Activated TGF-beta receptors by its ligand, TGF-beta downregulates cell division. You designed a mutant version of a downstream effector of TGF-beta receptor named Smad3. The mutation consists in the replacement of the three key serine residues were replaced with aspartic acid. In normal conditions, these serine residues are phosphorylated by the TGF-beta receptor. In contrast with the wild-type cells, the cells containing the mutant version of Smad3 quickly stop dividing in a culture independently whether or not TGF-beta is added to the cell media.explain this observation.