Under construction: building the macromolecular superstructure and signaling components of an electrical synapse

A great deal is now known about the protein components of tight junctions and adherens junctions, as well as how these are assembled. Less is known about the molecular framework of gap junctions, but these also have membrane specializations and are subject to regulation of their assembly and turnover. Thus, it is reasonable to consider that these three types of junctions may share macromolecular commonalities. Indeed, the tight junction scaffolding protein zonula occluden-1 (ZO-1) is also present at adherens and gap junctions, including neuronal gap junctions. On the basis of these earlier observations, we more recently found that two additional proteins, AF6 and MUPP1, known to be associated with ZO-1 at tight and adherens junctions, are also components of neuronal gap junctions in rodent brain and directly interact with connexin36 (Cx36) that forms these junctions. Here, we show by immunofluorescence labeling that the cytoskeletal-associated protein cingulin, commonly found at tight junctions, is also localized at neuronal gap junctions throughout the central nervous system. In consideration of known functions related to ZO-1, AF6, MUPP1, and cingulin, our results provide a context in which to examine functional relationships between these proteins at Cx36-containing electrical synapses in brain--specifically, how they may contribute to regulation of transmission at these synapses, and how they may govern gap junction channel assembly and/or disassembly.

The structure of the PDZ3-SH3-GuK tandem of ZO-1 protein suggests a supramodular organization of the membrane-associated guanylate kinase (MAGUK) family scaffold protein core

Membrane-associated guanylate kinases (MAGUKs) are a large family of scaffold proteins that play essential roles in tethering membrane receptors, adhesion molecules, and macromolecular signaling complexes for tissue developments, cell-cell communications, and intracellular signal transductions. The defining feature of the MAGUK family scaffolds is that each member contains a conserved core consisting of a PSD-95/Dlg/ZO-1 (PDZ) domain, an Src homology 3 (SH3) domain, and a catalytically inactive guanylate kinase (GuK) domain arranged in tandem, although the structural features and functional implications of the PDZ-SH3-GuK tandem arrangement are unclear. The structure of the ZO-1 PDZ3-SH3-GuK tandem solved in this study reveals that the PDZ domain directly interacts with the SH3-GuK module, forming a structural supramodule with distinct target binding properties with respect to the isolated domains. Structure-based sequence analysis suggests that the PDZ-SH3-GuK tandems of other members of the MAGUK family also form supramodules.

The plastic energy landscape of protein folding: a triangular folding mechanism with an equilibrium intermediate for a small protein domain

Protein domains usually fold without or with only transiently populated intermediates, possibly to avoid misfolding, which could result in amyloidogenic disease. Whether observed intermediates are productive and obligatory species on the folding reaction pathway or dispensable by-products is a matter of debate. Here, we solved the crystal structure of a small protein domain, SAP97 PDZ2 I342W C378A, and determined its folding pathway. The presence of a folding intermediate was demonstrated both by single and double-mixing kinetic experiments using urea-induced (un)folding as well as ligand-induced folding. This protein domain was found to fold via a triangular scheme, where the folding intermediate could be either on- or off-pathway, depending on the experimental conditions. Furthermore, we found that the intermediate was present at equilibrium, which is rarely seen in folding reactions of small protein domains. The folding mechanism observed here illustrates the roughness and plasticity of the protein folding energy landscape, where several routes may be employed to reach the native state. The results also reconcile the folding mechanisms of topological variants within the PDZ domain family.

Unusual binding interactions in PDZ domain crystal structures help explain binding mechanisms

PDZ domains most commonly bind the C-terminus of their protein targets. Typically the C-terminal four residues of the protein target are considered as the binding motif, particularly the C-terminal residue (P0) and third-last residue (P-2) that form the major contacts with the PDZ domain's "binding groove". We solved crystal structures of seven human PDZ domains, including five of the seven PDLIM family members. The structures of GRASP, PDLIM2, PDLIM5, and PDLIM7 show a binding mode with only the C-terminal P0 residue bound in the binding groove. Importantly, in some cases, the P-2 residue formed interactions outside of the binding groove, providing insight into the influence of residues remote from the binding groove on selectivity. In the GRASP structure, we observed both canonical and noncanonical binding in the two molecules present in the asymmetric unit making a direct comparison of these binding modes possible. In addition, structures of the PDZ domains from PDLIM1 and PDLIM4 also presented here allow comparison with canonical binding for the PDLIM PDZ domain family. Although influenced by crystal packing arrangements, the structures nevertheless show that changes in the positions of PDZ domain side-chains and the alpha B helix allow noncanonical binding interactions. These interactions may be indicative of intermediate states between unbound and fully bound PDZ domain and target protein. The noncanonical "perpendicular" binding observed potentially represents the general form of a kinetic intermediate. Comparison with canonical binding suggests that the rearrangement during binding involves both the PDZ domain and its ligand.

Endothelial cell specific adhesion molecule (ESAM) localizes to platelet-platelet contacts and regulates thrombus formation in vivo

BACKGROUND

In resting platelets, endothelial cell specific adhesion molecule (ESAM) is located in alpha granules, increasing its cell surface expression following platelet activation. However, the function of ESAM on platelets is unknown.

OBJECTIVE

To determine whether ESAM has a role in thrombus formation.

METHODS AND RESULTS

We found that following platelet activation ESAM localizes to the junctions between adjacent platelets, suggesting a role for this protein in contact-dependent events that regulate thrombus formation. To test this hypothesis we examined the effect of ESAM deletion on platelet function. In vivo, ESAM(-/-) mice achieved more stable hemostasis than wild-type mice following tail transection, and developed larger thrombi following laser injury of cremaster muscle arterioles. In vitro, ESAM(-/-) platelets aggregated at lower concentrations of G protein-dependent agonists than wild-type platelets, and were more resistant to disaggregation. In contrast, agonist-induced calcium mobilization, alpha(IIb)beta(3) activation, alpha-granule secretion and platelet spreading, were normal in ESAM-deficient platelets. To understand the molecular mechanism by which ESAM regulates platelet activity, we utilized a PDZ domain array to identify the scaffold protein NHERF-1 as an ESAM binding protein, and further demonstrated that it associates with ESAM in both resting and activated platelets.

