Signaling Systems of Serine Protein KinasesMitogen Activated Protein Kinase MAPK Cascade. Lalitha Ma

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Signaling Systems of Serine Protein
Kinases-Mitogen Activated Protein Kinase (MAPK)
Cascade.Lalitha Madhavan.
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INTRODUCTION

• MAPK evolutionarily conserved enzymes
  connecting cell surface receptors
• to critical regulatory targets withing the cell
• The name MAPK-acknowledges that they had first
  been detected as
• mitogen-stimulated tyrosine phospho-proteins in
  the early 1980s when there
• was an intense search for tyrosine kinases.
• Ray and Sturgill (1988 ) 42 kDa
  serine-threonine protein kinase (MAPK),
• isolated from insulin stimulated 3T3-L1 cells
  which was phosphorylated both on
• threonine and tyrosine.
• Functions of MAPK
• Cell proliferation
• Cell differentiation
• Apoptosis
• Immune responses

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Activators
MAPK pathway module
MKKK
MKK
MAPK
Substrates
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Regulation and Properties of MAPK

• The basic assembly of MAPK pathways three
  component module (3
• kinases), activated sequentially and conserved
  from yeast to humans.
• The first kinase-MAPK kinase kinase (MKKK).
• Activated by
• Phosphorylation by a MKKKK
• Interaction with a small GTP-binding protein of a
  Ras or Rho family.
• Oligomerization
• Subcellular relocalization.
• The second kinase-MAPK kinase (MKK)
• Recognize and phosphorylate a Thr-X-Tyr motif in
  the activation loop of
• MAPK- dual specificity kinases.
• Final kinase-MAPK
• Phosphorylates substrates on serine and threonine
  residues.
• Vast majority of defined substrates for MAPK are
  transcription factors other
• substrates are protein kinases, phospholipases
  and cytoskeletal associated
• proteins.

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Activation
of MAPK

• Dual phosphorylation- specifically, threonine
  and tyrosine.

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Unphosphorylated and Active, Dual phosphorylated
MAPK-erk2 Crystal structures
Low activity
Active
Nature, 367 704-711 (1994)
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• 
  Inactivation of MAPK
• Duration and amplitude of MAPK
  activation-balance between activation
• and inactivation mechanisms.
• Both mechanisms are influenced by negative
  feedback triggered by
• activation signal upstream of the MAP kinase.
• Evidence for dual specificity phosphatases known
  as MAPK phosphatases.
• Specificity of phosphatases is dictated by their
  intracellular localization thus
• clear cut evidence has been difficult to obtain.

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www.cellsignal.com
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www.cellsignal.com
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Evolution of MAPK pathways

• The evolutionary model-based on gene
  duplications that have occurred
• since the divergence of animals from yeast.
• JNK and p38 pathways arose from an ancestral
  hyperosmolarity pathway
• after being split from yeast and before split
  from C.Elegans
• These co-duplications on interacting proteins at
  the MAPK and MEK level
• have since evolved toward substrate specificity,
  thus giving distinct
• pathways.
• Experimentally defined cross-talk between the
  yeast pheromone and
• hyperosmolarity pathways is mirrored with
  corresponding cross-talk in the
• mammalian pathways-suggestive of existence of
  ancient orthologous cross-
• talk.

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www.cellsignal.com
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Mammalian MAPK pathway interactions
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Specificity of MAPK activation and function

• Scaffolding
• Sequential physical interaction between members
  of a given cascade.
• Indirect regulation of the expression of both
  ligands and inhibitors for
• cell surface receptors that feed into MAPK
  cascades.
• 4. Bipartite enzyme-substrate interaction.

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Nature 410, march 1, 2001, 37-40.
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Scaffolding proteins in routing MAPK modules
Science 1998, Vol 281 1625-26
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Pbs2p
Science 1997, Vol 276, 1702-05
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Pbs2p
Science 1997, Vol 276, 1702-05
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Science 1997, Vol 276, 1702-05
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Science1997, Vol 276, 1702-05
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MP-1
Science 1998, Vol 281, 1668-74
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Science 1998, Vol 281, 1668-74
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Science1998, Vol 281, 1668-74
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Science 1998, Vol 281, 1668-74
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Functions of scaffold proteins

• Organization of MAPK cascades for the effiecient
  serial activation of
• the components.
• Restriction of signal reception by recognizing
  signals from only a subset
• of possible receptor systems.
• Restriction of the specificity of signal
  transmission by interacting with a
• limited repertoire of potential components of MAP
  kinase cascades.
• Determination of the output signal , not only as
  a consequence of
• selectivity among MAP kinases, but also by
  localizing the cascade to
• selected sites of action, eg., transcription
  machinery, microtubule skeleton.

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Signal Amplification

• Can occur if each successive protein in the
  cascade is more abundant
• than its regulator.
• --MAP kinase module of the yeast pheromone
  response pathway.
• --ERK1/2 pathway- the Raf-MEK step (MEKK-MEK
  step).
• Establishment of threshold.
• --Due to dual phosphorylation of MAPK by MEK.
• --MEK-MAPK step-enhances cooperativity of
  activation of MAPK and to
• allow modulation by other signaling events in
  addition to or rather than
• amplifying the MEK signal.
• Optimal scaffold concentration can
  significantly increase signaling output.

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Conclusion
Thus a few MAPKs can control diverse cellular
processes in response to a plethora of
extracellular stimuli due to these inbuilt
mechanisms of specificity and signal
amplification.