Cell Cycle Regulation

Cdks

Cyclin-dependent kinases

• control the progression through the cell cycle

Phosphorylation of target proteins by proteins kinases and…

dephosphorylation by protein phosphatases is a common mechanisms for controlling the cell cycle

cell cycle progression is driven by…

protein kinases that are active only when bound to a cyclin

Mitotic cyclines

required for the G2 → M transitino and bind mitotic Cdks

MPF (mitosis-promoting factor)

when mitotic cyclins bind mitotic Cdks

G1 cyclins

required for passage through the G1 restriction point, the Cdks to which they are bind are called G1 Cdks

S cyclins

required for DNA replication

One level of control on Cdk-cyclin activity is….

availability of cyclin molecules

Concentration of cyclin varies with…

phase of the cell cycle

Cyclin B

mitotic cylin

cyclin D2

G1 cyclin

Cdk phosphorylation

type of cdk regulation that is a combination of multiple kinases (two inhibitory and one activating) and a phosphatase (activating), regulated even in the presence of correct cyclin

Once activated, the Cdk-cyclin phosphorylates…

lamin proteins of the nuclear lamina

What does the phosphorylation of lamin proteins do?

causes the lamina breakdown and destabilization of the nuclear envelope

Mitotic Cdk-cyclin also phosphorylates condensin, which is involved in…

chromosome condensation, and phosphorylates microtubule-associated proteins to facilitate assembly of the mitotic spindle

anaphase-promoting complex

functions as a ubiquitin ligase, it adds ubiquitin to proteins to target them for destruction

what activates anaphase-promoting complex?

mitotic Cdk-cyclin being phosphorylated

What is a target for destruction of anaphase-promoting complex?

securin, which is an inhibitor of sister chromatid separation

What holds sister chromatids together prior to anaphase?

adhesive proteins called cohesins

Securin maintiains attachment by inhibiting the…

separase protein that would otherwise degrade the cohesins

When anaphase-promoting complex triggers destruction of securin…

separae cleaves cohesin, freeing the sister chromatids

The anaphase-promoting complex targets mitotic cyclin for destruction…

which causes the kinase activity of mitotic Cdk to fall

What changes associated with exit from mitosis depend on degradation of mitotic cyclin?

cytokinesis, chromosome decondensation, and nuclear envelope reassembly

If cells proceeded from one phase of the cell cycle to the next without…

completing each step, daughter cells might be abnormal

aneuploidy

incorrect number of chromosomes

cells use a series of checkpoint mechanisms to…

ensure that each phase is completed properly before the next one begins

G1 Cdk-cyclin phosphorylates the….

key target Rb protein (retinoblastoma protein)

Nonphosphorylated Rb binds…

the E2F transcription factor

Unbound E2F activates transcription of genes coding…

for proteins that initiate DNA replication

While Rb remains bound to E2F, what does E2F do?

the molecule is inactive, and the cell cannot enter S phase

When cells are stimulated to divide by growth factors, what happens?

G1 Cdk-cyclin is activated and phosphorylates Rb

Once Rb is phosphorylated, Rb releases E2F, which does what?

which initiates transcription of genes needed for the entry of S phase

DNA replication checkpoint

ensures that DNA synthesis is complete before the cell exits G2 and begins mitosis

Cells that are prevented from completing DNA replication fail to undergo the…

final dephosphorylation stepp in the activation of mitotic Cdk-cyclin

The mitotic spindle checkpoint

prevents anaphase from beginning before the chromosomes are all attached to the spindle

Kinetochores that remain unattached to microtubules produce a…

“wait” signal that inhibits the anaphase-promoting complex, members of the Mad and Bub protein families are involved

Mad and Bub protiens accumulate at…

unattached kinetochores

What do Mad and Bub proteins become part of?

A multiprotein complex that inhibits the anaphase-promoting complex by blocking the action of Cdc20 protein

Once all the chromosomes are attached, Mad and Bud no longer..

persist at kinetochores, and the inhibition is lifted

A multiple series of DNA damage checkpoints monitor DNA for damage and…

halt the cell cycle at various points (lat G1, S, and late G2) by inhibiting different Cdk-cyclin complexees

p53 protein

“guardian of the genome” plays a central role in these checkpoint pathways

Breaks in DNA trigger the activation of what?

an enzyme called the ATM (ataxia telangiectasia mutated) protein kinase

ATM phosphorylates checkpoint kinases what does what?

it phosphorylates p53 (and other targets)

Phosphorylated p53 is unable to what?

bind to Mdm2, Mdm2 marks p53 for destruction

Phosphorylated p53 is protected from degradation and activates…

two types of events: cell cycle arrest, and cell death

p53 activates the gene coding for p21, a protein that does what?

halts progression of the cell cycle by inhibiting activity of different Cdk-cyclins

ATR acts similarly as it does what?

