23.3
MAPK-signaleringscascades
Door mitogeen geactiveerde proteïnekinase, of MAPK-route, activeert drie opeenvolgende kinasen om cellulaire reacties zoals proliferatie, differentiat…
Mitogen-activated protein kinase or MAPK pathways relay signals downstream of RTKs to activate transcription factors.
In the Ras-MAPK signaling pathway, Raf, the initial kinase of this pathway, binds the 14-3-3 protein dimer and remains inactive in the cytosol.
Mitogen binding on the RTKs activates Ras GTP, which then binds the Raf/14-3-3 complex.
Phosphorylation of multiple sites on Raf disassembles the complex allowing Raf to dimerize with a nearby phosphorylated monomer.
The dimeric Raf now phosphorylates and activates another kinase, MEK , which in turn phosphorylates the cascade's final kinase, ERK.
The activated ERK is transported to the nucleus to phosphorylate transcription factors such as c-Jun and c-Fos and initiate cellular processes such as cell proliferation, survival, differentiation, or apoptosis.
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Q1: How does the Ras-MAPK signaling pathway activate downstream kinases?
Mitogen binding activates receptor tyrosine kinases, which stimulate Ras GTP. Activated Ras recruits the Raf/14-3-3 complex to the membrane. Phosphorylation causes the complex to disassemble, allowing Raf to dimerize and phosphorylate MEK, which then activates ERK, the final kinase in the cascade.
Q2: What role do scaffold proteins play in MAPK signaling?
Scaffold proteins like KSR channel signals to the correct MAPK module, ensuring specificity and preventing crosstalk between parallel pathways. They localize kinases and substrates near each other, enhancing signal relay efficiency. This organized positioning allows cells to activate different effector proteins without interference between signaling pathways.
Q3: What are the three main types of MAPK pathways in mammals?
The three MAPK pathway types are ERK, activated by growth factors to promote cell growth and differentiation; JNK, induced by environmental stresses to cause cell death and inflammation; and P38/SAPKs, also stress-activated to promote inflammation, cell death, differentiation, and cell cycle regulation.
Q4: How does activated ERK trigger cellular responses?
Activated ERK translocates to the nucleus where it phosphorylates transcription factors such as c-Jun and c-Fos. These phosphorylated factors trigger gene expression changes, including activation of cyclin D1, which promotes cell cycle progression and initiates cellular processes like proliferation, survival, differentiation, or apoptosis.
Q5: Why is the 14-3-3 protein important in Raf activation?
The 14-3-3 protein dimer binds Raf in the cytosol, keeping it inactive until a signal arrives. Upon mitogen stimulation and Ras activation, the Raf/14-3-3 complex is recruited to the membrane. Phosphorylation at multiple sites causes the complex to disassemble, freeing Raf to dimerize and become catalytically active.
Q6: How do cells prevent crosstalk between different MAPK modules?
Eukaryotes use pathway-specific scaffold proteins that channel incoming signals to the correct MAPK module. These scaffolds localize and orient protein complexes near their substrates, ensuring signal specificity. Although different MAPK pathways often share the same kinases, scaffold proteins direct them to activate different effector proteins without interference.
Q7: What cellular responses result from MAPK pathway activation?
MAPK pathway activation regulates diverse cellular responses including proliferation, differentiation, survival, and apoptosis. ERK pathways promote growth and differentiation; JNK and P38 pathways respond to environmental stresses by triggering inflammation and cell death. The specific response depends on which MAPK module is activated and which transcription factors are phosphorylated.