PDZ domains and the formation of protein networks at the plasma membrane

Comparative Study of the Collective Dynamics of Proteins and Inorganic Nanoparticles

Molecular dynamics simulations of ubiquitin in water/glycerol solutions are used to test the suggestion by Karplus and coworkers that proteins in their biologically active state should exhibit a dynamics similar to 'surface-melted' inorganic nanoparticles (NPs). Motivated by recent studies indicating that surface-melted inorganic NPs are in a 'glassy' state that is an intermediate dynamical state between a solid and liquid, we probe the validity and significance of this proposed analogy. In particular, atomistic simulations of ubiquitin in solution based on CHARMM36 force field and pre-melted Ni NPs (Voter-Chen Embedded Atom Method potential) indicate a common dynamic heterogeneity, along with other features of glass-forming (GF) liquids such as collective atomic motion in the form of string-like atomic displacements, potential energy fluctuations and particle displacements with long range correlations ('colored' or 'pink' noise), and particle displacement events having a power law scaling in magnitude, as found in earthquakes. On the other hand, we find the dynamics of ubiquitin to be even more like a polycrystalline material in which the α-helix and β-sheet regions of the protein are similar to crystal grains so that the string-like collective atomic motion is concentrated in regions between the α-helix and β-sheet domains.

A high-grain diet alters the omasal epithelial structure and expression of tight junction proteins in a goat model

The omasal epithelial barrier plays important roles in maintaining nutrient absorption and immune homeostasis in ruminants. However, little information is currently available about the changes in omasal epithelial barrier function at the structural and molecular levels during feeding of a high-grain (HG) diet. Ten male goats were randomly assigned to two groups, fed either a hay diet (0% grain; n = 5) or HG diet (65% grain; n = 5). Changes in omasal epithelial structure and expression of tight junction (TJ) proteins were determined via electron microscopy and Western blot analysis. After 7 weeks on each diet, omasal contents in the HG group showed significantly lower pH (P <0.001) and significantly higher concentrations of free lipopolysaccharides (LPS; P = 0.001) than the hay group. The goats fed a HG diet showed profound alterations in omasal epithelial structure and TJ proteins, corresponding to depression of thickness of total epithelia, stratum granulosum, and the sum of the stratum spinosum and stratum basale, marked epithelial cellular damage, erosion of intercellular junctions and down-regulation in expression of the TJ proteins, claudin-4 and occludin. The study demonstrates that feeding a HG diet is associated with omasal epithelial cellular damage and changes in expression of TJ proteins. These research findings provide an insight into the possible significance of diet on the omasal epithelial barrier in ruminants.

Analysis of protein-protein interaction using proteinchip array-based SELDI-TOF mass spectrometry

Protein-protein interactions are key elements in the assembly of cellular regulatory and signaling protein complexes that integrate and transmit signals and information in controlling and regulating various cellular processes and functions. Many conventional methods of studying protein-protein interaction, such as the immuno-precipitation and immuno-blotting assay and the affinity-column pull-down and chromatographic analysis, are very time-consuming and labor intensive and lack accuracy and sensitivity. We have developed a simple, rapid, and sensitive assay using a ProteinChip array and SELDI-TOF mass spectrometry to analyze protein-protein interactions and map the crucial elements that are directly involved in these interactions. First, a purified "bait" protein or a synthetic peptide of interest is immobilized onto the preactivated surface of a PS10 or PS20 ProteinChip and the unoccupied surfaces on the chip are protected by application of a layer ethanolamine to prevent them from binding to other non-interactive proteins. Then, the target-containing cellular protein lysate or synthetic peptide containing the predicted amino acid sequence of protein-interaction motif is applied to the protected array with immobilized bait protein/peptide. The nonspecific proteins/peptides are washed off under various stringent conditions and only the proteins specifically interacting with the bait protein/peptide remain on the chip. Last, the captured interacting protein/peptide complexes are then analyzed by SELDI-TOF mass spectrometry and their identities are confirmed by their predicted distinctive masses. This method can be used to unambiguously detect the specific protein-protein interaction of known proteins/peptides, to easily identify potential cellular targets of proteins of interest, and to accurately analyze and map the structural elements of a given protein and its target proteins using synthetic peptides with the predicted potential protein interaction motifs.

