MDA-9/syntenin interacts with ubiquitin via a novel ubiquitin-binding motif.

Ubiquitination appears to be involved in proteasome-dependent proteolysis and in the membrane trafficking system including endocytosis and exocytosis. In this study, we identified MDA-9/syntenin as a novel ubiquitin-binding protein by a yeast two-hybrid system using modified ubiquitin in which lysine 48 is substituted by arginine. It has been reported that MDA-9/syntenin is a membrane-associated protein and regulates a cellular process involving endocytosis and intracellular transport. We found that MDA-9/syntenin binds to ubiquitin by a non-covalent bond and is ubiquitinated covalently. MDA-9/syntenin has no ubiquitin-binding motifs that have so far been reported, suggesting that MDA-9/syntenin physically interacts with ubiquitin via a novel binding motif. MDA-9/syntenin is stable in the cell, suggesting that ubiquitin binding of MDA-9/syntenin or ubiquitination of MDA-9/syntenin is not related to proteolysis. Furthermore, we showed that overexpression of wild-type MDA-9/syntenin enhances formation of filopodia, whereas MDA-9/syntenin lacking the PDZ domain inhibits the formation of filopodia, suggesting that MDA-9/syntenin plays an important role via interaction with ubiquitin in the regulation of cancer metastasis and invasion.