USP4 inhibits SMAD4 monoubiquitination and promotes activin and BMP signaling

SMAD4 is a common intracellular effector for TGF‐β family cytokines, but the mechanism by which its activity is dynamically regulated is unclear. We demonstrated that ubiquitin‐specific protease (USP) 4 strongly induces activin/BMP signaling by removing the inhibitory monoubiquitination from SMAD4. This modification was triggered by the recruitment of the E3 ligase, SMURF2, to SMAD4 following ligand‐induced regulatory (R)‐SMAD–SMAD4 complex formation. Whereas the interaction of the negative regulator c‐SKI inhibits SMAD4 monoubiquitination, the ligand stimulates the recruitment of SMURF2 to the c‐SKI‐SMAD2 complex and triggers c‐SKI ubiquitination and degradation. Thus, SMURF2 has a role in termination and initiation of TGF‐β family signaling. An increase in monoubiquitinated SMAD4 in USP4‐depleted mouse embryonic stem cells (mESCs) decreased both the BMP‐ and activin‐induced changes in the embryonic stem cell fate. USP4 sustained SMAD4 activity during activin‐ and BMP‐mediated morphogenic events in early zebrafish embryos. Moreover, zebrafish depleted of USP4 exhibited defective cell migration and slower coordinated cell movement known as epiboly, both of which could be rescued by SMAD4. Therefore, USP4 is a critical determinant of SMAD4 activity.     

The IKK‐related kinase TBK1 activates mTORC1 directly in response to growth factors and innate immune agonists

The innate immune kinase TBK1 initiates inflammatory responses to combat infectious pathogens by driving production of type I interferons. TBK1 also controls metabolic processes and promotes oncogene‐induced cell proliferation and survival. Here, we demonstrate that TBK1 activates mTOR complex 1 (mTORC1) directly. In cultured cells, TBK1 associates with and activates mTORC1 through site‐specific mTOR phosphorylation (on S2159) in response to certain growth factor receptors (i.e., EGF‐receptor but not insulin receptor) and pathogen recognition receptors (PRRs) (i.e., TLR3; TLR4), revealing a stimulus‐selective role for TBK1 in mTORC1 regulation. By studying cultured macrophages and those isolated from genome edited mTOR S2159A knock‐in mice, we show that mTOR S2159 phosphorylation promotes mTORC1 signaling, IRF3 nuclear translocation, and IFN‐β production. These data demonstrate a direct mechanistic link between TBK1 and mTORC1 function as well as physiologic significance of the TBK1‐mTORC1 axis in control of innate immune function. These data unveil TBK1 as a direct mTORC1 activator and suggest unanticipated roles for mTORC1 downstream of TBK1 in control of innate immunity, tumorigenesis, and disorders linked to chronic inflammation.