| Abstract |
Transforming growth factor β (TGF-β) is the prototypic member of a
large superfamily of cytokines that control key biological processes
including cell proliferation, differentiation, apoptosis, wound healing,
immunological response and inflammation. TGF-β regulates transcriptional
responses via activation of cytoplasmic effector proteins termed Smads.
The essential features of Smad function include TGF-β-stimulated Smad
phosphorylation, homo- and hetero-oligomerization, nuclear translocation
and transcriptional control of TGF-β target genes in co-operation with
nuclear co-factors.
Our previous structure-function analysis of human Smad3 protein, a
key mediator of TGF-β signalling in mammalian cells revealed that the
middle, non-conserved linker domain has an autonomous and potent
transactivation function. The minimal region required for transactivation
was the 200-230 region and a Smad3 mutant bearing an internal deletion
of the 200-230 region had severe defects in oligomerization and
transcriptional activation of target promoters.
In the present study we characterized further the transcriptionally
active 200-230 region of Smad3 protein and its role in TGF-β signalling.
Closer inspection of this region revealed the presence of only two amino
acids (glutamine 222 and proline 229), which are conserved among all RSmads
and Smad4. We hypothesized that the high degree of conservation
in these two specific amino acids could be indicative of their significance
for Smad3 function. To test this hypothesis, we constructed Smad3
mutants bearing single amino acid substitutions at these positions (Smad3
Q222A and P229A), as well as a Smad3 mutant bearing a substitution at a
non-conserved amino acid (Smad3 N218A) as a control. The mutant
Smad3 cDNAs were cloned as fusions with the 6-myc and bio epitope tags
as well as with the DNA binding domain of the yeast transactivator GAL4.
Using a protein-protein interaction assay, which is based on the
biotinylation of proteins bearing the bio epitope tag in vivo, we showed that
the mutation in the conserved residue Q222 abolished both homo- and
hetero-oligomerization, whereas the mutation in the conserved residue
3
P229 abolished interaction with wt Smad3, but retained interaction with
Smad2 and Smad4. The mutation in the non-conserved amino acid
residue N218 had no effect on the oligomerization properties of Smad3. In
control experiments we showed that all mutants retained their ability to
associate with the well characterized MH2-interacting protein c-ski in vivo.
In transactivation assays we showed that the Smad3 mutants
Q222A and P229A had severe defects in the transcriptional activation of
artificial and natural promoters compared with wild type Smad3. The
transcriptional activity of both mutants could not be rescued by the histone
acetyltransferase p/CAF. In contrast, the Smad3 mutant N218A behaved
similarly to the wild type Smad3 protein both in transactivation and coactivation
by p/CAF.
Finally, using immunofluorescence we showed that mutations in the
conserved residues Q222 and P229, but not in the non-conserved residue
N218 reduced the nuclear accumulation of the corresponding mutant
proteins.
Our data support an essential role of the previously uncharacterized
middle linker region of Smad3 for cytoplasmic and nuclear functions, such
as oligomerization, nuclear translocation and transcriptional activation of
TGF-β target genes.
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Ο ρόλος της περιοχής του linker στη λειτουργία της ανθρώπινης πρωτεΐνης SMAD3, που δρα στο μονοπάτι του TGF-β
