In Wnt‐stimulated cells, β‐catenin becomes stabilized in the cytoplasm, enters the nucleus and interacts with HMG box transcription factors of the lymphoid‐enhancing factor‐1 (LEF‐1)/T‐cell factor (TCF) family, thereby stimulating the transcription of specific target genes. We recently identified Pontin52 as a nuclear protein interacting with β‐catenin and the TATA‐box binding protein (TBP), suggesting its involvement in regulating β‐catenin‐mediated transactivation. Here, we report the identification of Reptin52 as an interacting partner of Pontin52. Highly homologous to Pontin52, Reptin52 likewise binds β‐catenin and TBP. Using reporter gene assays, we show that the two proteins antagonistically influence the transactivation potential of the β‐catenin–TCF complex. Furthermore, we demonstrate the evolutionary conservation of this mechanism in Drosophila . dpontin and dreptin are essential genes that act antagonistically in the control of Wingless signalling in vivo . These results indicate that the opposite action of Pontin52 and Reptin52 on β‐catenin‐mediated transactivation constitutes an additional mechanism for the control of the canonical Wingless/Wnt pathway.

Dishevelled (Dsh) induces a secondary axis and can translocate to the membrane when activated by Frizzleds; however, dominant‐negative approaches have not supported a role for Dsh in primary axis formation. We demonstrate that the Dsh protein is post‐translationally modified at the dorsal side of the embryo: timing and position of this regulation suggests a role of Dsh in dorsal–ventral patterning in Xenopus . To create functional links between these properties of Dsh we analyzed the influence of endogenous Frizzleds and the Dsh domain dependency for these characteristics. Xenopus Frizzleds phosphorylate and translocate Xdsh to the membrane irrespective of their differential ectopic axes inducing abilities, showing that translocation is insufficient for axis induction. Dsh deletion analysis revealed that axis inducing abilities did not segregate with Xdsh membrane association. The DIX region and a short stretch at the N‐terminus of the DEP domain are necessary for axis induction while the DEP region is required for Dsh membrane association and its phosphorylation. In addition, Dsh forms homomeric complexes in embryos suggesting that multimerization is important for its proper function.

Mesoderm induction assays in Xenopus have implicated growth factors such as activin, Vg1, Xwnt-8, and noggin as important in directing the formation of dorsal mesoderm (Spemann's organizer). Because these growth factors are structurally very different, they presumably act through distinct cell surface receptors that initiate different intracellular signaling cascades. A consequence of all of these signaling pathways, however, seems to be the induction of goosecoid (gsc) gene expression. To understand how integration of these different signaling pathways results in formation of Spemann's organizer, we sought to identify growth factor-responsive elements within the gsc promoter. Through microinjection of reporter genes we have identified two cis-acting elements, a distal element (DE) and a proximal element (PE), that are required for activin/BVg1 and Wnt induction, respectively. We have shown that the DE mediates activin induction in the absence of protein synthesis and therefore constitutes the first activin response element identified to interpret transforming growth factor-beta (TGF-beta) superfamily member signaling directly. Using a reporter gene construct containing a multimerized DE, we find that an activin/BVg1-type signaling cascade is active throughout the vegetal hemisphere and marginal zone but not in the animal hemisphere. We demonstrate further that both the distal and proximal elements are essential for high-level transcription of the gsc gene, specifically in dorsal mesoderm, strongly suggesting that establishment of Spemann's organizer requires synergistic input from activin/BVg1-like and Wnt signaling pathways. Finally, mechanisms of establishing the organizer are likely to be conserved throughout vertebrate evolution.