Abstract
The TGFβ pathway is critical for embryonic development and adult tissue homeostasis. On ligand stimulation, TGFβ and BMP receptors phosphorylate receptor-activated SMADs (R-SMADs), which then associate with SMAD4 to form a transcriptional complex that regulates gene expression through specific DNA recognition1,2. Several ubiquitin ligases serve as inhibitors of R-SMADs3,4, yet no deubiquitylating enzyme (DUB) for these molecules has so far been identified. This has left unexplored the possibility that ubiquitylation of R-SMADs is reversible and engaged in regulating SMAD function, in addition to degradation5. Here we identify USP15 as a DUB for R-SMADs. USP15 is required for TGFβ and BMP responses in mammalian cells and Xenopus embryos. At the biochemical level, USP15 primarily opposes R-SMAD monoubiquitylation, which targets the DNA-binding domains of R-SMADs and prevents promoter recognition. As such, USP15 is critical for the occupancy of endogenous target promoters by the SMAD complex. These data identify an additional layer of control by which the ubiquitin system regulates TGFβ biology.
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Acknowledgements
We thank W. Dubiel, P. ten Dijke, C. Hill, A. Moustakas and E. De Robertis for gifts of reagents, and E. Maspero and G. Fontemaggi for help with enzyme purification and ChIP. We are grateful to M. Aragona, F. Zanconato and O. Wessely for comments on the manuscript. This work is supported by grants from the following agencies: AIRC (Italian Association for Cancer Research) PI and AIRC Special Program Molecular Clinical Oncology ‘5 per mille’, University of Padua strategic grant, excellence grant IIT and Comitato Promotore Telethon grant to S.P., and AIRC and MIUR grants to S.D. and M.C. M.I. was a recipient of a Toyobo and Uehara Memorial Foundations grant and is a Marie Curie FP-7 IIF fellow.
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M.I. and S.P. designed research; M.I. and A. Manfrin carried out all the biochemical and functional assays in cells and Xenopus embryos. A. Mamidi and S.D. purified SMADs and carried out in vitro binding assays; G.M., S.S. and M.C. helped with Xenopus assays; L.M. and E.E. helped with mesenchymal stem cells and immunofluorescence; S.M. carried out the modelling analysis; S.P. prepared recombinant E3 ligases; S.P. coordinated the work; M.I. and S.P. wrote the manuscript.
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Inui, M., Manfrin, A., Mamidi, A. et al. USP15 is a deubiquitylating enzyme for receptor-activated SMADs. Nat Cell Biol 13, 1368–1375 (2011). https://doi.org/10.1038/ncb2346
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DOI: https://doi.org/10.1038/ncb2346