K. Yagi et al., Alternatively spliced variant of Smad2 lacking exon 3 - Comparison with wild-type Smad2 and Smad3, J BIOL CHEM, 274(2), 1999, pp. 703-709
An alternatively spliced variant of Smad2 with a deletion of exon 3 (Smad2
Delta exon3) is found in various cell types. Here, we studied the function
of Smad2 Delta exon3 and compared it with those of wild-type Smad2 containi
ng exon 3 (Smad2(wt)) and Smad3. When transcriptional activity was measured
using the p3TP-lux construct, Smad2 Delta exon3 was more potent than Smad2
(wt), and had activity similar to Smad3. Transcriptional activation of the
activin-responsive element (ARE) of Mix.2 gene promoter by Smad2 Delta hexo
n3 was also similar to that by Smad3, and slightly less potent than that by
Smad2(wt). Phosphorylation by the activated transforming growth factor-bet
a type I receptor and heteromer formation with Smad4 occurred to similar ex
tents in Smad2 Delta exon3, Smad2(wt), and Smad3. However, DNA binding to t
he activating protein-1 sites of p3TP-lux was observed in Smad2 Delta exon3
as well as in Smad3, but not in Smad2(wt). In contrast, Smad2(wt), Smad2 D
elta exon3, and Smad3 efficiently formed ARE-binding complexes with Smad4 a
nd FAST1, although Smad2(wt) did not directly bind to ARE. These results su
ggest that exon 3 of Smad2 interferes with the direct DNA binding of Smad2,
and modifies the function of Smad2 in transcription of certain target gene
s.