Binding of the PH and polybasic C-terminal domains of ARNO to phosphoinositides and to acidic lipids

The small molecule inhibitor NAV-2729 has a complex target profile including multiple ADP-ribosylation factor regulatory proteins

The ADP-ribosylation factor (Arf) GTPases and their regulatory proteins are implicated in cancer progression. NAV-2729 was previously identified as a specific inhibitor of Arf6 that reduced progression of uveal melanoma in an orthotopic xenograft. Here, our goal was to assess the inhibitory effects of NAV-2729 on the proliferation of additional cell types. We found NAV-2729 inhibited proliferation of multiple cell lines, but Arf6 expression did not correlate with NAV-2729 sensitivity, and knockdown of Arf6 affected neither cell viability nor sensitivity to NAV-2729. Furthermore, binding to native Arf6 was not detected; however, we determined that NAV-2729 inhibited both Arf exchange factors and Arf GTPase-activating proteins. ASAP1, a GTPase-activating protein linked to cancer progression, was further investigated. We demonstrated that NAV-2729 bound to the PH domain of ASAP1 and changed ASAP1 cellular distribution. However, ASAP1 knockdown did not fully recapitulate the cytoskeletal effects of NAV-2729 nor affect cell proliferation. Finally, our screens identified 48 other possible targets of NAV-2729. These results illustrate the complexities of defining targets of small molecules and identify NAV-2729 as a model PH domain-binding inhibitor.

Molecular principles of bidirectional signalling between membranes and small GTPases

Most small GTPases actuate their functions on subcellular membranes, which are increasingly seen as integral components of small GTPase signalling. In this review, we used the highly studied regulation of Arf GTPases by their GEFs to categorize the molecular principles of membrane contributions to small GTPase signalling, which have been highlighted by integrated structural biology combining in vitro reconstitutions in artificial membranes and high-resolution structures. As an illustration of how this framework can be harnessed to better understand the cooperation between small GTPases, their regulators and membranes, we applied it to the activation of the small GTPase Rac1 by DOCK-ELMO, identifying novel contributions of membranes to Rac1 activation. We propose that these structure-based principles should be considered when interrogating the mechanisms whereby small GTPase systems ensure spatial and temporal control of cellular signalling on membranes.

ELMO1 deficiency enhances platelet function

Phosphatidylinositol 3-kinase is an important signaling molecule that, once activated, leads to the generation of phosphatidylinositol (3,4,5)-trisphosphate (PIP₃). We performed a proteomic screen to identify PIP₃-interacting proteins in human platelets_. Among these proteins, we found engulfment and cell motility 1 (ELMO1), a scaffold protein with no catalytic activity. ELMO1 is expressed in platelets and interacts with active RhoG. However, the function of ELMO1 in platelets is not known. The focus of this study was to determine the function of ELMO1 in platelets utilizing ELMO1^-/- mice. Platelet aggregation, granule secretion, integrin αIIbβ3 activation, and thromboxane generation were enhanced in ELMO1^-/- platelets in response to glycoprotein VI (GPVI) agonists but unaltered when a protease-activated receptor 4 agonist was used. The kinetics of spreading on immobilized fibrinogen was enhanced in ELMO1^-/- platelets compared with wild-type (WT) littermate controls. This suggests that ELMO1 plays a role downstream of the GPVI and integrin αIIbβ3 pathway. Furthermore, whole blood from ELMO1^-/- mice perfused over collagen exhibited enhanced thrombus formation compared with WT littermate controls. ELMO1^-/- mice showed reduced survival compared with control following pulmonary embolism. ELMO1^-/- mice also exhibited a shorter time to occlusion using the ferric-chloride injury model and reduced bleeding times compared with WT littermate controls. These results indicate that ELMO1 plays an important role in hemostasis and thrombosis in vivo. RhoG activity was enhanced in ELMO1^-/- murine platelets compared with WT littermate controls in response to GPVI agonist. Together, these data suggest that ELMO1 negatively regulates GPVI-mediated thrombus formation via RhoG.

