The association of the C-terminal region of beta I sigma II spectrin to brain membranes is mediated by a PH domain, does not require membrane proteins, and coincides with a inositol-1,4,5 triphosphate binding site

ATP Consumption Is Coupled with Endocytosis in Exudated Neutrophils

Neutrophil energy metabolism during phagocytosis has been previously reported, and adenosine triphosphate (ATP) plays a crucial role in endocytosis. Neutrophils are prepared by intraperitoneal injection of thioglycolate for 4 h. We previously reported a system established for measuring particulate matter endocytosis by neutrophils using flow cytometry. In this study, we utilized this system to investigate the relationship between endocytosis and energy consumption in neutrophils. A dynamin inhibitor suppressed ATP consumption triggered by neutrophil endocytosis. In the presence of exogenous ATP, neutrophils behave differently during endocytosis depending on ATP concentration. The inhibition of ATP synthase and nicotinamide adenine dinucleotide phosphate oxidase but not phosphatidylinositol-3 kinase suppresses neutrophil endocytosis. The nuclear factor kappa B was activated during endocytosis and inhibited by I kappa B kinase (IKK) inhibitors. Notably, IKK inhibitors restored endocytosis-triggered ATP consumption. Furthermore, data from the NLR family pyrin domain containing three knockout mice suggest that inflammasome activation is not involved in neutrophil endocytosis or concomitant ATP consumption. To summarize, these molecular events occur via endocytosis, which is closely related to ATP-centered energy metabolism.

FAS: assessing the similarity between proteins using multi-layered feature architectures

MOTIVATION

Protein sequence comparison is a fundamental element in the bioinformatics toolkit. When sequences are annotated with features such as functional domains, transmembrane domains, low complexity regions or secondary structure elements, the resulting feature architectures allow better informed comparisons. However, many existing schemes for scoring architecture similarities cannot cope with features arising from multiple annotation sources. Those that do fall short in the resolution of overlapping and redundant feature annotations.

RESULTS

Here, we introduce FAS, a scoring method that integrates features from multiple annotation sources in a directed acyclic architecture graph. Redundancies are resolved as part of the architecture comparison by finding the paths through the graphs that maximize the pair-wise architecture similarity. In a large-scale evaluation on more than 10 000 human-yeast ortholog pairs, architecture similarities assessed with FAS are consistently more plausible than those obtained using e-values to resolve overlaps or leaving overlaps unresolved. Three case studies demonstrate the utility of FAS on architecture comparison tasks: benchmarking of orthology assignment software, identification of functionally diverged orthologs, and diagnosing protein architecture changes stemming from faulty gene predictions. With the help of FAS, feature architecture comparisons can now be routinely integrated into these and many other applications.

AVAILABILITY AND IMPLEMENTATION

FAS is available as python package: https://pypi.org/project/greedyFAS/.

PI(4,5)P2 diffuses freely in the plasma membrane even within high-density effector protein complexes

The lipid phosphatidyl-D-myo-inositol-4,5-bisphosphate [PI(4,5)P2] is a master regulator of plasma membrane (PM) function. Its effector proteins regulate transport, signaling, and cytoskeletal processes that define PM structure and function. How a single type of lipid regulates so many parallel processes is unclear. We tested the hypothesis that spatially separate PI(4,5)P2 pools associate with different PM complexes. The mobility of PI(4,5)P2 was measured using biosensors by single-particle tracking. We found that PM lipids including PI(4,5)P2 diffuse rapidly (∼0.3 µm2/s) with Brownian motion, although they spend one third of their time diffusing more slowly. Surprisingly, areas of the PM occupied by PI(4,5)P2-dependent complexes did not slow PI(4,5)P2 lateral mobility. Only the spectrin and septin cytoskeletons showed reduced PI(4,5)P2 diffusion. We conclude that even structures with high densities of PI(4,5)P2 effector proteins, such as clathrin-coated pits and focal adhesions, do not corral unbound PI(4,5)P2, questioning a role for spatially segregated PI(4,5)P2 pools in organizing and regulating PM functions.

Epitope mapping of an anti-diacylglycerol kinase delta monoclonal antibody DdMab-1

Diacylglycerol kinase δ (DGKδ) is a type II DGK, which catalyzes diacylglycerol phosphorylation to produce phosphatidic acid. DGKδ is expressed in several types of tissues and organs including the stomach, testis, bone marrow, and lymph node. Here, we established an anti-human DGKδ (hDGKδ) mAb, DdMab-1 (mouse IgG_2a, kappa), which is useful for Western blot analysis. We also introduced deletion or point mutations to hDGKδ, and performed western blotting to determine the binding epitope of DdMab-1. DdMab-1 reacted with the dN670 mutant, but not with the dN680 mutant, indicating that the N-terminus of the DdMab-1 epitope is mainly located between amino acids 670 and 680 of the protein. Further analysis using point mutants demonstrated that R675A, R678A, K679A, and K682A mutants were not detected, and V680A was only weakly detected by DdMab-1, indicating that Arg675, Arg678, Lys679, Val680 and Lys682 are important for binding of DdMab-1 to hDGKδ.

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DGKδ catalyzes diacylglycerol phosphorylation to produce phosphatidic acid.

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We established a novel anti-hDGKδ mAb, DdMab-1.

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DdMab-1 is useful for Western blot analysis.

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R675, R678, K679, V680, and K682 are important for binding of DdMab-1 to hDGKδ.

Insight into Mechanobiology: How Stem Cells Feel Mechanical Forces and Orchestrate Biological Functions

The cross-talk between stem cells and their microenvironment has been shown to have a direct impact on stem cells' decisions about proliferation, growth, migration, and differentiation. It is well known that stem cells, tissues, organs, and whole organisms change their internal architecture and composition in response to external physical stimuli, thanks to cells' ability to sense mechanical signals and elicit selected biological functions. Likewise, stem cells play an active role in governing the composition and the architecture of their microenvironment. Is now being documented that, thanks to this dynamic relationship, stemness identity and stem cell functions are maintained. In this work, we review the current knowledge in mechanobiology on stem cells. We start with the description of theoretical basis of mechanobiology, continue with the effects of mechanical cues on stem cells, development, pathology, and regenerative medicine, and emphasize the contribution in the field of the development of ex-vivo mechanobiology modelling and computational tools, which allow for evaluating the role of forces on stem cell biology.

Identification of a PH domain-containing protein which is localized to crystalloid bodies of Plasmodium ookinetes

Background

For the success of the malaria control and eradication programme it is essential to reduce parasite transmission by mosquito vectors. In the midguts of mosquitoes fed with parasite-infected blood, sexual-stage parasites fertilize to develop into motile ookinetes that traverse midgut epithelial cells and reside adjacent the basal lamina. Therefore, the ookinete is a promising target of transmission-blocking vaccines to break the parasite lifecycle in mosquito vectors. However, the molecular mechanisms of ookinete formation and invasion of epithelial cells have not been fully elucidated. A unique structure called the crystalloid body has been identified in the ookinete cytoplasm by electron microscopy, but its biological functions remain unclear.

