Mysterious tasks of tyrosines in syndecan-1 cytoplasmic tail.

Syndecans are transmembrane proteoglycan receptors that interact with a wide variety of extracellular molecules such as adhesion receptors and extracellular matrix components. There are four syndecans in mammals, which are expressed in a development-, cell-type-, and tissue-specific manner, and function either as coreceptors that cooperate with other cell surface receptors or as cell adhesion receptors that independently mediate cell signaling. Cell signaling is supported through their short cytoplasmic tail that contains four tyrosine residues, which are conserved among all syndecan family members. In this commentary, we report and discuss data showing that the receptor syndecan-1 interacts with the carboxy-terminal LG4/5 domain in laminin-332 to participate in cell adhesion and spreading. Remarkably, cell adhesion to LG4/5 is associated with a rapid dephosphorylation of tyrosine in syndecan-1. These results unveil for the first time that one "turn on" signal for syndecan-1 upon LG4/5 recognition may not be phosphorylation, but tyrosine dephosphorylation, an unexpected outcome. How this regulatory event may take place and which tyrosine residues are concerned are questions tackled in this report.