Regulation of the discs large tumor suppressor by a phosphorylation-dependent interaction with the β-TrCP ubiquitin ligase receptor

The discs large (hDlg) tumor suppressor is intimately involved in the control of cell contact, polarity, and proliferation by interacting with several components of the epithelial junctional complex and with the APC tumor suppressor protein. In epithelial cells, hDlg protein stability is regulated through the ubiquitin-proteasome pathway: hDlg is actively degraded in isolated cells, whereas it accumulates upon cell-cell contact. During neoplastic transformation of epithelial cells, loss of the differentiated morphology and progression toward a metastatic phenotype correlate with down-regulation of hDlg levels and loss of contact-dependent stabilization. Here we show that upon hyperphosphorylation, hDlg interacts with the beta-TrCP ubiquitin ligase receptor through a DSGLPS motif within its Src homology 3 domain. As a consequence, overexpression of beta-TrCP enhances ubiquitination of Dlg protein and decreases its stability, whereas a dominant negative beta-TrCP mutant inhibits this process. Furthermore, a mutant Dlg protein that is unable to bind beta-TrCP displays a higher protein stability and is insensitive to beta-TrCP. Using RNA interference, we also demonstrate that endogenous beta-TrCP regulates hDlg protein levels in epithelial cells. Finally, we show that beta-TrCP selectively induces the degradation of the membrane-cytoplasmic pool, without affecting the nuclear pool of hDlg.