Chapter 8 - pRb and Control of the Cell Cycle Clock Flashcards
Afferent signals programming the cell cycle
Growth Factor Receptors, Monitors of Genome Integrity, TGF-B Receptors, Integrins, Monitors of Cell Metabolism
Growth versus Proliferation (coupled or uncoupled)
Normally coupled - but may become uncoupled in cancer cells
Checkpoint at the END of G1
DNA damage checkpoint, decides to either halt the cycle or results in progression into S phase depending on genome integrity
Checkpoint IN S Phase
SECOND DNA damage checkpoint, DNA replication is halted if genome integrity is lost
Checkpoint at the END of G2
Replication and growth checkpoint, entrance into M Phase is blocked if DNA replication is incomplete
Checkpoint during M Phase
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
During what point of the cell cycle is the cell responsive to mitogenic GFs and to TGF-B?
Cells are responsive to growth factors through most of G1, leading up to and stopping response upon reaching R-Point
R-Point
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
Decatenation Checkpoint
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
CDKs
Cyclin-Dependent Kinases are Serine/Threonine kinases that are activated by “Cyclins”
Cyclins
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
Cyclin-CDK Complex Structure
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
CDK4/6
think G1 PHASE!
guided by cyclins D1, D2, and D3 aka D-type cyclins up until the R-POINT
CDK2 part ONE
think RPOINT to LATE S PHASE!
guided by cyclins E1 and E2 to enable entrance into S-Phase
CDK2 part TWO
think progress S!
E-type cyclins are replaced by A1 and A2 to progress towards the middle of S-Phase
CDC2 (or CDK1)
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
CDK3 (rarely used)
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
PSTAIRE a-helix
present in the structure of all CDKs and is essential for the role of cyclin binding to CDK
How is the level of cyclin D1 controlled to result in progression of G1?
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.
p15(^INK4B), p16(^INK4A), p18(^INK4C), p19(^INK4D)
inhibits cyclin D - CDK4/6 complexes – induced strongly by TGF-B
p21(^Cip1), p57(^Kip2), p27(^Cip1)
inhibits cyclin E - CDK2, cyclin A - CDK2, cyclin A - CDC2, cyclin B - CDC2 complexes
What is the paradoxical function of P21 and p27?
Though they work to inhibit E-CDK2, B-CDC2 and A-CDC2, they STIMULATE the formation of D-CDK4/6 complexes
How does phosphorylation of pRb correlate to the progression of the cell cycle?
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
What enzyme strips or removes the phosphate groups off of pRb?
protein phosphatase I (PP1)
What is the significance of hyperphosphorylation of pRb at the R-Point?
This allows us to assume that pRb may assist in governing the passage of a cell through the R-Point transition
How does phosphorylation regulate the activity of pRb and result in the R-Point transition occuring?
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
Do cyclin-CDK complexes play a role in regulating the phosphorylation of pRb?
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
E2Fs
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
Histone deacetylase
(HDAC) places chromatin in an untranscribable conformation
Histone acetylase
places chromatin in a configuration that is conducive to transcription
