The PDZ Domain of the Guanine Nucleotide Exchange Factor PDZGEF Directs Binding to Phosphatidic Acid during Brush Border Formation

PDZGEF is a guanine nucleotide exchange factor for the small G protein Rap. It was recently found that PDZGEF contributes to establishment of intestinal epithelial polarity downstream of the kinase Lkb1. By binding to phosphatidic acid enriched at the apical membrane, PDZGEF locally activates Rap2a resulting in induction of brush border formation via a pathway that includes the polarity players TNIK, Mst4 and Ezrin. Here we show that the PDZ domain of PDZGEF is essential and sufficient for targeting PDZGEF to the apical membrane of polarized intestinal epithelial cells. Inhibition of PLD and consequently production of phosphatidic acid inhibitis targeting of PDZGEF to the plasma membrane. Furthermore, localization requires specific positively charged residues within the PDZ domain. We conclude that local accumulation of PDZGEF at the apical membrane during establishment of epithelial polarity is mediated by electrostatic interactions between positively charged side chains in the PDZ domain and negatively charged phosphatidic acid.     

Apical-basal polarity in Drosophila neuroblasts is independent of vesicular trafficking

The possession of apical-basal polarity is a common feature of epithelia and neural stem cells, so-called neuroblasts (NBs). In Drosophila, an evolutionarily conserved protein complex consisting of atypical protein kinase C and the scaffolding proteins Bazooka/PAR-3 and PAR-6 controls the polarity of both cell types. The components of this complex localize to the apical junctional region of epithelial cells and form an apical crescent in NBs. In epithelia, the PAR proteins interact with the cellular machinery for polarized exocytosis and endocytosis, both of which are essential for the establishment of plasma membrane polarity. In NBs, many cortical proteins show a strongly polarized subcellular localization, but there is little evidence for the existence of distinct apical and basolateral plasma membrane domains, raising the question of whether vesicular trafficking is required for polarization of NBs. We analyzed the polarity of NBs mutant for essential regulators of the main exocytic and endocytic pathways. Surprisingly, we found that none of these mutations affected NB polarity, demonstrating that NB cortical polarity is independent of plasma membrane polarity and that the PAR proteins function in a cell type-specific manner.     

Cooperative phosphoinositide and peptide binding by PSD-95/discs large/ZO-1 (PDZ) domain of polychaetoid, Drosophila zonulin

PDZ domains are well known protein-protein interaction modules that, as part of multidomain proteins, assemble molecular complexes. Some PDZ domains have been reported to interact with membrane lipids, in particular phosphatidylinositol phosphates, but few studies have been aimed at elucidating the prevalence or the molecular details of such interactions. We screened 46 Drosophila PDZ domains for phosphoinositide-dependent cellular localization and discovered that the second PDZ domain of polychaetoid (Pyd PDZ2) interacts with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) at the plasma membrane. Surface plasmon resonance binding experiments with recombinant protein established that Pyd PDZ2 interacts with phosphatidylinositol phosphates with apparent affinities in the micromolar range. Electrostatic interactions involving an extended positively charged surface of Pyd PDZ2 are crucial for the PtdIns(4,5)P(2)-dependent membrane interactions as shown by a combination of three-dimensional modeling, mutagenesis, binding, and localization studies. In vivo localization studies further suggested that both lipid and peptide binding contribute to membrane localization. We identified the transmembrane protein Crumbs as a Pyd PDZ2 ligand and probed the relation between peptide and PtdIns(4,5)P(2) binding. Contrary to the prevalent view on PDZ/peptide/lipid binding, we did not find competition between peptide and lipid ligands. Instead, preloading the protein with the 10-mer Crb3 peptide increased the apparent affinity of Pyd PDZ2 for PtdIns(4,5)P(2) 6-fold. Our results suggest that membrane localization of Pyd PDZ2 may be driven by a combination of peptide and PtdIns(4,5)P(2) binding, which raises the intriguing possibility that the domain may coordinate protein- and phospholipid-mediated signals.     