Detailed regulatory mechanism of the interaction between ZO-1 PDZ2 and connexin43 revealed by MD simulations

The gap junction protein connexin43 (Cx43) binds to the second PDZ domain of Zonula occludens-1 (ZO-1) through its C-terminal tail, mediating the regulation of gap junction plaque size and dynamics. Biochemical study demonstrated that the very C-terminal 12 residues of Cx43 are necessary and sufficient for ZO-1 PDZ2 binding and phosphorylation at residues Ser (-9) and Ser (-10) of the peptide can disrupt the association. However, only a crystal structure of ZO-1 PDZ2 in complex with a shorter 9 aa peptide of connexin43 was solved experimentally. Here, the interactions between ZO-1 PDZ2 and the short, long and phosphorylated Cx43 peptides were studied using molecular dynamics (MD) simulations and free energy calculation. The short peptide bound to PDZ2 exhibits large structural variations, while the extension of three upstream residues stabilizes the peptide conformation and enhanced the interaction. Phosphorylation at Ser(-9) significantly weakens the binding and results in conformational flexibility of the peptide. Glu210 of ZO-1 PDZ2 was found to be a key regulatory point in Cx43 binding and phosphorylation induced dissociation.

An experimental platform for systemic drug delivery to the retina

Degenerative retinopathies, including age-related macular degeneration, diabetic retinopathy, and hereditary retinal disorders--major causes of world blindness--are potentially treatable by using low-molecular weight neuroprotective, antiapoptotic, or antineovascular drugs. These agents are, however, not in current systemic use owing to, among other factors, their inability to passively diffuse across the microvasculature of the retina because of the presence of the inner blood-retina barrier (iBRB). Moreover, preclinical assessment of the efficacies of new formulations in the treatment of such conditions is similarly compromised. We describe here an experimental process for RNAi-mediated, size-selective, transient, and reversible modulation of the iBRB in mice to molecules up to 800 Da by suppression of transcripts encoding claudin-5, a protein component of the tight junctions of the inner retinal vasculature. MRI produced no evidence indicative of brain or retinal edema, and the process resulted in minimal disturbance of global transcriptional patterns analyzed in neuronal tissue. We show that visual function can be improved in IMPDH1(-/-) mice, a model of autosomal recessive retinitis pigmentosa, and that the rate of photoreceptor cell death can be reduced in a model of light-induced retinal degeneration by systemic drug delivery after reversible barrier opening. These findings provide a platform for high-throughput drug screening in models of retinal degeneration, and they ultimately could result in the development of a novel "humanized" approach to therapy for conditions with little or no current forms of treatment.

Signaling pathways of PDZ2 domain: a molecular dynamics interaction correlation analysis

PDZ domains are found in many signaling proteins. One of their functions is to provide scaffolds for forming membrane-associated protein complexes by binding to the carboxyl termini of their partners. PDZ domains are thought also to play a signal transduction role by propagating the information that binding has occurred to remote sites. In this study, a molecular dynamics (MD) simulation-based approach, referred to as an interaction correlation analysis, is applied to the PDZ2 domain to identify the possible signal transduction pathways. A residue correlation matrix is constructed from the interaction energy correlations between all residue pairs obtained from the MD simulations. Two continuous interaction pathways, starting at the ligand binding pocket, are identified by a hierarchical clustering analysis of the residue correlation matrix. One pathway is mainly localized at the N-terminal side of helix alpha1 and the adjacent C-terminus of loop beta1-beta2. The other pathway is perpendicular to the central beta-sheet and extends toward the side of PDZ2 domain opposite to the ligand binding pocket. The results complement previous studies based on multiple sequence analysis, NMR, and MD simulations. Importantly, they reveal the energetic origin of the long-range coupling. The PDZ2 results, as well as the earlier rhodopsin analysis, show that the interaction correlation analysis is a robust approach for determining pathways of intramolecular signal transduction.

