PDZ tandem of human syntenin: crystal structure and functional properties

Syntenin, a 33 kDa protein, interacts with several cell membrane receptors and with merlin, the product of the causal gene for neurofibromatosis type II. We report a crystal structure of the functional fragment of human syntenin containing two canonical PDZ domains, as well as binding studies for full-length syntenin, the PDZ tandem, and isolated PDZ domains. We show that the functional properties of syntenin are a result of independent interactions with target peptides, and that each domain is able to bind peptides belonging to two different classes: PDZ1 binds peptides from classes I and III, while PDZ2 interacts with classes I and II. The independent binding of merlin by PDZ1 and syndecan-4 by PDZ2 provides direct evidence for the coupling of syndecan-mediated signaling to actin regulation by merlin.