Formation of a Bazooka–Stardust complex is essential for plasma membrane polarity in epithelia

Apical–basal polarity in Drosophila melanogaster epithelia depends on several evolutionarily conserved proteins that have been assigned to two distinct protein complexes: the Bazooka (Baz)–PAR-6 (partitioning defective 6)–atypical protein kinase C (aPKC) complex and the Crumbs (Crb)–Stardust (Sdt) complex. These proteins operate in a functional hierarchy, in which Baz is required for the proper subcellular localization of all other proteins. We investigated how these proteins interact and how this interaction is regulated. We show that Baz recruits Sdt to the plasma membrane by direct interaction between the Postsynaptic density 95/Discs large/Zonula occludens 1 (PDZ) domain of Sdt and a region of Baz that contains a phosphorylation site for aPKC. Phosphorylation of Baz causes the dissociation of the Baz–Sdt complex. Overexpression of a nonphosphorylatable version of Baz blocks the dissociation of Sdt from Baz, causing phenotypes very similar to those of crb and sdt mutations. Our findings provide a molecular mechanism for the phosphorylation-dependent interaction between the Baz–PAR-3 and Crb complexes during the establishment of epithelial polarity.     

Drosophila PAR-1 and 14-3-3 inhibit Bazooka/PAR-3 to establish complementary cortical domains in polarized cells

PAR-1 kinases are required for polarity in diverse cell types, such as epithelial cells, where they localize laterally. PAR-1 activity is believed to be transduced by binding of 14-3-3 proteins to its phosphorylated substrates, but the relevant targets are unknown. We show that PAR-1 phosphorylates Bazooka/PAR-3 on two conserved serines to generate 14-3-3 binding sites. This inhibits formation of the Bazooka/PAR-6/aPKC complex by blocking Bazooka oligomerization and binding to aPKC. In epithelia, this complex localizes apically and defines the apical membrane, whereas Bazooka lacking PAR-1 phosphorylation/14-3-3 binding sites forms ectopic lateral complexes. Lateral exclusion by PAR-1/14-3-3 cooperates with apical anchoring by Crumbs/Stardust to restrict Bazooka localization, and loss of both pathways disrupts epithelial polarity. PAR-1 also excludes Bazooka from the posterior of the oocyte, and disruption of this regulation causes anterior-posterior polarity defects. Thus, antagonism of Bazooka by PAR-1/14-3-3 may represent a general mechanism for establishing complementary cortical domains in polarized cells.     

The prevalence and significance of PDZ domain-phosphoinositide interactions

PDZ domains predominate in multi-cellular organisms. They are ubiquitous protein-interaction modules recognizing short peptide sequences generally situated at the C-terminal end of plasma membrane proteins. They contribute to the formation and spatial confinement of protein complexes and thereby play an essential role in the control of cell signaling. Recent studies indicate that PDZ domains can also interact with phosphoinositides (PIPs), signaling lipids with key-roles in receptor signal transduction, membrane trafficking, cytoskeleton remodeling and nuclear processes. In particular the PDZ domains of syntenin-1 and syntenin-2 bind to phosphatidylinositol 4, 5-bisphosphate (PIP2) with high-affinity. Syntenin-1/PIP2 interaction is important for receptor cargo recycling and syntenin-2 plays a role in the organization of nuclear PIP2. In addition, other lower-affinity PDZ domain/PIPs interactions are documented. Here, we summarize and discuss the present knowledge about the occurrence, the biochemistry and the biology of PDZ domain-lipid interactions.     

Magi Is Associated with the Par Complex and Functions Antagonistically with Bazooka to Regulate the Apical Polarity Complex

The mammalian MAGI proteins play important roles in the maintenance of adherens and tight junctions. The MAGI family of proteins contains modular domains such as WW and PDZ domains necessary for scaffolding of membrane receptors and intracellular signaling components. Loss of MAGI leads to reduced junction stability while overexpression of MAGI can lead to increased adhesion and stabilization of epithelial morphology. However, how Magi regulates junction assembly in epithelia is largely unknown. We investigated the single Drosophila homologue of Magi to study the in vivo role of Magi in epithelial development. Magi is localized at the adherens junction and forms a complex with the polarity proteins, Par3/Bazooka and aPKC. We generated a Magi null mutant and found that Magi null mutants were viable with no detectable morphological defects even though the Magi protein is highly conserved with vertebrate Magi homologues. However, overexpression of Magi resulted in the displacement of Baz/Par3 and aPKC and lead to an increase in the level of PIP3. Interestingly, we found that Magi and Baz functioned in an antagonistic manner to regulate the localization of the apical polarity complex. Maintaining the balance between the level of Magi and Baz is an important determinant of the levels and localization of apical polarity complex.     

