The novel E3 ubiquitin ligase Tiul1 associates with TGIF to target Smad2 for degradation

Ubiquitin-dependent degradation plays an important role in the negative regulation of TGF-beta signaling. Here, we identify Tiul1 (for TGIF interacting ubiquitin ligase 1), a novel E3 ubiquitin ligase that inhibits TGF-beta signaling by targeting both the activated receptor and SMAD2 for degradation. Tiul1 associates constitutively with Smad7 and induces degradation of the activated type I receptor without affecting the expression levels of Smad7. Tiul1 can also interact with SMAD2 and the nuclear corepressor TGIF upon activation of TGF-beta signaling. Like Smad7, the steady-state levels of TGIF are not affected by Tiul1, but the interaction of Tiul1 with TGIF allows this ubiquitin ligase to target SMAD2 for degradation. Consistent with this, overexpression of Tiul1 suppressed TGF-beta-induced growth arrest and transcriptional responses. In addition, silencing of Tiul1 or TGIF genes by siRNA resulted in suppression of the TGF-beta-dependent degradation of SMAD2 and an enhancement of TGF-beta-mediated gene expression. These results reveal a new role for TGIF as a component of a ubiquitin ligase complex that mediates the degradation of SMAD2 in response to TGF-beta signaling.