Signaling networks assembled by oncogenic EGFR and c-Met

A major question regarding the sensitivity of solid tumors to targeted kinase inhibitors is why some tumors respond and others do not. The observation that many tumors express EGF receptor (EGFR), yet only a small subset with EGFR-activating mutations respond clinically to EGFR inhibitors (EGFRIs), suggests that responsive tumors uniquely depend on EGFR signaling for their survival. The nature of this dependence is not understood. Here, we investigate dependence on EGFR signaling by comparing non-small-cell lung cancer cell lines driven by EGFR-activating mutations and genomic amplifications using a global proteomic analysis of phospho-tyrosine signaling. We identify an extensive receptor tyrosine kinase signaling network established in cells expressing mutated and activated EGFR or expressing amplified c-Met. We show that in drug sensitive cells the targeted tyrosine kinase drives other RTKs and an extensive network of downstream signaling that collapse with drug treatment. Comparison of the signaling networks in EGFR and c-Met-dependent cells identify a "core network" of approximately 50 proteins that participate in pathways mediating drug response.

Domain swapping within PDZ2 is responsible for dimerization of ZO proteins

ZO-1 is a multidomain protein involved in cell-cell junctions and contains three PDZ domains, which are necessary for its function in vivo. PDZ domains play a central role in assembling diverse protein complexes through their ability to recognize short peptide motifs on other proteins. We determined the structure of the second of the three PDZ domains of ZO-1, which is known to promote dimerization as well as bind to C-terminal sequences on connexins. The dimer is stabilized by extensive symmetrical domain swapping of beta-strands, which is unlike any other known mechanism of PDZ dimerization. The canonical peptide-binding groove remains intact in both subunits of the PDZ2 dimer and is created by elements contributed from both monomers. This unique structure reveals an additional example of how PDZ domains dimerize and has multiple implications for both peptide binding and oligomerization in vivo.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Homer/Vesl proteins and their roles in CNS neurons

Since their initial discovery in 1997, Homer/Vesl proteins have become increasingly investigated as putative regulators of receptor and ion-channel function in the central nervous system. Within a relatively brief period, numerous research reports have described manifold effects of Homer proteins, including the modulation of the trafficking of type I metabotropic glutamate receptors (mGluRs), axonal pathfinding, mGluR coupling to calcium and potassium channels, agonist-independent mGluR activity, ryanodine receptor regulation, locomotor activity, and behavioral plasticity. This review summarizes our current knowledge on the induction, expression, and structure of the various forms of Homer proteins, as well as their roles in neuronal function. In addition, we provide an outlook on novel developments with regard to the involvement of Homer-1a in hippocampal synaptic function.

Solution structure of the PDZ2 domain from cytosolic human phosphatase hPTP1E complexed with a peptide reveals contribution of the beta2-beta3 loop to PDZ domain-ligand interactions

The solution structure of the second PDZ domain from human phosphatase hPTP1E in complex with a C-terminal peptide from the guanine nucleotide exchange factor RA-GEF-2 has been determined using 2D and 3D heteronuclear NMR experiments. Compared to previously solved structures, the hPTP1E complex shows an enlarged interaction surface with the C terminus of the bound peptide. Novel contacts were found between the long structured beta2/beta3 loop of the PDZ domain and the sixth amino acid residue from the C terminus of the peptide. This work underlines the importance of the beta2/beta3 loop for ligand selection by PDZ domains.

mARVCF cellular localisation and binding to cadherins is influenced by the cellular context but not by alternative splicing

