cell bio exam 3: regulation of cdks and cell cycle

cdk is activated by

cak phosphorylation on the t-loop

when cyclin is not bound to cdks, the active site of cdks is blocked by a

t-loop; cycling binding to cdk moves the t-loop out of the active site

cyclin binding to cdk causes the t-loop to move out of the way and (?) of the cdk

partial activation

what causes full activation of a cycling/cdk complex by the end of G2?

phosphorylation of the t-loop by cdk-activating kinase (CAK)

why is the cdk/m-cyclin complex still inactive at the end of G2, even though CAK has phosphorylated the t-loop?

inhibitory kinase Wee1 phosphorylates the m-cdk and prevents activation

what is responsible for removing inhibitory phosphates placed on m-cdk by wee1?

cdc25 phosphotase

cdk inhibitor protein (cki)

blocks cdk activity by direct binding to cdks and cyclins→ prevents ATP from binding and substrates from binding

p27

• cdk inhibitor (CKI)

• binds to an active acyclin-cdk complex to inhibit phosphorylation

• prevents progression of G1 to S

cell cycle control system

• pauses the cell cycle at several transition points

• checkpoints/transition points = “molecular brakes”

• between G1 and S, between S and M, at the end of M

G0

• a prolonged non proliferative state

• can be permanent or may be temporary

• at the G1 to S checkpoint: evaluates whether or not conditions are right for completing another cell cycle

what causes sudden degradation of s and m cycling partway through m phase?`

ubiquitin mediated proteolysis and anaphase promoting complex apc/c

mitogens

extracellular signals that stimulate cells and induce division; stimulate synthesis of G1/S cyclins

if cells are deprived of mitogen,

they will arrest in G1; if for long enough, they will withdraw from the cell cycle

how do mitogens affect retinoblastoma (Rb) protein?

• normally, Rb inhibits transcriptional regulators by binding to them

• mitogens → signal cascade → pohsphorylate Rb

• Rb inactive → transcription regulators can activate genes

p53

• critical gene for preventing cancer! regulates G1/S transition

• normal cells express little p53 because it is rapidly degraded by ubiquitin-mediated proteolysis

• in high stress, cells are normally able to increase p53 levels to stop the cell cycle or start apoptosis

what effect does mutated p53 have on cells?

in response to cellular distress, p53 is not able to cause cell cycle arrest or apoptosis, so cells continue dividing even if DNA is damaged

how is p53 activated?

DNA damage → protein kinase activate and phosphorylate p53 → p53 stabilized and not degraded → p53 promotes p21 (CKI) → p21 binds to and inhibits G1/S cdk’s → cell cycle stops in G1 → DNA repair

errors in DNA replication can arrest the cell cycle in G2 by

inhibiting CDC 25th so Wee1 phosphorylation s can’t be removed; m-cdk remains inactive

active m cdk promotes two positive feedback loops via phosphorylation:

1. activates its activator (cdc25)

2. inhibits its inhibitor (Wee1)

causes a rapid increase in active m-cdk, carrying the cell from G2 to M phase