Arf GTPase interplay with Rho GTPases in regulation of the actin cytoskeleton

The Arf and Rho subfamilies of small GTPases are nucleotide-dependent molecular switches that act as master regulators of vesicular trafficking and the actin cytoskeleton organization. Small GTPases control cell processes with high fidelity by acting through distinct repertoires of binding partners called effectors. While we understand a great deal about how these GTPases act individually, relatively little is known about how they cooperate, especially in the control of effectors. This review highlights how Arf GTPases collaborate with Rac1 to regulate actin cytoskeleton dynamics at the membrane via recruiting and activating the Wave Regulatory Complex (WRC), a Rho effector that underpins lamellipodia formation and macropinocytosis. This provides insight into Arf regulation of the actin cytoskeleton, while putting the spotlight on small GTPase cooperation with emerging evidence of its importance in fundamental cell biology and interactions with pathogenic bacteria.

Specialized structural and functional roles of residues selectively conserved in subfamilies of the pleckstrin homology domain family

Homologous domains embedded in multidomain proteins of different domain architectures (DA) may exhibit subtle, but important, differences in their structure and function. Here, we consider two multidomain proteins, Arf nucleotide binding site opener (ARNO) and G protein‐coupled receptor kinase 2 (GRK2), which have very different DAs, but both contain pleckstrin homology (PH) domains. We analyzed the roles of residues selectively conserved in these subfamilies of PH domains from ARNO and GRK2 proteins. DA‐specific residues in PH domain are found to contribute to structural and functional specialization of ARNO and GRK2 in terms of (a) specific intra‐ and interprotein interactions; (b) specificity for phospholipids; and (c) participation in conformational excursions, leading to various functional forms. Our approach can also be applied to subfamilies of other protein families to identify subfamily‐specific residues and their specialized roles.

Pallidin is a novel interacting protein for cytohesin-2 and regulates the early endosomal pathway and dendritic formation in neurons

Cytohesin-2 is a member of the guanine nucleotide exchange factors for ADP ribosylation factor 1 (Arf1) and Arf6, which are small GTPases that regulate membrane traffic and actin dynamics. In this study, we first demonstrated that cytohesin-2 localized to the plasma membrane and vesicles in various subcellular compartment in hippocampal neurons by immunoelectron microscopy. Next, to understand the molecular network of cytohesin-2 in neurons, we conducted yeast two-hybrid screening of brain cDNA libraries using cytohesin-2 as bait and isolated pallidin, a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1) involved in endosomal trafficking. Pallidin interacted specifically with cytohesin-2 among cytohesin family members. Glutathione S-transferase pull-down and immunoprecipitation assays further confirmed the formation of a protein complex between cytohesin-2 and pallidin. Immunofluorescence demonstrated that cytohesin-2 and pallidin partially colocalized in various subsets of endosomes immunopositive for EEA1, syntaxin 12, and LAMP2 in hippocampal neurons. Knockdown of pallidin or cytohesin-2 reduced cytoplasmic EEA1-positive early endosomes. Furthermore, knockdown of pallidin increased the total dendritic length of cultured hippocampal neurons, which was rescued by co-expression of wild-type pallidin but not a mutant lacking the ability to interact with cytohesin-2. In contrast, knockdown of cytohesin-2 had the opposite effect on total dendritic length. The present results suggested that the interaction between pallidin and cytohesin-2 may participate in various neuronal functions such as endosomal trafficking and dendritic formation in hippocampal neurons. Cover Image for this issue: doi: 10.1111/jnc.14197.

