PDZ domain proteins: scaffolds for signaling complexes

A forward genetic screen in Drosophila melanogaster to identify mutations affecting INAD localization in photoreceptor cells

In Drosophila photoreceptors, the multivalent PDZ protein INAD interacts with multiple signaling components and localizes complexes to the rhabdomere, a subcellular compartment specialized for phototransduction. Since this localization is critical for signaling, we conducted a genetic screen of the third chromosome for mutations that result in mislocalization of an INAD-GFP fusion protein. We identified seven mutant lines that fall into two complementation groups, idl (INAD localization)-A and idl-B. We show that idl-A mutants fail to complement with chaoptic (chp) mutants. Since chaoptin is a structural component of the rhabdomere, mislocalization of INAD may be a secondary effect of the retinal degeneration in chp and idl-A mutants. Genetic complementation and DNA sequencing reveal that the two idl-B mutants represent new alleles of trp, a gene encoding the major light-activated channel. The molecular change in each allele affects a highly conserved residue in either an ankyrin domain on the N-terminus or in the S6 transmembrane domain of TRP. These changes lead to the loss of TRP protein. TRP has previously been shown to anchor INAD in the rhabdomeres, therefore the independent identification of two trp alleles validates our screen for INAD-GFP localization. One possibility is that a limited number of proteins are required for localizing INAD-signaling complexes. A similar screen of the X and second chromosomes may be required to find the remaining players involved.

Bioinformatic search of plant microtubule-and cell cycle related serine-threonine protein kinases

A bioinformatic search was carried for plant homologues of human serine-threonine protein kinases involved in regulation of cell division and microtubule protein phosphorylation (SLK, PAK6, PAK7, MARK1, MAST2, TTBK1, TTBK2, AURKA, PLK1, PLK4 and PASK). A number of SLK, MAST2 and AURKA plant homologues were identified. The closest identified homologue of human AURKA kinase was a protein of unknown function, A7PY12/GSVIVT00026259001 from Vitis vinifera (herein named as "STALK", Serine-Threonine Aurora-Like Kinase). Analysis of STALK's three-dimensional structure confirmed its relationship to the subgroup of AURKA-like protein kinases.

Pathogenic Bacterial Proteins and their Anti-Inflammatory Effects in the Eukaryotic Host

Bacteria use multiple strategies to bypass the inflammatory responses in order to survive in the host cells. In this review, we discuss the mechanism of the bacerial proteins in inhibiting inflammation. We highlight the anti-inflammatory roles of the type three secretion proteins including Salmonella AvrA, Enteropathogenic Escherichia coli Cif, and Yersinia YopJ, Staphylococcus aureus extracellular adherence protein, and Chlamydia proteins. We also discuss the research progress on the structures of these anti-inflammatory bacterial proteins. The current therapeutic methods for diseases, such as inflammatory bowel diseases, sclerosis, lack influence on the course of chronic inflammation and infection. Therefore, based on the molecular mechanism of the anti-inflammatory bacterial proteins and their 3-Dimension structure, we can design new peptides or non-peptidic molecules that serve as anti-inflammatory drugs without the possible side effect of promoting bacterial infection.

Human papillomavirus and molecular considerations for cancer risk

Human papillomaviruses (HPVs) are a major cause of cancer globally, including cervical cancer. The HPV 'early' proteins, E6 and E7, are the chief oncoproteins involved in cancer progression. These oncoproteins are more highly expressed in high-grade dysplasias and invasive cancer coincident with reduced viral DNA replication and reduced production of infective progeny virions. The E6 and E7 oncoproteins interact with several cellular proteins-classically TP53 and RB1, respectively-leading to the degradation of several of these proteins, although all interactions do not necessarily result in the degradation of a cellular protein. HPV infection is also associated with viral and host DNA methylation changes, many of which also occur in cancer types not associated with HPV infection. The E6 and E7 interactions with cellular proteins and DNA methylation changes are associated with changes in the integrity of key cellular pathways that regulate genomic integrity, cell adhesion, the immune response, apoptosis, and cell cycle control. The alterations in key cellular pathways may provide useful biomarkers to improve the sensitivity of current cancer screening methods, such as the Papanicolaou test. This review provides a detailed summary of the interactions of E6 and E7 with cellular proteins and alterations in cellular DNA methylation associated with HPV infection. The importance of molecular biomarkers to the clinical setting, underserved populations, and general public health is discussed.

Multiple signals regulate phospholipase CBeta3 in human myometrial cells

Phospholipase CB3 (PLCB3) serine(1105) (S(1105)), a substrate for multiple protein kinases, represents a potential point of convergence of several signaling pathways in the myometrium. To explore this hypothesis, the regulation of PLCB3-S(1105) phosphorylation (P-S(1105)) was studied in immortalized and primary human myometrial cells. 8-[4-chlorophenylthio] (CPT)-cAMP and calcitonin gene-related peptide (CALCA) transiently increased P-S(1105). Relaxin also stimulated P-S(1105); this effect was partially blocked by the protein kinase A (PRKA) inhibitor, Rp-8-CPT-cAMPS. Oxytocin, which stimulates Galphaq-mediated pathways, also rapidly increased P-S(1105), as did prostaglandin F2alpha and ATP. Oxytocin-stimulated phosphorylation was blocked by protein kinase C (PRKC) inhibitor Go6976 and by pretreatment overnight with a phorbol ester. Cypermethrin, a PP2B phosphatase inhibitor, but not okadaic acid, a PP1/PP2A inhibitor, prolonged the effect of CALCA on P-S(1105), whereas the reverse was the case for the oxytocin-stimulated increase in P-S(1105). PLCB3 was the predominant PLC isoform expressed in the myometrial cells and PLCB3 short hairpin RNA constructs significantly attenuated oxytocin-stimulated increases in intracellular calcium. oxytocin-induced phosphatidylinositol (PI) turnover was inhibited by CPT-cAMP and okadaic acid, but was enhanced by pretreatment with Go6976. CPT-cAMP inhibited oxytocin-stimulated PI turnover in the presence of overexpressed PLCB3, but not overexpressed PLCB3-S(1105)A. These data demonstrate that both negative crosstalk from the cAMP/PRKA pathway and a negative feedback loop in the oxytocin/G protein/PLCB pathway involving PRKC operate in myometrial cells and suggest that different protein phosphatases predominate in mediating P-S(1105) dephosphorylation in these pathways. The integration of multiple signal components at the level of PLCB3 may be important to its function in the myometrium.

