Transcriptional coactivator which acts as a downstream regulatory target in the Hippo signaling pathway that plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis (PubMed: 11118213, PubMed: 18227151). The core of this pathway is composed of a kinase cascade wherein STK3/MST2 and STK4/MST1, in complex with its regulatory protein SAV1, phosphorylates and activates LATS1/2 in complex with its regulatory protein MOB1, which in turn phosphorylates and inactivates YAP1 oncoprotein and WWTR1/TAZ (PubMed: 18227151). WWTR1 enhances PAX8 and NKX2-1/TTF1-dependent gene activation (PubMed: 19010321). In conjunction with YAP1, involved in the regulation of TGFB1-dependent SMAD2 and SMAD3 nuclear accumulation (PubMed: 18568018). Plays a key role in coupling SMADs to the transcriptional machinery such as the mediator complex (PubMed: 18568018). Regulates embryonic stem-cell self-renewal, promotes cell proliferation and epithelial-mesenchymal transition (PubMed: 18227151, PubMed: 18568018).