CONCLUSIONS

These findings support a model in which ESAM localizes to platelet contacts following platelet activation in order to limit thrombus growth and stability so that the optimal hemostatic response occurs following vascular injury.

Direct association of connexin36 with zonula occludens-2 and zonula occludens-3

The gap junction protein connexin36 (Cx36) is widely expressed in neurons and was previously shown to interact with the PDZ domain-containing protein zonula occludens-1 (ZO-1). We investigated whether Cx36 is also able to interact with other members of zonula occludens family of proteins, namely, ZO-2 and ZO-3, the former of which was reported to be co-localized with Cx36 at gap junctions in mouse retina. HeLa cells transfected with Cx36 and cultured betaTC-3 cells were found to express ZO-2 and ZO-3, and both of these ZO proteins were co-localized with Cx36 at gap junctional cell-cell contacts. In lysates of Cx36-transfected HeLa cells, ZO-2 and ZO-3 were shown to co-immunoprecipitate with Cx36, whereas Cx36/ZO-2 association was absent in cells transfected with truncated Cx36 lacking its C-terminus SAYV PDZ interaction motif. In vitro pull-down assays revealed that Cx36 interacts with the PDZ1, but not with the other two PDZ domains in ZO-2 or ZO-3. Truncated Cx36 lacking its PDZ binding motif failed to bind the PDZ1 domain of either ZO-2 or ZO-3. A 14 amino acid peptide corresponding to the C-terminus of Cx36 was also shown to interact with the PDZ1 domains of ZO-2 and ZO-3, and this peptide inhibited the association of Cx36 with the PDZ1 domains of these ZO proteins. These results indicate that Cx36 associates with the first PDZ domain of ZO-2 and ZO-3 and that this association requires the C-terminus SAYV sequence in Cx36. These findings, together with the known association of ZO-2 with a variety of proteins, including transcription factors, suggest that ZO-2 may serve to anchor regulatory proteins at gap junctions composed of Cx36.

The BTB/kelch protein, KRIP6, modulates the interaction of PICK1 with GluR6 kainate receptors

Neuronal proteins of the BTB/kelch and PDZ domain families interact with different regions of the cytoplasmic C-terminal domain of the GluR6 kainate receptor subunit. The BTB/kelch protein KRIP6 binds within a 58 amino acid segment of GluR6 proximal to the plasma membrane. In contrast, PDZ domain proteins, such as PICK1 and PSD95, interact with the last 4 residues of the GluR6 C-terminus. KRIP6 reduces peak currents mediated by recombinant GluR6 receptors and by native kainate receptors in neurons, whereas PICK1 stabilizes kainate receptors at synapses. Thus, protein-protein interactions at the C-terminal domain of GluR6 are important for regulating kainate receptor physiology. Here, we show by co-clustering and co-immunoprecipitation that KRIP6 interacts with PICK1 in heterologous cells. In addition, we demonstrate a novel modulation of GluR6 receptors by PICK1 resulting in increased peak current and relative desensitization of GluR6-mediated currents, phenotypes opposite to those produced by KRIP6. Importantly, these effects cancel out when KRIP6 and PICK1 are co-expressed together with GluR6. KRIP6 and PICK1 strongly co-cluster and co-immunoprecipitate regardless of the presence of GluR6. Immunofluorescence analysis reveals that GluR6 can either join the KRIP6-PICK1 clusters or remain separate; however, co-expression of KRIP6 reduces the fraction of PICK1 that co-immunoprecipitates with GluR6. Taken together, these results indicate that, in addition to a previously demonstrated direct interaction with the GluR6 C-terminal domain, KRIP6 regulates kainate receptors by inhibiting PICK1 modulation via competition or a mutual blocking effect.

Design of protein function leaps by directed domain interface evolution

Most natural proteins performing sophisticated tasks contain multiple domains where an active site is located at the domain interface. Comparative structural analyses suggest that major leaps in protein function occur through gene recombination events that connect two or more protein domains to generate a new active site, frequently occurring at the newly created domain interface. However, such functional leaps by combination of unrelated domains have not been directly demonstrated. Here we show that highly specific and complex protein functions can be generated by joining a low-affinity peptide-binding domain with a functionally inert second domain and subsequently optimizing the domain interface. These directed evolution processes dramatically enhanced both affinity and specificity to a level unattainable with a single domain, corresponding to >500-fold and >2,000-fold increases of affinity and specificity, respectively. An x-ray crystal structure revealed that the resulting "affinity clamp" had clamshell architecture as designed, with large additional binding surface contributed by the second domain. The affinity clamps having a single-nanomolar dissociation constant outperformed a monoclonal antibody in immunochemical applications. This work establishes evolutionary paths from isolated domains with primitive function to multidomain proteins with sophisticated function and introduces a new protein-engineering concept that allows for the generation of highly functional affinity reagents to a predefined target. The prevalence and variety of natural interaction domains suggest that numerous new functions can be designed by using directed domain interface evolution.

Reassessing a sparse energetic network within a single protein domain

Understanding the molecular principles that govern allosteric communication is an important goal in protein science. One way allostery could be transmitted is via sparse energetic networks of residues, and one such evolutionary conserved network was identified in the PDZ domain family of proteins by multiple sequence alignment [Lockless SW, Ranganathan R (1999) Science 286:295-299]. We have reassessed the energetic coupling of these residues by double mutant cycles together with ligand binding and stability experiments and found that coupling is not a special property of the coevolved network of residues in PDZ domains. The observed coupling for ligand binding is better explained by a distance relationship, where residues close in space are more likely to couple than distal residues. Our study demonstrates that statistical coupling from sequence analysis is not necessarily a reporter of energetic coupling and allostery.