Human platelet 12-lipoxygenase, new findings about its activity, membrane binding and low-resolution structure

Human platelet 12-lipoxygenase (hp-12LOX, 662 residues+iron nonheme cofactor) and its major metabolite 12S-hydroxyeicosatetraenoic acid have been implicated in cardiovascular and renal diseases, many types of cancer and inflammatory responses. However, drug development is slow due to a lack of structural information. The major hurdle in obtaining a high-resolution X-ray structure is growing crystals, a process that requires the preparation of highly homogenous, reproducible and stable protein samples. To understand the properties of hp-12LOX, we have expressed and studied the behavior, function and low-resolution structure of the hp-12LOX His-tagged recombinant enzyme and its mutants in solution. We have found that it is a dimer easily converted into bigger aggregates, which are soluble/covalent-noncovalent/reversible. The heavier oligomers show a higher activity at pH 8, in contrast to dimers with lower activity showing two maxima at pH 7 and pH 8, indicating the existence of two different conformers. In the seven-point C-->S mutant, aggregation is diminished, activity has one broad peak at pH 8 and there is no change in specificity. Truncation of the N(t)-beta-barrel domain (PLAT, residues 1-116) reduces activity to approximately 20% of that shown by the whole enzyme, does not affect regio- or stereospecificity and lowers membrane binding by a factor of approximately 2. "NoPLAT" mutants show strong aggregation into oligomers containing six or more catalytic domains regardless of the status of the seven cysteine residues tested. Time-of-flight mass spectrometry suggests two arachidonic acid molecules bound to one molecule of enzyme. Small angle X-ray scattering studies (16 A resolution, chi approximately 1) suggest that two hp-12LOX monomers are joined by the catalytic domains, with the PLAT domains floating on the flexible linkers away from the main body of the dimer.

Degradation of the tumor suppressor PML by Pin1 contributes to the cancer phenotype of breast cancer MDA-MB-231 cells

Promyelocytic leukemia protein (PML) is an important regulator due to its role in numerous cellular processes including apoptosis, viral infection, senescence, DNA damage repair, and cell cycle regulation. Despite the role of PML in many cellular functions, little is known about the regulation of PML itself. We show that PML stability is regulated through interaction with the peptidyl-prolyl cis-trans isomerase Pin1. This interaction is mediated through four serine-proline motifs in the C terminus of PML. Binding to Pin1 results in degradation of PML in a phosphorylation-dependent manner. Furthermore, our data indicate that sumoylation of PML blocks the interaction, thus preventing degradation of PML by this pathway. Functionally, we show that in the MDA-MB-231 breast cancer cell line modulating levels of Pin1 affects steady-state levels of PML. Furthermore, degradation of PML due to Pin1 acts both to protect these cells from hydrogen peroxide-induced death and to increase the rate of proliferation. Taken together, our work defines a novel mechanism by which sumoylation of PML prevents Pin1-dependent degradation. This interaction likely occurs in numerous cell lines and may be a pathway for oncogenic transformation.

Light-induced recruitment of INAD-signaling complexes to detergent-resistant lipid rafts in Drosophila photoreceptors

Here, we reveal a novel feature of the dynamic organization of signaling components in Drosophila photoreceptors. We show that the multi-PDZ protein INAD and its target proteins undergo light-induced recruitment to detergent-resistant membrane (DRM) rafts. Reduction of ergosterol, considered to be a key component of lipid rafts in Drosophila, resulted in a loss of INAD-signaling complexes associated with DRM fractions. Genetic analysis demonstrated that translocation of INAD-signaling complexes to DRM rafts requires activation of the entire phototransduction cascade, while constitutive activation of the light-activated channels resulted in recruitment of complexes to DRM rafts in the dark. Mutations affecting INAD and TRP showed that PDZ4 and PDZ5 domains of INAD, as well as the INAD-TRP interaction, are required for translocation of components to DRM rafts. Finally, selective recruitment of phosphorylated, and therefore activatable, eye-PKC to DRM rafts suggests that DRM domains are likely to function in signaling, rather than trafficking.

Molecular characterization of angiomotin/JEAP family proteins: interaction with MUPP1/Patj and their endogenous properties

We have previously shown that MUPP1, which has an MRE domain and 13 PDZ domains, is expressed in epithelial cells and localize at tight junctions (TJs) and apical membranes. Using yeast two-hybrid screening, we found here that MUPP1 interacts with angiomotin (Amot), JEAP/Amot-like 1 and MASCOT/Amot-like 2, which we refer to as Amot/JEAP family proteins. PDZ2 and -3 were responsible for MUPP1's interaction with Amot and MASCOT, whereas only PDZ3 was responsible for its interaction with JEAP. All the Amot/JEAP family proteins also interacted with Patj, a close relative of MUPP1. The C-terminal PDZ-binding motives of the Amot/JEAP family were required for these interactions. We successfully generated specific antibodies for these proteins and analyzed the endogenous molecular properties of the family in parallel. Immunofluorescence microscopy of cultured epithelial cells showed that in subcellular distribution, the Amot/JEAP family proteins were indistinguishable; they were apparent at TJs as well as apical membranes, and mostly co-localized with MUPP1. They were also located at TJs in several mouse tissues, but each protein showed a distinct tissue distribution. In biochemical fractionation assays, the Amot/JEAP family behaved not as transmembrane but as peripheral membrane proteins. Unexpectedly, the PDZ-binding motives were not necessarily required for their localization to TJs, and dominant negative MUPP1 or Patj did not affect the localization of Amot/JEAP family proteins, suggesting that the interaction with MUPP1/Patj is not necessarily responsible for their proper subcellular distribution.

Vascular permeability in ocular disease and the role of tight junctions

Vascular permeability is closely linked with angiogenesis in a number of pathologies. In the retina, the normally well-developed blood-retinal barrier is altered in a host of eye diseases preceding or commensurate with angiogenesis. This review examines the literature regarding the tight junction complex that establishes the blood-retinal barrier focusing on the transmembrane proteins occludin and the claudin family and the membrane associated protein zonula occludens. The changes observed in these proteins associated with vascular and epithelial permeability is discussed. Finally, novel literature addressing the link between the tight junction complex and angiogenesis is considered.

Phospholipase Cbeta 3 mediates the scratching response activated by the histamine H1 receptor on C-fiber nociceptive neurons

Phospholipase Cbeta (PLCbeta) isozymes represent a family of molecules that link G protein-coupled receptors (GPCRs) to an intracellular signaling network. Here, we investigated the function of PLCbeta isozymes in sensory neurons by using mutant mice deficient for specific PLCbeta family members. Expression analysis indicated that PLCbeta3, one of the four isoforms, is predominantly expressed in a subpopulation of C-fiber nociceptors. A subset of these neurons expressed the histamine H1 receptor. Ca(2+) imaging studies revealed that PLCbeta3 specifically mediates histamine-induced calcium responses through the histamine H1 receptor in cultured sensory neurons. In line with this, we found that PLCbeta3(-/-) mice showed significant defects in scratching behavior induced by histamine; histamine-trifluoromethyl-toluidine (HTMT), a selective H1 agonist; and compound 48/80, a mast cell activator. These results demonstrate that PLCbeta3 is required to mediate "itch" sensation in response to histamine acting on the histamine H1 receptor in C-fiber nociceptive neurons.