Extensions of PSD-95/discs large/ZO-1 (PDZ) domains influence lipid binding and membrane targeting of syntenin-1

Syntenin-1 is a PDZ protein involved in receptor recycling and clustering. Its two PDZ domains interact with various receptors and phosphoinositides, and are flanked by N- and C-terminal regions. Here, we report the identification of an autoinhibitory peptide stretch in the N-terminus that might be regulated by phosphorylation. We further establish that basic residues in the C-terminal region mediate electrostatic interactions with reconstituted liposomes and contribute to the plasma membrane targeting. Our study adds new components to the multi-dentate membrane targeting mechanism and highlights the role of N- and C-terminal PDZ extensions in the regulation of syntenin-1 plasma membrane localization.     

TIP-1 has PDZ scaffold antagonist activity

PDZ proteins usually contain multiple protein-protein interaction domains and act as molecular scaffolds that are important for the generation and maintenance of cell polarity and cell signaling. Here, we identify and characterize TIP-1 as an atypical PDZ protein that is composed almost entirely of a single PDZ domain and functions as a negative regulator of PDZ-based scaffolding. We found that TIP-1 competes with the basolateral membrane mLin-7/CASK complex for interaction with the potassium channel Kir 2.3 in model renal epithelia. Consequently, polarized plasma membrane expression of Kir 2.3 is disrupted resulting in pronounced endosomal targeting of the channel, similar to the phenotype observed for mutant Kir 2.3 channels lacking the PDZ-binding motif. TIP-1 is ubiquitously expressed, raising the possibility that TIP-1 may play a similar role in regulating the expression of other membrane proteins containing a type I PDZ ligand.     

Proteomics identification of sorting nexin 27 as a diacylglycerol kinase zeta-associated protein: new diacylglycerol kinase roles in endocytic recycling

Diacylglycerol kinase zeta is a member of the diacylglycerol kinase family of enzymes, which generate phosphatidic acid through diacylglycerol phosphorylation. In addition to the catalytic and cysteine-rich domains found in all diacylglycerol kinases, diacylglycerol kinase zeta has a MARCKS domain as well as a C-terminal region containing four ankyrin repeats and a PDZ-binding motif. Previous reports demonstrated that diacylglycerol kinase zeta interaction with several proteins is an important mechanism for modulating the localization and activity of this enzyme. Here we used a proteomics approach to search for novel diacylglycerol kinase zeta-interacting proteins and identified sorting nexin 27 (SNX27), a recently described member of a protein family involved in intracellular trafficking, which has a PDZ domain in addition to the phox homology domain characteristic of SNX proteins. Co-immunoprecipitation studies and two-hybrid analysis confirmed physical, PDZ-dependent association between SNX27 and diacylglycerol kinase zeta. Because diacylglycerol kinase zeta is expressed abundantly in T lymphocytes, we characterized SNX27 expression and subcellular localization in these cells. SNX27 co-localized with transferrin receptor-positive vesicles, pointing to its participation in T cell endocytic recycling. Expression of deletion mutants revealed that in addition to the phox homology domain the SNX27 PDZ domain contributed to vesicle localization of this protein, suggesting that interaction with diacylglycerol kinase zeta regulates SNX27 localization. Analysis of cells with RNA interference-mediated knockdown of diacylglycerol kinase zeta showed accelerated transferrin receptor exit from the lymphocyte endocytic recycling compartment back to the plasma membrane, further confirming diacylglycerol kinase zeta-dependent control of vesicle trafficking. These data support a previously unreported role for diacylglycerol kinase zeta in the modulation of membrane trafficking, which may also help to define SNX27 function.     

