Genetic exchange also happens between distantly related organisms by horizontal gene transfer, or HGT.

HGT is particularly important to prokarytotes and is the most common way by which they acquire new genes.

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#Unit 4. Cell Communication and Cell Signaling

In cytokine receptors, 

the receptor and kinase are separate polypeptides, encoded by different genes, yet are bound tightly together.

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#Unit 4. Cell Communication and Cell Signaling

In cytokine receptors, the tightly bound kinase is known as a JAK kinase.

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#Unit 4. Cell Communication and Cell Signaling

Cytokine receptors are stably associated with one or two of the four known JAKs:

JAK1, JAK2, JAK3, and Tyk2

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#Unit 4. Cell Communication and Cell Signaling

Each members of JAK family of kinases contains:

An N-terminal domain that binds to the receptor

A C-terminal kinase domain 

A middle pseudokinase domain

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#Unit 4. Cell Communication and Cell Signaling

JAK kinases become activated after ligand binding and

receptor dimerization

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#Unit 4. Cell Communication and Cell Signaling

Ligand binding triggers a conformational change in the JAKs & they phosphorylate each other on a critical tyrosine in the activation loop which greatly enhances kinase activity