Regulation of Smad-Mediated Gene Transcription by RGS3

Regulator of G protein signaling (RGS) proteins are united into a family by the presence of the homologous RGS domain that binds the subunits of heterotrimeric G proteins and accelerates their GTPase activity. A member of this family, RGS3 regulates the signaling mediated by Gq and Gi proteins by binding the corresponding G subunits. Here we show that RGS3 interacts with the novel partners Smad2, Smad3, and Smad4—the transcription factors that are activated through a transforming growth factor- (TGF-) receptor signaling. This interaction is mediated by the region of RGS3 outside of the RGS domain and by Smad’s Mad homology 2 domain. Overexpression of RGS3 results in inhibition of Smad-mediated gene transcription. RGS3 does not affect TGF--induced Smad phosphorylation, but it prevents heteromerization of Smad3 with Smad4, which is required for transcriptional activity of Smads. This translates to functional inhibition of TGF--induced myofibroblast differentiation by RGS3. In conclusion, this study identifies a novel, noncanonical role of RGS3 in regulation of TGF- signaling through its interaction with Smads and interfering with Smad heteromerization.