Methods

A recombinant protein of a novel molecule, designated as crystalloid body specific PH domain-containing protein of Plasmodium yoelii (PyCryPH), was synthesized using a wheat germ cell-free system. Specific rabbit antibodies against PyCryPH were obtained to characterize the expression and localization of PyCryPH during sexual-stage parasite development. In addition, PyCryPH knockout parasites were generated by targeted gene disruption to examine PyCryPH function in mosquito-stage parasite development.

Results

Western blot and immunofluorescence assays using specific antibodies showed that PyCryPH is specifically expressed in zygotes and ookinetes. By immunoelectron microscopy it was demonstrated that PyCryPH is localized within crystalloid bodies. Parasites with a disrupted PyCryPH gene developed normally into ookinetes and formed oocysts on the basal lamina of midguts. In addition, the number of sporozoites residing in salivary glands was comparable to that of wild-type parasites.

Conclusions

CryPH, containing a signal peptide and PH domain, is predominantly expressed in zygotes and ookinetes and is localized to crystalloid bodies in P. yoelii. CryPH accumulates in vesicle-like structures prior to the appearance of typical crystalloid bodies. Unlike other known crystalloid body localized proteins, CryPH does not appear to have a multiple domain architecture characteristic of the LAP/CCp family proteins. Although CryPH is highly conserved among Plasmodium, Babesia, Theileria, and Cryptosporidium, PyCryPH is dispensable for the development of invasive ookinetes and sporozoites in mosquito bodies.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2617-6) contains supplementary material, which is available to authorized users.

The Effect of SH2B1 Variants on Expression of Leptin- and Insulin-Induced Pathways in Murine Hypothalamus

OBJECTIVE

We aimed to determine the effect of human SH2B1 variants on leptin and insulin signaling, major regulators of energy homeostasis, on the RNA level.

METHODS

We analyzed the expression of infrequent alleles of seven SH2B1 variants (Arg67Cys, Lys150Arg, Thr175Ala, Thr343Met, Thr484Ala, Ser616Pro and Pro689Leu) in response to insulin or leptin cell stimulation. Two of these were identified in own mutation screens, the others were predicted to be deleterious or to serve as controls. The variants were analyzed in a homologous system of mouse hypothalamic cells. Changes in expression of downstream genes were measured. Student’s t-test for independent samples was applied and effect sizes using Cohen’s d were calculated.

RESULTS

In 34 of 54 analyzed genes involved in leptin (JAK/STAT or AKT) signaling, variants nominally changed expression. The expression of three genes was considerably increased (p values ≤ 0.001: Gbp2b (67Cys; d = 25.11), Irf9 (689Leu; d = 44.65) and Isg15 (150Arg; d = 20.35)). Of 32 analyzed genes in the insulin signaling pathway, the expression of 10 genes nominally changed (p ≤ 0.05), three resulted in p values ≤ 0.01 ( Cap1 (150Arg; d = 7.48), Mapk1 (343Met; d = –6.80) and Sorbs1 (689Leu; d = 7.82)).

CONCLUSION

The increased expression of genes in leptin (JAK/STAT or AKT) signaling implies that the main mode of action for human SH2B1 mutations might affect leptin signaling rather than insulin signaling.

Midbody localization of vinexin recruits rhotekin to facilitate cytokinetic abscission

Vinexin is a SH3 domain-containing adaptor protein that has diverse roles in cell adhesion, signal transduction, gene regulation and stress granule assembly. In this study, we found that vinexin localizes at the midbody during cell division and facilitates cytokinesis. Knockdown of vinexin in HeLa cells delayed the mitotic cell cycle progression and increased the time of cell abscission and the failure to resolve the cytoplasmic bridge. Midbody-localized vinexin is essential for recruiting rhotekin to this structure for cytokinesis because overexpression of a vinexin mutant without a rhotekin-binding motif or knockdown of rhotekin also impaired cytokinetic abscission and increased the number of cells arrested at the midbody stage. Aberrant expression of vinexin and rhotekin in various cancers has been implicated to promote metastasis because of their functions in cell adhesion and signaling. Our findings reveal a novel role of vinexin and rhotekin in cytokinetic abscission and provide another perspective of how both molecules may affect oncogenic transformation via this fundamental cell cycle process.

Vav1: A Dr. Jekyll and Mr. Hyde protein--good for the hematopoietic system, bad for cancer

Many deregulated signal transducer proteins are involved in various cancers at numerous stages of tumor development. One of these, Vav1, is normally expressed exclusively in the hematopoietic system, where it functions as a specific GDP/GTP nucleotide exchange factor (GEF), strictly regulated by tyrosine phosphorylation. Vav was first identified in an NIH3T3 screen for oncogenes. Although the oncogenic form of Vav1 identified in the screen has not been detected in clinical human tumors, its wild-type form has recently been implicated in mammalian malignancies, including neuroblastoma, melanoma, pancreatic, lung and breast cancers, and B-cell chronic lymphocytic leukemia. In addition, it was recently identified as a mutated gene in human cancers of various origins. However, the activity and contribution to cancer of these Vav1 mutants is still unclear. This review addresses the physiological function of wild-type Vav1 and its activity as an oncogene in human cancer. It also discusses the novel mutations identified in Vav1 in various cancers and their potential contribution to cancer development as oncogenes or tumor suppressor genes.

Two sites of action for PLD2 inhibitors: The enzyme catalytic center and an allosteric, phosphoinositide biding pocket

Phospholipase D (PLD) has been implicated in many physiological functions, such as chemotaxis and phagocytosis, as well as pathological functions, such as cancer cell invasion and metastasis. New inhibitors have been described that hamper the role of PLD in those pathologies but their site of action is not known. We have characterized the biochemical and biological behavior of the PLD1/2 dual inhibitor 5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), and the specific PLD2 inhibitor, N-[2-[1-(3-Fluorophenyl)-4-oxo-1,3,-8-triazaspiro[4.5]dec-8-yl]ethyl]-2-naphthalenecarboxamide (NFOT), and found that both FIPI and NFOT are mixed-kinetics inhibitors. Mutagenesis studies indicate that FIPI binds at S757 of PLD2, which is within the HKD2 catalytic site of the enzyme, whereas NFOT binds to PLD2 at two different sites, one being at S757/S648 and another to an allosteric site that is a natural site occupied by PIP2 (R210/R212). This latter site, along with F244/L245/L246, forms a hydrophobic pocket in the PH domain. The mechanism of action of FIPI is a direct effect on the catalytic site (and as such inhibits both PLD1 and PLD2 isoforms), whereas PLD2 affects both the catalytic site (orthosteric) and blocks PIP2 binding to PLD2 (allosteric), which negates the natural enhancing role of PIP2. Moreover, NFOT prevents cell invasion of cancer cells, which does not occur in cells overexpressing PLD2-F244A/L245A/L246A, or PLD2-R210A/R212A, or PLD2-S757/S648 mutants. This study provides new specific knowledge of enzyme regulation and mechanisms of activation and inhibition of PLD2 that are necessary to understand its role in cell signaling and to develop new inhibitors for cancer cell invasion and metastasis.

