Principle of one-way conduction at chemical synapses 

always transmit the signals in one direction i.e., from the neuron that secretes the neurotransmitter called the presynaptic neuron, to the neuron on which the transmitter acts, called the postsynaptic neuron

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Active Raf phosphorylates MEK on one or two serine residues in its activation loop, activating MEK kinase activity

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#Unit 4. Cell Communication and Cell Signaling

Constitutively active Raf kinase

a glutamic acid substitution for the valine at position 600

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#Unit 4. Cell Communication and Cell Signaling

MEK is a dual-specificity protein kinase

phosphorylates its target proteins on both tyrosine and serine/threonine

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#Unit 4. Cell Communication and Cell Signaling

In humans, there are three RAS, three Raf, two MEK, and two MAP proteins.

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#Unit 4. Cell Communication and Cell Signaling

Mammalian MAP kinases include  Jun N-terminal kinases and p38 kinases, which are activated by signaling pathways in response to various types of stresses

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#Unit 4. Cell Communication and Cell Signaling

Phosphorylation by MEK at Y-185 and T-183 in activation loop of MAP kinase leads to a marked conformational change & promotes MAP kinase activation and dimerization.