A complex of Armadillo, Legless, and Pygopus coactivates dTCF to activate wingless target genes

Upon receiving a Wnt signal, cells accumulate beta-catenin (Armadillo in Drosophila), which binds directly to TCF transcription factors, leading to the transcription of Wnt target genes. It is generally thought that beta-catenin/Armadillo is a transcriptional coactivator when bound to TCF in the nucleus and that this function is mediated by its C terminus. However, recent findings in Drosophila indicated that Armadillo may activate dTCF in the cytoplasm.

Here, I reexamine the mechanism of Armadillo's signaling function in light of Legless and Pygopus, two nuclear factors recently discovered to be essential for this function. I show that Armadillo, in order to activate dTCF, must enter the nucleus and form a complex with Legless and Pygopus. The ability of this complex to stimulate TCF-mediated transcription can be altered by linkage of a strong transcriptional activator or repressor to Armadillo. Furthermore, Armadillo is a strong transcriptional activator when fused to the yeast GAL4 DNA binding domain-an activity that depends on regions of the Armadillo repeat domain that mediate binding to Legless and to chromatin modifying and remodeling factors. Finally, linkage of the N-terminal region of Pygopus, but not the C terminus of Armadillo, to dominant-negative dTCF can restore its signaling activity in transgenic flies.

My evidence argues in favor of a revised coactivator factor model in which Armadillo's coactivator function depends on regions within its Armadillo repeat domain to which Legless/Pygopus and other transcriptional coactivators can bind. In contrast, the C terminus of Armadillo plays a less direct role in this function.