The syndecans are transmembrane proteoglycans that place structurally heterogeneous heparan sulfate chains at the cell surface and a highly conserved polypeptide in the cytoplasm. Their versatile heparan sulfate moieties support various processes of molecular recognition, signaling, and trafficking. Here we report the identification of a protein that binds to the cytoplasmic domains of the syndecans in yeast two-hybrid screens, surface plasmon resonance experiments, and ligand-overlay assays. This protein, syntenin, contains a tandem repeat of PDZ domains that reacts with the FYA C-terminal amino acid sequence of the syndecans. Recombinant enhanced green f luorescent protein (eGFP)-syntenin fusion proteins decorate the plasmamembrane and intracellular vesicles, where they colocalize and cosegregate with syndecans. Cells that overexpress eGFP-syntenin show numerous cell surface extensions, suggesting effects of syntenin on cytoskeleton-membrane organization. We propose that syntenin may function as an adaptor that couples syndecans to cytoskeletal proteins or cytosolic downstream signaleffectors.The cell surface heparan sulfate proteoglycans are at the cross-section of several different pathways (1, 2). The heparan sulfate moieties of these molecules bind various differentiation, growth, and scatter factors, facilitate the occupation and activation of the corresponding signal-transducing receptors, and are involved in the internalization and clearance of the signaling complexes from the cell surface (3). They assist receptors that are involved in cell-cell (4) and cell-matrix (5) adhesion and assist scavenging receptors that are involved in the endocytosis and transcytosis of lipoproteins and lipases (6). They also bind and activate serine proteinase inhibitors and accelerate the reactions of these inhibitors with their targets (7). Proteolysis, lipolysis, mesoderm induction, gastrulation, angiogenesis, and neuritogenesis all appear to be regulated by or to depend on heparan sulfate because this glycosaminoglycan is needed for the allosteric activation, approximation, and compartmentalization of the reactants that are engaged in these processes.In most cells syndecans represent the major source of cell surface heparan sulfate. The four known vertebrate syndecans are small type I membrane proteins, with similar and simple domain organizations: a single ectodomain, membrane span, and cytoplasmic domain (1). The ectodomains of the different syndecans have little in common, except for the presence of three or four consensus sites for heparan sulfate attachment near the N termini of the proteins and a dibasic, presumably protease-sensitive site at the junction with the membrane spanning segment. The structures of these ectodomains have not been evolutionary conserved, except again for these shared structural elements. The membrane-spanning and the small cytoplasmic domains of the syndecans, in contrast, show extensive structural similarity (Ϸ60% sequence identity) and have been highly conserved during evolutio...

PDZ proteins organize multiprotein signaling complexes. According to current views, PDZ domains engage in protein-protein interactions. Here we show that the PDZ domains of several proteins bind phosphatidylinositol 4,5-bisphosphate (PIP(2)). High-affinity binding of syntenin to PIP(2)-containing lipid layers requires both PDZ domains of this protein. Competition and mutagenesis experiments reveal that the protein and the PIP(2) binding sites in the PDZ domains overlap. Overlay assays indicate that the two PDZ domains of syntenin cooperate in binding to cognate peptides and PIP(2). Experiments on living cells demonstrate PIP(2)-dependent and peptide-dependent modes of plasma membrane association of the PDZ domains of syntenin. These observations suggest that local changes in phosphoinositide concentration control the association of PDZ proteins with their target receptors at the plasma membrane.

Syntenin is a PDZ protein that binds the cytoplasmic C-terminal FYA motif of the syndecans. Syntenin is widely expressed. In cell fractionation experiments, syntenin partitions between the cytosol and microsomes. Immunofluorescence microscopy localizes endogenous and epitope-tagged syntenin to cell adhesion sites, microfilaments, and the nucleus. Syntenin is composed of at least three domains. Both PDZ domains of syntenin are necessary to target reporter tags to the plasma membrane. The addition of a segment of 10 amino acids from the N-terminal domain of syntenin to these PDZ domains increases the localization of the tags to stress fibers and induces the formation of long, branching plasma membrane extensions. The addition of the complete N-terminal region, in contrast, reduces the localization of the tags to plasma membrane/adhesion sites and stress fibers, and reduces the morphotypical effects. Recombinant domains of syntenin with the highest plasma membrane localization display the lowest nuclear localization. Syndecan-1, E-cadherin, beta-catenin, and alpha-catenin colocalize with syntenin at cell-cell contacts in epithelial cells, and coimmunoprecipitate with syntenin from extracts of these cells. These results suggest a role for syntenin in the composition of adherens junctions and the regulation of plasma membrane dynamics, and imply a potential role for syntenin in nuclear processes.