(ATM-related) acts similarly in response to single-stranded DNA breaks

p53 stimulates production of enzymes involved in what?

DNA repair

If DNA damage cannot be repaired, p53 activates genes needed to….

trigger cell death by apoptosis

A key protein in the pathway of p53 unregulated modulator of apoptosis is…

Puma, which inactivates Bcl-2, an apoptosis inhibitor

The “machine” that regulates the eukaryotic cell cylce involved two interacting mechanisms, what are they?

1. autonomous clock goes through a fixed cycle over and over again via the synthesis and degradation of cyclins

2. clock is adjusted as needed

Apoptosis does what?

programmed cell death

• damaged or diseased cells need to be eliminated

• process cannot damage surrounding cells

necrosis

different type of cell death

• involved swelling and rupture of injured cells

difference between apoptosis and necrosis?

apoptosis - specific series of events that lead to dismanteling of cell contents

necrosis - swelling and rupture of injured cells

Steps of Apoptosis: Step 1

the cell’s DNA segregates near the periphery of the nucleus, and the cytoplasm decreases

Steps of Apoptosis: Step 2

the cell produces small cytoplasmic extensions, and the nucleus begins to fragment. DNA is cleaved by an apoptosis-specific endonuclease (DNase), and the cell is dismantled into small pieces called apoptotic bodies

Steps of Apoptosis: Step 3

Inactivation of a phospholipid translocator (flippase) causes accumulation of phosphatidylserine in the outer leaflet of the plasma membrane. This serves as a signal for the remnants of the affected cell to be engulfed by nearby cells via phagocytosis

How does apoptosis proceed?

through the activation of a series of enzymes called caspases

caspases are produced as…

inactive procaspases and are cleaved to create active enzymes

There are two main ways a cell can enter apoptosis

• tumor necrosis factor

• CD95/Fas

CD95

a protein on the surface of infected cells; lymphocytes have a protein on their sufaces that bind CD95 causing it to aggregate

cytotoxic T lymphocytes are activated when

cells are infected by certain viruses, and induce the infected cells to undergo apoptosis

Adaptor proteins attach to CD95 which does what?

which recruits procaspase-8 to the sites of clustering

When procaspase is activated…

it acts as an initiator caspase, initiating the cascade

Initiator caspases also activate an…

executioner caspase, caspase-3, which is important for activating many steps in apoptosis

When survival factors are withdrawn…

a cell may enter apoptosis

Site of action of survival factors

is mitochondrion

healthy cells have several…

anti-apoptotic proteins in the outer mitochondrial membrane

The proteins are structurally related to a protein called Bcl-2, which does what?

together with other proteins, counteracts proteins that promote apoptosis (pro-apoptotic proteins)

When cellular signals shift in balance toward pro-apoptotic proteins…

the cell is more likely to undergo apoptosis

Bad (Bcl-2-associated eath promotorer)

a type of pro-apoptotic protein

Mitochondria trigger apoptosis by releasing cytochrome c into…

the cytosol after accumulation of pro-apoptotic proteins leads to formation of channles in the outer mitochondrial membrane

cytochrome c stimulates the calcium release from…

mitochondria and ER, where it binds IP₃ resceptors

Cytochrom c also activates an initiator procaspase…

procaspase-9, by recruiting a cytosolic adaptor protein, Apaf-1

Apaf-1 assembles procaspase-9…

into a complex called an apoptosome

The apoptosome promotes…

production of active caspase-9

caspase-9 activates the…

exacutioner caspase, caspase-3

both mechanisms lead to activation of a…

common caspase that sets apoptosis in motion

If a cell suffers such damage that it can’t repair itself…

it may trigger its own demise

A cell can enter apoptosis through the actiity of p53 which acts…

through the protein Puma, which binds and inhibits Bcl-2