PDZ Domain-containing Scaffold Proteins Research Articles

Biomimetic divalent ligands for the acute disruption of synaptic AMPAR stabilization

The interactions of the AMPA receptor (AMPAR) auxiliary subunit Stargazin with PDZ domain-containing scaffold proteins such as PSD-95 are critical for the synaptic stabilization of AMPARs. To investigate these interactions, we have developed biomimetic competing ligands that are assembled from two Stargazin-derived PSD-95/DLG/ZO-1 (PDZ) domain-binding motifs using 'click' chemistry. Characterization of the ligands in vitro and in a cellular FRET-based model revealed an enhanced affinity for the multiple PDZ domains of PSD-95 compared to monovalent peptides. In cultured neurons, the divalent ligands competed with transmembrane AMPAR regulatory protein (TARP) for the intracellular membrane-associated guanylate kinase resulting in increased lateral diffusion and endocytosis of surface AMPARs, while showing strong inhibition of synaptic AMPAR currents. This provides evidence for a model in which the TARP-containing AMPARs are stabilized at the synapse by engaging in multivalent interactions. In light of the prevalence of PDZ domain clusters, these new biomimetic chemical tools could find broad application for acutely perturbing multivalent complexes.

Role of synectin in lymphatic development in zebrafish and frogs

AbstractThe molecular basis of lymphangiogenesis remains incompletely characterized. Here, we document a novel role for the PDZ domain-containing scaffold protein synectin in lymphangiogenesis using genetic studies in zebrafish and tadpoles. In zebrafish, the thoracic duct arises from parachordal lymphangioblast cells, which in turn derive from secondary lymphangiogenic sprouts from the posterior cardinal vein. Morpholino knockdown of synectin in zebrafish impaired formation of the thoracic duct, due to selective defects in lymphangiogenic but not angiogenic sprouting. Synectin genetically interacted with Vegfr3 and neuropilin-2a in regulating lymphangiogenesis. Silencing of synectin in tadpoles caused lymphatic defects due to an underdevelopment and impaired migration of Prox-1+ lymphatic endothelial cells. Molecular analysis further revealed that synectin regulated Sox18-induced expression of Prox-1 and vascular endothelial growth factor C–induced migration of lymphatic endothelial cells in vitro. These findings reveal a novel role for synectin in lymphatic development.

An Arginine Stretch Limits ADAM10 Exit from the Endoplasmic Reticulum

A disintegrin and metalloproteinase 10 (ADAM10) is a type I transmembrane glycoprotein responsible for the ectodomain shedding of a number of proteins implicated in the pathogenesis of diseases ranging from cancer to Alzheimer Disease. ADAM10 is synthesized in an inactive form, which is proteolytically activated during its forward transport along the secretory pathway and at the plasma membrane. Therefore, modulation of its trafficking could provide a mechanism to finely tune its shedding activity. Here we report the identification of an endoplasmic reticulum (ER) retention motif within the ADAM10 intracellular C-terminal tail. Sequential deletion/mutagenesis analyses showed that an arginine-rich ((723)RRR) sequence was responsible for the retention of ADAM10 in the ER and its inefficient surface trafficking. Mutating the second arginine to alanine was sufficient to allow ER exit and surface expression in both heterologous cells and hippocampal neurons. As synapse-associated protein 97 (SAP97) binds ADAM10 at its cytoplasmic tail and facilitates forward ADAM10 trafficking in neurons, we tested whether SAP97 could modulate ER export. However, neither expression nor Ser-39 phosphorylation of SAP97 in heterologous cells or hippocampal neurons were sufficient to allow the ER exit of ADAM10, suggesting that other signaling pathways or alternative binding partners are responsible for ADAM10 ER exit. Together, these results identify a novel mechanism regulating the intracellular trafficking and membrane delivery of ADAM10.

