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Smad6 inhibits BMP/Smad1 signaling by specifically competing with the Smad4 tumor suppressor
1Cell Biology Program, Howard Hughes Medical Institute and The Sloan-Kettering Division of the Cornell University Graduate School of Medical Sciences, Memorial Sloan-Kettering Cancer Center, The Rockefeller University, New York, NY 10021, USA.
Abstract:
Bone morphogenetic protein (BMP) receptors signal by phosphorylating Smad1, which then associates with Smad4; this complex moves into the nucleus and activates transcription. Here we report the existence of a natural inhibitor of this process, Smad6, a longer version of the previously reported JV15-1. In Xenopus embryos and in mammalian cells, Smad6 specifically blocks signaling by the BMP/Smad1 pathway. Smad6 inhibits BMP/Smad1 signaling without interfering with receptor-mediated phosphorylation of Smad1. Smad6 specifically competes with Smad4 for binding to receptor-activated Smad1, yielding an apparently inactive Smad1-Smad6 complex. Therefore, Smad6 selectively antagonizes BMP-activated Smad1 by acting as a Smad4 decoy.
Insights
A newly discovered protein, Smad6 (Smad6), acts as a natural inhibitor of Bone Morphogenetic Protein (BMP) signaling. It selectively blocks BMP/Smad1 pathway activation by preventing Smad4 from binding to Smad1.
Area of Science:
- Molecular Biology
- Cell Signaling
- Developmental Biology
Background:
- Bone Morphogenetic Protein (BMP) receptors initiate signaling cascades by phosphorylating Smad1.
- Phosphorylated Smad1 forms a complex with Smad4, translocating to the nucleus to regulate gene transcription.
Purpose of the Study:
- To identify natural inhibitors of the BMP/Smad1 signaling pathway.
- To characterize the mechanism of action of the identified inhibitor, Smad6.
Main Methods:
- Expression analysis in Xenopus embryos and mammalian cell lines.
- Investigation of Smad protein interactions using co-immunoprecipitation assays.
- Assessment of Smad6's effect on BMP/Smad1 pathway activity.
Main Results:
- Smad6, a longer isoform of JV15-1, was identified as a specific inhibitor of BMP/Smad1 signaling.
- Smad6 antagonizes BMP signaling without affecting Smad1 phosphorylation.
- Smad6 competes with Smad4 for binding to activated Smad1, forming an inactive complex.
Conclusions:
- Smad6 acts as a selective negative regulator of BMP/Smad1 signaling.
- Smad6 functions as a Smad4 decoy, preventing the formation of the transcriptionally active Smad1-Smad4 complex.
- This mechanism highlights a novel regulatory point in BMP-mediated cellular processes.