N-terminal polybasic motifs are required for plasma membrane localization of Galpha(s) and Galpha(q)

Heterotrimeric G proteins typically localize at the cytoplasmic face of the plasma membrane where they interact with heptahelical receptors. For G protein alpha subunits, multiple membrane targeting signals, including myristoylation, palmitoylation, and interaction with betagamma subunits, facilitate membrane localization. Here we show that an additional membrane targeting signal, an N-terminal polybasic region, plays a key role in plasma membrane localization of non-myristoylated alpha subunits. Mutations of N-terminal basic residues in alpha(s) and alpha(q) caused defects in plasma membrane localization, as assessed through immunofluorescence microscopy and biochemical fractionations. In alpha(s), mutation of four basic residues to glutamine was sufficient to cause a defect, whereas in alpha(q) a defect in membrane localization was not observed unless nine basic residues were mutated to glutamine or if three basic residues were mutated to glutamic acid. betagamma co-expression only partially rescued the membrane localization defects; thus, the polybasic region is also important in the context of the heterotrimer. Introduction of a site for myristoylation into the polybasic mutants of alpha(s) and alpha(q) recovered strong plasma membrane localization, indicating that myristoylation and polybasic motifs may have complementary roles as membrane targeting signals. Loss of plasma membrane localization coincided with defects in palmitoylation. The polybasic mutants of alpha(s) and alpha(q) were still capable of assuming activated conformations and stimulating second messenger production, as demonstrated through GST-RGS4 interaction assays, cAMP assays, and inositol phosphate assays. Electrostatic interactions with membrane lipids have been found to be important in plasma membrane targeting of many proteins, and these results provide evidence that basic residues play a role in localization of G protein alpha subunits.     

Caveolin scaffolding region and the membrane binding region of SRC form lateral membrane domains

Formation of domains by the membrane binding motifs of caveolin and src were studied in large unilamellar vesicles using fluorescence digital imaging microscopy. Caveolin, a major structural protein of caveolae, contains a scaffolding region (residues 82-101) that contributes to the binding of the protein to the plasma membrane. A caveolin peptide (82-101) corresponding to this scaffolding region induced the formation of membrane domains enriched in the acidic lipids phosphatidylserine and phosphatidylinositol-4,5-bisphosphate. Cholesterol, another predominant component of caveolae, was also enriched in these domains. Caveolae also contain many different signaling molecules including src family tyrosine kinases. Src proteins bind to the plasma membrane via a N-terminal myristate chain and a cluster of basic residues that can interact electrostatically with negatively charged lipids. A peptide corresponding to the src membrane binding motifs (residues myr-2-19) sequestered acidic lipids into lateral membrane domains. Both the src and the caveolin peptides colocalized together with acidic lipids in the domains. Control experiments show the domains are not the result of vesicle aggregation. Two-photon fluorescence correlation spectroscopy experiments suggest diffusion in the domains was slower, but the domains were dynamic. Protein kinase C phosphorylated src in its N-terminal membrane binding region; however, the caveolin scaffolding peptide inhibited this activity. Consequently, protein-induced membrane domains may affect cell signaling by organizing signal transduction components within the membrane and changing reaction rates.     

Role of the diacylglycerol kinase alpha-conserved domains in membrane targeting in intact T cells

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to phosphatidic acid, modifying the cellular levels of these two lipid mediators. Ten DGK isoforms, grouped into five subtypes, are found in higher organisms. All contain a conserved C-terminal domain and at least two cysteine-rich motifs of unknown function. DGKalpha is a type I enzyme that acts as a negative modulator of diacylglycerol-based signals during T cell activation. Here we studied the functional role of the DGKalpha domains using mutational analysis to investigate membrane binding in intact cells. We show that the two atypical C1 domains are essential for plasma membrane targeting of the protein in intact cells but unnecessary for catalytic activity. We also identify the C-terminal sequence of the protein as essential for membrane binding in a phosphatidic acid-dependent manner. Finally we demonstrate that, in the absence of the calcium binding domain, receptor-dependent translocation of the truncated protein is regulated by phosphorylation of Tyr(335). This functional study provides new insight into the role of the so-called conserved domains of this lipid kinase family and demonstrates the existence of additional domains that confer specific plasma membrane localization to this particular isoform.     