Mechanisms of Outside‐in Signaling at the Tight Junction by Junctional Adhesion Molecule A

Junctional adhesion molecule A (JAM-A) is a tight junction-associated, PDZ binding domain containing transmembrane protein that forms cis-homodimers in endothelial and epithelial cells. In vivo, the function of JAM-A in colonic mucosa has been examined using JAM-A knockout mice, which have increased intestinal permeability, inflammation and cellular proliferation compared to wild-type controls. In vitro studies have revealed that downregulation of JAM-A leads to altered cell migration secondary to diminished levels of beta1 integrin on the cell surface. Similar findings have been observed after transfection of epithelial cells with mutant JAM-A, which is defective in dimerization or lacks the PDZ binding domain. The dominant-negative effects of these mutant JAM-A proteins are most likely secondary to the inability of mutant JAM-A to form signaling complexes, the lack of which results in decreases in active or GTP-bound Rap1. This review highlights findings that support a hypothetical model for JAM-A mediated outside-in signaling. In this model, JAM-A dimerization is required for close cytoplasmic apposition of complexes containing specific PDZ domain-containing scaffold proteins that activate signaling molecules to serve as effectors for the regulation of cellular functions. The possibility of interactions of JAM-A cis-dimers between cells in trans is also discussed.

Normal Hepatic Cell Surface Localization of the High Density Lipoprotein Receptor, Scavenger Receptor Class B, Type I, Depends on All Four PDZ Domains of PDZK1

PDZK1 is a four PDZ domain-containing scaffold protein that binds to scavenger receptor class B, type I (SR-BI), the high density lipoprotein receptor, by its first PDZ domain (PDZ1). PDZK1 knock-out mice exhibit a >95% decrease in hepatic SR-BI protein and consequently an approximately 70% increase in plasma cholesterol in abnormally large high density lipoprotein particles. These defects are corrected by hepatic overexpression of full-length PDZK1 but not the PDZ1 domain alone, which partially restores SR-BI protein abundance but not cell surface expression or function. We have generated PDZK1 knock-out mice with hepatic expression of four PDZK1 transgenes encoding proteins with nested C-terminal truncations: pTEM, which lacks the three C-terminal residues (putative PDZ-binding motif), and PDZ1.2, PDZ1.2.3, or PDZ1.2.3.4, which contain only the first two, three, or four N-terminal PDZ domains, respectively, but not the remaining C-terminal sequences. Hepatic overexpression of pTEM restored normal hepatic SR-BI abundance, localization, and function. Hepatic overexpression of PDZ1.2 or PDZ1.2.3 partially restored SR-BI abundance ( approximately 12 or approximately 30% of wild type, respectively) but did not (PDZ1.2) or only slightly (PDZ1.2.3) restored hepatic SR-BI cell surface localization and function. Hepatic overexpression of PDZ1.2.3.4 completely restored SR-BI protein abundance, cell surface expression, and function (normalization of plasma cholesterol levels). Thus, all four PDZ domains in PDZK1, but not PDZ1-3 alone, are sufficient for its normal control of the abundance, localization, and therefore function of hepatic SR-BI, whereas the residues C-terminal to the PDZ4 domain, including the C-terminal putative PDZ-binding domain, are not required.

Par-3-mediated Junctional Localization of the Lipid Phosphatase PTEN Is Required for Cell Polarity Establishment

PDZ domain-containing scaffold protein Par-3 is the central organizer of the evolutionarily conserved cell polarity-regulatory Par-3.Par-6.atypical protein kinase C complex. The PDZ domains of Par-3 have also been implicated as potential phosphoinositide signaling integrators, since its second PDZ domain binds to phosphoinositides, and the third PDZ interacts with phosphoinositide phosphatase PTEN. However, the molecular basis of Par-3/PTEN interaction is still poorly understood. Additionally, it is not known whether the regulatory function of PTEN in cell polarity is specifically mediated by its interaction with Par-3. The structures of Par-3 PDZ3 in both its free and PTEN tail peptide-bound forms determined in this work reveal that Par-3 PDZ3 binds to PTEN with two discrete binding sites: a canonical PDZ-ligand interaction site and a distal, opposite charge-charge interaction site. This distinct target recognition mechanism confers the interaction specificity of the Par-3.PTEN complex. We show that the Par-3 PDZ3-PTEN binding is required for the enrichment of PTEN at the junctional membranes of Madin-Darby canine kidney cells. Finally, we demonstrate that the junctional membrane-localized PTEN is specifically required for the polarization of Madin-Darby canine kidney cells. These results, together with earlier data, firmly establish that Par-3 functions as a scaffold in integrating phosphoinositide signaling events during cellular polarization.

