Practice question

In resting cells GSK-3 beta resides in destruction complex with scaffold Axin and tumor suppressor APC, phosphorylating N-terminal residues Ser33, Ser37, Thr41 of beta-catenin. Phosphorylated beta-catenin recognized by beta-TrCP E3 ubiquitin ligase leading to ubiquitination and proteasomal degradation, keeping cytoplasmic levels low. Wnt signaling displaces complex via Dishevelled and GBP inhibiting GSK-3, preventing phosphorylation. Thus GSK-3 normally functions by degrading beta-catenin ventrally. Dorsal Wnt11 signaling inhibits GSK-3 allowing beta-catenin accumulation, nuclear translocation and organizer gene activation, fundamental regulatory mechanism of dorsal axis specification.