ARVCF, a member of the catenin family, is thought to contribute to the morphoregulatory function of the cadherin-catenin complex. Recently, we reported the isolation and characterisation of murine ARVCF (mARVCF), particularly its interaction with M-cadherin. Here, we describe the identification of novel mARVCF isoforms that arise by alternative splicing. At the N-terminus, alternative splicing results in the inclusion or omission of a coiled-coil region probably important for protein-protein interactions. At the C-terminus, four isoforms also differ by domains potentially important for selective protein-protein interaction. The eight putative mARVCF isoforms were expressed as EGFP-fusion proteins in six different cell lines that exhibit a distinct pattern of cadherins. Apparently, binding of the mARVCF isoforms to M-, N-, or E-cadherin is generally unaffected by their altered N- and C-termini, as revealed by the MOM recruitment assay. However, mARVCF isoforms reproducibly exhibit differential localisation in distinct cellular environments. For example, mARVCF isoforms are unable to colocalise with N-cadherin in EJ28 carcinoma cells but do so in HeLa cells. Our results suggest that the subcellular localisation of mARVCF may be determined not only by the presence or absence of an appropriate interaction partner, in this case cadherins, but also by the cellular context.

Mechanism and role of PDZ domains in signaling complex assembly

PDZ domains are protein-protein recognition modules that play a central role in organizing diverse cell signaling assemblies. These domains specifically recognize short C-terminal peptide motifs, but can also recognize internal sequences that structurally mimic a terminus. PDZ domains can therefore be used in combination to bind an array of target proteins or to oligomerize into branched networks. Several PDZ-domain-containing proteins play an important role in the transport, localization and assembly of supramolecular signaling complexes. Examples of such PDZ-mediated assemblies exist in Drosophila photoreceptor cells and at mammalian synapses. The predominance of PDZ domains in metazoans indicates that this highly specialized scaffolding module probably evolved in response to the increased signaling needs of multicellular organisms.

Interaction with mLin-7 alters the targeting of endocytosed transmembrane proteins in mammalian epithelial cells

To investigate the targeting mechanism for proteins bound to the mammalian Lin-7 (mLin-7) PDZ domain, we created receptor protein chimeras composed of the carboxyl-terminal amino acids of LET-23 fused to truncated nerve growth factor receptor/P75. mLin-7 bound to the chimera with a wild-type LET-23 carboxyl-terminal tail (P75t-Let23WT), but not a mutant tail (P75t-Let23MUT). In Madin-Darby canine kidney (MDCK) cells, P75t-Let23WT localized to the basolateral plasma membrane domain, whereas P75t-Let23MUT remained apical. Furthermore, mutant mLin-7 constructs acted as dominant interfering proteins and inhibited the basolateral localization of P75t-Let23WT. The mechanisms for this differential localization were examined further, and, initially, we found that P75t-Let23WT and P75t-Let23MUT were delivered equally to the apical and basolateral plasma membrane domains. Although basolateral retention of P75t-Let23WT, but not P75t-Let23MUT, was observed, the greatest difference in receptor localization was seen in the rapid trafficking of P75t-Let23WT to the basolateral plasma membrane domain after endocytosis, whereas P75t-Let23MUT was degraded in lysosomes, indicating that mLin-7 binding can alter the fate of endocytosed proteins. Altogether, these data support a model for basolateral protein targeting in mammalian epithelial cells dependent on protein-protein interactions with mLin-7, and also suggest a dynamic role for mLin-7 in endosomal sorting.

Characterization of syntenin, a syndecan-binding PDZ protein, as a component of cell adhesion sites and microfilaments

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.

Inactivation of MMAC1 in bladder transitional-cell carcinoma cell lines and specimens

We recently limited the location of a candidate tumor suppressor gene in invasive (T3a/b) bladder transitional-cell carcinoma (TCC) to a 2.5-cM region at chromosome 10q23.3. This region harbors the MMAC1/PTEN/TEP1 gene (referred to hereafter as MMAC1), a dual-phosphatase tumor-suppressor gene frequently inactivated in variety of malignant tumors. In the present study, we examined whether MMAC1 is a target for inactivation by mutations and deletions in bladder TCC cell lines and specimens. MMAC1 was inactivated by homozygous deletions and mutations in three (27%) of 11 bladder cancer cell lines. One cell line, UC-3, had homozygous deletions, and two other cell lines, T-24 and UC-9, had missense mutations. T-24 had also a nonsense mutation. However, none of the 33 bladder TCC specimens examined had a mutation or deletion in the coding region. These results suggest that MMAC1 is not the primary target for inactivation in bladder TCC and that another gene, in close proximity to the MMAC1 locus, within this region of frequent allelic losses, may be the target for inactivation.

