| β-catenin functions as the central effector of the canonical Wnt signaling pathway, operating both as a structural component of adherens junctions through its interaction with α-catenin and E-cadherin and as a transcriptional coactivator once released into the cytoplasm and nucleus, with phosphorylation at Ser33 marking the protein for degradation and thereby controlling which of these two roles it can perform. In the absence of Wnt ligand, cytosolic β-catenin is captured by the destruction complex, a multiprotein assembly built around Axin as the central scaffold, which directly binds APC, casein kinase 1 alpha, GSK3, and β-catenin itself, with Axin acting as the rate-limiting factor whose self-polymerization is required for efficient β-catenin recruitment into the complex. Before phosphorylation can proceed, α-catenin must first dissociate from β-catenin, an event triggered by tyrosine phosphorylation at Tyr142 mediated by the Src-family kinases FER and FYN, which grants casein kinase 1 alpha access to its substrate. Casein kinase 1 alpha then phosphorylates β-catenin at Ser45, and this modification creates a priming site that is necessary and sufficient for GSK3 to carry out sequential phosphorylation of the more N-terminal residues Thr41, then Ser37, and finally Ser33, generating a cluster of phosphorylated residues collectively referred to as the phosphodegron. Crystal structures of APC bound to phosphorylated versus nonphosphorylated β-catenin reveal that CK1 and GSK3 substrate sequences within APC itself undergo mutual priming phosphorylation, generating a phosphorylation-dependent binding motif that increases APC's affinity for β-catenin and stabilizes the destruction complex during the phosphorylation cycle. Once Ser33 and Ser37 are phosphorylated, these two residues together create the recognition site engaged by the F-box protein β-TrCP, the substrate-recognition subunit of the SCF ubiquitin ligase complex, and β-TrCP binding triggers polyubiquitination of β-catenin, targeting it for proteasomal degradation and keeping cytosolic β-catenin levels low under resting conditions. Reconstitution of the full human destruction complex confirms that a subpopulation of complex-bound β-catenin carries authentic phosphorylation at both the CK1-targeted Ser45 site and the GSK3-targeted Ser33, Ser37, and Thr41 sites, validating the sequential priming mechanism in a purified biochemical system. Mutations at Ser33, Ser37, or Thr41 prevent this phosphorylation-dependent degradation cycle, stabilizing β-catenin protein independent of upstream Wnt signal, and such mutations are recurrently found in tumor cell lines, making loss of Ser33 phosphorylation a defined molecular readout of destruction complex failure and aberrant Wnt pathway activation in cancer. |
