Sigma factor has four binding regions over DNA. 

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#Unit 4. Cell Communication and Cell Signaling

Ski recruits N-CoR, which binds directly to mSin3A; mSin3A interacts with HDAC, that promotes histone deacetylation on nearby promoters, repressing gene expression

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#Unit 4. Cell Communication and Cell Signaling

Smad7 binds to the activated RI receptors and blocks its ability to phosphorylate Smad2 or Smad3. 

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#Unit 4. Cell Communication and Cell Signaling

Like SOCS proteins in the cytokine signaling, 

Smad7 also recruits an E3 ubiquitin ligase that targets TGF-β receptors for degradation.

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#Unit 4. Cell Communication and Cell Signaling

The cytokines form a family of relatively small, secreted signaling molecules:

Contain about 160–200 AA

4 long, conserved α helices

Control growth and differentiation

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#Unit 4. Cell Communication and Cell Signaling

Large family of cytokines:

Interleukins

Interferons

Growth hormone

Prolactin

G-CSF

Thrombopoietin

Erythropoietin

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#Unit 4. Cell Communication and Cell Signaling

Receptor Homodimerization 

Cytokines (erythropoietin) activate receptors by forming dimers of two identical cytokine receptor proteins