Differential coupling of beta3A- and beta3B-adrenergic receptors to endogenous and chimeric Galphas and Galphai

Chimeric G proteins made by replacing the COOH-terminal heptapeptide of G(alpha)q with the COOH-terminal heptapeptide of G(alpha)s or G(alpha)i were used to assess the relative coupling of beta(3)-adrenergic receptor (beta(3)-AR) splice variants (beta(3A) and beta(3B)) to G(alpha)s and G(alpha)i. The G(alpha)q/s and G(alpha)q/i chimeras transformed the response to receptor activation from regulation of adenylyl cyclase to mobilization of intracellular calcium (Ca(2+)(i)). Complementary high-throughput and single-cell approaches were used to evaluate agonist-induced coupling of the receptor to the G protein chimeras. In cells stably transformed with rat beta(3)-AR, transfected with the G protein chimeras, and evaluated using a scanning fluorometer, beta(3)-AR-induced coupling to G(alpha)q/s produced a rapid eightfold increase in Ca(2+)(i) followed by a slow decay to levels 25% above baseline. G(alpha)q/i also linked rat beta(3)-AR to mobilization of Ca(2+)(i) in a similar time- and agonist-dependent manner, but the net 2.5-fold increase in Ca(2+)(i) was only 30% of the response obtained with G(alpha)q/s. Activation of the rat beta(3)-AR also increased GTP binding to endogenous G(alpha)i threefold in membranes from CHO cells stably transformed with the receptor. A complementary single-cell imaging approach was used to assess the relative coupling of mouse beta(3A)- and beta(3B)-AR to G(alpha)i under conditions established to produce equivalent agonist-dependent coupling of the receptor splice variants to G(alpha)q/s and to increases in intracellular cAMP through endogenous G(alpha)s. The beta(3A)- and beta(3B)-AR coupled equivalently to G(alpha)q/i, but the temporal patterns of Ca(2+)(i) mobilization indicated that coupling was significantly less efficient than coupling to G(alpha)q/s. Collectively, these findings indicate less efficient but equivalent coupling of beta(3A)- and beta(3B)-AR to G(alpha)i vs. G(alpha)s and suggest that differential expression of the splice variants would not produce local differences in signaling networks linked to beta(3)-AR activation.

Complex interaction of Drosophila GRIP PDZ domains and Echinoid during muscle morphogenesis

Glutamate receptor interacting protein (GRIP) homologues, initially characterized in synaptic glutamate receptor trafficking, consist of seven PDZ domains (PDZDs), whose conserved arrangement is of unknown significance. The Drosophila GRIP homologue (DGrip) is needed for proper guidance of embryonic somatic muscles towards epidermal attachment sites, with both excessive and reduced DGrip activity producing specific phenotypes in separate muscle groups. These phenotypes were utilized to analyze the molecular architecture underlying DGrip signaling function in vivo. Surprisingly, removing PDZDs 1-3 (DGripDelta1-3) or deleting ligand binding in PDZDs 1 or 2 convert DGrip to excessive in vivo activity mediated by ligand binding to PDZD 7. Yeast two-hybrid screening identifies the cell adhesion protein Echinoid's (Ed) type II PDZD-interaction motif as binding PDZDs 1, 2 and 7 of DGrip. ed loss-of-function alleles exhibit muscle defects, enhance defects caused by reduced DGrip activity and suppress the dominant DGripDelta1-3 effect during embryonic muscle formation. We propose that Ed and DGrip form a signaling complex, where competition between N-terminal and the C-terminal PDZDs of DGrip for Ed binding controls signaling function.

Diverse membrane‐associated proteins contain a novel SMP domain

We have analyzed the sequence of a mitochondrial integral membrane protein, Mdm12, and found that it forms the prototype for a novel domain, designated the SMP domain, that is common to an extended family of membrane-associated proteins. Comprehensive sequence searches using protein alignment models of SMP proteins were cross-validated by statistical resampling; providing strong support for these relationships. No consensus of 3-dimensional structure was reached upon threading sequences through known folds. SMP proteins are widespread amongst eukaryotic species with a particular enrichment in plants and features suggestive of species-specific functional variations. Members of 2 SMP families, the mitochore and tricalbin proteins, are essential components of protein complexes involved in mitochondrial inheritance and receptor endocytosis while a third SMP protein family, HT008, is associated with the Rvs161-Rvs167 complex, a known regulator of sphingolipid metabolism. In addition, HT008 and PDZK8 SMP proteins possess additional protein-protein interaction domains in domain architectures that are typical of protein scaffolds and adaptors. We therefore predict that the SMP domain is an important link between these distinct membrane-associated proteins and a key regulatory hub for unidentified global regulators.-Lee, I., Hong, W. Diverse membrane-associated proteins contain a novel SMP domain.

Assembly of a perivascular astrocyte protein scaffold at the mammalian blood–brain barrier is dependent on α-syntrophin

alpha-Syntrophin, a member of the dystrophin-associated protein complex, is required for proper localization of the water channel aquaporin-4 at the blood-brain barrier. Mice lacking alpha-syntrophin have reduced levels of aquaporin-4 in perivascular astroglial endfeet. Consequently, they exhibit reduced edema and infarct volume in brain trauma models and reduced K+ clearance from the neuropil, leading to increased seizure susceptibility. We have used the alpha-syntrophin null mice to investigate whether alpha-syntrophin is required for proper localization of other components of the dystrophin complex at the blood-brain barrier. We find that alpha-syntrophin is required for the full recruitment of gamma2-syntrophin and alpha-dystrobrevin-2 to glial endfeet in adult cerebellum. In contrast, the localization of beta1- and beta2-syntrophin and alpha-dystrobrevin-1 at the blood-brain barrier is not dependent on the presence of alpha-syntrophin. The localization patterns of alpha-dystrobrevin-1 and -2 in wild type cerebellum are strikingly different; while alpha-dystrobrevin-1 is present in glial endfeet throughout the cerebellum, alpha-dystrobrevin-2 is restricted to glial endfeet in the granular layer alone. Finally, we show that the enrichment of dystrophin in glial endfeet depends on the presence of alpha-syntrophin. This finding is the first demonstration that dystrophin localization is dependent on syntrophin. Since the localization of gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin is contingent on alpha-syntrophin, we conclude that alpha-syntrophin is a central organizer of the astrocyte dystrophin complex, an important molecular scaffold for localization of aquaporin-4 at the blood-brain barrier.

