Wnt-Dependent Inactivation of the Groucho/TLE Co-repressor by the HECT E3 Ubiquitin Ligase Hyd/UBR5

Abstract

SummaryExtracellular signals are transduced to the cell nucleus by effectors that bind to enhancer complexes to operate transcriptional switches. For example, the Wnt enhanceosome is a multiprotein complex associated with Wnt-responsive enhancers through T cell factors (TCF) and kept silent by Groucho/TLE co-repressors. Wnt-activated β-catenin binds to TCF to overcome this repression, but how it achieves this is unknown. Here, we discover that this process depends on the HECT E3 ubiquitin ligase Hyd/UBR5, which is required for Wnt signal responses in Drosophila and human cell lines downstream of activated Armadillo/β-catenin. We identify Groucho/TLE as a functionally relevant substrate, whose ubiquitylation by UBR5 is induced by Wnt signaling and conferred by β-catenin. Inactivation of TLE by UBR5-dependent ubiquitylation also involves VCP/p97, an AAA ATPase regulating the folding of various cellular substrates including ubiquitylated chromatin proteins. Thus, Groucho/TLE ubiquitylation by Hyd/UBR5 is a key prerequisite that enables Armadillo/β-catenin to activate transcription.

The Wnt Transcriptional Switch: TLE Removal or Inactivation?

“…If β‐catenin does not simply displace TLEs at W‐CRMs, how does Wnt signaling inactivate these co‐repressors? Another recent report from the Flack et al offers an intriguing mechanism. They found that the E3 ubiquitin‐protein ligase UBR5 (UBR5) is required for efficient Wnt signaling in HEK293 cells.…”

“…C) This model postulated that these factors are also associated with TCF in Wnt‐stimulated cells, along with β‐catenin and other co‐activators. There is also evidence that TLEs are inactivated by ubiquitination via UBR5 (not shown) . In addition, newly discovered negative regulatory elements (NREs) are proposed to be bound by TCFs and β‐catenin, repressing transcription upon Wnt stimulation, fine‐tuning the transcriptional response of traditional W‐CRMs .…”

“…How important are TLEs in regulating Wnt targets? While there are several examples of derepression of Wnt target gene expression in Drosophila gro mutants, the functional evidence implicating these co‐repressors in vertebrate systems is largely based on overexpression . However, ChIP‐seq data from early Xenopus gastrulae, reveals an 86% correlation between β‐catenin and TLE‐bound sites .…”

“…UBR5/Hyd functioned downstream of β‐catenin stabilization. A mass‐spectrometry screen identified an interaction between UBR5 and TLE3, and UBR5 ubiquitinated TLE3 on its C‐terminal WD40 domain . Activation of Wnt/β‐catenin signaling stimulated UBR5 ubiquitination of TLE3.…”

“…There are four TLEs in mammalian genomes, also known as Groucho related genes (Grgs), along with a fifth family member lacking the WD40 domain that is thought to act as a naturally occurring dominant negative . Several mammalian TLEs have been linked to repression of Wnt target genes …”