-cdk. Cdk1 associates with M-cyclin as the levels of M-cyclin gradually rise. The resulting M-cdk complex is

phosphorylated on an activating site by the cdk activating kinase, and on a pair of inhibitory sites by Wee1. The resulting inactive M-cdk complex is activated at the end of G2 by cdc25. The positive feedback is enhanced by the ability of M-cdk to inhibit Wee1.

The condensation and resolution of sister chromatids depends, on a 5-subunit protein complex called condensin. Condensin structure is

related to the cohesin complex that holds sister chromatids together. It uses the energy provided by ATP hydrolysis to promote the

compaction and resolution of sister chromatids. Phosphorylation of condensin subunits by M-Cdk stimulates this coiling activity, providing

one mechanism by which M-Cdk may promote chromosome restructuring in early mitosis.

In multicellular animals, cell size, cell division, and cell death are carefully controlled to ensure that the organism and its organs achieve and maintain an appropriate size.

Mitogens stimulate the rate of cell division by removing intracellular molecular brakes that restrain cell-cycle progression in G1.

Growth factors promote cell growth by stimulating the synthesis and inhibiting the degradation of macromolecules.

For proliferating cells to maintain a constant cell size, they employ multiple mechanisms to ensure that cell growth is coordinated with cell division.

Animals maintain the normal size of their tissues and organs by adjusting cell size to compensate for changes in cell number, or vice versa. The mechanisms that make this possible are not known.

The resulting G1/S-Cdk and S-Cdk activities further enhance

Rb protein phosphorylation forming a positive feedback

loop. E2F proteins also stimulate the transcription of

their own genes, forming another positive feedback loop.

Mechanisms controlling cell-cycle entry and S-phase initiation in animal cells. Mitogens bind to cell-surface receptors to initiate intracellular signaling pathways, including the gene encoding the gene

regulatory protein Myc. Myc increases the expression of many delayed-response genes, including some that lead to increased G1-Cdk

activity (cyclinD-Cdk4), which triggers the phosphorylation of members of the Rb family of proteins. This inactivates the Rb proteins, freeing the gene regulatory protein E2F to activate the transcription of G1/S genes,

including the genes for a G1/S-cyclin (cyclin-E ) and S-cyclin ( cyclin-A).

How DNA damage arrests the cell cycle in

G1. When DNA is damaged, protein kinases are recruited to the site of damage, e.g. ATM or ATR.

Activation of protein kinases Chkl 1 and Chk2

result in the phospho-rylation of the gene

regulatory protein p53. Phosphorylation of p53 blocks its bindingto Mdm2. As a result, p53 accumulates to

high levels and stimulate transcription of the gene that encodes the CK1 protein p21, which

binds and inactivates G1/S-Cdk and S-Cdk complexes, resting the cell in G1. DNA

damage also induces either the phosphorylation of Mdm2 or a decrease in Mdm2 production, which causes a further

increase in p53.

http://highered.mcgraw-hill.com/olc/dl/120082/bio34b.swf

Abnormally high levels of Myc cause the activation of Arf,

which binds and inhibits Mdm2 and

increases p53 levels. Depending on the cell type and extracellular conditions, p53 then causes either cell-

cycle arrest or active Mdm2 apoptosis.

Cell-cycle arrest or apoptosis induced by excessive stimulation

of mitogenic pathways.

Extracellular nutrients (e.g., amino acids), and binding of cell-surface receptors and growth factors leads to the

activation of Pl3-kinase, which activates a

signaling pathway that leads to the activation of the protein kinase TOR.

Growth factors also stimulate increased

production of the gene regulatory protein Myc

(not shown), which activates the

transcription of various genes that promote cell metabolism and growth. 4E-BP is an inhibitor of

the translation initiation factor elF4E.

Stimulation of cell growth

by growth factors

and extra-

cellular nutrients.

(A) In many cell types (e.g. yeast), the rate of cell division is governed by the rate of cell growth, so that division occurs only when growth rate

achieve some minimal threshold. In yeasts, it is mainly the levels of nutrients that regulate the rate of cell growth and cell division. (B) In

some animal cell types, growth and division can each be controlled by separate extracellular factors ( growth factors and mitogens,

respectively) and cell size depends on the relative levels of the two types of factors.

(C) Some extracellular factors can stimulate both cell growth and cell

division by simultaneousaly activating signaling pathways that promote growth and other pathways that promote cell-cycle progression.

Potential mechanisms

for coordinating cell growth

and division.

CANCER IS GROWTH

GONE BERSERK!