The Wnt/beta-catenin signaling pathway is responsible for the establishment of dorsoventral axis of Xenopus embryos. The recent finding of the F-box/WD40-repeat protein slimb in Drosophila, whose loss-of-function mutation causes ectopic activation of wingless signaling (Jiang, J., Struhl, G., 1998. Nature 391, 493-496), led us to examine the role of its vertebrate homolog betaTrCp in the Wnt/beta-catenin signaling and dorsal axis formation in Xenopus embryos. Co-injection of betaTrCp mRNA diminished Xwnt8 mRNA-induced axis formation and expression of Siamois and Xnr3, suggesting that betaTrCP is a negative regulator of the Wnt/beta-catenin signaling pathway. An mRNA for a betaTrCp mutant construct (DeltaF), which lacked the F-box domain, induced an ectopic axis and expression of Siamois and Xnr3. Because this activity of DeltaF was suppressed by co-injection of DeltaF TrCP mRNA, DeltaF likely acts in a dominant negative fashion. The activity of DeltaF was diminished by C-cadherin, glycogen synthase kinase 3 and Axin, but not by a dominant negative dishevelled. These results suggest that betaTrCp can act as a negative regulator of dorsal axis formation in Xenopus embryos.
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