Which of the Following Mechanisms is NOT Employed by I-Smads

Question

Which of the following mechanisms is NOT employed by I-Smads to negatively regulate Smad signaling?
A) Competing with R-Smads for binding sites on the receptor, decreasing R-Smad phosphorylation.
B) Recruiting the ubiquitin ligase Smurf, leading to receptor degradation.
C) Binding to caveolin, inhibiting the formation of caveolae.
D) Binding to the co-Smad, Smad4, inhibiting it.
E) Recruiting protein phosphatases, leading to receptor inactivation.

Accepted Answer

Activated TGFβ receptors can be endocytosed by two distinct routes: one is dependent on clathrin-coated vesicles and leads to further activation, whereas the other one depends on caveolae and leads to inactivation. Inhibiting the formation of caveolae can therefore activate, rather than, inhibit TGFβ signaling.