Dorsal determinant Hwa stabilizes β-catenin through direct inhibition of GSK3.

Stabilization of β-catenin on the dorsal side of the embryo is critical for the formation of the dorsal organizer. The novel transmembrane protein Huluwa (Hwa) has recently been identified as the maternal dorsal determinant responsible for β-catenin stabilization in dorsal organizer formation. The molecular mechanism by which Hwa induces WNT-independent β-catenin stabilization remains elusive. In this study, we demonstrate that the conserved PPNSP motif of Hwa is phosphorylated by GSK3 and that the phosphorylated PPNSP motif potently inhibits GSK3, leading to β-catenin stabilization. Notably, the phosphorylated PPNSP motif of Hwa has stronger GSK3 inhibitory activity than the phosphorylated PPPSP motif of LRP6. Molecular dynamics simulations suggest that the PPNpSP peptide has stronger affinity for GSK3 than the PPPpSP peptide, facilitated by the hydrogen bonding capacity of the asparagine residue. Consistent with Hwa's GSK3 inhibitory activity, Hwa enhances SIAH1-dependent degradation of AXIN. Hwa-induced β-catenin stabilization and AXIN degradation are significantly enhanced by oligomerization. Thus, Hwa stabilizes β-catenin through a molecular mechanism similar to that of LRP6 in mediating WNT signaling, representing a striking example of molecular convergence.