A short amino-acid sequence in MH1 domain is responsible for functional differences between Smad2 and Smad3

Smad proteins are essential components of the signalling cascade initiated by members of the Transforming Growth Factor-beta family. TGF beta binding to heteromeric complexes of transmembrane Ser/Thr kinases induces Smad2 and Smad3 phosphorylation on their C terminus residues. This phosphorylation l eads to oligomerization with Smad4, a common mediator of TGF-beta, activin and BMP signalling. The Smad complexes then translocate to the nucleus wher e they play transcription regulator roles. Even if they share 92% identity, the two TGF beta restricted Smad2 and Smad3 are not functionally equivalen t. As we have previously shown, Smad3 acts as a transcription factor by bin ding to a TGF beta-responsive sequence termed CAGA box whereas Smad2 does n ot. Smad2 differs from Smad3 mainly in the N-terminal MH1 domain where it c ontains two additional stretches of amino acids that are lacking in Smad3. Here, we show that one of these domains corresponding to exon 3 is responsi ble for the absence of Smad2 transcriptional activity in CAGA box-containin g promoters. Furthermore, in vitro studies indicate that this domain preven ts Smad2 from binding to this DNA sequence. This suggests that Smad2 and Sm ad3 may have different subsets of target genes participating thus in distin ct responses among TGF beta pleiotropic effects.