Roles of mono-ubiquitinated Smad4 in the formation of Smad transcriptional complexes.
TGF-beta activates receptor-regulated Smad (R-Smad) through phosphorylation by type I receptors. Activated R-Smad binds to Smad4 and the complex translocates into the nucleus and stimulates the transcription of target genes through association with co-activators including p300. It is not clear, however, how activated Smad complexes are removed from target genes. In this study, we show that TGF-beta enhances the mono-ubiquitination of Smad4. Smad4 mono-ubiquitination was promoted by p300 and suppressed by the c-Ski co-repressor. Smad4 mono-ubiquitination disrupted the interaction with Smad2 in the presence of constitutively active TGF-beta type I receptor. Furthermore, mono-ubiquitinated Smad4 was not found in DNA-binding Smad complexes. A Smad4-Ubiquitin fusion protein, which mimics mono-ubiquitinated Smad4, enhanced localization to the cytoplasm. These results suggest that mono-ubiquitination of Smad4 occurs in the transcriptional activator complex and facilitates the turnover of Smad complexes at target genes.
Wang
Bei
B
Department of Experimental Pathology, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Ibaraki 305-8575, Japan.
Suzuki
Hiroyuki
H
Kato
Mitsuyasu
M
D000818:Animals; D002460:Cell Line; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D051899:Smad2 Protein / Q000378:metabolism; D051901:Smad4 Protein / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014162:Transfection; D016211:Transforming Growth Factor alpha / Q000494:pharmacology; D025801:Ubiquitin / Q000378:metabolism; D054875:Ubiquitination / Q000187:drug effects; D050880:p300-CBP Transcription Factors / Q000378:metabolism
D004268:DNA-Binding Proteins; D011518:Proto-Oncogene Proteins; D011993:Recombinant Fusion Proteins; D051899:Smad2 Protein; D051901:Smad4 Protein; D016211:Transforming Growth Factor alpha; D025801:Ubiquitin; C060614:SKI protein, human; D004247:DNA; D050880:p300-CBP Transcription Factors