Chapter 8 - pRb and Control of the Cell Cycle Clock

Question 1

Afferent signals programming the cell cycle

Answer 1

Growth Factor Receptors, Monitors of Genome Integrity, TGF-B Receptors, Integrins, Monitors of Cell Metabolism

Question 2

Growth versus Proliferation (coupled or uncoupled)

Answer 2

Normally coupled - but may become uncoupled in cancer cells

Question 3

Checkpoint at the END of G1

Answer 3

DNA damage checkpoint, decides to either halt the cycle or results in progression into S phase depending on genome integrity

Question 4

Checkpoint IN S Phase

Answer 4

SECOND DNA damage checkpoint, DNA replication is halted if genome integrity is lost

Question 5

Checkpoint at the END of G2

Answer 5

Replication and growth checkpoint, entrance into M Phase is blocked if DNA replication is incomplete

Question 6

Checkpoint during M Phase

Answer 6

Ensures proper alignment of chromosomes assembled on the mitotic spindle at the end of metaphase, to ensure no uneven separation of chromosomes occurs during anaphase

Question 7

During what point of the cell cycle is the cell responsive to mitogenic GFs and to TGF-B?

Answer 7

Cells are responsive to growth factors through most of G1, leading up to and stopping response upon reaching R-Point

Question 8

R-Point

Answer 8

AKA restriction point – a point during the cell cycle where the cell must commit to advance through the cell cycle, remain in G1, or remove itself from the active cell cycle into G0

Question 9

Decatenation Checkpoint

Answer 9

Occurs in late G2 phase and prevents entrance into M phase until the pair of DNA helices replicated during S phase have been unwound from one another

Question 10

CDKs

Answer 10

Cyclin-Dependent Kinases are Serine/Threonine kinases that are activated by “Cyclins”

Question 11

Cyclins

Answer 11

regulatory protein subunits capable of activating the catalytic function when associated with its corresponding CDK forming a bimolecular complex – serve as a “guide dog” for CDKs helping the cyclin-CDK complex to recognize the correct protein substrate within the cell

Question 12

Cyclin-CDK Complex Structure

Answer 12

cyclin activates a CDK by binding to it -resulting in sterochemical shifts of the catalytic site resulting in CDK-activating kinase phosphorylation on a threonine residue of the activation loop - and results in catalytic activity by further locating target proteins for the now active CDK to act upon

Question 13

CDK4/6

Answer 13

think G1 PHASE!
guided by cyclins D1, D2, and D3 aka D-type cyclins up until the R-POINT

Question 14

CDK2 part ONE

Answer 14

think RPOINT to LATE S PHASE!
guided by cyclins E1 and E2 to enable entrance into S-Phase

Question 15

CDK2 part TWO

Answer 15

think progress S!
E-type cyclins are replaced by A1 and A2 to progress towards the middle of S-Phase

Question 16

CDC2 (or CDK1)

Answer 16

think BACK HALF of S to END!
A-type cyclins jump from CDK2 to CDC2 and are replaced by B1 and B2 at the transition from G2 to M Phase

Question 17

CDK3 (rarely used)

Answer 17

think G0 to ACTIVE G1!
guided by C-type cyclins to move a cell from the G0 state to G1 Phase of the active cell cycle

Question 18

PSTAIRE a-helix

Answer 18

present in the structure of all CDKs and is essential for the role of cyclin binding to CDK

Question 19

How is the level of cyclin D1 controlled to result in progression of G1?

Answer 19

extracellular mitogens activate GF and ECM cascades involving RTKs and integrins to lead to the AP-1 TF that is a modulator of cyclin D1 transcription. AP-1 is a TF formed from the Jun-Fos subunits into a complex. There are many other cascades capable of activating transcription of the cyclin D1 gene involving mitogenic signals.

Question 20

p15(^INK4B), p16(^INK4A), p18(^INK4C), p19(^INK4D)

Answer 20

inhibits cyclin D - CDK4/6 complexes – induced strongly by TGF-B

Question 21

p21(^Cip1), p57(^Kip2), p27(^Cip1)

Answer 21

inhibits cyclin E - CDK2, cyclin A - CDK2, cyclin A - CDC2, cyclin B - CDC2 complexes

Question 22

What is the paradoxical function of P21 and p27?

Answer 22

Though they work to inhibit E-CDK2, B-CDC2 and A-CDC2, they STIMULATE the formation of D-CDK4/6 complexes

Question 23

How does phosphorylation of pRb correlate to the progression of the cell cycle?

Answer 23

pRb is dephosphorylated during the M Phase to G1 transition, however upon entering G1, pRb is phosphorylated once resulting in “hypophosphorylated pRb” and then upon reaching and progressing through the R-Point transition, pRb is hyperphosphorylated where it remains until after M Phase where is is a gain consequently stripped of its phosphate groups leaving it dephosphorylated

Question 24

What enzyme strips or removes the phosphate groups off of pRb?

Answer 24

protein phosphatase I (PP1)

Question 25

What is the significance of hyperphosphorylation of pRb at the R-Point?

Answer 25

This allows us to assume that pRb may assist in governing the passage of a cell through the R-Point transition

Question 26

How does phosphorylation regulate the activity of pRb and result in the R-Point transition occuring?

Answer 26

hypophosphorylation of pRb allows pRb to actively inhibit growth where after the R-Point transition hyperphosphorylation inactivates pRb - disabling its inhibitory powers resulting in continuation of cell growth = hyperphosphorylation inactivates pRb

Question 27

Do cyclin-CDK complexes play a role in regulating the phosphorylation of pRb?

Answer 27

yes, D-type cyclins and CDk4/6 drives the hypophosphorylation of pRb and E-CDK2 drives the hypo to hyper phosphorylation transition of pRb following r-Point

Question 28

E2Fs

Answer 28

a group of transcription factors regulated by pRb (and two cousins p107 and p130) where while bound to pRb, is unable to activate transcription of the DNA they are bound to

Question 29

Histone deacetylase

Answer 29

(HDAC) places chromatin in an untranscribable conformation

Question 30

Histone acetylase

Answer 30

places chromatin in a configuration that is conducive to transcription