Structural and functional analysis of the PDZ domains of human HtrA1 and HtrA3

High-temperature requirement A (HtrA) and its homologs contain a serine protease domain followed by one or two PDZ domains. Bacterial HtrA proteins and the mitochondrial protein HtrA2/Omi maintain cell function by acting as both molecular chaperones and proteases to manage misfolded proteins. The biological roles of the mammalian family members HtrA1 and HtrA3 are less clear. We report a detailed structural and functional analysis of the PDZ domains of human HtrA1 and HtrA3 using peptide libraries and affinity assays to define specificity, structural studies to view the molecular details of ligand recognition, and alanine scanning mutagenesis to investigate the energetic contributions of individual residues to ligand binding. In common with HtrA2/Omi, we show that the PDZ domains of HtrA1 and HtrA3 recognize hydrophobic polypeptides, and while C-terminal sequences are preferred, internal sequences are also recognized. However, the details of the interactions differ, as different domains rely on interactions with different residues within the ligand to achieve high affinity binding. The results suggest that mammalian HtrA PDZ domains interact with a broad range of hydrophobic binding partners. This promiscuous specificity resembles that of bacterial HtrA family members and suggests a similar function for recognizing misfolded polypeptides with exposed hydrophobic sequences. Our results support a common activation mechanism for the HtrA family, whereby hydrophobic peptides bind to the PDZ domain and induce conformational changes that activate the protease. Such a mechanism is well suited to proteases evolved for the recognition and degradation of misfolded proteins.

Structure of PICK1 and other PDZ domains obtained with the help of self-binding C-terminal extensions

PDZ domains are protein-protein interaction modules that generally bind to the C termini of their target proteins. The C-terminal four amino acids of a prospective binding partner of a PDZ domain are typically the determinants of binding specificity. In an effort to determine the structures of a number of PDZ domains we have included appropriate four residue extensions on the C termini of PDZ domain truncation mutants, designed for self-binding. Multiple truncations of each PDZ domain were generated. The four residue extensions, which represent known specificity sequences of the target PDZ domains and cover both class I and II motifs, form intermolecular contacts in the expected manner for the interactions of PDZ domains with protein C termini for both classes. We present the structures of eight unique PDZ domains crystallized using this approach and focus on four which provide information on selectivity (PICK1 and the third PDZ domain of DLG2), binding site flexibility (the third PDZ domain of MPDZ), and peptide-domain interactions (MPDZ 12th PDZ domain). Analysis of our results shows a clear improvement in the chances of obtaining PDZ domain crystals by using this approach compared to similar truncations of the PDZ domains without the C-terminal four residue extensions.

Erbin enhances voltage-dependent facilitation of Ca(v)1.3 Ca2+ channels through relief of an autoinhibitory domain in the Ca(v)1.3 alpha1 subunit

Ca(v)1.3 (L-type) voltage-gated Ca2+ channels have emerged as key players controlling Ca2+ signals at excitatory synapses. Compared with the more widely expressed Ca(v)1.2 L-type channel, relatively little is known about the mechanisms that regulate Ca(v)1.3 channels. Here, we describe a new role for the PSD-95 (postsynaptic density-95)/Discs large/ZO-1 (zona occludens-1) (PDZ) domain-containing protein, erbin, in directly potentiating Ca(v)1.3. Erbin specifically forms a complex with Ca(v)1.3, but not Ca(v)1.2, in transfected cells. The significance of erbin/Ca(v)1.3 interactions is supported by colocalization in somatodendritic domains of cortical neurons in culture and coimmunoprecipitation from rat brain lysates. In electrophysiological recordings, erbin augments facilitation of Ca(v)1.3 currents by a conditioning prepulse, a process known as voltage-dependent facilitation (VDF). This effect requires a direct interaction of the erbin PDZ domain with a PDZ recognition site in the C-terminal domain (CT) of the long variant of the Ca(v)1.3 alpha1 subunit (alpha1 1.3). Compared with Ca(v)1.3, the Ca(v)1.3b splice variant, which lacks a large fraction of the alpha1 1.3 CT, shows robust VDF that is not further affected by erbin. When coexpressed as an independent entity with Ca(v)1.3b or Ca(v)1.3 plus erbin, the alpha1 1.3 CT strongly suppresses VDF, signifying an autoinhibitory function of this part of the channel. These modulatory effects of erbin, but not alpha1 1.3 CT, depend on the identity of the auxiliary Ca2+ channel beta subunit. Our findings reveal a novel mechanism by which PDZ interactions and alternative splicing of alpha1 1.3 may influence activity-dependent regulation of Ca(v)1.3 channels at the synapse.

Binding of proteins to the PDZ domain regulates proteolytic activity of HtrA1 serine protease

HtrA1, a member of the mammalian HtrA (high temperature requirement A) serine protease family, has a highly conserved protease domain followed by a PDZ domain. Accumulating evidence has indicated that PDZ domains regulate protease activity of HtrA proteins. We searched for binding partners of the PDZ domain of mouse HtrA1 by yeast two-hybrid screening, and isolated proteins that were recognized by the HtrA1 PDZ domain through their C-terminal ends with a core consensus Phi-X-Phi-[V/L/F/A]-COOH sequence (where Phi is a hydrophobic/non-polar amino acid). C-propeptides of fibrillar collagens were most frequently isolated. Type III procollagen alpha1 C-propeptide, which was used as a model protein, was digested by HtrA1. HtrA1 cleavage of the collagen C-propeptide was enhanced by reductive denaturation of the C-propeptide and partly inhibited by removal of the C-terminal four amino acids from the C-propeptide, suggesting that the substrate recognition was facilitated by the binding of the free C-terminal ends of substrates to the PDZ domain of HtrA1. The synthetic oligopeptide (GM130Pep) that fitted the consensus recognition sequence bound to HtrA1 with a high affinity (K(d)=6.0 nM). GM130Pep stimulated HtrA1 protease activity 3- to 4-fold, but did not efficiently stimulate the activity of an HtrA1 mutant lacking the PDZ domain, supporting the notion that the PDZ domain enhances protease activity upon ligand binding. The peptide derived from Type III collagen alpha1 C-propeptide specifically stimulated protease activity of HtrA1, but did not stimulate nor significantly bind to HtrA2, suggesting that the collagen C-propeptide is a specific physiological regulator of HtrA1.