The role of Pak-interacting exchange factor-β phosphorylation at serines 340 and 583 by PKCγ in dopamine release

Protein kinase C (PKC) has been implicated in the control of neurotransmitter release. The AS/AGU rat, which has a nonsense mutation in PKCγ, shows symptoms of parkinsonian syndrome, including dopamine release impairments in the striatum. Here, we found that the AS/AGU rat is PKCγ-knock-out (KO) and that PKCγ-KO mice showed parkinsonian syndrome. However, the PKCγ substrates responsible for the regulated exocytosis of dopamine in vivo have not yet been elucidated. To identify the PKCγ substrates involved in dopamine release, we used PKCγ-KO mice and a phosphoproteome analysis. We found 10 candidate phosphoproteins that had decreased phosphorylation levels in the striatum of PKCγ-KO mice. We focused on Pak-interacting exchange factor-β (βPIX), a Cdc42/Rac1 guanine nucleotide exchange factor, and found that PKCγ directly phosphorylates βPIX at Ser583 and indirectly at Ser340 in cells. Furthermore, we found that PKC phosphorylated βPIX in vivo. Classical PKC inhibitors and βPIX knock-down (KD) significantly suppressed Ca(2+)-evoked dopamine release in PC12 cells. Wild-type βPIX, and not the βPIX mutants Ser340 Ala or Ser583 Ala, fully rescued the decreased dopamine release by βPIX KD. Double KD of Cdc42 and Rac1 decreased dopamine release from PC12 cells. These findings indicate that the phosphorylation of βPIX at Ser340 and Ser583 has pivotal roles in Ca(2+)-evoked dopamine release in the striatum. Therefore, we propose that PKCγ positively modulates dopamine release through β2PIX phosphorylation. The PKCγ-βPIX-Cdc42/Rac1 phosphorylation axis may provide a new therapeutic target for the treatment of parkinsonian syndrome.

Ankyrin-G palmitoylation and βII-spectrin binding to phosphoinositide lipids drive lateral membrane assembly

Ankyrin-G and βII-spectrin colocalize at sites of cell-cell contact in columnar epithelial cells and promote lateral membrane assembly. This study identifies two critical inputs from lipids that together provide a rationale for how ankyrin-G and βII-spectrin selectively localize to Madin-Darby canine kidney (MDCK) cell lateral membranes. We identify aspartate-histidine-histidine-cysteine 5/8 (DHHC5/8) as ankyrin-G palmitoyltransferases required for ankyrin-G lateral membrane localization and for assembly of lateral membranes. We also find that βII-spectrin functions as a coincidence detector that requires recognition of both ankyrin-G and phosphoinositide lipids for its lateral membrane localization. DHHC5/8 and βII-spectrin colocalize with ankyrin-G in micrometer-scale subdomains within the lateral membrane that are likely sites for palmitoylation of ankyrin-G. Loss of either DHHC5/8 or ankyrin-G-βII-spectrin interaction or βII-spectrin-phosphoinositide recognition through its pleckstrin homology domain all result in failure to build the lateral membrane. In summary, we identify a functional network connecting palmitoyltransferases DHHC5/8 with ankyrin-G, ankyrin-G with βII-spectrin, and βII-spectrin with phosphoinositides that is required for the columnar morphology of MDCK epithelial cells.

Pleckstrin homology domain containing 6 protein (PLEKHA6) polymorphisms are associated with psychopathology and response to treatment in schizophrenic patients

Pleckstrin homology domain (PH domain) comprises approximately 120 amino acids and is integrated in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton. This domain can bind phosphatidylinositol (3,4,5)-triphosphate and phosphatidylinositol (4,5)-biphosphate and proteins such as the βγ-subunits of heterotrimeric G proteins and protein kinase C. Associations with psychiatric diseases have not been investigated yet. To identify genes involved in response to antipsychotics, mice were treated with haloperidol (1mg/kg, n = 11) or saline (n = 12) for one week. By analyzing microarray data, we observed an increase of pleckstrin homology domain containing 6 (PLEKHA6) gene expression. Furthermore, we genotyped 263 schizophrenic patients, who were treated monotherapeutically with different antipsychotics within randomized-controlled trials. Psychopathology was measured weekly using the PANSS for a minimum of four and a maximum of twelve weeks. Correlations between PANSS subscale scores at baseline and PANSS improvement scores after four weeks of treatment and genotypes were calculated by using a linear model for all investigated SNPs. We found associations between four PLEKHA6 polymorphisms (rs17333933 (T/G), rs3126209 (C/T), rs4951338 (A/G) and rs100900571 (T/C)) and different PANSS subscales at baseline. Furthermore two different polymorphisms (rs7513240 (T/C), rs4951353 (A/G)) were found to be associated with therapy response in terms of a significant correlation with different PANSS improvement subscores after four weeks of antipsychotic treatment. Our observation of an association between genetic polymorphisms of a protein of the PH domain and psychopathology data in schizophrenic patients might be indicative for an involvement of PLEKHA6 in the pathophysiology of schizophrenia and the therapy response towards antipsychotics.

A genome- and phenome-wide association study to identify genetic variants influencing platelet count and volume and their pleiotropic effects

