E-cadherin phosphorylation occurs during its biosynthesis to promote its cell surface stability and adhesion

E-cadherin is highly phosphorylated within its beta-catenin-binding region, and this phosphorylation increases its affinity for beta-catenin in vitro. However, the identification of key serines responsible for most cadherin phosphorylation and the adhesive consequences of modification at such serines have remained unknown. In this study, we show that as few as three serines in the beta-catenin-binding domain of E-cadherin are responsible for most radioactive phosphate incorporation. These serines are required for binding to beta-catenin and the mutual stability of both E-cadherin and beta-catenin. Cells expressing a phosphodeficient (3S>A) E-cadherin exhibit minimal cell-cell adhesion due to enhanced endocytosis and degradation through a lysosomal compartment. Conversely, negative charge substitution at these serines (3S>D) antagonizes cadherin endocytosis and restores wild-type levels of adhesion. The cadherin kinase is membrane proximal and modifies the cadherin before it reaches the cell surface. Together these data suggest that E-cadherin phosphorylation is largely constitutive and integral to cadherin-catenin complex formation, surface stability, and function.