Collenchyma cells

Collenchyma cells are generally elongated cells that have thicker primary walls than parenchyma cells,

Collenchyma cells provide flexible support without restraining growth

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In cytokine receptors, 

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

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

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

JAK1, JAK2, JAK3, and Tyk2

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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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JAK kinases become activated after ligand binding and

receptor dimerization

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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