Cyclins Flashcards
What are cyclins?
Cylincs are cyclically activated proteins that are expressed at different levels during the cell cycle, when present, they bind to specific kinases to activate them
What do cyclin dependent kinases do?
They phosphorylate proteins that are specific to certain stages of the cell cycle
What gives the cell cycle directionality?
Its created via the sequential appearance and destruction of the cyclins
What do cyclins do?
They direct kinases towards different substrates at different stages of the cell cycle
they allow the transition of a particular cell cycle phase
What experiment a long time ago identified cyclins/cdka?
Experiment on a particular virus which fused cells together
they found that when you fused cells at different cell cycle phases- mitotic cells will try and induce mitosis in the other cells
identified via the observation of condensed chromosomes in the non mitotic cells
the result= death of the chromosomes
In summary- what do mitotic cells do in terms of the cell cycle?
Mitotic cells cause interphase cells to join mitosis
Mitosis is dominant
What was the critical experiment concerning frogs eggs and meiosis?
Injected the cytoplasm of a mature from egg (in meiotic metaphase) into an immature oocyte (arrested at G2)
this induced the maturation of the oocyte- could see the presence of a meiotic spindle
Something in the mature egg is inducing entry into M phase
How can you tell meiosis has occurred in frogs eggs?
A little white spot on top of the egg is the meiotic spindle
What did the maturation factor turn out to be- how did they find out?
They purified the factor from the other proteins via chromatography
The protein turned out to be a kinase- due to its ability to phosphorylate histone H1
What critical experiment was carried out in sea urchin eggs?
Incubated the eggs with radioactive methionine (methionine would be incorporated into newly made proteins)
can see the radioactivity in the proteins via electrophoresis and autoradiography
What did they find in the sea urchin eggs experiment?
Found a few proteins that were appearing and then disappearing
(unusual as most proteins were accumulating)
they plotted the data for the proteins and found that they were being created and destroyed in accordance to the cell cycle - synchrony to the cell cycle
CYCLINS*
What experiment was carried out on linear yeast?
The creation of cell division cycle mutants
observed that CDC2 mutants were very long- they kept growing in G2
the same was observed in CDC25 mutants
observed that wee1 mutants, however, were very small
hypothesis was= cdc mutants are mutations which slow down the timing taken to induce mitosis
similiarly, wee1 mutants decrease this time
What was concluded when they sequenced the genes in the linear yeast?
Cdc25 is a positive regulator of cdc2
Wee1 is a negative regulator of cdc2
cdc2 induces mitosis
Cdc2 was found to = Cdk1
MAKES SENSE- lose cdc2- wont go into mitosis- stay in G2= longer cells
lose wee1- go into mitosis quicker- shorter cells
After sequencing- what was Cdc2 in yeast similar to?
The maturation promoting factor found in the mature frog eggs
What did all 3 cell cycle driver experiments come together to conclude?
MPF is a protein kinase controlled by cyclins which are made and destroyed on a cyclical basis
What was interesting about MPF and cyclin b activity?
The peaks of cyclin B activity didn’t exactly match up to MPF activity- which you would’ve thought it did considering cyclin B activates its Cdk
clearly something else is involved
What does cyclin D interact with?
Cdk 4/6 = G1 Cdk
What does cyclin E interact with?
Cdk2 = G1/S Cdk
What does cyclin A interact with?
Cdk2/1 = S Cdk
What does cyclin B interact with?
Cdk1 = M Cdk
What are the two main functions of cyclins?
Targeting their catalytic subunit in the cell
activating the catalytic subunit
How are Cdks activated?
Cyclins
CAKs- complex which phosphorylates a threonine residue on the cdk- activates proper catalytic activity
Cdc25- dephosphorylates the inhibitory phosphorylation what wee1 induces- activates the Cdk as a result
How are Cdks inhibited?
CKIs- mainly operate in G1 and S
Wee1= phosphorylates adjacent threonine and tyrosine residues which inhibits the kinase
Examples of CKIs and the cyclins they inhibit
P16 and 15 inhibits Cdk4/6
P21 + P27 inhibits Cdk2 and Cdk2/1
How can antiproliferative signalling induce inhibitor expression?
TGFbeta binds to its receptor and induces the transcription of P15
P15 inhibits Cdk4/6- this prevents the transition from G1 to S
Neighbouring cells can also stop proliferation via contact inhibition
How do mitogens regulate CKI function?
They inhibit the P21/27 system- so stops the inhibition of cell cycle progression
Akt (which is stimulated by the mitogen binding to its receptor) prevents P21 from reaching the nucleus
What does cytoplasmic P27 do in low grade cancers?
Inhibits cell cycle progression
How does nuclear and cytoplasmic P27 have different prognostic indications?
just nuclear P27= better prognostic outcome
nuclear and cytoplasmic p27= worse prognostic outcome
What is the restriction point?
The specialised position within G1 where cells make the decision to exit the cell cycle into G0- or with the presence of proliferating signals- carry on through the cycle and divide
What does Rb do and what are its implications in the absence of mitogens?
Rb is a DNA binding protein
inactivates E2F- transcription factor required for S phase entry
binds to a histone deacetylase which results in chromatin compaction
What is the effect of mitogens on cyclin D, and thus Rb?
Mitogens induce the production of cyclin D
Cyclin D leads to the hyperphosphorylation of Rb- which prevents it from binding to the DNA and E2F
In the presence of mitogens, what does Rb do?
Rb is hyperphosphorylated, can no longer bind to E2F or histone deacetylases
E2F is able to interact with RNA polymerase 2= transcription of genes required for S phase entry
= cell cycle progression