The interaction between the adaptor protein APS and Enigma is involved in actin organisation

APS (adaptor protein with PH and SH2 domains) is an adaptor protein phosphorylated by several tyrosine kinase receptors including the insulin receptor. To identify novel binding partners of APS, we performed yeast two-hybrid screening. We identified Enigma, a PDZ and LIM domain-containing protein that was previously shown to be associated with the actin cytoskeleton. In HEK 293 cells, Enigma interacted specifically with APS, but not with the APS-related protein SH2-B. This interaction required the NPTY motif of APS and the LIM domains of Enigma. In NIH-3T3 cells that express the insulin receptor, Enigma and APS were partially co-localised with F-actin in small ruffling structures. Insulin increased the complex formation between APS and Enigma and their co-localisation in large F-actin containing ruffles. While in NIH-3T3 and HeLa cells the co-expression of both Enigma and APS did not modify the actin cytoskeleton organisation, expression of Enigma alone led to the formation of F-actin clusters. Similar alteration in actin cytoskeleton organisation was observed in cells expressing both Enigma and APS with a mutation in the NPTY motif. These results identify Enigma as a novel APS-binding protein and suggest that the APS/Enigma complex plays a critical role in actin cytoskeleton organisation.

Pleiotrophin stimulates tyrosine phosphorylation of β-adducin through inactivation of the transmembrane receptor protein tyrosine phosphatase β/ζ

Pleiotrophin (PTN the protein, Ptn the gene) signals through a unique mechanism; it inactivates the tyrosine phosphatase activity of its receptor, the transmembrane receptor protein tyrosine phosphatase (RPTP)beta/zeta, and increases tyrosine phosphorylation of the substrates of RPTPbeta/zeta through the continued activity of a yet to be described protein tyrosine kinase(s) in PTN-stimulated cells. We have now found that the cytoskeletal protein beta-adducin interacts with the intracellular domain of RPTPbeta/zeta in a yeast two-hybrid system, that beta-adducin is a substrate of RPTPbeta/zeta, that beta-adducin is phosphorylated in tyrosine in cells not stimulated by PTN, and that tyrosine phosphorylation of beta-adducin is sharply increased in PTN-stimulated cells, suggesting that beta-adducin is a downstream target of and regulated by the PTN/RPTPbeta/zeta signaling pathway. beta-Catenin was the first downstream target of the PTN/RPTPbeta/zeta signaling pathway to be identified; these data thus also suggest that PTN coordinately regulates steady state levels of tyrosine phosphorylation of the important cytoskeletal proteins beta-adducin and beta-catenin and, through PTN-stimulated tyrosine phosphorylation, beta-adducin may contribute to the disruption of cytoskeletal structure, increased plasticity, and loss of homophilic cell-cell adhesion that are the consequences of PTN stimulation of cells and a characteristic feature of different malignant cells with mutations that activate constitutive expression of the endogenous Ptn gene.

Frodo proteins: modulators of Wnt signaling in vertebrate development

The Frodo/dapper (Frd) proteins are recently discovered signaling adaptors, which functionally and physically interact with Wnt and Nodal signaling pathways during vertebrate development. The Frd1 and Frd2 genes are expressed in dynamic patterns in early embryos, frequently in cells undergoing epithelial-mesenchymal transition. The Frd proteins function in multiple developmental processes, including mesoderm and neural tissue specification, early morphogenetic cell movements, and organogenesis. Loss-of-function studies using morpholino antisense oligonucleotides demonstrate that the Frd proteins regulate Wnt signal transduction in a context-dependent manner and may be involved in Nodal signaling. The identification of Frd-associated factors and cellular targets of the Frd proteins should shed light on the molecular mechanisms underlying Frd functions in embryonic development and in cancer.

Evolution of metazoan cell junction proteins: the scaffold protein MAGI and the transmembrane receptor tetraspanin in the demosponge Suberites domuncula

Until recently the positioning of the sponges (phylum Porifera) within the metazoan systematics was hampered by the lack of molecular evidence for the existence of junctional structures in the surface cell layers. In this study two genes related to the tight junctions are characterized from the demosponge Suberites domuncula: tetraspanin (SDTM4SF), a cell surface receptor, and MAGI (SDMAGI), a MAGUK (membrane-associated guanylate kinase homologue) protein. Especially the MAGI protein is known in other metazoan animal phyla to exist exclusively in tight junctions. The characteristic domains of MAGI proteins (six PDZ domains, two WW domains, and a truncated guanylate kinase motif) are conserved in the sponge protein. The functional analysis of SDMAGI done by in situ hybridization shows its expression in the surface epithelial layers (exopinacoderm and endopinacoderm). Northern blot studies reveal that expression of SDMAGI and SDTM4SF increases after formation of the pinacoderm layer in the animals as well as in primmorphs. These results support earlier notions that sponges contain junctional structures. We conclude that sponges contain epithelia whose cells are organized by cell junctions.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Binding proteins from alternative scaffolds

The use of so-called protein scaffolds for the generation of novel binding proteins via combinatorial engineering has recently emerged as a powerful alternative to natural or recombinant antibodies. This concept requires an extraordinary stable protein architecture tolerating multiple substitutions or insertions at the primary structural level. With respect to broader applicability it should involve a type of polypeptide fold which is observed in differing natural contexts and with distinct biochemical functions, so that it is likely to be adaptable to novel molecular recognition purposes. The quickly growing number of approaches can be classified into three groups: carrier proteins for the display of single variegated loops, scaffolds providing rigid elements of secondary structure, and protein frameworks supporting a group of conformationally variable loops in a fixed spatial arrangement. Generally, such artificial receptor proteins should be based on monomeric and small polypeptides that are robust, easily engineered, and efficiently produced in inexpensive prokaryotic expression systems. Today, progress in protein library technology allows for the parallel development of immunoglobulin (Ig) as well as scaffold-based affinity reagents. Both biomolecular tools have the potential to complement each other, thus expanding the possibility to find an affinity reagent suitable for a given application. The repertoire of protein scaffolds hitherto recruited for combinatorial protein engineering purposes will probably be further expanded in the future, including both additional natural proteins and de novo designed proteins, contributing to the collection of libraries available at present. In this review both the structural features and the practical use of scaffold proteins will be discussed and exemplified.