Inhibition of WAVE Regulatory Complex Activation by a Bacterial Virulence Effector Counteracts Pathogen Phagocytosis

To establish pathogenicity, bacteria must evade phagocytosis directed by remodeling of the actin cytoskeleton. We show that macrophages facilitate pathogen phagocytosis through actin polymerization mediated by the WAVE regulatory complex (WRC), small GTPases Arf and Rac1, and the Arf1 activator ARNO. To establish extracellular infections, enteropathogenic (EPEC) and enterohaemorrhagic (EHEC) Escherichia coli hijack the actin cytoskeleton by injecting virulence effectors into the host cell. Here, we find that the virulence effector EspG counteracts WRC-dependent phagocytosis, enabling EPEC and EHEC to remain extracellular. By reconstituting membrane-associated actin polymerization, we find that EspG disabled WRC activation through two mechanisms: EspG interaction with Arf6 blocked signaling to ARNO while EspG binding of Arf1 impeded collaboration with Rac1, thereby inhibiting WRC recruitment and activation. Investigating the mode of EspG interference revealed sites in Arf1 required for WRC activation and a mechanism facilitating pathogen evasion of innate host defenses.

Allosteric regulation of Arf GTPases and their GEFs at the membrane interface

Arf GTPases assemble protein complexes on membranes to carry out major functions in cellular traffic. An essential step is their activation by guanine nucleotide exchange factors (GEFs), whose Sec7 domain stimulates GDP/GTP exchange. ArfGEFs form 2 major families: ArfGEFs with DCB, HUS and HDS domains (GBF1 and BIG1/BIG2 in humans), which act at the Golgi; and ArfGEFs with a C-terminal PH domain (cytohesin, EFA6 and BRAG), which function at the plasma membrane and endosomes. In addition, pathogenic bacteria encode an ArfGEF with a unique membrane-binding domain. Here we review the allosteric regulation of Arf GTPases and their GEFs at the membrane interface. Membranes contribute several regulatory layers: at the GTPase level, where activation by GTP is coupled to membrane recruitment by a built-in structural device; at the Sec7 domain, which manipulates this device to ensure that Arf-GTP is attached to membranes; and at the level of non-catalytic ArfGEF domains, which form direct or GTPase-mediated interactions with membranes that enable a spectacular diversity of regulatory regimes. Notably, we show here that membranes increase the efficiency of a large ArfGEF (human BIG1) by 32-fold by interacting directly with its N-terminal DCB and HUS domains. The diversity of allosteric regulatory regimes suggests that ArfGEFs can function in cascades and circuits to modulate the shape, amplitude and duration of Arf signals in cells. Because Arf-like GTPases feature autoinhibitory elements similar to those of Arf GTPases, we propose that their activation also requires allosteric interactions of these elements with membranes or other proteins.

PI3K regulates endocytosis after insulin secretion by mediating signaling crosstalk between Arf6 and Rab27a

In secretory cells, endocytosis is coupled to exocytosis to enable proper secretion. Although endocytosis is crucial to maintain cellular homeostasis before and after secretion, knowledge about secretagogue-induced endocytosis in secretory cells is still limited. Here, we searched for proteins that interacted with the Rab27a GTPase-activating protein (GAP) EPI64 (also known as TBC1D10A) and identified the Arf6 guanine-nucleotide-exchange factor (GEF) ARNO (also known as CYTH2) in pancreatic β-cells. We found that the insulin secretagogue glucose promotes phosphatidylinositol (3,4,5)-trisphosphate (PIP3) generation through phosphoinositide 3-kinase (PI3K), thereby recruiting ARNO to the intracellular side of the plasma membrane. Peripheral ARNO promotes clathrin assembly through its GEF activity for Arf6 and regulates the early stage of endocytosis. We also found that peripheral ARNO recruits EPI64 to the same area and that the interaction requires glucose-induced endocytosis in pancreatic β-cells. Given that GTP- and GDP-bound Rab27a regulate exocytosis and the late stage of endocytosis, our results indicate that the glucose-induced activation of PI3K plays a pivotal role in exocytosis-endocytosis coupling, and that ARNO and EPI64 regulate endocytosis at distinct stages.