The organization of tight junctions in epithelia: implications for mammary gland biology and breast tumorigenesis

Tight junctions (TJs), the most apical components of the cell-cell junctional complexes, play a crucial role in the establishment and maintenance of cell polarity within tissues. In secretory glandular tissues, such as the mammary gland, TJs are crucial for separating apical and basolateral domains. TJs also create the variable barrier regulating paracellular movement of molecules through epithelial sheets, thereby maintaining tissue homeostasis. Recent advances reveal that TJs exist as macromolecular complexes comprised of several types of membrane proteins, cytoskeletal proteins, and signaling molecules. Many of these components are regulated during mammary gland development and pregnancy cycles, and several have received much attention as possible "tumor suppressors" during progression to breast cancer.

Intracellular membrane targeting and suppression of Ser880 phosphorylation of glutamate receptor 2 by the linker I-set II domain of AMPA receptor-binding protein

AMPA receptor-binding protein (ABP) is a multi-postsynaptic density-95/discs large/zona occludens (PDZ) protein that binds to the glutamate receptor 2/3 (GluR2/3) subunits of the AMPA receptor and is implicated in receptor membrane anchorage. A palmitoylated form of ABP localizes to spine heads, whereas a nonpalmitoylated form is found in intracellular clusters. Here, we investigate intracellular cluster formation by ABP and the ability of ABP to associate with GluR2 while in these clusters. We show that ABP interacts with intracellular membranes via the ABP linker I (LI)-set II (SII) subdomain, a region consisting of ABP linker 1 and PDZ4, -5, and -6. This suggests that cluster formation results from LI-SII ABP association with the membrane of a vesicular structure. We present evidence that ABP can self-associate at intracellular membrane surfaces via interactions involving SII. ABP in such membrane clusters can bind and retain GluR2 that has trafficked endocytotically from the plasma membrane. Phosphorylation of GluR2 at serine 880, proximal to the ABP binding site, has been implicated by others in the release of ABP from GluR2 and the mobilization of AMPA receptors for trafficking. We show that binding of GluR2 to ABP blocks phosphorylation of serine 880. This suggests that ABP can stabilize its own association with GluR2. We discuss a model in which ABP can form a protein scaffold at a vesicular membrane that is capable of binding GluR2, leading to formation of an intracellular AMPA receptor pool. Receptors in such a pool may contribute to receptor endocytotic and exocytotic trafficking and recycling.

Congenic mapping of alcohol and pentobarbital withdrawal liability loci to a <1 centimorgan interval of murine chromosome 4: identification of Mpdz as a candidate gene

Risk for onset of alcoholism is related to genetic differences in acute alcohol withdrawal liability. We previously mapped a locus responsible for 26% of the genetic variance in acute alcohol withdrawal convulsion liability to a >35 centimorgan (cM) interval of murine chromosome 4. Here, we narrow the position of this locus to a <1 cM interval (approximately 1.8 megabase, containing 15 genes and/or predicted genes) using a combination of novel, interval-specific congenic strains and recombinant progeny testing. We report the development of a small-donor-segment congenic strain, which confirms capture of a gene affecting alcohol withdrawal within the <1 cM interval. We also confirm a pentobarbital withdrawal locus within this interval, suggesting that the same gene may influence predisposition to physiological dependence on alcohol and a barbiturate. This congenic strain will be invaluable for determining whether this interval also harbors a gene(s) underlying other quantitative trait loci mapped to chromosome 4, including loci affecting voluntary alcohol consumption, alcohol-induced ataxia, physical dependence after chronic alcohol exposure, and seizure response to pentylenetetrazol or an audiogenic stimulus. To date, Mpdz, which encodes the multiple PSD95/DLG/ZO-1 (PDZ) domain protein (MPDZ), is the only gene within the interval shown to have allelic variants that differ in coding sequence and/or expression. Sequence analysis of 15 standard inbred mouse strains identifies six Mpdz haplotypes that predict three MPDZ protein variants. These analyses, and evidence using interval-specific congenic lines, show that alcohol withdrawal severity is genetically correlated with MPDZ status, indicating that MPDZ variants may influence alcohol withdrawal liability.

Proteins involved in the trafficking and functional synaptic expression of AMPA and KA receptors

Alpha-amino-3-hydroxy-5-methylisoxazolepropionate receptors (AMPARs) mediate the majority of fast synaptic transmission in the mammalian central nervous system, play a central role in synapse stabilisation and plasticity, and their prolonged activation is potently neurotoxic. The functional roles of kainate receptors (KARs) are less well defined but they play a role in some forms of synaptic plasticity. Both receptor types have been shown to be highly developmentally and activity-dependently regulated and their functional synaptic expression is under tight cellular regulation. The molecular and cellular mechanisms that regulate the synaptic localisation and functional expression of AMPARs and KARs are objects of concerted research. There has been significant progress towards elucidating some of the processes involved with the discovery of an array of proteins that selectively interact with individual AMPAR and KAR subunits. These proteins have been implicated in, among other things, the regulation of post-translational modification, targeting and trafficking, surface expression, and anchoring. The aim of this review is to present an overview of the major interacting proteins and suggest how they may fit into the hierarchical series of events controlling the trafficking of AMPARs and KARs.

Tamalin, a PDZ domain-containing protein, links a protein complex formation of group 1 metabotropic glutamate receptors and the guanine nucleotide exchange factor cytohesins

In this investigation, we report identification and characterization of a 95 kDa postsynaptic density protein (PSD-95)/discs-large/ZO-1 (PDZ) domain-containing protein termed tamalin, also recently named GRP1-associated scaffold protein (GRASP), that interacts with group 1 metabotropic glutamate receptors (mGluRs). The yeast two-hybrid system and in vitro pull-down assays indicated that the PDZ domain-containing, amino-terminal half of tamalin directly binds to the class I PDZ-binding motif of group 1 mGluRs. The C-terminal half of tamalin also bound to cytohesins, the members of guanine nucleotide exchange factors (GEFs) specific for the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. Tamalin mRNA is expressed predominantly in the telencephalic region and highly overlaps with the expression of group 1 mGluR mRNAs. Both tamalin and cytohesin-2 were enriched and codistributed with mGluR1a in postsynaptic membrane fractions. Importantly, recombinant and native mGluR1a/tamalin/cytohesin-2 complexes were coimmunoprecipitated from transfected COS-7 cells and rat brain tissue, respectively. Transfection of tamalin and mutant tamalin lacking a cytohesin-binding domain caused an increase and decrease in cell-surface expression of mGluR1a in COS-7 cells, respectively. Furthermore, adenovirus-mediated expression of tamalin and dominant-negative tamalin facilitated and reduced the neuritic distribution of endogenous mGluR5 in cultured hippocampal neurons, respectively. The results indicate that tamalin plays a key role in the association of group 1 mGluRs with the ARF-specific GEF proteins and contributes to intracellular trafficking and the macromolecular organization of group 1 mGluRs at synapses.