Intramolecular interactions regulate SAP97 binding to GKAP

Membrane-associated guanylate kinase homologs (MAGUKs) are multidomain proteins found to be central organizers of cellular junctions. In this study, we examined the molecular mechanisms that regulate the interaction of the MAGUK SAP97 with its GUK domain binding partner GKAP (GUK-associated protein). The GKAP-GUK interaction is regulated by a series of intramolecular interactions. Specifically, the association of the Src homology 3 (SH3) domain and sequences situated between the SH3 and GUK domains with the GUK domain was found to interfere with GKAP binding. In contrast, N-terminal sequences that precede the first PDZ domain in SAP97, facilitated GKAP binding via its association with the SH3 domain. Utilizing crystal structure data available for PDZ, SH3 and GUK domains, molecular models of SAP97 were generated. These models revealed that SAP97 can exist in a compact U-shaped conformation in which the N-terminal domain folds back and interacts with the SH3 and GUK domains. These models support the biochemical data and provide new insights into how intramolecular interactions may regulate the association of SAP97 with its binding partners.

Multi-PDZ domain protein MUPP1 is a cellular target for both adenovirus E4-ORF1 and high-risk papillomavirus type 18 E6 oncoproteins

A general theme that has emerged from studies of DNA tumor viruses is that otherwise unrelated oncoproteins encoded by these viruses often target the same important cellular factors. Major oncogenic determinants for human adenovirus type 9 (Ad9) and high-risk human papillomaviruses (HPV) are the E4-ORF1 and E6 oncoproteins, respectively, and although otherwise unrelated, both of these viral proteins possess a functional PDZ domain-binding motif that is essential for their transforming activity and for binding to the PDZ domain-containing and putative tumor suppressor protein DLG. We report here that the PDZ domain-binding motifs of Ad9 E4-ORF1 and high-risk HPV-18 E6 also mediate binding to the widely expressed cellular factor MUPP1, a large multi-PDZ domain protein predicted to function as an adapter in signal transduction. With regard to the consequences of these interactions in cells, we showed that Ad9 E4-ORF1 aberrantly sequesters MUPP1 within the cytoplasm of cells whereas HPV-18 E6 targets this cellular protein for degradation. These effects were specific because mutant viral proteins unable to bind MUPP1 lack these activities. From these results, we propose that the multi-PDZ domain protein MUPP1 is involved in negatively regulating cellular proliferation and that the transforming activities of two different viral oncoproteins depend, in part, on their ability to inactivate this cellular factor.

Identification and characterization of genes upregulated in cells transformed by v-Jun

The transcription factor Jun (c-Jun) functions as a recipient of extracellular growth signals and converts them into patterns of gene expression. An oncogenic variant of c-Jun was isolated from the acutely transforming retrovirus ASV17. Overexpression of this viral Jun (v-Jun) induces transformation of chicken embryo fibroblasts (CEF) in culture and fibrosarcomas in chickens. v-Jun is a constitutively active form of c-Jun and transforms cells presumably by deregulating the expression of specific target genes. In this report, we describe six genes whose transcripts are upregulated in v-Jun-transformed CEF. Three of these genes show homology to known mammalian genes, to MAP kinase phosphatase 2 (MKP-2), to reversion-induced LIM protein (RIL) and to cytokine-inducible SH2-containing protein (CIS). Northern blot analysis, using CEF infected with various Jun mutants or an estrogen-regulatable Jun chimera, revealed distinct induction patterns of individual targets by v-Jun. The chicken RIL homolog showed an expression pattern tightly correlated with the activity of v-Jun. Its expression is also transformation-dependent, suggesting a role for this gene in v-Jun transformation. The newly identified v-Jun targets can serve as molecular markers in the v-Jun transformation process. Oncogene (2000) 19, 3537 - 3545