Platelets are enucleated cell fragments derived from megakaryocytes that play key roles in hemostasis and in the pathogenesis of atherothrombosis and cancer. Platelet traits are highly heritable and identification of genetic variants associated with platelet traits and assessing their pleiotropic effects may help to understand the role of underlying biological pathways. We conducted an electronic medical record (EMR)-based study to identify common variants that influence inter-individual variation in the number of circulating platelets (PLT) and mean platelet volume (MPV), by performing a genome-wide association study (GWAS). We characterized genetic variants associated with MPV and PLT using functional, pathway and disease enrichment analyses; we assessed pleiotropic effects of such variants by performing a phenome-wide association study (PheWAS) with a wide range of EMR-derived phenotypes. A total of 13,582 participants in the electronic MEdical Records and GEnomic network had data for PLT and 6,291 participants had data for MPV. We identified five chromosomal regions associated with PLT and eight associated with MPV at genome-wide significance (P < 5E-8). In addition, we replicated 20 SNPs [out of 56 SNPs (α: 0.05/56 = 9E-4)] influencing PLT and 22 SNPs [out of 29 SNPs (α: 0.05/29 = 2E-3)] influencing MPV in a published meta-analysis of GWAS of PLT and MPV. While our GWAS did not find any new associations, our functional analyses revealed that genes in these regions influence thrombopoiesis and encode kinases, membrane proteins, proteins involved in cellular trafficking, transcription factors, proteasome complex subunits, proteins of signal transduction pathways, proteins involved in megakaryocyte development, and platelet production and hemostasis. PheWAS using a single-SNP Bonferroni correction for 1,368 diagnoses (0.05/1368 = 3.6E-5) revealed that several variants in these genes have pleiotropic associations with myocardial infarction, autoimmune, and hematologic disorders. We conclude that multiple genetic loci influence interindividual variation in platelet traits and also have significant pleiotropic effects; the related genes are in multiple functional pathways including those relevant to thrombopoiesis.

Focal adhesions function as a mechanosensor

Focal adhesions (FAs) are complex plasma membrane-associated macromolecular assemblies that engage with the surrounding extracellular matrix (ECM) via integrin receptors and physically connect with the actin cytoskeleton through the recruitment of numerous FA-associated proteins. FAs undergo a maturation process, which is known to be induced by biochemical or physical cues, to grow and change composition. Varying FA size, distribution, dynamics, and compositions during maturation process is required for transducing the specific signaling networks that reflect the requirements of a cell to sense, adapt, and response to a variety of the environments. While advances have been demonstrated in understanding how important FAs are in mediating various biological processes, less is known about how FA composition is regulated and coordinately transduces the specific signals in mediating the distinct biological outcomes, especially cell migration.

Vps1 in the late endosome-to-vacuole traffic

Vacuolar protein sorting 1 (Vps1), the yeast homolog to human dynamin, is a GTP hydrolyzing protein, which plays an important role in protein sorting and targeting between the Golgi and late endosomal compartments. In this study, we assessed the functional significance of Vps1 in the membrane traffic towards the vacuole. We show here that vps1 delta cells accumulated FM4-64 to a greater extent than wild-type (WT))cells, suggesting slower endocytic degradation traffic toward the vacuole. In addition, we observed that two endosome-to-vacuole traffic markers, DsRed-FYVE and Ste2-GFP, were highly accumulated in Vps1-deficient cells, further supporting Vps1's implication in efficient trafficking of endocytosed materials to the vacuole. Noteworthy, a simultaneous imaging analysis in conjunction with FM4-64 pulse-chase experiment further revealed that Vps1 plays a role in late endosome to the vacuole transport. Consistently, our subcellular localization analysis showed that Vps1 is present at the late endosome. The hyperaccumulation of endosomal intermediates in the vps1 mutant cells appears to be caused by the disruption of integrity of HOPS tethering complexes, manifested by mislocalization of Vps39 to the cytoplasm. Finally, we postulate that Vps1 functions together with the Endosomal Sorting Complex Required for Transport (ESCRT) complex at the late endosomal compartments, based on the observation that the double mutants, in which VPS1 along with singular ESCRT I, II and III genes have been disrupted, exhibited synthetic lethality. Together, we propose that Vps1 is required for correct and efficient trafficking from the late endosomal compartments to the vacuole.

Nesprins: from the nuclear envelope and beyond

Nuclear envelope spectrin-repeat proteins (Nesprins), are a novel family of nuclear and cytoskeletal proteins with rapidly expanding roles as intracellular scaffolds and linkers. Originally described as proteins that localise to the nuclear envelope (NE) and establish nuclear-cytoskeletal connections, nesprins have now been found to comprise a diverse spectrum of tissue specific isoforms that localise to multiple sub-cellular compartments. Here, we describe how nesprins are necessary in maintaining cellular architecture by acting as essential scaffolds and linkers at both the NE and other sub-cellular domains. More importantly, we speculate how nesprin mutations may disrupt tissue specific nesprin scaffolds and explain the tissue specific nature of many nesprin-associated diseases, including laminopathies.

Mechanotransduction at focal adhesions: integrating cytoskeletal mechanics in migrating cells

Focal adhesions (FAs) are complex plasma membrane-associated macromolecular assemblies that serve to physically connect the actin cytoskeleton to integrins that engage with the surrounding extracellular matrix (ECM). FAs undergo maturation wherein they grow and change composition differentially to provide traction and to transduce the signals that drive cell migration, which is crucial to various biological processes, including development, wound healing and cancer metastasis. FA-related signalling networks dynamically modulate the strength of the linkage between integrin and actin and control the organization of the actin cytoskeleton. In this review, we have summarized a number of recent investigations exploring how FA composition is affected by the mechanical forces that transduce signalling networks to modulate cellular function and drive cell migration. Understanding the fundamental mechanisms of how force governs adhesion signalling provides insights that will allow the manipulation of cell migration and help to control migration-related human diseases.

Phosphoinositide lipids and the Legionella pathogen vacuole

Subversion of vesicle trafficking is vital for intracellular survival of Legionella pneumophila within host cells. L. pneumophila produces several type IV-translocated effector proteins that modify components of the phagosomal membrane, in particular the phosphoinositide (PI) lipids. Within eukaryotic cells PIs co-define subcellular compartments and membrane dynamics. The generation, half-life, and localization of PI lipids are not only tightly regulated by the host cell, but also targeted and modulated by a number of L. pneumophila effectors. These effectors either anchor to PIs, directly modify the lipids, or recruit PI-metabolizing enzymes to the LCV membrane. Together, PI-subverting L. pneumophila effectors act jointly to promote the formation of a replication-permissive niche inside the host.

Novel mechanism for negatively regulating Rho-kinase (ROCK) signaling through Coronin1B protein in neuregulin 1 (NRG-1)-induced tumor cell motility

Although many mechanisms that activate ROCK are known, corresponding negative regulatory mechanisms required for cytoskeletal plasticity are poorly understood. We have discovered that Coronin1B is a novel attenuator of ROCK signaling. We initially identified Coronin1A in a proteomics screen for ROCK2-binding proteins, and here we demonstrate that Coronin1A/B bind directly to ROCK2 through its PH (Pleckstrin Homology) domain. The consequence of the ROCK2-Coronin1B interaction was tested and revealed that increased expression of Coronin1B inhibited, whereas knockdown of Coronin1B stimulated, phosphorylation of the ROCK substrate myosin light chain phosphatase and subsequently, myosin light chain. Thus, Coronin1B is a previously unrecognized inhibitor of ROCK signaling to myosin. Furthermore, we found that the phosphatase Slingshot IL (SSH1L) was required for Coronin1B to inhibit ROCK signaling. To test the significance of this novel mechanism in tumor cell motility, we investigated its role in neuregulin 1 (NRG-1)-induced cell scattering. Importantly, we found that attenuation of the ROCK signaling by Coronin1B was required for NRG-1 stimulated scattering. Our data support a model in which Coronin1B fine-tunes ROCK signaling to modulate myosin activity, which is important for tumor cell motility.