Interaction of synaptic scaffolding molecule and Beta -catenin

Synaptic scaffolding molecule (S-SCAM) is a synaptic membrane-associated guanylate kinase with inverted domain organization (MAGI) that interacts with NMDA receptor subunits and neuroligin. In epithelial cells, the non-neuronal isoform of S-SCAM (MAGI-1) is localized at tight or adherens junctions. Recent studies have revealed that the polarized targeting of MAGI-1 to the lateral membrane is mediated by its C-terminal region and that MAGI-1 interacts with beta-catenin in epithelial cells. In this article, we report that S-SCAM interacts with beta-catenin in neurons. beta-Catenin is coimmunoprecipitated with S-SCAM from rat brain. Both S-SCAM and beta-catenin are localized at synapses and are partially colocalized. The C-terminal region of S-SCAM binds to the C-terminal region of beta-catenin. We have tested how the interaction between S-SCAM and beta-catenin plays a role in the synaptic targeting of S-SCAM and beta-catenin. S-SCAM is targeted to synapses via the C-terminal postsynaptic density-95/Dlg-A/ZO-1 (PDZ) domain. beta-Catenin is targeted to synapses with armadillo repeats. The overexpressed C-terminal region of beta-catenin blocks the synaptic targeting of S-SCAM. The overexpressed C-terminal region of S-SCAM is partially targeted to synapses and forms a small number of clusters. In the presence of overexpressed beta-catenin, the C-terminal region of S-SCAM forms more clusters at synapses. These data suggest that the synaptic targeting of S-SCAM is mediated by the interaction with beta-catenin.

Delphilin: a novel PDZ and formin homology domain-containing protein that synaptically colocalizes and interacts with glutamate receptor delta 2 subunit

The glutamate receptor delta2 (GluRdelta2) subunit is selectively expressed in cerebellar Purkinje cells and plays an important role in cerebellar long-term depression, motor learning, motor coordination, and synapse development. We identified a novel GluRdelta2-interacting protein, named Delphilin, that contains a single PDZ domain and formin homology (FH) domains FH1 and FH2 plus coiled-coil structure. As far as we know, this is the first reported protein that contains both PDZ and FH domains. Yeast two-hybrid and surface plasmon resonance (SPR) analyses indicated that Delphilin interacts with the GluRdelta2 C terminus via its PDZ domain. This was also supported by coimmunoprecipitation experiments using a heterologous expression system in mammalian cells. Yeast cell and SPR analyses also demonstrated the possibility that the FH1 proline-rich region of Delphilin interacts with profilin, an actin-binding protein, and with the Src homology 3 domain of neuronal Src protein tyrosine kinase. In situ hybridization demonstrated the highest expression of Delphilin mRNA in Purkinje cells. Delphilin polypeptide was highly enriched in the synaptosomal membrane fraction of the cerebellum and coimmunoprecipitated with the GluRdelta2 subunit. The post-embedding immunogold technique demonstrated that Delphilin is selectively localized at the postsynaptic junction site of the parallel fiber-Purkinje cell synapse and colocalized with GluRdelta2. Thus, Delphilin is a postsynaptic scaffolding protein at the parallel fiber-Purkinje cell synapse, where it may serve to link GluRdelta2 with actin cytoskeleton and various signaling molecules.

PICK1 targets activated protein kinase Calpha to AMPA receptor clusters in spines of hippocampal neurons and reduces surface levels of the AMPA-type glutamate receptor subunit 2

The PICK1 protein interacts in neurons with the AMPA-type glutamate receptor subunit 2 (GluR2) and with several other membrane receptors via its single PDZ domain. We show that PICK1 also binds in neurons and in heterologous cells to protein kinase Calpha (PKCalpha) and that the interaction is highly dependent on the activation of the kinase. The formation of PICK1-PKCalpha complexes is strongly induced by TPA, and PICK1-PKCalpha complexes are cotargeted with PICK1-GluR2 complexes to spines, where GluR2 is found to be phosphorylated by PKC on serine 880. PICK1 also reduces the plasma membrane levels of the GluR2 subunit, consistent with a targeting function of PICK1 and a PKC-facilitated release of GluR2 from the synaptic anchoring proteins ABP and GRIP. This work indicates that PICK1 functions as a targeting and transport protein that directs the activated form of PKCalpha to GluR2 in spines, leading to the activity-dependent release of GluR2 from synaptic anchor proteins and the PICK1-dependent transport of GluR2 from the synaptic membrane.

Laminar organization of the NMDA receptor complex within the postsynaptic density

The NR2 subunit is an essential component of the NMDA receptor. Recent biochemical research has identified a number of molecules that can bind directly or indirectly to its cytoplasmic tail. These postsynaptic density (PSD) proteins play a role in intracellular signal transduction, and are implicated in synaptic plasticity and memory mechanisms. We performed systematic electron microscopic immunogold analysis in rat neocortex to determine the spatial organization of NR2, in relation to six other proteins thought to be involved in the NMDA receptor complex. Peak concentrations of each protein were within the PSD but in different "layers" of the density. In the axodendritic axis, gold particles coding for PSD-95 lay an average of 12 nm cytoplasmic to the extracellular face of the plasma membrane, very close to the C terminal of NR2. Nitric oxide synthase lay 18 nm inside the membrane; the scaffolding proteins guanylate kinase-associated protein and Shank lay 24-26 nm inside the membrane; and CRIPT and dynein light chain, proteins that may link the complex to cytoskeletal elements, lay on the cytoplasmic side of the PSD, 29-32 nm inside the plasma membrane and extending into the spine cytoplasm. The supramolecular organization of these molecules may modulate intracellular transduction of NMDA-mediated signals.