Specific antagonists of NMDA receptors prevent osteoclast sealing zone formation required for bone resorption

N-Methyl-d-aspartate (NMDA) glutamate receptors, widely distributed in the nervous system, have recently been identified in bone. They are expressed and are functional in osteoclasts. In the present work, we have studied the effects of specific antagonists of NMDA receptors on osteoclast activation and bone resorption. Using an in vitro assay of bone resorption, we showed that several antagonists of NMDA receptors binding to different sites of the receptor inhibit bone resorption. Osteoclast activation requires adhesion to the bone surface, cytoskeletal reorganization and survival. We demonstrated by autoradiography that the specific NMDA receptor channel blocker, MK 801, binds to osteoclasts. This antagonist had no effect on osteoclast attachment to bone and did not induce osteoclast apoptosis. In contrast, MK 801 rapidly decreased the percentage of osteoclasts with actin ring structures that are associated with actively resorbing osteoclasts. These results suggest that NMDA receptors expressed by osteoclasts may be involved in adhesion-induced formation of the sealing zone required for bone resorption.

Molecular characterisation of two structurally distinct groups of human homers, generated by extensive alternative splicing

Homer proteins bind specifically to the C termini of the metabotropic glutamate receptor mGluR1alpha/a and mGluR5, play a role in their targeting and modulate their synaptic properties. We have discovered that extensive alternative splicing generates a family of 17 Homer proteins. These fall into two distinct groups of 12 "long" Homers, which all have a coiled-coil domain at their C termini, and five "short" Homers, which lack such a domain. All Homers contain the N-terminal sequence responsible for their binding to mGluR1alpha/a receptors and can be co-localised with the recombinantly expressed mGluR1alpha/a protein in HEK-293 cells. The existence of the long and the short variants of each of the Homer-1, Homer-2 and Homer-3 proteins reflects the fundamental principles of Homer functions.

Growth factor signaling pathways in vascular development

Recent research on the formation and maintenance of the vasculature in the embryo and in the adult has provided a greater understanding of the cellular signals involved in these processes. With this understanding comes the potential means of controlling vascularization in pathological situations such as tumorigenesis and wounding. For the purpose of this review, we will discuss the key receptor tyrosine kinases involved in vascular function and the molecules which relay signals downstream of receptor activation. The receptor tyrosine kinases discussed include the vascular endothelial cell growth factor receptors, Eph receptors, Tie1, and Tie2, all of which are expressed on vascular endothelial cells. We also discuss the roles of the platelet derived growth factor receptors which are expressed on vascular smooth muscle cells. While all of these receptor tyrosine kinases activate many similar effector molecules, some of the signals initiated appear to be distinct. This may explain, at least in part, how different receptor tyrosine kinases expressed in overlapping patterns on the developing vasculature, direct unique biological functions.

Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations

Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

The syndecans, tuners of transmembrane signaling

Syndecans, a family of transmembrane proteoglycans, are putative integrators of extracellular signals. The interaction of syndecans with extracellular ligands via particular motifs in their heparan sulfate chains, their clustering, association with particular cytoskeletal structures, binding to cytoplasmic effectors, and intracellular phosphorylation represent as many means to bring this role to a successful conclusion. In this review, we will briefly address the characteristics of syndecans as heparan sulfate proteoglycans (HSPGs) and focus mainly on the properties, binding interactions, and potential signaling functions of the cytoplasmic domains of these molecules.

