BMS2021 Lecture Notes - Lecture 25: Old Age, Cytokinesis, Proteasome
30 May 2018
School
Department
Course
Professor
Week 9. Regulation of cell cycle and Cell
death/ Apoptosis
REGULATION OF CELL CYCLE AND THE ROLE OF CYCLINS
• Cell cycle control is regulated by checkpoints at G1/S, G2/M and the metaphase to anaphase
transition
• Checkpoints sense possible defects during DNA synthesis and chromosome segregation
• Activation of checkpoints induce cell cycle arrest through modification of CDK activity and allows
cells to properly repair defects and thus preventing transmission to resulting daughter cells:
o DNA damage checkpoint is at G2/M -> spindle assembly checkpoint (chromosome
kinetochores/segregation)
• Progression of the cell cycle is regulated by CDK (cyclin dependent kinases) and cyclins
-kinases phosphorylates proteins
-cyclins must bind to CDK for active site to open -> conformation change to become active
(if not bound then not active)
-cyclins aid transition between cycle phases
• CDKs: engine that drives cell cycle progression
o Contain a serine/threonine specific catalytic core -> kinase phosphorylates
serine/threonine
o Phosphorylate proteins that regulate chromatin condensation, nuclear envelope
breakdown, spindle assembly etc
Interphase CDKs
Mitotic CDKs
CDK2, CDK4, CDK6
o Mediated by kinase activity of CDK
via phosphorylation
CDK1, M-CDK (master regulator)
o Triggers centrosome separation,
nuclear envelope breakdown,
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chromosome segregation etc.
o Regulated by cyclin degradation
• Principle of mitotic checkpoint signalling:
o APC/C is a ubiquitin ligase that degrades targets including cycling by 26s proteasome
-> adds ubiquitin -> targets to proteasome -> degrades cyclin
o Unattached kinetochore (prometaphase) emits a checkpoint signal that inhibits APC/C
o When all chromosomes have attached and congressed to the metaphase plate, APC/C is
activated following the transition to anaphase
o Activation of checkpoint = APC/C inhibited -> cyclin B remains high and CDK remains
active -> apoptosis
• Cyclins: regulatory subunits
o Activating proteins
o Bind to CDK and control ability to phosphorylate target proteins
o Expression changes with the cell cycle
o Expression is regulated by its mRNA and protein stability and degradation
o There are 4 classes
G1/S
S
M
G1
o Activate CDKs in
late G1 to trigger
cell cycle entry
o Levels fall in S
phase
o Activate CDKs just
after entry to
promote DNA
replication and
early mitotic
events
o Activate CDKs to
stimulate entry
into mitosis
o Are degraded at
mid-mitosis (Met-
Ana)
o Coordinate with
G1/S
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• Role of CDK1- cyclin B in chromosome segregation:
o Complex is active in early mitosis and late G2 to stimulate chromatin condensation (chromatids
associate via cohesion ring)
o During kinetochore and chromosome alignment, APC/C degrades cyclin B and securing thereby
inhibiting CDK1
o CDK inhibition leads to separase activation and thus cleavage of cohesion ring
-> allows sister chromatid segregation and transition to anaphase
-> cohesion ring must be degraded in order to form centromere structure
o Inactivation of CDK 1 is also required for:
Chromosome decondensation
Reformation of the nuclear envelope
Cytokinesis
o If CDK1 is recovered after chromosome segregation and decondensation, the chromosomes re-
condense and the cells are unable to exit mitosis
-has to remain low during M-A phase or else DNA cant enter anaphase
• Regulatory mechanisms of CDK activity is controlled by 4 mechanisms:
-CDKs are composed of two lobes, a catalytic cleft and a flexible H-loop (substrate binding) which
in the absence of cyclins, the H-loop blocks the catalytic cleft and prevents ATP binding
1
2
3
4
o Must be bound to
a cyclin (+ve)
o Primary regulatory
mechanism
o Causes a
conformational
change in CDK
allowing ATP
binding
o Phosphorylation of
CDK H-loop Thr
160 residue (+ve)
by CAK (CDK
activating kinase)
o Phosphorylation of
CDK at Tyr 15 by
Wee1 which
inhibits CDK
(opposed by
Cdc25)
o Binding of CDI
(CDK inhibitor)
proteins induces
an inactive CDK
formation
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Regulation of cell cycle and cell death/ apoptosis. Cyclins must bind to cdk for active site to open -> conformation change to become active (if not bound then not active) Cyclins aid transition between cycle phases: cdks: engine that drives cell cycle progression, contain a serine/threonine specific catalytic core -> kinase phosphorylates serine/threonine, phosphorylate proteins that regulate chromatin condensation, nuclear envelope breakdown, spindle assembly etc. G1/s: activate cdks in late g1 to trigger cell cycle entry, levels fall in s phase. G1: coordinate with after entry to promote dna replication and early mitotic events stimulate entry into mitosis, are degraded at mid-mitosis (met- > allows sister chromatid segregation and transition to anaphase. > cohesion ring must be degraded in order to form centromere structure. Inactivation of cdk 1 is also required for: If cdk1 is recovered after chromosome segregation and decondensation, the chromosomes re- condense and the cells are unable to exit mitosis.
