R-SMAD

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R-Smads or receptor regulated Smads are a class of proteins that include SMAD1, SMAD2, SMAD3, SMAD5, and SMAD9. In response to signals by the TGF-β superfamily of ligands these proteins associate with receptor kinases and are phosphorylated at an SSXS motif at their extreme C-terminus. These proteins then typically bind to the common mediator Smad or co-SMAD SMAD4. Smad complexes then accumulate in the cell nucleus where they regulate transcription of specific target genes. SMAD2 and SMAD3 are activated in response to TGF-β or Activin signals, whereas SMAD1, SMAD5 and SMAD9 are activated in response to BMP or bone morphogenetic protein signals.