Translocation of protein kinase C-betaII in astrocytes requires organized actin cytoskeleton and is not accompanied by synchronous RACK1 relocation

Protein kinase C (PKC)-betaII is the most abundant PKC isoform in astrocytes. Upon activation, this isoform of PKC translocates from the cytosol to the plasma membrane (PM). In this study, we investigated in astrocytes the modality of PKC-betaII translocation as far as the participation of the receptor for activated C kinase-1 (RACK1) and the requirement for intact cytoskeleton in the process. In astrocytes, Western blots and immunocytochemistry coupled to confocal microscopic quantitative analysis showed that after 5 min of phorbol-12-myristate-13-acetate (PMA) exposure, native PKC-betaII, but not PKC-betaI, is relocated efficiently from the cytosol to the PM. Translocation of PKC-betaII was not associated with synchronous RACK1 relocation. Furthermore, the quantity of PM-associated PKC-betaII that co-immunoprecipitated with PM-bound RACK1 increased following PMA exposure, indicating a post activation binding of the two proteins in the PM. Because RACK1 and PKC-betaII relocation seemed not to be synchronous, we hypothesized that an intermediate interaction with the cytoskeleton was taking place. In fact, we were able to show that pharmacological disruption of actin-based cytoskeleton greatly deranged PKC-betaII translocation to the PM. The requirement for intact actin cytoskeleton was specific, because depolymerization of tubulin had no effect on the ability of the kinase to translocate to the PM. These results indicate that in astrocytes, RACK1 and PKC-betaII synchronous relocation is not essential for relocation of PKC-betaII to the PM. In addition, we show for the first time that the integrity of the actin cytoskeleton plays a specific role in PKC-betaII movements in these cells. We hypothesize that in glial cells, rapidly occurring changes of actin cytoskeleton arrangement may be involved in the fast reprogramming of PKC targeting to specific PM location to phosphorylate substrates in different cellular locations.

Stimulation by Parathyroid Hormone of a NHERF-1-assembled Complex Consisting of the Parathyroid Hormone I Receptor, Phospholipase Cβ, and Actin Increases Intracellular Calcium in Opossum Kidney Cells

Parathyroid hormone (PTH) binds its cognate G-protein-coupled receptor (PTH1R) and signals through both adenylyl cyclase and phospholipase C (PLC). C-terminal determinants of the PTH1R interact with the Na+/H+ exchanger regulatory factor 1 (NHERF-1) by binding the first of two PDZ (psd95, discs-large, ZO-1) domains. Compared with wild-type opossum kidney (OK) cells, OKH cells, a sub-clone, do not display PTH-mediated increases of [Ca2+]i and express NHERF-1 at markedly lower levels. Stable expression of NHERF-1 in the OKH parent (OKH-N1) restores the PTH-mediated increase of [Ca2+]i that arises from an influx of extracellular calcium and is both PLC-dependent and pertussis toxin-sensitive. From a morphological perspective, NHERF-1 and the PTH1R co-localize to apical patches of OKH-N1 cells, an expression pattern that is absent in OKH cells and depends on a direct NHERF-1-PTH1R interaction in OKH-N1 cells. Actin and PLCbeta1 and -beta3 co-localize with NHERF-1 and the PTH1R in OKH-N1 cell apical patches. Actin is also an integral component of the NHERF-1-assembled complex because cytochalasin D disrupts apical localization of both NHERF-1 and the PTH1R and inhibits the PTH-mediated increase of [Ca2+]i. Expression of the first PDZ domain of NHERF-1 acts as a dominant-negative interactor by blocking apical localization of the PTH1R and inhibiting PTH-elicited increases of [Ca2+]i. Thus, NHERF-1 assembles a signaling complex in the apical domains of OK cells that contains the PTH1R, PLCbeta, and the actin cytoskeleton. Disruption of this complex blocks the PTH mediated increases of intracellular calcium.

Type I collagen-mediated changes in gene expression and function of prostate cancer cells

In this study, cDNA microarrays were used to characterize gene expression changes elicited in prostate cancer cells by plating them on type I collagen. The results clearly reveal changes in the expression of genes associated with cellular signaling, cellular metabolism, gene transcription and gene translation which are indicative of cells that are actively proliferating. Together these results suggest that these changes in the gene expression profiles mediated by type I collagen may influence the proliferative capacity of prostate cancer cells in the bone microenvironment and facilitate development of prostate cancer bone metastases. Additionally, the microarray approach provides an invaluable tool to determine and track changes in gene expression in numerous disease states including prostate cancer. This technology is certain to facilitate discovery of new therapeutic gene targets.

Molecular effects of volume expansion on the renal sodium phosphate cotransporter

BACKGROUND

Volume Expansion (VE) results in both natriuresis and a phosphaturia. In previous studies, Sprague-Dawley rats were infused with a modified saline solution. The expansion procedure resulted in a 70% increase in the phosphorylation of a 72 kDa proximal tubular brush border membrane (BBM) protein. In recent experiments, Sprague-Dawley rats were subjected to the same short term VE. For both control and VE animals, brush border membrane vesicles (BBMV) were obtained.

METHODS AND RESULTS

Mass spectrometry of 3 proteins in the size range of our phosphoprotein resulted in the identification of ezrin/villin2, moesin, and PDZ domain-containing 1 (PDZ-dc1). Diphor-1 (currently renamed PDZ-dc1) is involved in regulation of the type II Na/Pi cotransporter. Ezrin and moesin are membrane-cytoskeletal linking proteins that are involved in the regulation of the sodium-hydrogen exchanger (NHE3) via interactions with another PDZ protein identified as sodium-hydrogen exchanger regulatory factor (EBP50, NHERF). Ezrin, moesin, and PDZ-dc1 protein levels were not increased following short term VE. Two-dimensional electrophoresis of our phosphorylated BBM proteins, followed by MALDI/MS analysis resulted in the identification of a protein mixture containing ezrin/moesin, alkaline phosphatase, and an unknown protein. Based on Western and immunoprecipitation data for ezrin, moesin, and PDZ-dc1 we believe that it is unlikely that our phosphoprotein is any of these 3 proteins. Parallels between NHE3 regulation (through EBP50/ERM proteins) and Na/Pi cotransporter regulation (through PDZ-dc1/ERM proteins) may be drawn.

CONCLUSION

These changes in proximal Na/Pi cotransport may involve a signal transduction cascade including PDZ-dc1, ezrin, moesin, our phosphoprotein, and possibly other proteins.

Dynamic behavior of paired claudin strands within apposing plasma membranes

The tight junction (TJ) strand is a linear proteinaceous polymer spanning plasma membranes, and each TJ strand associates laterally with another TJ strand in the apposing membranes of adjacent cells to form "paired" TJ strands. Claudins have been identified as the major constituents of TJ strands, and when exogenously expressed in L fibroblasts, they polymerize into paired strands, which are morphologically similar to paired TJ strands in epithelia. Here, we show that a fusion protein of GFP with claudin-1 can also form similar paired strands in L fibroblasts, allowing us to directly observe individual paired claudin strands in live cells in real time. These paired strands showed more dynamic behavior than expected; they were occasionally broken and annealed, and dynamically associated with each other in both an end-to-side and side-to-side manner. Through this behavior of individual paired claudin strands, the network of strands was reorganized dynamically. Furthermore, fluorescence recovery after photobleaching analyses revealed that claudin molecules were not mobile within paired strands. Although these observations are not necessarily representative of TJ strands per se in epithelial cells, they provide important information on the structural and kinetic properties of TJ strands in situ with significant implications for barrier function of TJs.