The PDZ Protein Mupp1 Promotes Gi Coupling and Signaling of the Mt1 Melatonin Receptor

Intracellular signaling events are often organized around PDZ (PSD-95/Drosophila Disc large/ZO-1 homology) domain-containing scaffolding proteins. The ubiquitously expressed multi-PDZ protein MUPP1, which is composed of 13 PDZ domains, has been shown to interact with multiple viral and cellular proteins and to play important roles in receptor targeting and trafficking. In this study, we show that MUPP1 binds to the G protein-coupled MT(1) melatonin receptor and directly regulates its G(i)-dependent signal transduction. Structural determinants involved in this interaction are the PDZ10 domain of MUPP1 and the valine of the canonical class III PDZ domain binding motif DSV of the MT(1) carboxyl terminus. This high affinity interaction (K(d) approximately 4 nm), which is independent of MT(1) activation, occurs in the ovine pars tuberalis of the pituitary expressing both proteins endogenously. Although the disruption of the MT(1)/MUPP1 interaction has no effect on the subcellular localization, trafficking, or degradation of MT(1), it destabilizes the interaction between MT(1) and G(i) and abolishes G(i)-mediated signaling of MT(1). Our findings highlight a previously unappreciated role of PDZ proteins in promoting G protein coupling to receptors.

Bringing It All Together

Polarization of various cell types depends on a complex containing the PDZ domain-containing scaffolding proteins Par-3 and Par-6 and atypical protein kinase C. Noting that phosphoinositides (PIPs) have also been implicated in cell polarization and that PDZ domains--although better known for mediating protein-protein interactions--can bind phosphatidylinositol (PI) lipids, Wu et al . investigated interactions between Par protein PDZ domains and PI lipids. The second Par-3 PDZ domain (PDZ2) bound to brain-derived liposomes with an affinity comparable to that of the PLCδ pleckstrin homology (PH) domain (a PI lipid-binding domain); strip-based assays and experiments with defined liposomes indicated that it bound directly to PIPs. Lipid-binding assays of 74 human PDZ domains and 14 PDZ tandems revealed that a subset bound PI lipids. Structural and biochemical analysis indicated that lipid binding by the Par-3 PDZ2 depended on a cluster of positively charged residues and that PDZ2 contained a PIP head group-binding pocket that overlapped its peptide ligand-binding site. In MDCK epithelial cells, Par-3 primarily localized to the cell membrane; mutational analysis indicated that membrane localization depended on PI lipid binding by PDZ2. Par-3 knockdown disrupted tight junction formation and reestablishment of polarization in MDCK cells. Epithelial repolarization was rescued by wild-type Par-3 or by a mutant in which the PLCδ PH domain replaced PDZ2 but not by mutants defective in lipid binding. The Par-3 PDZ3 domain bound to the lipid phosphatase PTEN. The authors conclude that lipid binding by the Par-3 PDZ2 domain is critical to its role in epithelial polarization and that the combination of PIP-binding by PDZ2 and PTEN-binding by PDZ3 could enable Par-3 to integrate PIP signaling. H. Wu, W. Feng, J. Chen, L.-N. Chan, S. Huang, M. Zhang, PDZ domains of Par-3 as potential phosphoinositide signaling integrators. Mol. Cell 28 , 886-898 (2007). [PubMed]