Shigella flexneri regulation of ARF6 activation during bacterial entry via an IpgD-mediated positive feedback loop

Entry into cells is critical for virulence of the human bacterial pathogens Shigella spp. Shigella spp. induce membrane ruffle formation and macropinocytic uptake, but the events instigating this process are incompletely understood. The host small GTPase ADP-ribosylation factor 6 (ARF6) functions in membrane trafficking at the plasma membrane and activates membrane ruffle formation. We demonstrate that ARF6 is required for efficient Shigella flexneri entry, is activated by S. flexneri dependent on the phosphatase activity of the type III secreted effector IpgD, and depends on cytohesin guanine nucleotide exchange factors (GEFs) for recruitment to entry sites. The cytohesin GEF ARF nucleotide binding site opener (ARNO) is recruited to these sites, also dependent on IpgD phosphatase activity. ARNO recruitment is independent of ARF6, indicating that, in addition to the described recruitment of ARNO by ARF6, ARNO is recruited upstream of ARF6. Our data provide evidence that ARF6, IpgD, phosphoinositide species, and ARNO constitute a previously undescribed positive feedback loop that amplifies ARF6 activation at bacterial entry sites, thereby promoting efficient S. flexneri uptake.

IMPORTANCE

Shigella spp. cause diarrhea and dysentery by infection of epithelial cells in the human colon. Critical to disease is the ability of Shigella to enter into cells, yet the mechanisms involved in entry are incompletely understood. We demonstrate that the small GTPase ADP-ribosylation factor 6 (ARF6) is required for efficient cellular entry of Shigella flexneri and that activation of ARF6 depends on the phosphatase activity of the Shigella protein IpgD, which is introduced into cells via the bacterial type III secretion system. We further show that IpgD phosphatase activity is required for recruitment of the ARF6 guanine nucleotide exchange factor (GEF) ARF nucleotide binding site opener (ARNO) to bacterial entry sites and that ARNO lies upstream of ARF6 activation. These relationships define a positive feedback loop that contributes to activation of ARF6 at S. flexneri entry sites and leads to local amplification of signals that promote bacterial entry.

Cytohesin-2 phosphorylation by protein kinase C relieves the constitutive suppression of platelet dense granule secretion by ADP-ribosylation factor 6

BACKGROUND

Protein kinase C (PKC) is a major regulator of platelet function and secretion. The underlying molecular pathway from PKC to secretion, however, is poorly understood. By a proteomics screen we identified the guanine nucleotide exchange factor cytohesin-2 as a candidate PKC substrate.

OBJECTIVES

We aimed to validate cytohesin-2 as a PKC substrate in platelets and to determine its role in granule secretion and other platelet responses.

METHODS AND RESULTS

Immunoprecipitation was performed with a phosphoserine PKC substrate antibody followed by mass spectrometry, leading to the identification of cytohesin-2. By western blotting we showed that different agonists induced cytohesin-2 phosphorylation by PKC. Protein function was investigated using a pharmacological approach. The cytohesin inhibitor SecinH3 significantly enhanced platelet dense granule secretion and aggregation, as measured by lumi-aggregometry. Flow cytometry data indicate that α-granule release and integrin αII b β3 activation were not affected by cytohesin-2 inhibition. Lysosome secretion was assessed by a colorimetric assay and was also unchanged. As shown by western blotting, ARF6 interacted with cytohesin-2 and was present in an active GTP-bound form under basal conditions. Upon platelet stimulation, this interaction was largely lost and ARF6 activation decreased, both of which could be rescued by PKC inhibition.

CONCLUSIONS

Cytohesin-2 constitutively suppresses platelet dense granule secretion and aggregation by keeping ARF6 in a GTP-bound state. PKC-mediated phosphorylation of cytohesin-2 relieves this inhibitory effect, thereby promoting platelet secretion and aggregation.