Phosphorylation of the AMPA receptor subunit GluR2 differentially regulates its interaction with PDZ domain-containing proteins

PSD-95, DLG, ZO-1 (PDZ) domain-mediated protein interactions have been shown to play important roles in the regulation of glutamate receptor function at excitatory synapses. Recent studies demonstrating the rapid regulation of AMPA receptor function during synaptic plasticity have suggested that AMPA receptor interaction with PDZ domain-containing proteins may be dynamically modulated. Here we show that PKC phosphorylation of the AMPA receptor GluR2 subunit differentially modulates its interaction with the PDZ domain-containing proteins GRIP1 and PICK1. The serine residue [serine-880 (Ser880)] in the GluR2 C-terminal sequence (IESVKI) critical for PDZ domain binding is a substrate of PKC and is phosphorylated in vivo. In vitro binding and coimmunoprecipitation studies show that phosphorylation of serine-880 within the GluR2 PDZ ligand significantly decreases GluR2 binding to GRIP1 but not to PICK1. Immunostaining of cultured hippocampal neurons demonstrates that the Ser880-phosphorylated GluR2 subunits are enriched and colocalized with PICK1 in the dendrites, with very little staining observed at excitatory synapses. Interestingly, PKC activation in neurons increases the Ser880 phosphorylation of GluR2 subunits and recruits PICK1 to excitatory synapses. Moreover, PKC stimulation in neurons results in rapid internalization of surface GluR2 subunits. These results suggest that GluR2 phosphorylation of serine-880 may be important in the regulation of the AMPA receptor internalization during synaptic plasticity.

PICK1 interacts with and regulates PKC phosphorylation of mGLUR7

The G-protein-coupled metabotropic glutamate receptor subtype 7a (mGluR7a) is a member of group III metabotropic glutamate receptors that plays an important role as a presynaptic receptor in regulating transmitter release at glutamatergic synapses. Here we report that the protein interacting with C-kinase (PICK1) binds to the C terminus (ct) of mGluR7a. In the yeast two-hybrid system, the extreme ct of mGluR7a was shown to interact with the PSD-95/Discs large/ZO-1 (PDZ) domain of PICK1. Pull-down assays indicated that PICK1 was retained by a glutathione S-transferase fusion of ct-mGluR7a. Furthermore, recombinant and native PICK1/mGluR7a complexes were coimmunoprecipitated from COS-7 cells and rat brain tissue, respectively. Confocal microscopy showed that both PICK1 and mGluR7a displayed synaptic colocalization in cultured hippocampal neurons. PICK1 has previously been shown to bind protein kinase C alpha-subunit (PKCalpha), and mGluR7a is known to be phosphorylated by PKC. We show a relationship between these three proteins using recombinant PICK1, mGluR7, and PKCalpha, where they were co-immunoprecipitated as a complex from COS-7 cells. In addition, PICK1 caused a reduction in PKCalpha-evoked phosphorylation of mGluR7a in in vitro phosphorylation assays. These results suggest a role for PICK1 in modulating PKCalpha-evoked phosphorylation of mGluR7a.

An intramolecular interaction between Src homology 3 domain and guanylate kinase-like domain required for channel clustering by postsynaptic density-95/SAP90

Members of the postsynaptic density-95 (PSD-95)/SAP90 family of membrane-associated guanylate kinase (MAGUK) proteins function as multimodular scaffolds that organize protein-signaling complexes at neuronal synapses. MAGUK proteins contain PDZ, Src homology 3 (SH3), and guanylate kinase (GK)-like domains, all of which can function as sites for specific protein-protein interactions. We report here a direct protein-protein interaction between the SH3 domain and the GK region in the PSD-95 family of MAGUKs. The SH3 domain of the PSD-95 family appears to have an atypical binding specificity, because the classical SH3 binding (-P-X-X-P-) motif is absent from the GK domain. Although SH3-GK binding can occur in either an intramolecular or intermolecular manner, the intramolecular mode is preferred, possibly because of additional tertiary interactions available when the SH3 and GK domains are adjacent in the same polypeptide. Mutations disrupting the intramolecular SH3-GK interaction do not interfere with PSD-95 association with the K(+) channel Kv1.4 or with the GK domain-binding protein GKAP. The same mutations, however, inhibit the clustering of Kv1.4 by PSD-95, suggesting that the intramolecular SH3-GK interaction may modulate the clustering activity of PSD-95.

Aczonin, a 550-kD putative scaffolding protein of presynaptic active zones, shares homology regions with Rim and Bassoon and binds profilin

Neurotransmitter exocytosis is restricted to the active zone, a specialized area of the presynaptic plasma membrane. We report the identification and initial characterization of aczonin, a neuron-specific 550-kD protein concentrated at the presynaptic active zone and associated with a detergent-resistant cytoskeletal subcellular fraction. Analysis of the amino acid sequences of chicken and mouse aczonin indicates an organization into multiple domains, including two pairs of Cys(4) zinc fingers, a polyproline tract, and a PDZ domain and two C2 domains near the COOH terminus. The second C2 domain is subject to differential splicing. Aczonin binds profilin, an actin-binding protein implicated in actin cytoskeletal dynamics. Large parts of aczonin, including the zinc finger, PDZ, and C2 domains, are homologous to Rim or to Bassoon, two other proteins concentrated in presynaptic active zones. We propose that aczonin is a scaffolding protein involved in the organization of the molecular architecture of synaptic active zones and in the orchestration of neurotransmitter vesicle trafficking.

