Constitutive and UV-induced fibronectin degradation is a ubiquitination-dependent process controlled by β-TrCP

Loss of fibronectin (FN) assembly in the extracellular matrix has long been recognized as a feature of cellular transformation. However, such assembly is regulated not only by FN synthesis but also by its post-translational modifications. The mechanism controlling FN protein stability has remained unclear so far. Recently it was demonstrated that FN matrix turnover occurs intracellularly at the lysosome following caveolin-1-dependent endocytosis. Although FN was reported to undergo ubiquitin-dependent degradation, the ubiquitin ligase responsible for FN ubiquitination is unknown. In this study, we have identified β-TrCP as the ubiquitin ligase for lysosomal degradation of FN. We found two conserved β-TrCP recognition motif (DSGVVYS and DSGSIVVS) in the primary amino acid sequence of human, mouse, and rat FN. Down-regulation of either β-TrCP1 or β-TrCP2 by small interference (siRNA) caused significant accumulation of FN. Immunolocalization studies showed intracellular accumulation of FN in β-TrCP siRNA-treated cells without showing much alteration in its matrix association. We also observed that exposure of cells to UV irradiation effectively down-regulated FN following increased ubiquitination, which was significantly inhibited either by lysosomal inhibitor or by siRNA-mediated down-regulation of β-TrCP. Taken together, constitutive FN degradation, as well as UV-induced degradation, is ubiquitination dependent and controlled by β-TrCP.