Membrane shaping by the Bin/amphiphysin/Rvs (BAR) domain protein superfamily

BAR domain superfamily proteins have emerged as central regulators of dynamic membrane remodeling, thereby playing important roles in a wide variety of cellular processes, such as organelle biogenesis, cell division, cell migration, secretion, and endocytosis. Here, we review the mechanistic and structural basis for the membrane curvature-sensing and deforming properties of BAR domain superfamily proteins. Moreover, we summarize the present state of knowledge with respect to their regulation by autoinhibitory mechanisms or posttranslational modifications, and their interactions with other proteins, in particular with GTPases, and with membrane lipids. We postulate that BAR superfamily proteins act as membrane-deforming scaffolds that spatiotemporally orchestrate membrane remodeling.

Yeast dynamin implicated in endocytic scission and the disassembly of endocytic components

The yeast dynamin-related GTPase Vps1 has been implicated in a range of cellular functions including vacuolar protein sorting, protein trafficking, organization of peroxisome and endocytosis.1,2 Vps1 is present at endocytic sites and may be directly involved in endocytic vesicle invagination through its membrane-tubulating activity. Here, evidence supporting the functional link between Vps1 and the yeast amphiphysin Rvs167 in vesicle invagination is discussed. Though the disassembly of endocytic factors from pinched-off endocytic vesicles appears to be tightly regulated in a spatiotemporal manner, we are far from having complete understanding of the underlying mechanism. In this study, we provide evidence that Vps1 plays a role in the uncoating of endocytic proteins from post-internalized vesicles, based on the observation of a quick disassembly of two endocytic coat proteins Ent1 and Ent2 in cells lacking Vps1.

Ankyrin protein networks in membrane formation and stabilization

In eukaryotic cells, ankyrins serve as adaptor proteins that link membrane proteins to the underlying cytoskeleton. These adaptor proteins form protein complexes consisting of integral membrane proteins, signalling molecules and cytoskeletal components. With their modular architecture and ability to interact with many proteins, ankyrins organize and stabilize these protein networks, thereby establishing the infrastructure of membrane domains with specialized functions. To this end, ankyrin collaborates with a number of proteins including cytoskeletal proteins, cell adhesion molecules and large structural proteins. This review addresses the targeting and stabilization of protein networks related to ankyrin interactions with the cytoskeletal protein β-spectrin, L1-cell adhesion molecules and the large myofibrillar protein obscurin. The significance of these interactions for differential targeting of cardiac proteins and neuronal membrane formation is also presented. Finally, this review concludes with a discussion about ankyrin dysfunction in human diseases such as haemolytic anaemia, cardiac arrhythmia and neurological disorders.

Ankyrin-G and β2-Spectrin Collaborate in Biogenesis of Lateral Membrane of Human Bronchial Epithelial Cells

Ankyrins are a family of adapter proteins required for localization of membrane proteins to diverse specialized membrane domains including axon initial segments, specialized sites at the transverse tubule/sarcoplasmic reticulum in cardiomyocytes, and lateral membrane domains of epithelial cells. Little is currently known regarding the molecular basis for specific roles of different ankyrin isoforms. In this study, we systematically generated alanine mutants of clusters of charged residues in the spectrin-binding domains of both ankyrin-B and -G. The corresponding mutants were evaluated for activity in either restoration of abnormal localization of the inositol trisphosphate receptor in the sarcoplasmic reticulum in mutant mouse cardiomyocytes deficient in ankyrin-B or in prevention of loss of lateral membrane in human bronchial epithelial cells depleted of ankyrin-G by small interfering RNA. Interestingly, ankyrin-B and -G share two homologous sites that result in loss of function in both systems, suggesting that common molecular interactions underlie diverse roles of these isoforms. Ankyrins G and B also exhibit differences; mutations affecting spectrin binding had no effect on ankyrin-B function but did abolish activity of ankyrin-G in restoring lateral membrane biogenesis. Depletion of beta(2)-spectrin by small interfering RNA phenocopied depletion of ankyrin-G and resulted in a failure to form new lateral membrane in interphase and mitotic cells. These results demonstrate that ankyrin-G and beta(2)-spectrin are functional partners in biogenesis of the lateral membrane of epithelial cells.

Spectrin functions upstream of ankyrin in a spectrin cytoskeleton assembly pathway

Prevailing models place spectrin downstream of ankyrin in a pathway of assembly and function in polarized cells. We used a transgene rescue strategy in Drosophila melanogaster to test contributions of four specific functional sites in beta spectrin to its assembly and function. (1) Removal of the pleckstrin homology domain blocked polarized spectrin assembly in midgut epithelial cells and was usually lethal. (2) A point mutation in the tetramer formation site, modeled after a hereditary elliptocytosis mutation in human erythrocyte spectrin, had no detectable effect on function. (3) Replacement of repetitive segments 4-11 of beta spectrin with repeats 2-9 of alpha spectrin abolished function but did not prevent polarized assembly. (4) Removal of the putative ankyrin-binding site had an unexpectedly mild phenotype with no detectable effect on spectrin targeting to the plasma membrane. The results suggest an alternate pathway in which spectrin directs ankyrin assembly and in which some important functions of spectrin are independent of ankyrin.

Continuous membrane-cytoskeleton adhesion requires continuous accommodation to lipid and cytoskeleton dynamics

The plasma membrane of most animal cells conforms to the cytoskeleton and only occasionally separates to form blebs. Previous studies indicated that many weak interactions between cytoskeleton and the lipid bilayer kept the surfaces together to counteract the normal outward pressure of cytoplasm. Either the loss of adhesion strength or the formation of gaps in the cytoskeleton enables the pressure to form blebs. Membrane-associated cytoskeleton proteins, such as spectrin and filamin, can control the movement and aggregation of membrane proteins and lipids, e.g., phosphoinositol phospholipids (PIPs), as well as blebbing. At the same time, lipids (particularly PIPs) and membrane proteins affect cytoskeleton and signaling dynamics. We consider here the roles of the major phosphatidylinositol-4,5-diphosphate (PIP2) binding protein, MARCKS, and PIP2 levels in controlling cytoskeleton dynamics. Further understanding of dynamics will provide important clues about how membrane-cytoskeleton adhesion rapidly adjusts to cytoskeleton and membrane dynamics.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

Ankyrin-B targets beta2-spectrin to an intracellular compartment in neonatal cardiomyocytes