Structural basis for membrane recruitment and allosteric activation of cytohesin family Arf GTPase exchange factors

Membrane recruitment of cytohesin family Arf guanine nucleotide exchange factors depends on interactions with phosphoinositides and active Arf GTPases that, in turn, relieve autoinhibition of the catalytic Sec7 domain through an unknown structural mechanism. Here, we show that Arf6-GTP relieves autoinhibition by binding to an allosteric site that includes the autoinhibitory elements in addition to the PH domain. The crystal structure of a cytohesin-3 construct encompassing the allosteric site in complex with the head group of phosphatidyl inositol 3,4,5-trisphosphate and N-terminally truncated Arf6-GTP reveals a large conformational rearrangement, whereby autoinhibition can be relieved by competitive sequestration of the autoinhibitory elements in grooves at the Arf6/PH domain interface. Disposition of the known membrane targeting determinants on a common surface is compatible with multivalent membrane docking and subsequent activation of Arf substrates, suggesting a plausible model through which membrane recruitment and allosteric activation could be structurally integrated.

The N termini of a-subunit isoforms are involved in signaling between vacuolar H+-ATPase (V-ATPase) and cytohesin-2

Previously, we reported an acidification-dependent interaction of the endosomal vacuolar H(+)-ATPase (V-ATPase) with cytohesin-2, a GDP/GTP exchange factor (GEF), suggesting that it functions as a pH-sensing receptor. Here, we have studied the molecular mechanism of signaling between the V-ATPase, cytohesin-2, and Arf GTP-binding proteins. We found that part of the N-terminal cytosolic tail of the V-ATPase a2-subunit (a2N), corresponding to its first 17 amino acids (a2N(1-17)), potently modulates the enzymatic GDP/GTP exchange activity of cytohesin-2. Moreover, this peptide strongly inhibits GEF activity via direct interaction with the Sec7 domain of cytohesin-2. The structure of a2N(1-17) and its amino acids Phe(5), Met(10), and Gln(14) involved in interaction with Sec7 domain were determined by NMR spectroscopy analysis. In silico docking experiments revealed that part of the V-ATPase formed by its a2N(1-17) epitope competes with the switch 2 region of Arf1 and Arf6 for binding to the Sec7 domain of cytohesin-2. The amino acid sequence alignment and GEF activity studies also uncovered the conserved character of signaling between all four (a1-a4) a-subunit isoforms of mammalian V-ATPase and cytohesin-2. Moreover, the conserved character of this phenomenon was also confirmed in experiments showing binding of mammalian cytohesin-2 to the intact yeast V-ATPase holo-complex. Thus, here we have uncovered an evolutionarily conserved function of the V-ATPase as a novel cytohesin-signaling receptor.

Finding a needle in a haystack: the role of electrostatics in target lipid recognition by PH domains

Interactions between protein domains and lipid molecules play key roles in controlling cell membrane signalling and trafficking. The pleckstrin homology (PH) domain is one of the most widespread, binding specifically to phosphatidylinositol phosphates (PIPs) in cell membranes. PH domains must locate specific PIPs in the presence of a background of approximately 20% anionic lipids within the cytoplasmic leaflet of the plasma membrane. We investigate the mechanism of such recognition via a multiscale procedure combining Brownian dynamics (BD) and molecular dynamics (MD) simulations of the GRP1 PH domain interacting with phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P₃). The interaction of GRP1-PH with PI(3,4,5)P₃ in a zwitterionic bilayer is compared with the interaction in bilayers containing different levels of anionic ‘decoy’ lipids. BD simulations reveal both translational and orientational electrostatic steering of the PH domain towards the PI(3,4,5)P₃-containing anionic bilayer surface. There is a payoff between non-PIP anionic lipids attracting the PH domain to the bilayer surface in a favourable orientation and their role as ‘decoys’, disrupting the interaction of GRP1-PH with the PI(3,4,5)P₃ molecule. Significantly, approximately 20% anionic lipid in the cytoplasmic leaflet of the bilayer is nearly optimal to both enhance orientational steering and to localise GRP1-PH proximal to the surface of the membrane without sacrificing its ability to locate PI(3,4,5)P₃ within the bilayer plane. Subsequent MD simulations reveal binding to PI(3,4,5)P₃, forming protein-phosphate contacts comparable to those in X-ray structures. These studies demonstrate a computational framework which addresses lipid recognition within a cell membrane environment, offering a link between structural and cell biological characterisation.

