Regulation of glycogen synthase kinase 3 beta and downstream Wnt signalingby axin

Axin is a recently identified protein encoded by the fused locus in mice th at is required for normal vertebrate axis formation. We have defined a 25-a mino-acid sequence in axin that comprises the glycogen synthase kinase 3 be ta (GSK9 beta) interaction domain (GID), In contrast to full-length axin, w hich has been shown to antagonize Wnt signaling, the GID inhibits GSK-3 bet a in vivo and activates Wnt signaling. Similarly, mutants of adn lacking ke y regulatory domains such as the RGS domain, which is required for interact ion with the adenomatous polyposis coli protein, bind and inhibit GSK-3 bet a in vivo, suggesting that these domains are critical for proper regulation of GSK-3 beta activity. We have identified a novel self-interaction domain in axin and have shown that formation of an axin regulatory complex in viv o is critical for axis formation and GSK-3 beta activity. Based on these da ta, we propose that the axin complex may directly regulate GSK-3 beta enzym atic activity in vivo. These observations also demonstrate that alternative inhibitors of GSK-3 beta can mimic the effect of lithium in developing Xen opus embryos.