Ankyrin-B is a spectrin-binding protein that is required for localization of inositol 1,4,5-trisphosphate receptor and ryanodine receptor in neonatal cardiomyocytes. This work addresses the interaction between ankyrin-B and beta(2)-spectrin in these cells. Ankyrin-B and beta(2)-spectrin are colocalized in an intracellular striated compartment overlying the M-line and distinct from T-tubules, sarcoplasmic reticulum, Golgi, endoplasmic reticulum, lysosomes, and endosomes. Beta(2)-Spectrin is absent in ankyrin-B-null cardiomyocytes and is restored to a normal striated pattern by rescue with green fluorescent protein-220-kDa ankyrin-B. We identified two mutants (A1000P and DAR976AAA) located in the ZU5 domain which eliminate spectrin binding activity of ankyrin-B. Ankyrin-B mutants lacking spectrin binding activity are normally targeted but do not reestablish beta(2)-spectrin in ankyrin-B(+/-) cardiomyocytes. However, both mutant forms of ankyrin-B are still capable of restoring inositol 1,4,5-trisphosphate receptor localization and normal contraction frequency of cardiomyocytes. Therefore, direct binding of beta(2)-spectrin to ankyrin-B is required for the normal targeting of beta(2)-spectrin in neonatal cardiomyocytes. In contrast, ankyrin-B localization and function are independent of beta(2)-spectrin. In summary, this work demonstrates that interaction between members of the ankyrin and beta-spectrin families previously established in erythrocytes and axon initial segments also occurs in neonatal cardiomyocytes with ankyrin-B and beta(2)-spectrin. This work also establishes a functional hierarchy in which ankyrin-B determines the localization of beta(2)-spectrin and operates independently of beta(2)-spectrin in its role in organizing membrane-spanning proteins.

Suprachiasmatic nucleus circadian oscillatory protein, a novel binding partner of K-Ras in the membrane rafts, negatively regulates MAPK pathway

Suprachiasmatic nucleus circadian oscillatory protein (SCOP) is a member of the leucine-rich repeat (LRR)-containing protein family. In addition to circadian expression in the rat hypothalamic suprachiasmatic nucleus, SCOP is constitutively expressed in neurons throughout the rat brain. Here we found that a substantial amount of SCOP was localized in the brain membrane rafts, in which only K-Ras was abundant among Ras isoforms. SCOP interacted directly through its LRR domain with a subset of K-Ras in the guanine nucleotide-free form that was present in the raft fraction. This interaction interfered with the binding of added guanine nucleotide to K-Ras in vitro. A negative regulatory role of SCOP for K-Ras function was examined in PC12 cell lines stably overexpressing SCOP or its deletion mutants. Overexpression of full-length SCOP markedly down-regulated ERK1/ERK2 activation induced by depolarization or phorbol ester stimulation, and this inhibitory effect of overexpressed SCOP was dependent on its LRR domain. These results strongly suggest that SCOP negatively regulates K-Ras signaling in the membrane rafts, identifying a novel mechanism for regulation of the Ras-MAPK pathway.

Occurrence of lipid receptors inferred from brain and erythrocyte spectrins binding NaOH-extracted and protease-treated neuronal and erythrocyte membranes

It was previously shown in model systems that brain spectrin binds membrane phospholipids. In the present study, we analysed binding of isolated brain spectrin and red blood cell spectrin to red blood or neuronal membranes which had been treated as follows: (1). extracted with low ionic-strength solution, (2). the above membranes extracted with 0.1 M NaOH, and (3). membranes treated as above, followed by protease treatment and re-extraction with 0.1 M NaOH. It was found that isolated, NaOH-extracted, protease-treated neuronal and red blood cell membranes bind brain and red blood cell spectrin with moderate affinities similar to those obtained in model phospholipid membrane-spectrin interaction experiments. Moreover, this binding was competitively inhibited by liposomes prepared from membrane lipids. The presented results indicate the occurrence of receptor sites for spectrins that are extraction- and protease-resistant, therefore most probably of lipidic nature, in native membranes.

Brain spectrin exerts much stronger effect on anionic phospholipid monolayers than erythroid spectrin

Red blood cell spectrin and its nonerythroid analogues are linked to integral proteins of the membrane by several skeletal protein receptors, such as ankyrin and protein 4.1 together with p55. However, there are also many reasons for believing that they are insufficient to engender all the properties that characterise the native membrane. Therefore, we are concerned with the mechanism by which brain spectrin interacts with phospholipids of the membrane bilayer. Brain and erythrocyte spectrin were shown previously to bind phospholipid vesicles as well as monolayers prepared from aminophospholipids: phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (PC). In the present study, it is shown that brain spectrin binds to monolayers prepared from anionic phospholipids, such as phosphatidylinositol (PI), phosphatidic acid (PA), phosphatidyl glycerol, diphosphatidylglycerol, and their mixtures with PC. Brain spectrin injected into the subphase to reach nanomolar concentration induced a substantial increase in the surface pressure of monolayers prepared from the phospholipids and their mixtures mentioned above, possibly by penetrating them. This effect is stronger in the case of monolayers prepared from anionic phospholipids alone and weaker when monolayers were prepared from mixtures with PC. The weakest effect was observed in the case of phosphatidylinositol-4,5-bisphosphate monolayers. An interaction of brain spectrin with monolayers prepared from anionic phospholipids (PI/PC 7:3 and PA/PC 7:3) was inhibited (PI/PC much stronger than PA/PC) by purified erythrocyte ankyrin, which indicates that the binding site for those lipids is located in the beta-subunit, possibly in, or in close proximity of, the ankyrin-binding site. In contrast, erythrocyte spectrin injected into the subphase induced a change in the surface pressure of monolayers prepared from anionic phospholipids, which was equal or smaller than the value of surface pressure change induced by protein without a monolayer. This effect was different from what had been observed previously for monolayers prepared from aminophospholipids and their mixtures with PC, and from the data for nonerythroid spectrin presented here.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

The muscle regulatory and structural protein MLP is a cytoskeletal binding partner of betaI-spectrin

Muscle LIM protein (MLP) is a striated muscle-specific factor that enhances myogenic differentiation and is critical to maintaining the structural integrity of the contractile apparatus. The ability of MLP to regulate myogenesis is particularly interesting since it exhibits multiple subcellular localizations, being found in both nuclear and cytoplasmic compartments. Despite extensive biochemical analyses on MLP, the mechanism(s) by which it influences the myogenic program remains largely undefined. To further examine the role of MLP as a positive myogenic regulator, a yeast two-hybrid screen was employed to identify cytoplasmic-associated MLP binding partners. From this screen, the cytoskeletal protein betaI-spectrin was isolated. Protein interaction assays demonstrate that MLP and betaI-spectrin associate with one another in vivo as well as when tested under several in vitro binding conditions. betaI-spectrin binds specifically to MLP but not to the MLP related proteins CRP1 and CRP2 or to other LIM domain containing proteins. The MLP:beta-spectrin interaction is mediated by the second LIM motif of MLP and by repeat 7 of beta-spectrin. Confocal microscopy studies also reveal that MLP co-localizes with beta-spectrin at the sarcolemma overlying the Z- and M-lines of myofibrils in both cardiac and skeletal muscle tissue. Given that beta-spectrin is a known costamere protein, we propose that sarcolemma-associated MLP also serves as a key costamere protein, stabilizing the association of the contractile apparatus with the sarcolemma by linking the beta-spectrin network to the alpha-actinin crosslinked actin filaments of the myofibril.

