Cohesins 

The protein ring complexes that establish linkages between sister chromatids

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#Unit 4. Cell Communication and Cell Signaling

Active type I receptor kinase phosphorylates a Smad

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#Unit 4. Cell Communication and Cell Signaling

Three types of Smad proteins function in the TGF-β signaling pathway: 

R-Smads (Smad2 and Smad3)

Co-Smads (Smad4)

I-Smads (Smad7)

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#Unit 4. Cell Communication and Cell Signaling

Smad2/3 become phosphorylated by active RI receptors and together with Smad4 move into the nucleus to activate gene transcription

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#Unit 4. Cell Communication and Cell Signaling

R-Smads comprising: 

An N-terminal MH1 domain

A central proline-rich linker 

A C-terminal MH2 domain

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#Unit 4. Cell Communication and Cell Signaling

MH1 domain contains:

DNA binding segment Nuclear-localization signal

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#Unit 4. Cell Communication and Cell Signaling

Phosphorylation of two serines in Ser-X-Ser motif at the extreme C-terminus of the MH2 domain by the activated RI separates the two domains, exposing the NLS for binding to importins