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SMAD 4, also called SMAD family member n°4 or Mothers against decapentaplegic homolog 4, is a protein involved in cell signaling in mammals. It belongs to the proteins of the SMAD family.
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SMAD 4 participates in chain reactions inside cells where it interacts with other members of the SMAD family such as SMAD2 or SMAD3. SMAD 4 forms with SMAD 3 a complex which can bind to DNA and modify the expression of several genes related to cellular activities such as proliferation or differentiation [1]. The complex SMAD 3-SMAD 4 is therefore a transcription factor.
Intracellular reactions involving SMAD 4 are triggered by the biding, on the surface of the cells, of growth factors from the TGFβ family. The sequence of intracellular reactions involving SMADS is called the SMAD pathway or the transforming growth factor beta (TGF-β) pathway, since the sequence starts with the recognition of TGF-β by cells.
SMAD 4 belongs to the co-SMAD group, the second class of the SMAD family. SMAD 4 is the only known co-SMAD in mammals.
It also belongs to the Darwin family of proteins that modulate members of the TGFβ superfamily, a family of proteins that all play a role in the regulation of cellular responses.
Mammalian SMAD 4 is a homolog of the Drosophila protein Mothers against decapentaplegic or MAD.