Association of AMPA receptors with a subset of glutamate receptor-interacting protein in vivo

The NMDA and AMPA classes of ionotropic glutamate receptors are concentrated at postsynaptic sites in excitatory synapses. NMDA receptors interact via their NR2 subunits with PSD-95/SAP90 family proteins, whereas AMPA receptors bind via their GluR2/3 subunits to glutamate receptor-interacting protein (GRIP), AMPA receptor-binding protein (ABP), and protein interacting with C kinase 1 (PICK1). We report here a novel cDNA (termed ABP-L/GRIP2) that is virtually identical to ABP except for additional GRIP-like sequences at the N-terminal and C-terminal ends. Like GRIP (which we now term GRIP1), ABP-L/GRIP2 contains a seventh PDZ domain at its C terminus. Using antibodies that recognize both these proteins, we examined the subcellular localization of GRIP1 and ABP-L/GRIP2 (collectively termed GRIP) and their biochemical association with AMPA receptors. Immunogold electron microscopy revealed the presence of GRIP at excitatory synapses and also at nonsynaptic membranes and within intracellular compartments. The association of native GRIP and AMPA receptors was confirmed biochemically by coimmunoprecipitation from rat brain extracts. A majority of detergent-extractable GluR2/3 was complexed with GRIP in the brain. However, only approximately half of GRIP was associated with AMPA receptors. Unexpectedly, immunocytochemistry of cultured hippocampal neurons and rat brain at the light microscopic level showed enrichment of GRIP in GABAergic neurons and in GABAergic nerve terminals. Thus GRIP is associated with inhibitory as well as excitatory synapses. Collectively, these findings support a role for GRIP in the synaptic anchoring of AMPA receptors but also suggest that GRIP has additional functions unrelated to the binding of AMPA receptors.

Proline-rich synapse-associated protein-1/cortactin binding protein 1 (ProSAP1/CortBP1) is a PDZ-domain protein highly enriched in the postsynaptic density

The postsynaptic density (PSD) is crucially involved in the structural and functional organization of the postsynaptic neurotransmitter reception apparatus. Using antisera against rat brain synaptic junctional protein preparations, we isolated cDNAs coding for proline-rich synapse-associated protein-1 (ProSAP1), a PDZ-domain protein. This protein was found to be identical to the recently described cortactin-binding protein-1 (CortBP1). Homology screening identified a related protein, ProSAP2. Specific antisera raised against a C-terminal fusion construct and a central part of ProSAP1 detect a cluster of immunoreactive bands of 180 kDa in the particulate fraction of rat brain homogenates that copurify with the PSD fraction. Transcripts and immunoreactivity are widely distributed in the brain and are upregulated during the period of synapse formation in the brain. In addition, two short N-terminal insertions are detected; they are differentially regulated during brain development. Confocal microscopy of hippocampal neurons showed that ProSAP1 is predominantly localized in synapses, and immunoelectron microscopy in situ revealed a strong association with PSDs of hippocampal excitatory synapses. The accumulation of ProSAP1 at synaptic structures was analyzed in the developing cerebral cortex. During early postnatal development, strong immunoreactivity is detectable in neurites and somata, whereas from postnatal day 10 (P10) onward a punctate staining is observed. At the ultrastructural level, the immunoreactivity accumulates at developing PSDs starting from P8. Both interaction with the actin-binding protein cortactin and early appearance at postsynaptic sites suggest that ProSAP1/CortBP1 may be involved in the assembly of the PSD during neuronal differentiation.

Cloning and characterization of neuropilin-1-interacting protein: a PSD-95/Dlg/ZO-1 domain-containing protein that interacts with the cytoplasmic domain of neuropilin-1

Neuropilin-1 (Npn-1), a receptor for semaphorin III, mediates the guidance of growth cones on extending neurites. The molecular mechanism of Npn-1 signaling remains unclear. We have used a yeast two-hybrid system to isolate a protein that interacts with the cytoplasmic domain of Npn-1. This Npn-1-interacting protein (NIP) contains a central PSD-95/Dlg/ZO-1 (PDZ) domain and a C-terminal acyl carrier protein domain. The physiological interaction of Npn-1 and NIP is supported by co-immunoprecipitation of these two proteins in extracts from a heterologous expression system and from a native tissue. The C-terminal three amino acids of Npn-1 (S-E-A-COOH), which is conserved from Xenopus to human, is responsible for interaction with the PDZ domain-containing C-terminal two-thirds of NIP. NIP as well as Npn-1 are broadly expressed in mice as assayed by Northern and Western analysis. Immunohistochemistry and in situ hybridization experiments revealed that NIP expression overlaps with that of Npn-1. NIP has been independently cloned as RGS-GAIP-interacting protein (GIPC), where it was identified by virtue of its interaction with the C terminus of RGS-GAIP and suggested to participate in clathrin-coated vesicular trafficking. We suggest that NIP and GIPC may participate in regulation of Npn-1-mediated signaling as a molecular adapter that couples Npn-1 to membrane trafficking machinery in the dynamic axon growth cone.

Direct interaction of CASK/LIN-2 and syndecan heparan sulfate proteoglycan and their overlapping distribution in neuronal synapses

CASK, the rat homolog of a gene (LIN-2) required for vulval differentiation in Caenorhabditis elegans, is expressed in mammalian brain, but its function in neurons is unknown. CASK is distributed in a punctate somatodendritic pattern in neurons. By immunogold EM, CASK protein is concentrated in synapses, but is also present at nonsynaptic membranes and in intracellular compartments. This immunolocalization is consistent with biochemical studies showing the presence of CASK in soluble and synaptosomal membrane fractions and its enrichment in postsynaptic density fractions of rat brain. By yeast two-hybrid screening, a specific interaction was identified between the PDZ domain of CASK and the COOH terminal tail of syndecan-2, a cell surface heparan sulfate proteoglycan (HSPG). The interaction was confirmed by coimmunoprecipitation from heterologous cells. In brain, syndecan-2 localizes specifically at synaptic junctions where it shows overlapping distribution with CASK, consistent with an interaction between these proteins in synapses. Cell surface HSPGs can bind to extracellular matrix proteins, and are required for the action of various heparin-binding polypeptide growth/differentiation factors. The synaptic localization of CASK and syndecan suggests a potential role for these proteins in adhesion and signaling at neuronal synapses.

