Week 8 (The Structure and Function of CDKs and Ubiquitin Ligases) Flashcards
What do all Kinases have?
All kinases have a pocket that holds ATP and an active site that holds the polypeptide chain that will receive the phosphate group
What must Cyclin-dependent kinases be bound to, to be active?
a cyclin
Do Cyclins have intrinsic enzymatic activity?
No
Cyclins Cycle but are CDK levels constant?
Yes
What processes do cyclins undergo in each cycle?
Cyclins undergo a cycle of synthesis and degradation in each cell cycle
How many Cdks do yeast use?
yeast use a single Cdk
How many Cdks do vertebrates use?
vertebrates use five Cdks
What do Cyclical changes in cyclins drive?
cyclical changes in
Cdk-cyclin activity.
What does S-Cdks denote?
Cdks bound to S cyclins
How are cyclins regulated?
-The transcription of cyclin mRNA controls when
cyclins appear in the cell cycle.
-Degrading cyclins is a way of ensuring that the
cell cycle doesn’t go backwards.
-All remaining cyclins are degraded upon entry
into anaphase so levels are very low at the start
of G1.
How do Cyclins Influence the Target Specificity of the CDK?
- G1/S-cyclins bind CDKs in G1 and commit the cell to DNA replication (some cells also require G1 cyclins to pass the restriction point/START in late G1).
- S-cyclins bind CDKs during S phase and are required to initiate DNA replication
- M-cyclins promote the events of mitosis
What is the T-loop?
-The T-loop is a “pseudosubstrate”. It inhibits the
activity of the CDK by partially occupying the
active site.
-When CAK phosphorylates the T-loop, the position
of the T-loop shifts and the CDK is able to bind
substrate.
-Since CAK is always active, phosphorylation of
the T-loop is not a key regulatory step.
How is the activity of CDK-Cyclins fine tuned?
-The activity of the Cdk-cyclin complex
is fine-tuned by the balance between
inhibitory kinases and activating phosphatases
-CKIs (CDK-inhibitors) are another way to
reversibly inhibit CDK-cyclin complexes
What determines CDK-cyclin Activation?
The Balance between Wee1 & Cdc25
What are p27 and p21?
CKIs which reversibly inhibit
CDK cyclins by preventing substrate binding
Why do cyclins need to be destroyed?
Destroying cyclins and other cell cycle proteins
prevents the cell cycle from going backwards.
How are Cyclins marked for destruction
by the
covalent attachment of chains of ubiquitin by
enzymes called ubiquitin ligases.
What is the primary
determinant of Cdk activity
The rise and fall of cyclins
Why do cyclins need to be destroyed?
Destroying cyclins and other cell cycle proteins
prevents the cell cycle from going backwards.
How are Cyclins marked for destruction
by the
covalent attachment of chains of ubiquitin by
enzymes called ubiquitin ligases.
What is the primary
determinant of Cdk activity
The rise and fall of cyclins
CDK also require phosphorylation of the T loop
**
Would activating checkpoints help the cell progress through the cell cycle?
No
Checkpoints are negative signals that inhibit progression
When are levels of G1/S cyclin low?
When are they high?
- Low at the start of G1 and at the beginning of S phase
- High in the middle of G1 phase
When are levels of S cyclin low?
When are they high?
- Low at the start-mid G1 phase and at the end of M phase (after metaphase-anaphase boundary)
- High from start of S phase to beginning of M phase
When are levels of M cyclin low?
When are they high?
- low throughout G1 and S
- start rising in G2
- peak at beginning of M
- falls at metaphase-anaphase boundary
How many CDK’s dp we have thoughout the cell cycle?
1 CDK
Many different cyclins can bind
What does binding of a cyclin to a CDK do?
Makes the active site more accessible
Only then can the CDK activating kinase (CAK can activate phosphorylate the T loop)
What does the Wee 1 kinase do?
Inhibits the cyclin- CDK complex by adding an extra phosphate
What removes the extra phosphate added by Wee 1 kinase?
Cdc 25 phosphatase
Which reactivates the cyclin- CDK complex
The ratio of which two enzymes determines if the CDK- cyclins are active or not?
Cdc 25 phosphatase
Wee1 kinase
The ratio of which two enzymes determines if the CDK- cyclins are active or not?
Cdc 25 phosphatase
Wee1 kinase
Which part of a protein is ubiquitin linked to?
A lysine side chain (by an isopeptide/ covalent bond)
Does the proteosome destroy CDKs?
No