Transmembrane proteins in the tight junction barrier

Three types of transmembrane proteins have been identified within the tight junction, but it remains to be determined how they provide the molecular basis for regulating the paracellular permeability for water, solutes, and immune cells. Several of these proteins localize specifically within the continuous cell-to-cell contacts of the tight junction. One of these, occludin, is a cell adhesion molecule that has been demonstrated to influence ion and solute permeability. The claudins are a family of four-membrane spanning proteins; unexpectedly, other members of this family have already been characterized without recognizing their relationship to tight junctions. Junction adhesion molecule, the most recently identified tight junction component, is a member of the Ig superfamily and influences the paracellular transmigration of immune cells. A plaque of cytoplasmic proteins under the junction may be responsible for scaffolding the transmembrane proteins, creating a link to the perijunctional actin cytoskeleton and transducing regulatory signals that control the paracellular barrier.

Recruitment of an alternatively spliced form of synaptojanin 2 to mitochondria by the interaction with the PDZ domain of a mitochondrial outer membrane protein

Synaptojanin 1 is an inositol 5'-phosphatase highly enriched in nerve terminals with a putative role in recycling of synaptic vesicles. We have previously described synaptojanin 2, which is more broadly expressed as multiple alternatively spliced forms. Here we have identified and characterized a novel mitochondrial outer membrane protein, OMP25, with a single PDZ domain that specifically binds to a unique motif in the C-terminus of synaptojanin 2A. This motif is encoded by the exon sequence specific to synaptojanin 2A. OMP25 mRNA is widely expressed in rat tissues. OMP25 is localized to the mitochondrial outer membrane via the C-terminal transmembrane region, with the PDZ domain facing the cytoplasm. Overexpression of OMP25 results in perinuclear clustering of mitochondria in transfected cells. This effect is mimicked by enforced expression of synaptojanin 2A on the mitochondrial outer membrane, but not by the synaptojanin 2A mutants lacking the inositol 5'-phosphatase domain. Our findings provide evidence that OMP25 mediates recruitment of synaptojanin 2A to mitochondria and that modulation of inositol phospholipids by synaptojanin 2A may play a role in maintenance of the intracellular distribution of mitochondria.

Molecular analysis of the X11-mLin-2/CASK complex in brain

A heterotrimeric complex containing Lin-10/X11alpha, Lin-2/CASK, and Lin-7 is evolutionarily conserved from worms to mammals. In Caenorhabditis elegans, it localizes Let-23, a receptor tyrosine kinase, to the basolateral side of vulval epithelium, a step crucial for proper vulva development. In mammals, the complex may also participate in receptor targeting in neurons. Accordingly, phosphotyrosine binding (PTB) and postsynaptic density-95/Discs large/Zona Occludens-1 domains found in X11alpha and mLin-2/CASK bind to cell-surface proteins, including amyloid precursor protein, neurexins, and syndecans. In this paper, we have further analyzed the X11alpha-mLin-2/CASK association that is mediated by a novel protein-protein interaction. We show that the mLin-2/CASK calmodulin kinase II (CKII) domain directly binds to a 63 amino acids peptide located between the Munc-18-1 binding site and the PTB domain in X11alpha. Ca2+/calmodulin association with mLin-2/CASK does not modify the X11alpha-mLin-2 interaction. A region containing the mLin-2/CASK guanylate kinase domain also interacts with X11alpha but with a lower affinity than the CKII domain. Immunostaining of X11alpha in the brain shows that the protein is expressed in areas shown previously to be positive for mLin-2/CASK staining. Together, our data demonstrate that the X11alpha-mLin-2 complex contacts many partners, creating a macrocomplex suitable for receptor targeting at the neuronal plasma membrane.

The tight junction protein ZO-1 establishes a link between the transmembrane protein occludin and the actin cytoskeleton