Proteome identification of binding-partners interacting with cell polarity protein Par3 in Jurkat cells

The evolutionarily conserved cell polarity protein Par3, a scaffold-like PDZreontaining protein, plays a critical role in the establishment and maintenance of epithelial cell polarity. Although the role of Par3 in establishing cell polarity in epithelial cells has been intensively explored, the function of Par3 in hematopoietic cells remains elusive. To address this issue, we generated GST-fusion proteins of Par3 PDZ domains. By combiningthe GST-pull-down approach with liquid chromatography-tandem mass spectrometry, we identified 10 potential novel binding proteins of PDZ domains of Par3 in Jurkat cells (a T-cell line). The interaction of Par3 with three proteins—nuclear transport protein importin-α4 and proteasome activators PA28β and PA28γ—was confirmed using in vitro binding assay, co-immunoprecipitation assay and immunofluorescence microscopy. Our results have the potential to uncover novel functions of the cell polarity protein Par3 in blood cells.     

Critical amino acid residues of maurocalcine involved in pharmacology, lipid interaction and cell penetration

Maurocalcine (MCa) is a 33-amino acid residue peptide that was initially identified in the Tunisian scorpion Scorpio maurus palmatus. This peptide triggers interest for three main reasons. First, it helps unravelling the mechanistic basis of Ca(2+) mobilization from the sarcoplasmic reticulum because of its sequence homology with a calcium channel domain involved in excitation-contraction coupling. Second, it shows potent pharmacological properties because of its ability to activate the ryanodine receptor. Finally, it is of technological value because of its ability to carry cell-impermeable compounds across the plasma membrane. Herein, we characterized the molecular determinants that underlie the pharmacological and cell-penetrating properties of maurocalcine. We identify several key amino acid residues of the peptide that will help the design of cell-penetrating analogues devoid of pharmacological activity and cell toxicity. Close examination of the determinants underlying cell penetration of maurocalcine reveals that basic amino acid residues are required for an interaction with negatively charged lipids of the plasma membrane. Maurocalcine analogues that penetrate better have also stronger interaction with negatively charged lipids. Conversely, less effective analogues present a diminished ability to interact with these lipids. These findings will also help the design of still more potent cell penetrating analogues of maurocalcine.     

Single-amino acid substitutions alter the specificity and affinity of PDZ domains for their ligands

PDZ domains are modular protein-protein interaction domains that bind to specific C-terminal sequences of membrane proteins and/or to other PDZ domains. Certain PDZ domains in PSD-95 and syntrophins interact with C-terminal peptide ligands and heterodimerize with the extended nNOS PDZ domain. The capacity to interact with nNOS correlates with the presence of a Lys residue in the carboxylate- binding loop of these PDZ domains. Here, we report that substitution of an Arg for Lys-165 in PSD-95 PDZ2 disrupted its interaction with nNOS, but not with the C terminus of the Shaker-type K(+) channel Kv1.4. The same mutation affected nNOS binding to alpha1- and beta1-syntrophin PDZ domains to a lesser extent, due in part to the stabilizing effect of tertiary interactions with the canonical nNOS PDZ domain. PDZ domains with an Arg in the carboxylate-binding loop do not bind nNOS; however, substitution with Lys or Ala was able to confer nNOS binding. Our results indicate that the carboxylate-binding loop Lys or Arg is a critical determinant of nNOS binding and that the identity of this residue can profoundly alter one mode of PDZ recognition without affecting another. We also analyzed the effects of mutating Asp-143, a residue in the alphaB helix of alpha1-syntrophin that forms a tertiary contact with the nNOS PDZ domain. This residue is important for both nNOS and C-terminal peptide binding and confers a preference for peptides with a positively charged residue at position -4. On this basis, we have identified the C terminus of the Kir2.1 channel as a possible binding partner for syntrophin PDZ domains. Together, our results demonstrate that single-amino acid substitutions alter the specificity and affinity of PDZ domains for their ligands.     