Controlling cytoskeleton structure by phosphoinositide-protein interactions: phosphoinositide binding protein domains and effects of lipid packing

Cell movement and resistance to mechanical forces are largely governed by the cytoskeleton, a three-dimensional network of protein filaments that form viscoelastic networks within the cytoplasm. The cytoskeleton underlying the plasma membrane of most cells is rich in actin filaments whose assembly and disassembly are regulated by actin binding proteins that are stimulated or inhibited by signals received and transmitted at the membrane/cytoplasm interface. Inositol phospholipids, or phosphoinositides, are potent regulators of many actin binding proteins, and changes in the phosphorylation of specific phosphoinositide species or in their spatial localization are associated with cytoskeletal remodeling in vitro. This review will focus on recent studies directed at defining the structural features of phosphoinositide binding sites in actin binding proteins and on the influence of the physical state of phosphoinositides on their ability to interact with their target proteins.

Phospholipid-binding protein domains

Research into cellular mechanisms for signal transduction is currently one of the most exciting and rapidly advancing fields of biological study. It has been known for some time that numerous intracellular signals are transmitted by specific protein-protein interactions, as exemplified by those involving the Src homology domains. However, after some controversy, it has recently been widely accepted that specific protein-phospholipid interactions also play key roles in many signal transduction pathways. In this review, landmark discoveries and recent advances describing protein domains known to associate with phospholipids are discussed. Particular emphasis is placed on the interactions of proteins with phospholipids acting as second messengers in signalling pathways. For this purpose, the pleckstrin homology (PH) domain is highlighted, since studies of this domain provided some of the earliest, detailed data about protein-phospholipid interactions occurring downstream of growth factor-mediated receptor stimulation. Moreover, studies of PH domains have given insight into the mechanisms of certain diseases, revealed a number of intriguing functional variations on a common structural theme and recently culminated in providing the missing links in erstwhile mysteries of phosphoinositide-dependent signal transduction pathways. Finally, a short discussion is devoted to the developing field of protein-phospholipid interactions that influence cytoskeletal organisation.

The PH domain and the polybasic c domain of cytohesin-1 cooperate specifically in plasma membrane association and cellular function

Recruitment of intracellular proteins to the plasma membrane is a commonly found requirement for the initiation of signal transduction events. The recently discovered pleckstrin homology (PH) domain, a structurally conserved element found in approximately 100 signaling proteins, has been implicated in this function, because some PH domains have been described to be involved in plasma membrane association. Furthermore, several PH domains bind to the phosphoinositides phosphatidylinositol-(4,5)-bisphosphate and phosphatidylinositol-(3,4,5)-trisphosphate in vitro, however, mostly with low affinity. It is unclear how such weak interactions can be responsible for observed membrane binding in vivo as well as the resulting biological phenomena. Here, we investigate the structural and functional requirements for membrane association of cytohesin-1, a recently discovered regulatory protein of T cell adhesion. We demonstrate that both the PH domain and the adjacent carboxyl-terminal polybasic sequence of cytohesin-1 (c domain) are necessary for plasma membrane association and biological function, namely interference with Jurkat cell adhesion to intercellular adhesion molecule 1. Biosensor measurements revealed that phosphatidylinositol-(3,4,5)-trisphosphate binds to the PH domain and c domain together with high affinity (100 nM), whereas the isolated PH domain has a substantially lower affinity (2-3 microM). The cooperativity of both elements appears specific, because a chimeric protein, consisting of the c domain of cytohesin-1 and the PH domain of the beta-adrenergic receptor kinase does not associate with membranes, nor does it inhibit adhesion. Moreover, replacement of the c domain of cytohesin-1 with a palmitoylation-isoprenylation motif partially restored the biological function, but the specific targeting to the plasma membrane was not retained. Thus we conclude that two elements of cytohesin-1, the PH domain and the c domain, are required and sufficient for membrane association. This appears to be a common mechanism for plasma membrane targeting of PH domains, because we observed a similar functional cooperativity of the PH domain of Bruton's tyrosine kinase with the adjacent Bruton's tyrosine kinase motif, a novel zinc-containing fold.

ADP ribosylation factor regulates spectrin binding to the Golgi complex

Homologues of two major components of the well-characterized erythrocyte plasma-membrane-skeleton, spectrin (a not-yet-cloned isoform, betaI Sigma* spectrin) and ankyrin (AnkG119 and an approximately 195-kDa ankyrin), associate with the Golgi complex. ADP ribosylation factor (ARF) is a small G protein that controls the architecture and dynamics of the Golgi by mechanisms that remain incompletely understood. We find that activated ARF stimulates the in vitro association of betaI Sigma* spectrin with a Golgi fraction, that the Golgi-associated betaI Sigma* spectrin contains epitopes characteristic of the betaI Sigma2 spectrin pleckstrin homology (PH) domain known to bind phosphatidylinositol 4,5-bisphosphate (PtdInsP2), and that ARF recruits betaI Sigma* spectrin by inducing increased PtdInsP2 levels in the Golgi. The stimulation of spectrin binding by ARF is independent of its ability to stimulate phospholipase D or to recruit coat proteins (COP)-I and can be blocked by agents that sequester PtdInsP2. We postulate that a PH domain within betaI Sigma* Golgi spectrin binds PtdInsP2 and acts as a regulated docking site for spectrin on the Golgi. Agents that block the binding of spectrin to the Golgi, either by blocking the PH domain interaction or a constitutive Golgi binding site within spectrin's membrane association domain I, inhibit the transport of vesicular stomatitis virus G protein from endoplasmic reticulum to the medial compartment of the Golgi complex. Collectively, these results suggest that the Golgi-spectrin skeleton plays a central role in regulating the structure and function of this organelle.

