The Cadherin Cytoplasmic Domain is Unstructured in the Absence of Beta-catenin. A Possible Mechanism for Regulating Cadherin Turnover

Cadherins are single pass transmembrane proteins that mediate Ca(2+)-dependent homophilic cell-cell adhesion by linking the cytoskeletons of adjacent cells. In adherens junctions, the cytoplasmic domain of cadherins bind to beta-catenin, which in turn binds to the actin-associated protein alpha-catenin. The physical properties of the E-cadherin cytoplasmic domain and its interactions with beta-catenin have been investigated. Proteolytic sensitivity, tryptophan fluorescence, circular dichroism, and (1)H NMR measurements indicate that murine E-cadherin cytoplasmic domain is unstructured. Upon binding to beta-catenin, the domain becomes resistant to proteolysis, suggesting that it structures upon binding. Cadherin-beta-catenin complex stability is modestly dependent on ionic strength, indicating that, contrary to previous proposals, the interaction is not dominated by electrostatics. Comparison of 18 cadherin sequences indicates that their cytoplasmic domains are unlikely to be structured in isolation. This analysis also reveals the presence of PEST sequences, motifs associated with ubiquitin/proteosome degradation, that overlap the previously identified beta-catenin-binding site. It is proposed that binding of cadherins to beta-catenin prevents recognition of degradation signals that are exposed in the unstructured cadherin cytoplasmic domain, favoring a cell surface population of catenin-bound cadherins capable of participating in cell adhesion.