Identification of two novel regulated serines in the N terminus of β-catenin

β-Catenin plays a key role in the Wnt signaling cascade. The levels of β-catenin within a cell are regulated via phosphorylation of the N terminus of β-catenin by GSK-3β. The phosphorylation leads to ubiquitination and subsequent degradation of the protein. Thus far three serines (S33, 37, 45) and one threonine (T41) are considered to be the substrates for GSK-3β phosphorylation. Indeed, these amino acids are regularly mutated in tumors, resulting in β-catenin molecules with enhanced transcriptional activity. Aligning N-terminal sequences of β-catenin homologues of different species revealed two other highly conserved serines (S23, 29), which have also been found mutated in tumors. We show that these serines are modified in the same fashion as that of the known regulatory residues. During embryogenesis, the phosphorylation status of S23 and S29 appears to be actively regulated. Nevertheless, constructs harboring the mutations found in tumors fail to show enhanced transcriptional activity or transforming properties.