Fodrin inhibits phospholipases A2, C, and D by decreasing polyphosphoinositide cell content

Brain fodrin inhibited in a dose dependent manner the GTPgammaS-stimulated cytosolic PLA2 (cPLA2), PLC, and PLD activities in differentiated HL-60 cells permeabilized with streptolysin O. cPLA2 and PLD were inhibited by the same concentrations of fodrin (IC50=1.5-2 nM) but PLC was inhibited by lower concentrations (IC50=0.3 nM). Moreover, the rates of inhibition were different between the phospholipases. Spectrin, which shares 50% homology with fodrin, had similar effects on the three phospholipases. However, using cytosol-depleted cells or recombinant PLD1, we showed that fodrin was not a direct inhibitor. Studying the potential mechanisms of these inhibitions, we demonstrated that a major decrease in membrane phosphatidylinositol 4-monophosphate (PtdIns(4)P) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) amounts was induced by fodrin. Exogenous PtdIns(4,5)P2 partly reversed fodrin inhibition of GTPgammaS-stimulated phospholipase C activity. Hence, inhibition of PLC, cPLA2, and PLD activities observed with fodrin could be related to the decrease of PtdIns(4,5)P2, substrate of PLC, a cofactor of PLD and an enhancer of cPLA2 activity.

Pleckstrin homology domains: a common fold with diverse functions

Pleckstrin homology (PH) motifs are approximately 100 amino-acid residues long and have been identified in nearly 100 different eukaryotic proteins, many of which participate in cell signaling and cytoskeletal regulation. Despite minimal sequence homology, the three-dimensional structures are remarkably conserved. This review gives an overview of the PH domain architecture and examines the best-studied examples in an attempt to understand their function.

Phosphoinositide 3-OH kinase activates the beta2 integrin adhesion pathway and induces membrane recruitment of cytohesin-1

Signal transduction through phosphoinositide 3-OH kinase (PI 3-kinase) has been implicated in the regulation of lymphocyte adhesion mediated by integrin receptors. Cellular phosphorylation products of PI 3-kinases interact with a subset of pleckstrin homology (PH) domains, a module that has been shown to recruit proteins to cellular membranes. We have recently identified cytohesin-1, a cytoplasmic regulator of beta2 integrin adhesion to intercellular adhesion molecule 1. We describe here that expression of a constitutively active PI 3-kinase is sufficient for the activation of Jurkat cell adhesion to intercellular adhesion molecule 1, and for enhanced membrane association of cytohesin-1. Up-regulation of cell adhesion by PI 3-kinase and membrane association of endogenous cytohesin-1 is abrogated by overexpression of the isolated cytohesin-1 PH domain, but not by a mutant of the PH domain which fails to associate with the plasma membrane. The PH domain of Bruton's tyrosine kinase (Btk), although strongly associated with the plasma membrane, had no effect on either membrane recruitment of cytohesin-1 or on induction of adhesion by PI 3-kinase. Having delineated the critical steps of the beta2 integrin activation pathway by biochemical and functional analyses, we conclude that PI 3-kinase activates inside-out signaling of beta2 integrins at least partially through cytohesin-1.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Targeting of Tiam1 to the plasma membrane requires the cooperative function of the N-terminal pleckstrin homology domain and an adjacent protein interaction domain

The Rho-like GTPases Cdc42, Rac, and Rho play key roles in the regulation of the actin cytoskeleton and are implicated in transcriptional activation and cell transformation. We have previously identified the invasion-inducing Tiam1 gene, which encodes an activator of Rac. In fibroblasts, Tiam1 induces Rac-mediated membrane ruffling, which requires the N-terminal pleckstrin homology (PHn) domain. Here we show that this PHn domain is part of a protein interaction domain, which mediates membrane localization of Tiam1. After subcellular fractionation, up to 50% of Tiam1 is recovered in the Triton X-100-insoluble high speed pellet that contains small protein complexes. The regions in Tiam1 that are responsible for these protein interactions comprise the PHn domain, an adjacent putative coiled coil region (CC), and an additional flanking region (Ex). Deletions in each of these regions abolish membrane localization of Tiam1 and membrane ruffling, suggesting that they function cooperatively. Indeed, only polypeptides encompassing the PHn-CC-Ex region, and not the PHn-CC or the Ex region, localize at the membrane. These results indicate that the N-terminal PH domain is part of a larger functional Tiam1 domain that mediates protein complex formation and membrane localization of Tiam1.

Na,K-ATPase transport from endoplasmic reticulum to Golgi requires the Golgi spectrin-ankyrin G119 skeleton in Madin Darby canine kidney cells

Spectrin (betaISigma*) and ankyrin (AnkG119) associate with Golgi membranes and the dynactin complex, but their role in vesicle trafficking remains uncertain. We find that the actin-binding domain and membrane-association domain 1 (MAD1) of betaI spectrin together form a constitutive Golgi targeting signal in transfected MDCK cells. Expression of this signal in transfected cells disrupts the endogenous Golgi spectrin skeleton and blocks transport of alpha- and beta-Na,K-ATPase and vesicular stomatitis virus-G protein from the endoplasmic reticulum (ER) but does not disrupt the formation of Golgi stacks, the distribution of beta-COP, or the transport and surface display of E-cadherin. The Golgi spectrin skeleton is thus required for the transport of a subset of membrane proteins from the ER to the Golgi. We postulate that together with polyfunctional adapter proteins such as AnkG119, Golgi spectrin forms a docking complex that acts prior to the cis-Golgi, presumably with vesicular-tubular clusters (VTCs or ERGIC), to sequester specific membrane proteins into vesicles transiting between the ER and Golgi, and subsequently (probably involving other isoforms of spectrin and ankyrin) to mediate cargo transport within the Golgi and to other membrane compartments. We hypothesize that this vesicular spectrin-ankyrin adapter-protein trafficking (or tethering) system (SAATS) mediates the capture and transport of many membrane proteins and acts in conjunction with vesicle-targeting molecules to effect the efficient transport of cargo proteins.

Membrane binding and enzymatic activation of a Dbl homology domain require the neighboring pleckstrin homology domain

Dbl-homology (DH) domains are invariably located immediately N-terminal to a pleckstrin homology (PH) domain. To understand the functional relationship between these two domains we expressed the DH domain alone, the PH domain alone, and the DH-PH combination of the invasion inducing protein Tiam-1 fused to glutathione-S-transferase (GST) or green fluorescent protein (GFP). We found that the GST-DH-PH and the GST-PH constructs bind to preparations of brain membranes and to the beta gamma subunits of trimeric G proteins in vitro, while the GST-DH and GST control do not. The GFP-DH-PH and GFP-PH constructs are localized to peripheral membranes of COS-7 cells in vivo, while GFP and GFP-DH domain constructs are found diffusely in the cytoplasm. The DH-PH domain combination activates Jun N-terminal kinase (JNK) strongly, but the DH domain alone and the PH domain alone have little effect. We conclude that membrane localization and enzymatic activation of the DH domain require the adjacent PH domain.