Cell Cycle Important Proteins and Terms

Quiescence

reversible arrest from cell cycle

Senescence

irreversible arrest from cell cycle

Cyclins

activators of CDKs, cell cycle stage specific

CDK(2), cdc28, cdc2

What the different cyclins bind to, is a kinase

Third phosphate

Which phosphate of a kinase is the one that is added/transferred?

MPF (Maturation Promoting Factor)

a diffusible factor that induces meiosis and mitosis

dimer of cyclin and CDK

wee1

inhibits cdc2 inducing G2 → mitosis

• kinase

cdc25

activates cdc2 and induces G2 → mitosis

• phosphatase

P-Cdc2

What is the active form of cdc2?

Cdc2

What is the intermediate form of Cdc2 (not fully active but not inactivated)

Cdc2-P

What is the inactive form of cdc2?

Cyclin E

Which cyclin drives DNA replication initiation
expressed in G1/S

Cyclin A

Which cyclin promotes DNA replication

expressed in S/G2

Cyclin B

Which cyclin initiates mitosis

expressed in G2/M

They have NES signal sequences so they are usually found in the cytosol

What do cyclins contain that help to spatially separate them from CDKs? Where are they usually found (inactive)?

Phosphorylation within NES signal sequence inhibits nuclear export → accumulation in nucleus

How are cyclins activated?

CAK (CDK activating kinase)

What fully activates cdc2, it activates via phosphorylating a different site

• kinase

1. inhibitory phosphate removed by cdc25 phosphatase

2. cyclin binding (low-activity dimer)

3. phosphorylation of Thr in activation loop (T-loop) by CAK (high-activity dimer)

What are the three steps to activate CDK?

p16

What blocks the phosphorylation by CAK on CDK

• tumor suppresor

CKI (cyclin kinase inhibitors), p27

group of proteins that inhibit CDKs

SCF (G1 → S)

One of the two major ubiquitination systems

regulates S-phase entry by degrading CKI (when CKI is phosphorylated) and G1/S phase cyclins

normally active, therefore SUBSTRATES need to be marked

APC/C (exit from mitosis → cytokinesis)

One of the two major ubiquitination systems

Spindle Assembly Checkpoint (SAC)

promotes anaphase

MUST be activated

Sic1

CKI in yeast, gets phosphorylated by CDK at six sites

once fully phosphorylated, becomes a substrate for SCF

Destruction of this allows for S-phase entry

Rb (retino blastoma)

S-phase suppressor, guards entry

recruits HDACs

gets phosphorylated by CDK → increased gene expression

phosphorylation of Rb by CDK-G1 → Rb exported out of nucleus

How do cells enter S-phase?

Cyclin D

Which cyclin initiates progression?

expressed in G1

E2F transcription factors which increased transcription of S-phase genes and S-phase cyclins

The exporting of Rb from the nucleus activates what?

ORC (origin recognition complex), always present on DNA

What marks origins of replication? Is it temporary or always present on DNA throughout cycle?

a DNA helicase

What is MCM?

During late M/early G1

When is the pre-RC formed?

non-phosphorylated Cdc6

non-phosphorylated Cdt1

What two proteins play an important role in assembly and loading of MCM onto Pre-RC?

1. Cdt1 and Cdc6 exit the preRC after loading MCM onto ORC

2. Cdc6 is phosphorylated and Cdt1 is destroyed (prevents re-initiation of origin)

3. S-phase CDK phosphorylated MCM and activates it

In S-phase, how is replication started?

after loading MCM, Cdt1 and cdc6 exit the preRC

What is the first step in the firing of origins?

cdc6 is phosphorylated and exported out of nucleus, Cdt1 is degraded
- this prevent re-initiation of origin

What happens to cdc6 and cdt1 after they leave the preRC?

Geminin

What protein binds to and inactivates residual Cdt1 in the nucleus?

phosphatases dephosphorylate cdc6 (activation)

Geminin is ubiquitinated by the then active APC/C (geminin destroyed)

In anaphase/mitotic exit, how are cdc6 and Cdt1 re-activated?

CHK1/2

What inhibitory phosphorylates Cdc25?

Spindle assembly checkpoint (SAC) and DNA damage checkpoint

What are the two examples of checkpoints in the cell cycle?

Anaphase

What does the SAC prevent until all chromosomes are properly attached?

Kinetochores

What sense occupation by microtubules and tension forces?

Aurora B

This protein generates the phosphorylation gradient in the middle of chromosomes

Aurora B is able to phosphorylate substrates around itself

What happens when kinetochores experience low tension (not attached on both sides)?

Aurora B is unable to phosphorylate substrates because they are positioned farther away

What happens when kinetochores experience high tension (attached on both sides)?

Cohesin

What protein in the centromeric region holds the two chromosomes together (looks like they are glued together)?

It is cleaved by Separase

What happens to cohesin when all kinetochores are connecting to opposing spindles and sense tension?

MAD and BUB

Which proteins bind to CDC20 when there are unoccupied kinetochores?

Blocks APC/C activation (prevent anaphase when kinetochores not properly attached and under tension)

What does MCC do?

Securin

What protein keeps separase inactive?

MCC (mitotic checkpoint complex)

The binding of MAD and BUB to CDC20 create what complex?

CDC20

When all kinetochores are occupied, what activates APC/C

p53 (tumor suppresor)

Known as the guardian of the genome

transcription factor that is activates in response to DNA damage

G1 or G2, can also cause programmed cell death in severe damage cases

In what phases will p53 arrest cells?

Formation of CKIs and DNA repair proteins

What does p53 promote?