Week 8 (The Structure and Function of CDKs and Ubiquitin Ligases)

Question 1

What do all Kinases have?

Answer 1

All kinases have a pocket that holds ATP and an active site that holds the polypeptide chain that will receive the phosphate group

Question 2

What must Cyclin-dependent kinases be bound to, to be active?

Answer 2

a cyclin

Question 3

Do Cyclins have intrinsic enzymatic activity?

Answer 3

No

Question 4

Cyclins Cycle but are CDK levels constant?

Answer 4

Yes

Question 5

What processes do cyclins undergo in each cycle?

Answer 5

Cyclins undergo a cycle of synthesis and degradation in each cell cycle

Question 6

How many Cdks do yeast use?

Answer 6

yeast use a single Cdk

Question 7

How many Cdks do vertebrates use?

Answer 7

vertebrates use five Cdks

Question 8

What do Cyclical changes in cyclins drive?

Answer 8

cyclical changes in

Cdk-cyclin activity.

Question 9

What does S-Cdks denote?

Answer 9

Cdks bound to S cyclins

Question 10

How are cyclins regulated?

Answer 10

-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.

Question 11

How do Cyclins Influence the Target Specificity of the CDK?

Answer 11

• 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

Question 12

What is the T-loop?

Answer 12

-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.

Question 13

How is the activity of CDK-Cyclins fine tuned?

Answer 13

-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

Question 14

What determines CDK-cyclin Activation?

Answer 14

The Balance between Wee1 & Cdc25

Question 15

What are p27 and p21?

Answer 15

CKIs which reversibly inhibit

CDK cyclins by preventing substrate binding

Question 16

Why do cyclins need to be destroyed?

Answer 16

Destroying cyclins and other cell cycle proteins

prevents the cell cycle from going backwards.

Question 17

How are Cyclins marked for destruction

Answer 17

by the
covalent attachment of chains of ubiquitin by
enzymes called ubiquitin ligases.

Question 18

What is the primary

determinant of Cdk activity

Answer 18

The rise and fall of cyclins

Question 19

Why do cyclins need to be destroyed?

Answer 19

Destroying cyclins and other cell cycle proteins

prevents the cell cycle from going backwards.

Question 20

How are Cyclins marked for destruction

Answer 20

by the
covalent attachment of chains of ubiquitin by
enzymes called ubiquitin ligases.

Question 21

What is the primary

determinant of Cdk activity

Answer 21

The rise and fall of cyclins

Question 22

CDK also require phosphorylation of the T loop

Answer 22

**

Question 23

Would activating checkpoints help the cell progress through the cell cycle?

Answer 23

No

Checkpoints are negative signals that inhibit progression

Question 24

When are levels of G1/S cyclin low?

When are they high?

Answer 24

• Low at the start of G1 and at the beginning of S phase

- High in the middle of G1 phase

Question 25

When are levels of S cyclin low?

When are they high?

Answer 25

• 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

Question 26

When are levels of M cyclin low?

When are they high?

Answer 26

• low throughout G1 and S
• start rising in G2
• peak at beginning of M
• falls at metaphase-anaphase boundary

Question 27

How many CDK’s dp we have thoughout the cell cycle?

Answer 27

1 CDK

Many different cyclins can bind

Question 28

What does binding of a cyclin to a CDK do?

Answer 28

Makes the active site more accessible

Only then can the CDK activating kinase (CAK can activate phosphorylate the T loop)

Question 29

What does the Wee 1 kinase do?

Answer 29

Inhibits the cyclin- CDK complex by adding an extra phosphate

Question 30

What removes the extra phosphate added by Wee 1 kinase?

Answer 30

Cdc 25 phosphatase

Which reactivates the cyclin- CDK complex

Question 31

The ratio of which two enzymes determines if the CDK- cyclins are active or not?

Answer 31

Cdc 25 phosphatase

Wee1 kinase

Question 32

The ratio of which two enzymes determines if the CDK- cyclins are active or not?

Answer 32

Cdc 25 phosphatase

Wee1 kinase

Question 33

Which part of a protein is ubiquitin linked to?

Answer 33

A lysine side chain (by an isopeptide/ covalent bond)

Question 34

Does the proteosome destroy CDKs?

Answer 34

No