Practice question

Glycogen synthase kinase-3 normally phosphorylates beta-catenin at N-terminal residues targeting it for beta-TrCP mediated ubiquitination in ventral cells. Dominant-negative GSK3 carries mutation abolishing kinase activity but retains binding to Axin complex, competitively blocking endogenous enzyme. When expressed on ventral side, it allows beta-catenin stabilization, nuclear entry, activation of Siamois and Twin and formation of ectopic Nieuwkoop center. Ectopic center induces secondary Spemann organizer ventrally that organizes host tissues into complete secondary embryonic axis with independent notochord and neural structures. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.