TAZ promotes cell proliferation, development, and tumorigenesis by regulating target gene transcription. However, how TAZ orchestrates the transcriptional responses remains poorly defined. Here we demonstrate that TAZ forms nuclear condensates via liquid-liquid phase separation to compartmentalize its DNA binding co-factor TEAD4, the transcription co-activators BRD4 and MED1 and the transcription elongation factor CDK9 for activation of gene expression.TAZ, but not its paralog YAP, forms phase-separated droplets in vitro and liquid-like nuclear condensates in vivo, and this ability is negatively regulated by Hippo signaling via LATS-mediated phosphorylation and mediated by the coiled-coil domain. Deletion of the TAZ coiled-coil domain or substitution with the YAP coiled-coil domain does not affect the interaction of TAZ with its partners, but prevents its phase separation and more importantly, its ability to induce target gene expression. Thus, our study identifies a novel mechanism for the transcriptional activation by TAZ and demonstrates for the first time that pathway-specific transcription factors also engage the phase separation mechanism for efficient transcription activation.

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The Hippo (Hpo) pathway controls tissue growth and organ size by regulating the activity of transcriptional co-activator Yorkie (Yki), which associates with transcription factor Scalloped (Sd) in the nucleus to promote downstream target gene expression. Here we identify a novel protein Sd-Binding-Protein (SdBP)/Tgi, which directly competes with Yki for binding to Sd through its TDU domains and inhibits the Sd-Yki transcriptional activity. We also find that SdBP retains Yki in the nucleus through the association with Yki WW domains via its PPXY motifs. Collectively, we identify SdBP as a novel component of the Hpo pathway, negatively regulating the transcriptional activity of SdYki to restrict tissue growth.