Binding to Syntenin-1 Protein Defines a New Mode of Ubiquitin-based Interactions Regulated by Phosphorylation

Syntenin-1 is a PDZ domain-containing adaptor that controls trafficking of transmembrane proteins including those associated with tetraspanin-enriched microdomains. We describe the interaction of syntenin-1 with ubiquitin through a novel binding site spanning the C terminus of ubiquitin, centered on Arg72, Leu73, and Arg74. A conserved LYPSL sequence in the N terminus, as well as the C-terminal region of syntenin-1, are essential for binding to ubiquitin. We present evidence for the regulation of this interaction through syntenin-1 dimerization. We have also established that syntenin-1 is phosphorylated downstream of Ulk1, a serine/threonine kinase that plays a critical role in autophagy and regulates endocytic trafficking. Importantly, Ulk1-dependent phosphorylation of Ser6 in the LYPSL prevents the interaction of syntenin-1 with ubiquitin. These results define an unprecedented ubiquitin-dependent pathway involving syntenin-1 that is regulated by Ulk1.