Lecture 5: Cell Cycle - cyclins, CDKs, CDKIs Flashcards
What is a CDK
Cyclin dependent kinase
What is a CDKI
Cyclin dependent kinase inhibitor
What are the key control proteins
CDK’s and cyclins.
Cyclins bind to CDKs and affect the latter’s ability to phosphorylate serine and threonine residues of their substrates
CDKs - 40% homology with each other
Cyclins - 100 AA residue long domain
How do CDK/cyclins interact with checkpoints
activity levels of particular CDK/cyclins allow the cell to pass through the different checkpoints (G1 to S, intra-S, G2 to M) of the cell cycle
ALSO A SPINDLE CHECKPOINT
What is a restriction point?
The G1 to S check point - cyclin D is responsive to growth factors (E.g., Ras pathway) which pushes the cell from G1 to S
(Before its too late. Once past it either happens or the cell self destructs)
How are CDKs regulated
Via protein - protein interactions and phosphorylation’s (can be +/-)
Results in conformational change (opening a substrate cleft)
Without binding to a cyclin (or other) its binding to substrates is poor
What are cyclns
‘Cyclin box’ - 100 aa to interact with kinase partner (CDK)
Transcriptional and post-transcriptional controls on cyclin levels:
1. transcription: TFs, activity of cyclin/CDK themselves (one cyclin/CDK will lead to increased transcription of the next)
2. Protein abundance: rapid degradation
3. Protein localisation
What are CDK concentrations like
VERY CONSTANT
What are cyclin concentrations like
VARIES, the concentrations correlate with check points
What are D type cyclins
3 types (D1, D2, D3)
Levels can be induced by growth factos (mitogens) = integration of multiple signlalling pathways through control of CDK4 & 6 activation
Control G0 progression to G1
ONCOGENE ACTIVATION DRIVES CYCLIN D EXPRESSION
Different cyclins bind to different CDKs - the right cyclin binding also impacts
its level of activity
Can a Cyclin-CDK complex be further regulated?
YES.
They are subject to further positive and negative regulation via phosphorylation
E.g., phosphorylation a C terminus threonine residue by CAK is required for activity
Phosphorylation of two residues near N terminus threonine tyrosine is inhibitory
CDC25 phosphatases remove inhibitory phosphorylation
What are the CDKs / cyclins for G1 check point
D1, 2, 3 cyclins
CDK4, 6
Cyclin E1, E2 & CDK2 - critical initiators of DNA replication, chromatin unwinding, DNA polymerases.
Cyclin E transcribed by E2F
What are the CDKs / cyclins for S/G2 check point
Cyclin A1, A2 & CDK2
- phosphorylates DP1 (part of E2F complex) inactivating it
What are the CDKs / cyclins for M check point
Cyclin B1, B2 & CDK1 (CDC2)
1. directs nearly all processes during mitosis including chromosome condensation and segregation
2. promotoes mitotic spindle formation
3. degradation of cyclin B essentail for exit of M phase
Why is CDK1 essential?
Key for mitotic progression
Only possible CDK capable of initiating M phase
CHK & WEE1 mediate DNA replication and M phase via CDK1 activitiy
How are CDKs activated by non-cyclin proteins
KSV: K cyclin (~30% identiyy to cyclin D2), binds to CDK6 & 4. Inactivates pRB
RINGO/Speedy proteins (Human Spy1) activate CDK1 & 2 (no similarity to cyclins)
How do cyclin/CDK alterations lead to cancer?
slow degradation or bad binding affintities can elgonate certain phases
How does Cyclin D1 cause cancer
too much Cyclin D1 = Breast carcinoma leukaemia’s
reduced degradation due to depressed GSK3 activity = diverse tumours
How does Cyclin E cause cancer
Over expression = breast carcinoma
Defective degradation due to lo,ss of hCDC4 = endometrial, breast, ovarian carcinomas
How does CDK4 cause cancer
Structural mutation (INK4 resistance) = melanoma
Amplification = several types
What are the two types of CDKIs
INK4 proteins:
Inhibit CDK4/6
Cip/Kip proteins:
inhibit all other cyclin-CDK complexes
TGF-beta = growth inhibitory by inducing p15^INK4B (& weakly induces p15^Cip1/Waf1)
DNA damage induces p21^Cip1/Waf1
How are CDKs regulated?
Cyclin (or RINGO/speedy) binding
CDKIs (these are also regulated)
+/- phosphorylations
What do mitogens do?
Inhibit CDKIs by activating PI3K->PKB/Akt (pathway)
What is the role of the PKN/Akt pathway?
phosphorylates p21^Cip1/Waf1:
Causes export from nucleus to cytoplasm, no longer engage and inhibit cyclin-CDK complexes
Phosphorylates p27^Kip1: cannot locate nucleus
Loss of CDKI activity leads to cancer
Loss of p16^INK4a or Rb allows prevents the cell from stopping proliferation
How might p21^Cip1/Waf1 have oncogenic potential
