MAP Kinase Pathways
■ Activated Ras triggers a kinase cascade in which Raf, MEK, and MAP kinase are sequentially phosphorylated and thus activated. Activated MAP kinase dimerizes and translocates to the nucleus (see Figure 14-21).
■ Phosphorylation of one or more residues in a conserved lip region activates MAP kinases and many other protein kinases involved in signal-transduction pathways.
■ Activation of MAP kinase following stimulation of a growth factor receptor leads to phosphorylation and activation of two transcription factors, TCF and SRF. These associate into a trimeric complex that promotes transcription of various early-response genes (see Figure 14-23).
■ Yeast and higher eukaryotes contain multiple MAP ki-nase pathways that are triggered by activation of various receptor classes including G protein-coupled receptors.
■ Different extracellular signals induce activation of different MAP kinases, which regulate diverse cellular processes (see Figure 14-25).
■ The upstream components of MAP kinase cascades assemble into large pathway-specific complexes stabilized by scaffold proteins (see Figure 14-24). This assures that activation of one pathway by a particular extracellular signal does not lead to activation of other pathways containing shared components.
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