Tyrosine Dephosphorylation of the Syndecan-1 PDZ Binding Domain Regulates Syntenin-1 Recruitment

Heparan sulfate proteoglycan receptor syndecan-1 interacts with the carboxyl-terminal LG4/5 domain in laminin 332 (alpha 3LG4/5) and participates in cell adhesion and spreading. To dissect the function of syndecan-1 in these processes, we made use of a cell adhesion model in which syndecan-1 exclusively interacts with a recombinantly expressed alpha 3LG4/5 fragment. Plating HT1080 cells on this fragment induces the formation of actin-containing protrusive structures in an integrin-independent manner. Here we show that syndecan-1-mediated formation of membrane protrusions requires dephosphorylation of tyrosine residues in syndecan-1. Accordingly, inhibition of phosphatases with orthovanadate decreases cell adhesion to the alpha 3LG4/5 fragment. We demonstrate that the PDZ-containing protein syntenin-1, known to connect cytoskeletal proteins, binds to syndecan-1 in cells plated on the alpha 3LG4/5 fragment and participates in the formation of membrane protrusions. We further show that syntenin-1 recruitment depends on the dephosphorylation of Tyr-309 located within syndecan-1 PDZ binding domain EFYA. We propose that tyrosine dephosphorylation of syndecan-1 may regulate its association with cytoskeleton components.