Cell signalling pathways are crucial for many biological processes including cell proliferation and survival. Signalling is governed by a complex network of interactions within the cell, and disruption of signalling can lead to a variety of human diseases. Often, a key event in the signalling cascade is the reversible recruitment of peripheral membrane proteins to the surface of the cell membrane, where they then bind to a specific lipid in order to perform their function. However, it is not clear how these proteins locate their target lipid in the complex multi-lipid environment of the plasma membrane. Here, we have used a combination of computational techniques to simulate the association of a signalling protein with the surface of the cell membrane. We demonstrate that the mechanism of membrane binding is dependent upon the lipid composition of the lipid bilayer, and the results show that orientational and positional steering of the protein is optimised when the anionic lipid content of our model membrane matches the physiological composition observed in cells.

Salmonella virulence effector SopE and Host GEF ARNO cooperate to recruit and activate WAVE to trigger bacterial invasion

Salmonella virulence effectors elicit host cell membrane ruffling to facilitate pathogen invasion. The WAVE regulatory complex (WRC) governs the underlying membrane-localized actin polymerization, but how Salmonella manipulates WRC is unknown. We show that Rho GTPase activation by the Salmonella guanine nucleotide exchange factor (GEF) SopE efficiently triggered WRC recruitment but not its activation, which required host Arf GTPase activity. Invading Salmonella recruited and activated Arf1 to facilitate ruffling and uptake. Arf3 and Arf6 could also enhance invasion. RNAi screening of host Arf-family GEFs revealed a key role for ARNO in pathogen invasion and generation of pathogen-containing macropinosomes enriched in Arf1 and WRC. Salmonella recruited ARNO via Arf6 and the phosphoinositide phosphatase effector SopB-induced PIP3 generation. ARNO in turn triggered WRC recruitment and activation, which was dramatically enhanced when SopE and ARNO cooperated. Thus, we uncover a mechanism by which pathogen and host GEFs synergize to regulate WRC and trigger Salmonella invasion.

Effect of phosphorylation of phosphatidylinositol on myelin basic protein-mediated binding of actin filaments to lipid bilayers in vitro

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocytes and is believed to be responsible for adhesion of these surfaces in the multilayered myelin sheath. It can also assemble actin filaments and tether them to lipid bilayers through electrostatic interactions. Here we investigate the effect of increased negative charge of the lipid bilayer due to phosphorylation of phosphatidylinositol (PI) on MBP-mediated binding of actin to the lipid bilayer, by substituting phosphatidylinositol 4-phosphate or phosphatidylinositol 4,5-bisphosphate for PI in phosphatidylcholine/phosphatidylglycerol lipid vesicles. Phosphorylation of PI caused dissociation of the MBP/actin complex from the lipid vesicles due to repulsion of the negatively charged complex from the negatively charged membrane surface. An effect of phosphorylation could be detected even if the inositol lipid was only 2mol% of the total lipid. Calcium-calmodulin dissociated actin from the MBP-lipid vesicles and phosphorylation of PI increased the amount dissociated. These results show that changes to the lipid composition of myelin, which could occur during signaling or other physiological events, could regulate the ability of MBP to act as a scaffolding protein and bind actin filaments to the lipid bilayer.

