Interaction among GSK-3, GBP, axin, and APC in Xenopus axis specification

Glycogen synthase kinase 3 (GSK-3) is a constitutively active kinase that n egatively regulates its substrates, one of which is beta-catenin, a downstr eam effector of the Wnt signaling pathway that is required for dorsal-ventr al axis specification in the Xenopus embryo, GSK-3 activity is regulated th rough the opposing activities of multiple proteins, Axin, GSK-3, and beta-c atenin form a complex that promotes the GSK3-mediated phosphorylation and s ubsequent degradation of beta-catenin. Adenomatous polyposis coli (APC) joi ns the complex and downregulates beta-catenin in mammalian cells, but its r ole in Xenopus is less clear. In contrast, GBP, which is required for axis formation in Xenopus, binds and inhibits GSK-3, We show here that GSK-3 bin ding protein (GBP) inhibits GSK-3, in part, by preventing Axin from binding GSK-3. Similarly, we present evidence that a dominant-negative GSK-3 mutan t, which causes the same effects as GBP, keeps endogenous GSK-3 from bindin g to Axin, We show that GBP also functions by preventing the GSK-3-mediated phosphorylation of a protein substrate without eliminating its catalytic a ctivity. Finally, we show that the previously demonstrated axis-inducing pr operty of overexpressed APC is attributable to its ability to stabilize cyt oplasmic beta-catenin levels, demonstrating that APC is impinging upon the canonical Wnt pathway in this model system. These results contribute to our growing understanding of how GSK-3 regulation in the early embryo leads to regional differences in beta-catenin levels and establishment of the dorsa l axis.