genetic studies in yeast identified single protein kinase that plays a
central role in progression through cell cycle- Cdc28 in s.
cerevisiae & Cdc2 in s.pombe- Cdc28/Cdc2 is required for
transitions in the cell cycle eg. G1/S & G2/M-Activity of kinases reg
by cyclins which are synthesised & degraded in each cell cycle
In HUMANS-have protein homologous to Cdc2=CDK1 & also have
related protein kinases= CDK2-6. For activity the CDKs must be in
physical association with a CYCLIN A-G
graph showing diff cyclins/kinases
1. Cyclin D/CDK4- G1-progression
through early or mid G1 2. Cyclin
D/CDK6-G1-progression through early or
mid G1 3. Cyclin
E/CDK2-G1/S-progression through late
G1 to S 4. Cyclin A/CDK2-S/M- initiation &
completion of DNA sun 5. Cyclin
A/CDC2-G2-link to M phase 6. Cyclin
B/CDC2-G2/M-initiation & maintainence of
mitosis
Substrates for G1/S cyclin/CDK kinases
pRB- release & activation of txn factors/ p53-
role in nuclear localisation/ RPA (rep protein
A)- Initiation of DNA rep/ RNA Pol II- basal txn
catalytic kinase activity can be controlled by cyclin dependent kinase inhibitors-
p21-inhibit cyclin/CDK complexes req for G1/S transition. Other CKI's are the related
proteins p27, p57, p15, p16, p18 & p19. CKI's act at diff stags of the cell cycle & their
action represents checkpoint controls
CHECKPOINT CONTROLS
1.monitors for DNA damage and arrest cells in G1/S
Ensures cells do not enter S-phase
with damaged DNA (DNSA rep on a
damaged substrate leads to mutation
& genomic rearrangement).
G1/S check point involves: pRB
(retinoblastoma protein) & control of E2F,
cyclin E/CDK2, CKI (p21), p53 & ATM
protein (Ataxia Telangiectasia mutated)
-G1: Rb protein is bound to E2F txn
factor -G1 to S: Levels of cyclin E rise so
that CDK2/Cyclin E complex
phosphorylates the Rb
protein-phosphorylated Rb unable to bind
E2F -Early S: Txn of E2F regulated genes
to rep DNA
WHEN DNA DAMAGED: causes
activation of txn factor p53 which
induces expression of p21 (CKI)=p21
binds to & inhibits CDK2/Cyclin E= no
longer able to phosphorylate Rb protein-
Rb remains bound to E2F=repression of
genes req for G1 to S transition
HOW IS p53 ACTIVATED? in normal
cells, p53 levels are low. Mdm2 protein
removes p53 from the nucleus leading to
its destruction by the proteosome.
When damage to DNA- DNA damage signals eg ATM-binds to DNA ds breaks/ ATR recognises stalled rep
forks. The binding of ATM to ds breaks activates its kinase activity & it phosphorylates a no. of proteins
including p53=p53 phosphorylated & activated=mdm2 cannot bind to modified p53 so its levels rise, the
modification of p53 activates it as a txn factor. Increased txn of certain genes follow e.g. Waf1 that encodes
CKI p21
ATM also phosphorylates the
NBS1 & BRCA1 proteins therby
activating their role in DNA repair.
ATR also phosphorylates NBS1 &
BRCA1
Errors that can contribute to cancer
1. Changes in the reg of Cyclin/CDK
complexes seen in cancers-so cells go
through stages when there not supposed to
2. Demonstration that Cyclin/CDK complexes
help regulate proteins important in tumourgenesis