Suppression of TGF-β/SMAD signaling by an inner nuclear membrane phosphatase complex

Area of Science:

• Cell biology
• Molecular biology
• Signal transduction

Background:

• Transforming growth factor-beta (TGF-β) superfamily cytokines regulate cell fate through receptor-regulated SMAD (R-SMAD) transcription factors.
• R-SMAD activation involves cytosolic phosphorylation, leading to nuclear accumulation and downstream signaling.
• Mechanisms of R-SMAD inactivation in the nucleus are critical for proper signal termination.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying R-SMAD inactivation by dephosphorylation within the cell nucleus.
• To identify the specific phosphatase responsible for R-SMAD dephosphorylation.
• To investigate the role of the scaffold protein MAN1 in this inactivation process.

Main Methods:

• Structural prediction and domain mapping of protein interactions.
• Site-directed mutagenesis to assess protein function.
• Analysis of R-SMAD localization and signaling activity in response to complex disruption.

Main Results:

• R-SMAD dephosphorylation is mediated by an inner nuclear membrane complex involving MAN1 and the CTDNEP1-NEP1R1 phosphatase.
• MAN1 acts as a scaffold, binding independently to both CTDNEP1-NEP1R1 and R-SMADs to facilitate R-SMAD inactivation.
• Disruption of the MAN1-CTDNEP1-NEP1R1 complex leads to aberrant nuclear R-SMAD accumulation and signaling, even without TGF-β stimulation.
• Disease-associated MAN1 mutations impair this inactivation process.

Conclusions:

• CTDNEP1-NEP1R1 is identified as the key R-SMAD phosphatase.
• The MAN1-CTDNEP1-NEP1R1 complex provides the mechanistic basis for TGF-β signaling inactivation.
• Dysfunction of this complex, particularly due to MAN1 mutations, contributes to aberrant cellular signaling and disease.