Structure of Cdc42 in a complex with the GTPase-binding domain of the cell polarity protein, Par6

Cdc42 is a small GTPase that is required for cell polarity establishment in eukaryotes as diverse as budding yeast and mammals. Par6 is also implicated in metazoan cell polarity establishment and asymmetric cell divisions. Cdc42.GTP interacts with proteins that contain a conserved sequence called a CRIB motif. Uniquely, Par6 possesses a semi-CRIB motif that is not sufficient for binding to Cdc42. An adjacent PDZ domain is also necessary and is required for biological effects of Par6. Here we report the crystal structure of a complex between Cdc42 and the Par6 GTPase-binding domain. The semi-CRIB motif forms a beta-strand that inserts between the four strands of Cdc42 and the three strands of the PDZ domain to form a continuous eight-stranded sheet. Cdc42 induces a conformational change in Par6, detectable by fluorescence resonance energy transfer spectroscopy. Nuclear magnetic resonance studies indicate that the semi-CRIB motif of Par6 is at least partially structured by the PDZ domain. The structure highlights a novel role for a PDZ domain as a structural scaffold.

Targeted disruption of the PDZK1 gene by homologous recombination

Proteins containing PDZ domains are involved in a large number of biological functions, including protein scaffolding, organization of ion channels, and signal transduction. We recently identified a novel PDZ domain-containing protein, PDZK1, that is selectively expressed in normal tissues, where it is associated and colocalized with MAP17, a small 17-kDa membrane-associated protein; cMOAT, an organic anion transporter implicated in multidrug resistance; and the type IIa Na/Pi cotransporter. The protein cluster formed by PDZK1, MAP17, and cMOAT is upregulated in a significant number of human carcinomas originating in the colon, breast, lung, and kidney. In order to better define the function of PDZK1 in the protein cluster and its potential role in the organization of ion channels, we generated a PDZK1 knockout mouse. While PDZK1-deficient mice developed normally, did not display any gross phenotypic abnormalities, and were fecund, lack of PDZK1 resulted in modulation of expression of selective ion channels in the kidney, as well as increased serum cholesterol levels. However, no significant redistribution of proteins known to interact with PDZK1, such as MAP17, cMOAT, and the type IIa Na/Pi cotransporter, was observed. The absence of a more significant phenotype in PDZK1-deficient mice may be due to functional compensation by other PDZ domain-containing proteins, which could be instrumental in determining the location of interacting proteins such as ion channels and other membrane-associated proteins in defined areas of the plasma membrane.

The cytoplasmic tail of large conductance, voltage- and Ca2+-activated K+ (MaxiK) channel is necessary for its cell surface expression

The large conductance, voltage- and Ca(2+)-activated K(+) channel (MaxiK) is expressed in several renal segments and functions in cell volume regulation and flow-mediated K(+) secretion. Previously, we cloned two MaxiK channel isoforms, named rbslo1 and rbslo2, from rabbit renal cells. rbslo1 has a 58-amino acid insertion after the S8 hydrophobic domain, whereas rbslo2 is truncated and cannot be activated. Here we use the sequence differences between the two variants to examine their plasma membrane processing. Plasma membrane localization of rbslo1 and 2 expressed in HEK293 cells was assayed by electrophysiology, immunocytochemistry, and biochemistry studies. Consistent with its functional silence, rbslo2 localized primarily within the cytoplasm, presumably in the endoplasmic reticulum and Golgi region. Coexpression with MaxiK beta subunits did not alter the cellular localization of either rbslo1 or rbslo2. When rbslo1 and 2 are cotransfected in non-polarized cells, they colocalized primarily within the cell with only rbslo1 detected at the plasma membrane. When transfected into polarized, medullary-thick ascending limb (mTAL) cells, rbslo1 is expressed at the apical membrane whereas the majority of rbslo2 localized throughout the cytoplasm. Given the high degree of similarity between the two isoforms, we conclude that the cytoplasmic tail of rbslo1 is important for the cell surface expression of MaxiK channels.

Phospholipase C-beta 2 interacts with mitogen-activated protein kinase kinase 3

Phospholipase C (PLC)-beta enzymes (isoenzymes beta 1-beta 4) are activated by G protein subunits, leading to the generation of intracellular messengers which mobilize calcium and activate protein kinase C. It has recently been recognized that these enzymes interact with and are regulated by proteins other than G proteins. Using the yeast two-hybrid technique to screen a leukocyte library we identified mitogen-activated protein kinase kinase 3 (MKK3) as a partner of PLC-beta 2. The interaction was confirmed by co-immunoprecipitation assays which indicated that MKK3 interacts with PLC-beta 2, but not with other PLC-betas. PLC-beta 2 interacted weakly with MKK6, which is related to MKK3, but not with the other MKK3 tested. The region of PLC-beta 2 involved in the interaction with MKK3 was mapped to the C-terminus of PLC-beta 2. p38MAPK also co-immunoprecipitated with PLC-beta 2. The data suggest that PLC-beta 2 serves an unappreciated role assembling components of the p38MAPK signaling module.

Identification of functional PDZ domain binding sites in several human proteins

TIP-15 was previously identified as a cellular protein that can bind to the C-terminal end of the HTLV-1 Tax protein via its two PDZ domains. The sequence of the N-terminal part of TIP-15 is identical to that of the synaptic protein PSD-95. Both proteins are likely to be produced from the same gene by alternative splicing. Whereas expression of the PSD-95 mRNA was detected only with brain RNAs, that of TIP-15 was detected with RNAs from thymus, brain, skeletal muscle and Jurkat cells. The TIP-15 protein exhibits an apparent molecular weight of 40 kD and is weakly expressed in T cell lines. A two-hybrid screen performed with TIP-15 as bait revealed the presence of a PDZ binding site (PDZ-BS) in the following proteins: Lysyl tRNA synthetase, 6-phosphogluconolactonase (6-GPL), Stress-activated protein kinase 3 (SAPK3), NET-1, Diacylglycerol kinase zeta, MTMR1, MCM7, and hSec8. The sequence at the C-terminal ends of these proteins matches the X-S/T-X-V-COOH consensus previously defined for PDZ-BSs, with the exception of 6-GPL and SAPK3 which include a leucine as the C-terminal residue. For Lysyl tRNA synthetase, NET1, MTMR1 and hSec8, binding to TIP-15 was confirmed by co-immunoprecipitation experiments performed with the extracts of transfected COS7 cells. These results show the existence of functional PDZ-BSs in these proteins, but future studies will be necessary to establish whether or not TIP-15 represents a physiological partner. The significance of the presence of a PDZ-BS in these various proteins is discussed with respect to their function.

