REGULATION OF CELL CYCLE BY PROTEIN KINASES

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REGULATION OF CELL CYCLE BY PROTEIN KINASES

• BIOL 306 -BIOCHEMISTRY II
• H.ESRA AKGÜL
• 01040604

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References

• LEHNINGER PRINCIPLES OF BIOCHEMISTRY
• www.biop.ox.ac.uk
• www.biochemj.org

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SUBJECTS

• AN INTRODUCTION
• CELL CYCLE
• IMPORTANCE OF PROTEIN KINASES
• LEVELS OF CYCLIN DEPENDENT PROTEIN KINASES
  OSCILLATES
• CRITICAL PROTEINS IN CDKs REGULATE

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• Cell division requires a ordered sequence of
  biochemical events that assures every daughter
  cell a full complement of molecules required
  for life.
• Control of cell division in eukaryotic cells
  have revealed regulatory mechanisms.
• Protein kinases and protein phosphorylation are
  central to the timing mechanism that determines
  entry into cell division and ensures orderly
  passage through

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Cell cycle
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Phases

• S PHASEDNA is replicated to produce copies for
  both daughter cells.
• G2 PHASENew proteins are synthesize and the cell
  double in size
• M PHASE (MITOTIC)maternal nuclear envelope
  breaks down,paierde chromosome are pulled to
  opposire of cell and cytokinesis ,producing two
  daughter cell
• G1 PHASEThe waiting period of again dividing.
• G0 PHASE Ceases phase,entering the quiscent.If
  cell starts to divide,it reenters to G1 .

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IMPORTANCE OF PROTEIN KINASES

• The timing of the cell cycle is controlled by
  protein kinases with activities that change in
  response to cellular signals.
• By phosphorylating of proteins,protein kinases
  the metabolic activities to produce cell division.

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WHAT IS CDKs?

• The kinases are heterodimers with a regulatory
  subunit, cyclin, and a catalytic subunit,
  cyclin-dependent protein kinase (CDK). In the
  absenceof cyclin, the catalytic subunit is
  virtually inactive.When cyclin binds, the
  catalytic site opens up, aresidue essential to
  catalysis becomes accessible

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Cdk2 structure
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• CDK activities show striking oscillations
• These oscillations are the result of four
  mechanisms for regulating CDK activity
• 1-phosphorylation or dephosphorylation of the
  CDK,
• 2- controlled degradation of the cyclin subunit,
• 3-periodic synthesis of CDKs and cyclins,
• 4-the action of specific CDKinhibiting proteins.

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Regulation of CDks by phosphorylation

• The activity of a CDK is strikingly affected by
  two critical residues in the protein
• Phosphorylation of Tyr15 near the amino
• terminus renders CDK2 inactive the P Tyr
  residue is
• in the ATP-binding site of the kinase, and the
  negatively
• charged phosphate group blocks the entry of ATP.
  A
• specific phosphatase dephosphorylates this P Tyr
• residue, permitting the binding of ATP.
• Phosphorylation of Thr160 in the T loop
  of CDK, forces the T loop out of the substrate
  binding cleft, permitting substrate binding and
  catalytic activity.

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Controlled degradation of cyclin

• Progress through mitosis requires first the
  activation then the destruction of cyclins A and
  B, which activate the catalytic subunit of the
  M-phase CDK. These cyclins contain near their
  amino terminus the sequence ArgThrAlaLeuGlyAs
  pIleGlyAsn, the destruction box, which
  targets them for degradation.The DBRP(destruction
  box recognizing protein.) recognize this protein.

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• UbiquitinCyclin and activated ubiquitin are
  covalently joined by the enzyme ubiquitin ligase
• Several more ubiquitin molecules are then
  appended, providing the signal for a proteolytic
  enzyme complex, or proteasome, to degrade cyclin.

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The feedback mechanism

• Increased CDK activity activates cyclin
  proteolysis. Newly synthesized cyclin associates
  with
• and activates CDK, which phosphorylates and
  activates
• DBRP. Active DBRP then causes proteolysis of
  cyclin.
• Lowered cyclin causes a decline in CDK
  activity, and
• the activity of DBRP also drops through slow,
  constant
• dephosphorylation and inactivation by a DBRP
  phosphatase.The cyclin level is ultimately
  restored by synthesisof new cyclin molecules.

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Regulated Synthesis of CDKs and Cyclins

• CyclinD, cyclin E, CDK2, and CDK4 are synthesized
  only when a specific transcription factor, E2F,
  is present in the nucleus to activate
  transcription of their genes. Synthesis of E2F is
  in turn regulated by extracellular signals such
  as growth factors and cytokines

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Inhibition of CDKs

• Specific protein inhibitors bind to and
  inactivate specific CDKs. One such protein
• is p21.

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Critical proteins
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• How does the activity of CDK control
• the cell cycle?
• BehindCDK regulation by inspecting the effect of
  CDKs on the structures of laminin and myosin and
  on the activity of retinoblastoma protein.
• pRb when DNA damage is detected, this protein
  participates in a mechanism that arrests cell
  division in G1

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• Breakdown of the nuclear envelope before
  segregation of the sisterchromatids in mitosis is
  partly due to the phosphorylationof laminin by a
  CDK, which causes laminin filaments to
  depolymerize.
• After the division, CDK phosphorylates a small
  regulatory subunit of myosin, causing
  dissociation of myosin from actin filaments and
  inactivating the contractile machinery

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• When pRb, is phosphorylated,itcannot bind and
  inactivate EF2, a transcription factor that
  promotes synthesis of enzymes essential to DNA
  synthesis. If the regulatory protein p53 is
  activated by ATM and ATR, protein kinases that
  detect damaged DNA, it stimulates the synthesis
  of p21, which can bind to and inhibit cyclin
  ECDK2 and thus prevent phosphorylation of pRb.
  Unphosphorylated pRb binds and inactivates E2F,
  blocking passage from G1 to S until the DNA has
  been repaired.