THE AXIS-INDUCING ACTIVITY, STABILITY, AND SUBCELLULAR-DISTRIBUTION OF BETA-CATENIN IS REGULATED IN XENOPUS EMBRYOS BY GLYCOGEN-SYNTHASE KINASE-3

The serine/threonine kinase Xgsk-3 and the intracellular protein beta- catenin are necessary for the establishment of the dorsal-ventral axis in Xenopus. Although genetic evidence from Drosophila indicates that Xgsk-3 is upstream of beta-catenin, direct interactions between these proteins have not been demonstrated. We demonstrate that phosphorylati on of beta-catenin in vivo requires an in vitro amino-terminal Xgsk-3 phosphorylation site, which is conserved in the Drosophila protein arm adillo. beta-Catenin mutants lacking this site are more active in indu cing an ectopic axis in Xenopus embryos and are more stable than wild- type beta-catenin in the presence of Xgsk-3 activity, supporting the h ypothesis that Xgsk-3 is a negative regulator of beta-catenin that act s through the amino-terminal site. Inhibition of endogenous Xgsk-3 fun ction with a dominant-negative mutant leads to an increase in the stea dy-state levels of ectopic beta-catenin, indicating that Xgsk-3 functi ons to destabilize beta-catenin and thus decrease the amount of beta-c atenin available for signaling. The levels of endogenous beta-catenin in the nucleus increases in the presence of the dominant-negative Xgsk -3 mutant, suggesting that a role of Xgsk-3 is to regulate the steady- state levels of beta-catenin within specific subcellular compartments. These studies provide a basis for understanding the interaction betwe en Xgsk-3 and beta-catenin in the establishment of the dorsal-ventral axis in early Xenopus embryos.