Multi-PDZ domain protein 1 (MUPP1) is concentrated at tight junctions through its possible interaction with claudin-1 and junctional adhesion molecule

Claudins, most of which end in valine at their COOH termini, constitute tight junction (TJ) strands, suggesting that TJ strands strongly attract PDZ-containing proteins. Indeed, ZO-1, -2, and -3, each of which contains three PDZ domains, were shown to directly bind to claudins. Using the yeast two-hybrid system, we identified ZO-1 and MUPP1 (multi-PDZ domain protein 1) as binding partners for the COOH terminus of claudin-1. MUPP1 has been identified as a protein that contains 13 PDZ domains, but it has not been well characterized. In vitro binding assays with recombinant MUPP1 confirmed the interaction between MUPP1 and claudin-1 and identified PDZ10 as the responsible domain for this interaction. A polyclonal antibody specific for MUPP1 was then generated. Immunofluorescence confocal microscopy as well as immunoelectron microscopy with this antibody revealed that in polarized epithelial cells MUPP1 was exclusively concentrated at TJs. Furthermore, in vitro binding and transfection experiments showed that junctional adhesion molecule, another TJ adhesion molecule, also bound to the PDZ9 domain of MUPP1. These findings suggested that MUPP1 is concentrated at TJs in epithelial cells through its binding to claudin and junctional adhesion molecule and that it may function as a multivalent scaffold protein that recruits various proteins to TJs.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

The carboxyl terminus of the prolactin-releasing peptide receptor interacts with PDZ domain proteins involved in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor clustering

PDZ domain proteins use the PDZ domain binding motif to bind to the C-terminal sequence of membrane proteins to help scaffold them and spatially organize the components of the intracellular signaling machinery. We have identified a sequence at the C terminus of a G protein-coupled receptor, the PrRP receptor, that shares similarities with the C-terminal sequence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R) subunits that interact with PDZ domain proteins. When coexpressed in human embryonic kidney 293 cells, PrRP receptor was able to coimmunoprecipitate the three PDZ domain proteins known to interact with AMPA receptors: glutamate receptor interacting protein (GRIP), AMPA binding protein (ABP), and protein that interacts with C-kinase (PICK1), but not the PDZ domain protein PSD-95, which does not interact with AMPA receptors. These interactions are sequence-selective as determined by mutagenesis. Furthermore, we show that PrRP receptor forms intracellular clusters when coexpressed with PICK1, and that this clustering effect is dependent on the interaction between the PICK1 PDZ domain and the last four amino acids of PrRP receptor. We found that PrRP receptor interaction with GRIP is not protein kinase C-regulated but may be regulated by other unidentified kinase because okadaic acid dramatically reduced GRIP interaction. By in situ hybridization, we show that the PrRP receptor is expressed in neurons that also express these PDZ domain proteins. We thus demonstrate that PrRP receptor interacts with the same PDZ domain proteins as the AMPA-Rs, raising the possibility that these two proteins could be scaffolded together at the synapse. These results may help to gain important insights into PrRP functions within the central nervous system.

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.

Scaffolding proteins organize multimolecular protein complexes for sensory signal transduction

Scaffolding proteins composed of protein-protein interaction domains have emerged as organizers of multiprotein complexes in diverse cellular compartments, including neuronal synapses, cell-cell junctions of epithelial cells, and the stimulus perceiving structures of sensory neurons. This review focuses on the INAD-assembled signalling complex of Drosophila photoreceptors, which organizes key components of the phototransduction cascade into a multiprotein signal transduction unit. The structure, the physiological consequences, and the assembly and targeting of the members of the INAD signalling complex will be described. In addition, the existence of signalling complexes in vertebrate photoreceptors, olfactory neurons and mechanosensitive hair cells will be discussed.

Intracellular and extracellular control of activin function by novel regulatory molecules

Activin signal transduction is regulated through multiple mechanisms. We have identified novel regulatory proteins that control activin functions either intracellularly or extracellularly. As intracellular molecules, PSD-95/Dlg/ZO-1 (PDZ) proteins that specifically associate with activin type II receptors (ActRIIs) were identified. We have named the molecules as activin receptor-interacting proteins (ARIPs). ARIP1 has two WW domains and five PDZ domains, associates not only with ActRIIs but also with Smads, and controls activin functions intracellularly in neuronal cells. Another ARIP we have found has only one PDZ domain, and is likely to be involved in intracellular trafficking and sorting of activin receptor complexes in the cell. As an extracellular regulatory protein, we have identified a novel follistatin-like protein, named follistatin-related gene (FLRG). Like follistatins, FLRG binds activins and bone morphogenetic proteins (BMPs) and controls their functions extracellularly. The mode of association of follistatin and FLRG with activins and their expression patterns are different, suggesting the distinct functions of follistatin and FLRG in vivo.

Sema4c, a transmembrane semaphorin, interacts with a post-synaptic density protein, PSD-95

Semaphorins are known to act as chemorepulsive molecules that guide axons during neural development. Sema4C, a group 4 semaphorin, is a transmembrane semaphorin of unknown function. The cytoplasmic domain of Sema4C contains a proline-rich region that may interact with some signaling proteins. In this study, we demonstrate that Sema4C is enriched in the adult mouse brain and associated with PSD-95 isoforms containing PDZ (PSD-95/DLG/ZO-1) domains, such as PSD-95/SAP90, PSD-93/chapsin110, and SAP97/DLG-1, which are concentrated in the post-synaptic density of the brain. In the neocortex, S4C is enriched in the synaptic vesicle fraction and Triton X-100 insoluble post-synaptic density fraction. Immunostaining for Sema4C overlaps that for PSD-95 in superficial layers I-IV of the neocortex. In neocortical culture, S4C is colocalized with PSD-95 in neurons, with a dot-like pattern along the neurites. Sema4C thus may function in the cortical neurons as a bi-directional transmembrane ligand through interacting with PSD-95.

The C. elegans gon-2 gene encodes a putative TRP cation channel protein required for mitotic cell cycle progression

The C. elegans gon-2 gene is required for the post-embryonic mitotic cell divisions of the gonadal precursor cells. A single major transcript of approximately 6.7 kb is derived from the gon-2 locus. This mRNA encodes a protein related to the TRP family of cation channels and has a high degree of similarity to several vertebrate genes, including melastatin. Mutant alleles of gon-2 affect evolutionarily conserved amino acid residues. Northern analyses suggest that gon-2 expression is not limited to gonadal tissues.