Heterogeneity in the molecular composition of excitatory postsynaptic sites during development of hippocampal neurons in culture

To determine their roles in the assembly of glutamatergic postsynaptic sites, we studied the distributions of NMDA- and AMPA-type glutamate receptors; the NMDA receptor-interacting proteins alpha-actinin-2, PSD-95, and chapsyn; and the PSD-95-associated protein GKAP during the development of hippocampal neurons in culture. NMDA receptors first formed nonsynaptic proximal dendrite shaft clusters within 2-5 d. AMPA receptors were diffuse at this stage and began to cluster on spines at 9-10 d. NMDA receptor clusters remained partially nonsynaptic and mainly distinct from AMPA receptor clusters until after 3 weeks in culture, when the two began to colocalize at spiny synaptic sites. Thus, the localization of NMDA and AMPA receptors must be regulated by different mechanisms. alpha-Actinin-2 colocalized with the NMDA receptor only at spiny synaptic clusters, but not at shaft nonsynaptic or synaptic clusters, suggesting a modulatory role in the anchoring of NMDA receptor at spines. PSD-95, chapsyn, and GKAP were present at some, but not all, nonsynaptic NMDA receptor clusters during the first 2 weeks, indicating that none is essential for NMDA receptor cluster formation. When NMDA receptor clusters became synaptic, PSD-95 and GKAP were always present, consistent with an essential function in synaptic localization of NMDA receptors. Furthermore, PSD-95 and GKAP clustered opposite presynaptic terminals several days before either NMDA or AMPA receptors clustered at these presumptive postsynaptic sites. These results suggest that synapse development proceeds by formation of a postsynaptic scaffold containing PSD-95 and GKAP in concert with presynaptic vesicle clustering, followed by regulated attachment of glutamate receptor subtypes to this scaffold.

Interaction of muscle and brain sodium channels with multiple members of the syntrophin family of dystrophin-associated proteins

Syntrophins are cytoplasmic peripheral membrane proteins of the dystrophin-associated protein complex (DAPC). Three syntrophin isoforms, alpha1, beta1, and beta2, are encoded by distinct genes. Each contains two pleckstrin homology (PH) domains, a syntrophin-unique (SU) domain, and a PDZ domain. The name PDZ comes from the first three proteins found to contain repeats of this domain (PSD-95, Drosophila discs large protein, and the zona occludens protein 1). PDZ domains in other proteins bind to the C termini of ion channels and neurotransmitter receptors containing the consensus sequence (S/T)XV-COOH and mediate the clustering or synaptic localization of these proteins. Two voltage-gated sodium channels (NaChs), SkM1 and SkM2, of skeletal and cardiac muscle, respectively, have this consensus sequence. Because NaChs are sarcolemmal components like syntrophins, we have investigated possible interactions between these proteins. NaChs copurify with syntrophin and dystrophin from extracts of skeletal and cardiac muscle. Peptides corresponding to the C-terminal 10 amino acids of SkM1 and SkM2 are sufficient to bind detergent-solubilized muscle syntrophins, to inhibit the binding of native NaChs to syntrophin PDZ domain fusion proteins, and to bind specifically to PDZ domains from alpha1-, beta1-, and beta2-syntrophin. These peptides also inhibit binding of the syntrophin PDZ domain to the PDZ domain of neuronal nitric oxide synthase, an interaction that is not mediated by C-terminal sequences. Brain NaChs, which lack the (S/T)XV consensus sequence, also copurify with syntrophin and dystrophin, an interaction that does not appear to be mediated by the PDZ domain of syntrophin. Collectively, our data suggest that syntrophins link NaChs to the actin cytoskeleton and the extracellular matrix via dystrophin and the DAPC.

Inhibition of patterned cell shape change and cell invasion by Discs large during Drosophila oogenesis

Drosophila Discs large (Dlg) is a tumor suppressor gene whose loss in epithelial tissues causes disrupted cell polarity and increased cell proliferation. A human Dlg homolog, hDlg, has been implicated in tumorigenic processes via its association with the product of the Adenomatous Polyposis Coli (APC) gene. We show for the first time that Drosophila Dlg is required to block cell invasion. Loss of dlg activity during oogenesis causes follicle cells to change shape and invade in a pattern similar to border cells, a small population of cells that break from the post-mitotic follicular epithelium during wild-type oogenesis, yet dlg mutant cells have not adopted a border cell fate. Both functional and morphological evidence indicates that cooperation between germ cell and follicle cell Dlg, probably mediated by Dlg PDZ domains, is crucial for regulating cell mixing, suggesting a novel developmental mechanism and mode of action for the Dlg family of molecules. These findings suggest that Dlg does not simply inhibit individual cell behaviors during oogenesis, but rather acts in a developmental pathway essential for blocking cell proliferation and migration in a spatio-temporally defined manner. A model for Dlg action in blocking cell invasion is presented.

Excitatory amino acid transporter 5, a retinal glutamate transporter coupled to a chloride conductance

Although a glutamate-gated chloride conductance with the properties of a sodium-dependent glutamate transporter has been described in vertebrate retinal photoreceptors and bipolar cells, the molecular species underlying this conductance has not yet been identified. We now report the cloning and functional characterization of a human excitatory amino acid transporter, EAAT5, expressed primarily in retina. Although EAAT5 shares the structural homologies of the EAAT gene family, one novel feature of the EAAT5 sequence is a carboxy-terminal motif identified previously in N-methyl-D-aspartate receptors and potassium channels and shown to confer interactions with a family of synaptic proteins that promote ion channel clustering. Functional properties of EAAT5 were examined in the Xenopus oocyte expression system by measuring radiolabeled glutamate flux and two-electrode voltage clamp recording. EAAT5-mediated L-glutamate uptake is sodium- and voltage-dependent and chloride-independent. Transporter currents elicited by glutamate are also sodium- and voltage-dependent, but ion substitution experiments suggest that this current is largely carried by chloride ions. These properties of EAAT5 are similar to the glutamate-elicited chloride conductances previously described in retinal neurons, suggesting that the EAAT5-associated chloride conductance may participate in visual processing.