The Par-3 NTD adopts a PB1-like structure required for Par-3 oligomerization and membrane localization

The evolutionarily conserved Par-3/Par-6/aPKC complex is essential for the establishment and maintenance of polarity of a wide range of cells. Both Par-3 and Par-6 are PDZ domain containing scaffold proteins capable of binding to polarity regulatory proteins. In addition to three PDZ domains, Par-3 also contains a conserved N-terminal oligomerization domain (NTD) that is essential for proper subapical membrane localization and consequently the functions of Par-3. The molecular basis of NTD-mediated Par-3 membrane localization is poorly understood. Here, we describe the structure of a monomeric form of the Par-3 NTD. Unexpectedly, the domain adopts a PB1-like fold with both type-I and type-II structural features. The Par-3 NTD oligomerizes into helical filaments via front-to-back interactions. We further demonstrate that the NTD-mediated membrane localization of Par-3 in MDCK cells is solely attributed to its oligomerization capacity. The data presented in this study suggest that the Par-3 NTD is likely to facilitate the assembly of higher-order Par-3/Par-6/aPKC complex with increased avidities in targeting the complex to the subapical membrane domain and in binding to other polarity-regulating proteins.     

Mutagenesis and selection of PDZ domains that bind new protein targets

PDZ domains are a recently characterized protein-recognition module. In most cases, PDZ domains bind to the C-terminal end of target proteins and are thought thereby to link these target proteins into functional signaling networks. We report the isolation of artificial PDZ domains selected via a mutagenesis screen in vivo, each recognizing a different C-terminal peptide. We demonstrate that the PDZ domains isolated can bind selectively to their target peptides in vitro and in vivo. Two of the target peptides chosen are the C-terminal ends of two cellular transmembrane proteins with which no known PDZ domains have been reported to interact. By targeting these artificial PDZ domains to the nucleus, interacting target peptides were efficiently transported to the same subcellular localization. One of the isolated PDZ domains was tested and shown to be efficiently directed to the plasma membrane when cotransfected with the full-length transmembrane protein in mammalian cells. Thus, artificial PDZ domains can be engineered and used to target intracellular proteins to different subcellular compartments.     

The phosphorylation code is implicated in cell type-specific trafficking of PIN-FORMEDs

The subcellular polarity of PIN-FORMEDs (PINs) is critical for directional cell-to-cell transport of auxin. Phosphorylation of PIN proteins plays an important role in generating and maintaining specific PIN polarity. In a recent study, we have shown that phosphorylation in certain conserved residues of the PIN3 hydrophilic loop (HL) modulates its subcellular localization and polarity in a cell type-specific manner in different root tissues. Here, we additionally show that the phosphorylation code of PIN3-HL is operational for the determination of PIN3 polarity in the Arabidopsis guard cell and is deciphered in a differential way even in a single tobacco cell for the intracellular trafficking of PIN3. On the other hand, PIN3 localization often remained unaltered in certain cell types irrespective of its phosphorylation status. These findings, together with previous reports, indicate that the phosphorylation code of the PIN-HL along with cell type-specific factors, kinases, and developmental/environmental cues is instrumental for the PIN trafficking to different subcellular compartments as well as different plasma membrane domains.     

Proteins and cholesterol-rich domains

Biological membranes are composed of many molecular species of lipids and proteins. These molecules do not mix ideally. In the plane of the membrane components are segregated into domains that are enriched in certain lipids and proteins. Cholesterol is a membrane lipid that is not uniformly distributed in the membrane. Proteins play an important role in determining cholesterol distribution. Certain types of protein lipidation are known to cause the lipoprotein to sequester with cholesterol and to stabilize cholesterol-rich domains. However, proteins that are excluded from such domains also contribute to the redistribution of cholesterol. One of the motifs that favor interaction with cholesterol is the CRAC motif. The role of the CRAC motif of the gp41 fusogenic protein of HIV is discussed. The distribution of the multianionic lipid, phosphatidylinositol(4,5)bis-phosphate (PtnIns(4,5)P2), is also not uniform in cell membranes. This lipid has several functions in the cell, including a morphological role in determining the sites of attachment of the actin cytoskeleton to the plasma membrane. PtnIns(4,5)P2 is sequestered by proteins having clusters of cationic residues in their sequence. Certain proteins containing cationic clusters also contain moieties such as myristoylation or a CRAC segment that would also endow them with the ability to sequester to a cholesterol-rich domain. These proteins interact with PtnIns(4,5)P2 in a cholesterol-dependent manner forming domains that are enriched in both cholesterol and in PtnIns(4,5)P2 but can also be distinct from liquid-ordered raft-like domains.