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Updated: Aug 8, 2026

Detection of Protein Ubiquitination
Published on: August 19, 2009
Ligand-dependent degradation of Smad3 by a ubiquitin ligase complex of ROC1 and associated proteins
M Fukuchi1, T Imamura, T Chiba
1Department of Biochemistry, the Cancer Institute of Japanese Foundation for Cancer Research, and Research for the Future Program, the Japan Society for the Promotion of Science, 1-37-1 Kami-ikebukuro, Toshima-ku, Tokyo 170-8455, Japan.
Abstract:
Smads are signal mediators for the members of the transforming growth factor-beta (TGF-beta) superfamily. Upon phosphorylation by the TGF-beta receptors, Smad3 translocates into the nucleus, recruits transcriptional coactivators and corepressors, and regulates transcription of target genes. Here, we show that Smad3 activated by TGF-beta is degraded by the ubiquitin-proteasome pathway. Smad3 interacts with a RING finger protein, ROC1, through its C-terminal MH2 domain in a ligand-dependent manner. An E3 ubiquitin ligase complex ROC1-SCF(Fbw1a) consisting of ROC1, Skp1, Cullin1, and Fbw1a (also termed betaTrCP1) induces ubiquitination of Smad3. Recruitment of a transcriptional coactivator, p300, to nuclear Smad3 facilitates the interaction with the E3 ligase complex and triggers the degradation process of Smad3. Smad3 bound to ROC1-SCF(Fbw1a) is then exported from the nucleus to the cytoplasm for proteasomal degradation. TGF-beta/Smad3 signaling is thus irreversibly terminated by the ubiquitin-proteasome pathway.
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