REGULATION OF SPEMANN ORGANIZER FORMATION BY THE INTRACELLULAR KINASEXGSK-3

Dorsal axis formation in the Xenopus embryo can be induced by the ecto pic expression of several Wnt family members. In Drosophila, the prote in encoded by the Wnt family gene, wingless, signals through a pathway that antagonizes the effects of the serine/threonine kinase zeste-whi te 3/shaggy. We describe the isolation and characterization of a Xenop us homolog of zeste-white 3/shaggy, Xgsk-3. A kinase-dead mutant of Xg sk-3, Xgsk-3K-->R, has a dominant negative effect and mimics the abili ty of Wnt to induce a secondary axis by induction of an ectopic Speman n organizer. Xgsk-3K-->R, like Wnt, induces dorsal axis formation when expressed in the deep vegetal cells, which do not contribute to the a xis. These results indicate that the dorsal fate is actively repressed by Xgsk-3, which must be inactivated for dorsal axis formation to occ ur. Furthermore, our work suggests that the effects of Xgsk-3K-->R are mediated by an additional intercellular signal.