Syntenin-syndecan binding requires syndecan-synteny and the co-operation of both PDZ domains of syntenin

Syntenin is an adaptor-like molecule that binds to the cytoplasmic domains of all four vertebrate syndecans, Syntenin-syndecan binding involves the C- terminal part of syntenin that contains a tandem of PDZ domains. Here we pr ovide evidence that each PDZ domain of syntenin can interact with a syndeca n, Isolated or combined mutations of the carboxylate binding lysines in the inter-beta A beta B loops and of the alpha B1 residues in either one or bo th the PDZ domains of syntenin all reduce syntenin-syndecan binding in yeas t two-hybrid, blot-overlay, and surface plasmon resonance assays. PDZ2 muta tions have more pronounced effects on binding: than PDZ1 mutations, but com plete abrogation of syntenin-syndecan binding requires the combination of b oth the lysine and the alpha B1 mutations in both the PDZ domains of synten in, Isothermal calorimetric titration of syntenin with syndecan peptide rev eals the presence of two binding sites in syntenin, Yet, unlike a tandem of two PDZ2 domains and a reconstituted PDZ1 + PDZ2 tandem, a tandem of two P DZ1 domains and isolated PDZ1 or PDZ2 domains do not interact with syndecan bait. We conclude to a co-operative binding mode whereby neither of these two PDZ domains is sufficient by itself but where PDZ2 functions as a "majo r" or "high affinity" syndecan binding domain, and PDZ1 functions as an "ac cessory" or "low affinity" syndecan binding domain. The paired, but not the isolated PDZ domains of syntenin bind also strongly to the immobilized cyt oplasmic domains of neurexin and B-class ephrins, By inference, these data suggest a model whereby recruitment of syntenin to membrane surfaces requir es two compatible types of bait that are in "synteny" (occurring together i n location) and engages both PDZ domains of syntenin, The synteny of compat ible bait may result from the assemblies and co-assemblies of syndecans and other similarly suited partners in larger supramolecular complexes, In gen eral, an intramolecular combination of PDZ domains that are weak, taken ind ividually, would appear to be designed to detect rather than drive the form ation of specific molecular assemblies.