The tight junction protein ZO-1 belongs to a family of multidomain proteins known as the membrane-associated guanylate kinase homologs (MAGUKs). ZO-1 has been demonstrated to interact with the transmembrane protein occludin, a second tight junction-specific MAGUK, ZO-2, and F-actin, although the nature and functional significance of these interactions is poorly understood. To further elucidate the role of ZO-1 within the epithelial tight junction, we have introduced epitope-tagged fragments of ZO-1 into cultured MDCK cells and identified domains critical for the interaction with ZO-2, occludin, and F-actin. A combination of in vitro and in vivo binding assays indicate that both ZO-2 and occludin interact with specific domains within the N-terminal (MAGUK-like) half of ZO-1, whereas the unique proline-rich C-terminal half of ZO-1 cosediments with F-actin. Consistent with these observations, we found that a construct encoding the N-terminal half of ZO-1 is specifically associated with tight junctions, whereas the unique C-terminal half of ZO-1 is distributed over the entire lateral surface of the plasma membrane and other actin-rich structures. In addition, we have identified a 244-amino acid domain within the N-terminal half of ZO-1, which is required for the stable incorporation of ZO-1 into the junctional complex of polarized MDCK cells. These observations suggest that one functional role of ZO-1 is to organize components of the tight junction and link them to the cortical actin cytoskeleton.

Emerging roles of Dlg-like PDZ proteins in the organization of the NMDA-type glutamatergic synapse

A group of proteins found at cell-cell junctions have a common structural domain, called PDZ-a stretch of 80-90 amino acid residues initially identified in the three proteins PSD-95, Dlg, and ZO-1. This domain is found in various proteins from bacteria to mammals and is involved in protein-protein interaction. Recently, many proteins containing this domain were identified in the nervous system by molecular cloning and shown to interact with other synaptic proteins, including various transmitter receptors, ion channels, and signal transducers. These PDZ-containing proteins are mostly located near the synaptic membrane and are, therefore, speculated to transport associated proteins to the synapse and/or anchor them at the synaptic sites. Alternatively, as a single molecule often contains multiple PDZ domains that can interact with each other, it may cluster all these synaptic molecules and facilitate their signaling at synaptic sites. This review focuses on the best characterized PDZ-containing proteins that interact with N-methyl-D-aspartate (NMDA)-type glutamate receptors and discusses their functions in synaptic organization.

Crystal structure of the hCASK PDZ domain reveals the structural basis of class II PDZ domain target recognition

PDZ domain containing proteins assist formation of cell-cell junctions and localization of membrane protein receptors and ion channels. PDZ domains interact with the C-terminal residues of a particular target membrane protein. Based on their binding specificities and sequence homologies, PDZ domains fall into two classes. The first crystal structure of a class II PDZ domain, that of hCASK, has been solved by multi-wavelength anomalous dispersion phasing. Complex formation with the C-terminus of a neighboring non-crystallographically related PDZ domain reveals interactions between hCASK and its ligand. Class II PDZ domains differ from class I domains by formation of a second hydrophobic binding pocket. The C-terminal carboxylate binding loop of the PDZ domain is structurally conserved in both classes suggesting a generalized carboxylate binding motif (h-Gly-h) where h is a hydrophobic residue.

Organizing a functional junctional complex requires specific domains of the Drosophila MAGUK Discs large

Discs large (Dlg) was the first identified member of an increasingly important class of proteins called membrane-associated guanylate kinase homologs (MAGUKs), which are often concentrated at cell junctions and contain distinct peptide domains named PDZ1-3, SH3, HOOK, and GUK. Dlg is localized at and required for the formation of both septate junctions in epithelial cells and synaptic junctions in neurons. In the absence of Dlg, epithelia lose their organization and overgrow. We tested the functions of each domain of Dlg in vivo by constructing transgenic flies expressing altered forms of the protein. In the first set of experiments each domain was examined for its ability to correctly target an epitope-tagged Dlg to pre-existing septate junctions. Based on these results the Hook domain is necessary for localization of the protein to the cell membrane and the PDZ2 is required for restricting the protein to the septate junction. In the second set of experiments each domain was tested for its role in growth regulation and organization of epithelial structure. These results show that PDZ1 and GUK are apparently dispensable for function, PDZ2 and PDZ3 are required for growth regulation but not for epithelial structure, and SH3 and HOOK are essential for both aspects of function. The results demonstrate the functional modularity of Dlg and clarify the functions of individual MAGUK domains in regulating the structure and growth of epithelial tissue.