The roles of claudin superfamily proteins in paracellular transport

The claudin superfamily consists of at least 18 homologous proteins in humans. These proteins are important structural and functional components of tight junctions in paracellular transport. Complexed with two other integral transmembrane proteins, occludin and junctional adhesion molecule, claudins are located in both epithelial and endothelial cells in all tight junction-bearing tissues. Claudins interact directly with tight junction-specific, membrane-associated guanylate kinase homologues, ZO-1, ZO-2, and ZO-3, and indirectly with AF-6 and the myosin-binding molecule cingulin. These protein-protein interactions promote scaffolding of the tight junction transmembrane proteins and provide a link to the actin cytoskeleton for transducing regulatory signals to and from tight junctions. The distinct permeability properties observed in different epithelia and endothelia seemingly result from the restricted tissue expression, variability of the homopolymer and heteropolymer assembly, regulated transcription and translation, and the subcellular localization of claudin family proteins. Defects in claudins are causatively associated with a variety of human diseases, demonstrating that claudins play important roles in human physiology. In conditions where the cell adhesion function contributed by tight junctions is essential, such as in altered paracellular transport, in proliferative diseases, and during morphogenesis, the claudin superfamily of homologous proteins provides the molecular basis for the uniqueness of tight junctions and emerges as a new target for intervention.

Several E4 region functions influence mammary tumorigenesis by human adenovirus type 9

Among oncogenic adenoviruses, human adenovirus type 9 (Ad9) is unique in eliciting exclusively estrogen-dependent mammary tumors in rats and in not requiring viral E1 region transforming genes for tumorigenicity. Instead, studies with hybrid viruses generated between Ad9 and the closely related nontumorigenic virus Ad26 have roughly localized an Ad9 oncogenic determinant(s) to a segment of the viral E4 region containing open reading frame 1 (E4-ORF1), E4-ORF2, and part of E4-ORF3. Although subsequent findings have shown that E4-ORF1 codes for an oncoprotein essential for tumorigenesis by Ad9, it is not known whether other E4 region functions may similarly play a role in this process. We report here that new results with Ad9/Ad26 hybrid viruses demonstrated that the minimal essential Ad9 E4-region DNA sequences include portions of both E4-ORF1 and E4-ORF2. Investigations with Ad9 mutant viruses additionally showed that the E4-ORF1 protein and certain E4-ORF2 DNA sequences are necessary for Ad9-induced tumorigenesis, whereas the E4-ORF2 and E4-ORF3 proteins are not. In fact, the E4-ORF3 protein was found to antagonize this process. Also pertinent was that certain crucial nucleotide differences between Ad9 and Ad26 within E4-ORF1 and E4-ORF2 were found to be silent with respect to the amino acid sequences of the corresponding proteins. Furthermore, supporting a prominent role for the E4-ORF1 oncoprotein in Ad9-induced tumorigenesis, an E1 region-deficient Ad5 vector that expresses the Ad9 but not the Ad26 E4-ORF1 protein was tumorigenic in rats and, like Ad9, promoted solely mammary tumors. These findings argue that the E4-ORF1 oncoprotein is the major oncogenic determinant of Ad9 and that an undefined regulatory element(s) within the E4 region represents a previously unidentified second function likewise necessary for tumorigenesis by this virus.

Transmembrane transforming growth factor-alpha tethers to the PDZ domain-containing, Golgi membrane-associated protein p59/GRASP55

Transforming growth factor-alpha (TGF-alpha) and related proteins represent a family of transmembrane growth factors with representatives in flies and worms. Little is known about the transport of TGF-alpha and other transmembrane growth factors to the cell surface and its regulation. p59 was purified as a cytoplasmic protein, which at endogenous levels associates with transmembrane TGF-alpha. cDNA cloning of p59 revealed a 452 amino acid sequence with two PDZ domains. p59 is myristoylated and palmitoylated, and associates with the Golgi system, where it co-localizes with TGF-alpha. Its first PDZ domain interacts with the C-terminus of transmembrane TGF-alpha and select transmembrane proteins. p59 is the human homolog of GRASP55, which is structurally related to GRASP65. GRASP55 and GRASP65 have been shown to play a role in stacking of the Golgi cisternae in vitro. C-terminal mutations of transmembrane TGF-alpha, which decrease or abolish the interaction with p59, also strongly impair cell surface expression of TGF-alpha. Our observations suggest a role for membrane tethering of p59/GRASP55 to select transmembrane proteins, including TGF-alpha, in maturation and transport to the cell surface.

G-protein signaling: satisfying the basic necessities of life

Recent studies have shed light on the role of G-protein signaling, and in particular the regulatory RGS proteins, in behavioral adaptations of the round worm Caenorhabditis elegans; similar signaling pathways underlie analogous physiology and behaviors in mammals.

The PDZ protein TIP-1 interacts with the Rho effector rhotekin and is involved in Rho signaling to the serum response element

The human T-cell lymphotrophic virus, type 1 Tax protein can interact via its C terminus with various proteins including a PDZ domain. In this work, one of them, TIP-1, is characterized as a cytoplasmic 14-kDa protein mainly corresponding to one PDZ domain. A two-hybrid screen performed with TIP-1 as bait showed that it interacts with the human homologue of rhotekin that was previously identified in mice as a Rho effector. Both human and mouse rhotekins exhibit at their C termini the sequence QSPV-COOH that matches the X(S/T)XV-COOH consensus known for proteins recognizing PDZ domains. Mutation of the serine and valine residues to alanine impairs interaction of rhotekin with TIP-1. Transient expression experiments with a reporter construct including the c-Fos serum response element (SRE) showed that coexpression of TIP-1 with the constitutively active RhoA.V14 mutant and human rhotekin caused a strong activation of the SRE. A negative mutant of Rho, RhoA.N19, was unable to cooperate with TIP-1 and rhotekin. The positive effect of TIP-1 was also lost when the C terminus of rhotekin was mutated. These data show that the complex of active Rho with its effector rhotekin bound to TIP-1 produces in the cytoplasm a signal that triggers strong activation of the SRE.

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.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

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

Interactions between AMPA receptors and intracellular proteins

alpha-Amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors mediate most fast excitatory synaptic transmission in the mammalian CNS. They play a central role in synapse stabilisation and plasticity and their prolonged activation is potently neurotoxic. Developmental and activity-dependent changes in the functional synaptic expression of these receptors are subject to tight cellular regulation. The molecular and cellular mechanisms which control the postsynaptic insertion and arrangement of individual AMPA receptor variants are therefore the subject of intense investigation and in the last two years there has been significant progress towards elucidating some of the processes involved. Much of the new information has come from the application of the yeast two-hybrid assay, which has led to the discovery of a hitherto unexpected complexity of proteins which selectively interact with individual AMPA receptor subunits. These proteins have been implicated in the regulation of AMPA receptor post-translational modification, targeting and trafficking, surface expression and anchoring. The aim of this article is to present an overview of the major interacting proteins described so far and to place these in the context of how they may participate in the well ordered series of events controlling the cell biology of AMPA receptors.

CFTR Chloride Channels: Binding Partners and Regulatory Networks

The cystic fibrosis gene encodes a chloride channel (CFTR) that regulates transepithelial salt and water transport. Two classes of CFTR-binding proteins appear to link the opposing cytoplasmic tails of this channel to distinct regulatory networks. Such interactions may constitute new paradigms for modulating CFTR activity in health and disease.