An Active Player

Scaffolding proteins that bind to and organize components of signaling pathways into macromolecular complexes are crucial to the efficiency, specificity, and speed of various signaling pathways. For instance, INAD (inactivation, no after-potential D), a PDZ domain-containing scaffolding protein that, together with three of its binding partners, constitutes the core complex of the Drosophila phototransduction cascade, is critical to the electrical response to light (see Montell). Using x-ray crystallography, Mishra et al . found that, in the oxidized state, the fifth INAD PDZ protein interaction domain (PDZ5) assumed a distorted conformation inconsistent with canonical ligand binding. This conformation depended on an intramolecular disulfide bond and, in the reduced state, PDZ5 structurally resembled other PDZ domains. In transgenic flies expressing wild-type INAD, light exposure led to formation of the disulfide bond (and thus the oxidized conformation), whereas subsequent darkness was associated with return to the reduced form; development of the oxidized form was attenuated in flies lacking the major isoform of rhodopsin. Electrophysiological analysis of isolated photoreceptor cells expressing a mutant form of INAD in which PDZ5 only existed in the reduced conformation indicated that the sensitivity of the mutant form to light was normal but that loss of a refractory period led to defective termination of the response to bright light. Moreover, flies with the mutant INAD showed a defective escape response to a brief exposure to darkness (mimicking the shadow of a predator). Thus, rather than performing a purely passive function in organizing signaling proteins, INAD appears to play a dynamic role in the Drosophila phototransduction pathway. P. Mishra, M. Socolich, M. A. Wall, J. Graves, Z. Wang, R. Ranganathan, Dynamic scaffolding in a G protein-coupled signaling system. Cell 131 , 80-92 (2007). [PubMed] C. Montell, Dynamic regulation of the INAD signaling scaffold becomes crystal clear. Cell 131 , 19-21 (2007). [PubMed]

The cytoplasmic domain of NrCAM binds to PDZ domains of synapse‐associated proteins SAP90/PSD95 and SAP97

NrCAM, a member of the L1 family of cell adhesion molecules, serves important functions during the development of the nervous system, e.g. in adhesion-dependent processes such as neurite outgrowth and axonal pathfinding. Complex homo- and heterophilic binding and several extracellular ligands of NrCAM have been described, but less is known about intracellular interaction partners. The cytoplasmic carboxy-terminus of NrCAM contains a typical sequence motif for binding to PDZ domains, making interactions with PDZ domain-containing scaffolding proteins quite conceivable. In this study, we identified specific interactions of the intracellular domain of NrCAM with class I PDZ domains of the membrane-associated guanylate kinases SAP90/PSD95 and SAP97. In contrast to NrCAM, the intracellular domains of the other mammalian L1 family molecules, e.g. L1, CHL1 and Neurofascin, did not interact with these PDZ domains. In transfected COS-7 cells, NrCAM-mediated recruitment of SAP97 to the plasma membrane was dependent on the PDZ binding motif. We show that NrCAM and SAP97 are colocalized, e.g. within photoreceptor terminals of the mammalian retina. In summary, our results confirm a functional PDZ domain binding motif at the carboxy-terminus of NrCAM and support potential functions of NrCAM during the assembly of highly organized protein complexes at the cell membrane.

Role of Unc51.1 and its binding partners in CNS axon outgrowth.

Previous studies showed that the serine/threonine kinase Unc51.1 is one of the earliest genes in neuronal differentiation and is required for granule cell axon formation. To examine the mechanism of Unc51.1 regulation of axon extension, we have identified two direct binding partners. The first, SynGAP, a negative regulator of Ras, is expressed within axons and growth cones of developing granule cells. Overexpression of SynGAP blocks neurite outgrowth by a mechanism that involves Ras-like GTPase cascade. The second binding partner is a PDZ domain-containing scaffolding protein, Syntenin, that binds Rab5 GTPase, the activity of which is attenuated by SynGAP. Thus, our results demonstrate that the Unc51.1-containing protein complex governs axon formation via Ras-like GTPase signaling and through regulation of the Rab5-mediated endocytic pathways within developing axons.

