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Ubiquitin-dependent degradation of TGF-β-activated Smad2

Abstract

SMAD proteins are phosphorylated by transforming growth factor-β (TGF-β) receptors and translocate to the nucleus, where they control transcription. Here we investigate the fate of activated Smad2. We show that receptor-mediated activation leads to multi-ubiquitination and subsequent degradation of Smad2 by the proteasome. Ubiquitination of Smad2 is a consequence of its accumulation in the nucleus. If degradation is averted, the phosphorylated Smad2 remains in the nucleus in an active state. By targeting Smad2 for destruction, TGF-β ensures the irreversible termination of its own signalling function.

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Figure 1: Proteasome inhibition blocks the time-dependent loss of endogenous receptor-phosphorylated Smad2.
Figure 2: Proteasome inhibitors protect Smad2 from a TGF-β-induced depletion.
Figure 3: Proteasome inhibition enhances TGF-β-induced nuclear accumulation of endogenous Smad2 and transcriptional complex assembly.
Figure 4: TGF-β induces proteasome-dependent loss of Smad2 involving the ubiquitination pathway.
Figure 5: Nuclear localization of Smad2 leads to Smad2 ubiquitination.
Figure 6: A model of ubiquitin-dependent degradation of Smad2 at the end of the TGF-β signalling pathway.

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Acknowledgements

We thank R. R. Kopito for ubiquitin expression constructs; K. Iwai and A. Ciechanover for E2 constructs; and E. Lai for Fast2. R.S.L. thanks Y-G. Chen, J. Seoane, C. Pouponnot, J. Doody, D. Wotton and S. H. Roan for help. This work was supported by an NIH grant to J.M. (CA34619) and to the Memorial Sloan-Kettering Cancer Center and by an NIH Medical Scientist Training Program (MSTP) grant to R.S.L. J.M. is an investigator of the Howard Hughes Medical Institute.

Correspondence and requests for materials should be addressed to J.M.

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Correspondence to Joan Massagué.

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Lo, R., Massagué, J. Ubiquitin-dependent degradation of TGF-β-activated Smad2. Nat Cell Biol 1, 472–478 (1999). https://doi.org/10.1038/70258

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