Cytoplasmic interactions of syndecan-4 orchestrate adhesion receptor and growth factor receptor signalling

Syndecan-4 is a ubiquitous transmembrane proteoglycan that localizes to the focal adhesions of adherent cells and binds to a range of extracellular ligands, including growth factors and extracellular-matrix proteins. Engagement of syndecan-4 is essential for adhesion formation in cells adhering via certain integrins, and for cell proliferation and migration in response to growth factors. The cytoplasmic domain of syndecan-4 interacts with a number of signalling and structural proteins, and both extracellular and cytoplasmic domains are necessary for regulated activation of associated transmembrane receptors. PDZ domain-containing scaffold proteins (syntenin and CASK) bind to the C-terminus of the syndecan-4 cytoplasmic domain and co-ordinate clustering of receptors and connection to the actin cytoskeleton. Syndecan-4 also binds and activates protein kinase Calpha in the presence of phosphatidylinositol 4,5-bisphosphate, and regulates signalling by Rho-family GTPases and focal adhesion kinase. This review discusses the cytoplasmic interactions of syndecan-4 and how they affect cell behaviour as a consequence of the interaction with extracellular ligands. These conclusions also offer an insight into the role of syndecan-4 in vivo, and are consistent with phenotypes generated as a consequence of abnormal syndecan-4 expression in pathologies and gene disruption studies.

Molecular organization of the postsynaptic specialization.

A specific set of molecules including glutamate receptors is targeted to the postsynaptic specialization of excitatory synapses in the brain, gathering in a structure known as the postsynaptic density (PSD). Synaptic targeting of glutamate receptors depends on interactions between the C-terminal tails of receptor subunits and specific PDZ domain-containing scaffold proteins in the PSD. These scaffold proteins assemble a specialized protein complex around each class of glutamate receptor that functions in signal transduction, cytoskeletal anchoring, and trafficking of the receptors. Among the glutamate receptor subtypes, the N-methyl-d-aspartate receptor is relatively stably integrated in the PSD, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor moves in and out of the postsynaptic membrane in highly dynamic fashion. The distinctive cell biological behaviors of N-methyl-d-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors can be explained by their differential interactions with cytoplasmic proteins.

β1-Adrenergic Receptor Association with PSD-95: INHIBITION OF RECEPTOR INTERNALIZATION AND FACILITATION OF β1-ADRENERGIC RECEPTOR INTERACTION WITHN-METHYL-d-ASPARTATE RECEPTORS

The beta(1)-adrenergic receptor (beta(1)AR) is the most abundant subtype of beta-adrenergic receptor in the mammalian brain and is known to potently regulate synaptic plasticity. To search for potential neuronal beta(1)AR-interacting proteins, we screened a rat brain cDNA library using the beta(1)AR carboxyl terminus (beta(1)AR-CT) as bait in the yeast two-hybrid system. These screens identified PSD-95, a multiple PDZ domain-containing scaffolding protein, as a specific binding partner of the beta(1)AR-CT. This interaction was confirmed by in vitro fusion protein pull-down and blot overlay experiments, which demonstrated that the beta(1)AR-CT binds specifically to the third PDZ domain of PSD-95. Furthermore, the full-length beta(1)AR associates with PSD-95 in cells, as determined by co-immunoprecipitation experiments and immunofluorescence co-localization studies. The interaction between beta(1)AR and PSD-95 is mediated by the last few amino acids of the beta(1)AR, and mutation of the beta(1)AR carboxyl terminus eliminated the binding and disrupted the co-localization of the beta(1)AR and PSD-95 in cells. Agonist-induced internalization of the beta(1)AR in HEK-293 cells was markedly attenuated by PSD-95 co-expression, whereas co-expression of PSD-95 has no significant effect on either desensitization of the beta(1)AR or beta(1)AR-induced cAMP accumulation. Furthermore, PSD-95 facilitated the formation of a complex between the beta(1)AR and N-methyl-d-aspartate receptors, as assessed by co-immunoprecipitation. These data reveal that PSD-95 is a specific beta(1)AR binding partner that modulates beta(1)AR function and facilitates physical association of the beta(1)AR with synaptic proteins, such as the N-methyl-d-aspartate receptors, which are known to be regulated by beta(1)AR stimulation.