Characterization of Mid1 domains for targeting and scaffolding in fission yeast cytokinesis

Division-site selection and contractile-ring assembly are two crucial steps in cytokinesis. In fission yeast, the anillin-like Mid1 protein specifies the division site at the cell equator by assembling cortical nodes, the precursors of the contractile ring. Thus, Mid1 is essential for linking the positional cues for the cleavage site to contractile-ring formation. However, how Mid1 domains cooperate to regulate cytokinesis is poorly understood. Here we unravel the functions of different Mid1 domains (motifs) by a series of truncations. We report that the conserved PH domain stabilizes Mid1 in nodes by binding to lipids and is required for Mid1 cortical localization during interphase in the absence of Cdr2 kinase. Mid1 lacking an internal region that is approximately one third of the full-length protein has higher nuclear and cortical concentration and suppresses the division-site positioning defects in cells with a deletion of the dual-specificity tyrosine-regulated kinase Pom1. The N-terminus of Mid1 physically interacts with cytokinesis node proteins. When fused to cortical node protein Cdr2, Mid1(1-100) is sufficient to assemble cytokinesis nodes and the contractile ring. Collectively, our study recognizes domains regulating Mid1 cortical localization and reveals domains sufficient for contractile-ring assembly.

Lateral dynamics of charged lipids and peripheral proteins in spatially heterogeneous membranes: comparison of continuous and Monte Carlo approaches

Biological membranes are complex environments whose physico-chemical properties are of utmost importance for the understanding of many crucial biological processes. Much attention has been given in the literature to the description of membranes along the z-axis perpendicular to the membrane. Here, we instead consider the lateral dynamics of lipids and peripheral proteins due to their electrostatic interaction. Previously, we constructed a Monte Carlo automaton capable of simulating mutual diffusive dynamics of charged lipids and associated positively charged peptides. Here, we derive and numerically analyze a system of Poisson-Boltzmann-Nernst-Planck (PBNP) equations that provide a mean-field approximation compatible with our Monte Carlo model. The thorough comparison between the mean-field PBNP equations and Monte Carlo simulations demonstrates that both the approaches are in a good qualitative agreement in all tested scenarios. We find that the two methods quantitatively deviate when the local charge density is high, presumably because the Poisson-Boltzmann formalism is applicable in the so-called weak coupling limit, whose validity is restricted to low charge densities. Nevertheless, we conclude that the mean-field PBNP approach provides a good approximation for the considerably more detailed Monte Carlo model at only a fraction of the associated computational cost and allows simulation of the membrane lateral dynamics on the space and time scales relevant for the realistic biological problems.

Kinetic studies of the Arf activator Arno on model membranes in the presence of Arf effectors suggest control by a positive feedback loop

Proteins of the cytohesin/Arno/Grp1 family of Arf activators are positive regulators of the insulin-signaling pathway and control various remodeling events at the plasma membrane. Arno has a catalytic Sec7 domain, which promotes GDP to GTP exchange on Arf, followed by a pleckstrin homology (PH) domain. Previous studies have revealed two functions of the PH domain: inhibition of the Sec7 domain and membrane targeting. Interestingly, the Arno PH domain interacts not only with a phosphoinositide (phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate) but also with an activating Arf family member, such as Arf6 or Arl4. Using the full-length membrane-bound forms of Arf1 and Arf6 instead of soluble forms, we show here that the membrane environment dramatically affects the mechanism of Arno activation. First, Arf6-GTP stimulates Arno at nanomolar concentrations on liposomes compared with micromolar concentrations in solution. Second, mutations in the PH domain that abolish interaction with Arf6-GTP render Arno completely inactive when exchange reactions are reconstituted on liposomes but have no effect on Arno activity in solution. Third, Arno is activated by its own product Arf1-GTP in addition to a distinct activating Arf isoform. Consequently, Arno activity is strongly modulated by competition with Arf effectors. These results show that Arno behaves as a bistable switch, having an absolute requirement for activation by an Arf protein but, once triggered, becoming highly active through the positive feedback effect of Arf1-GTP. This property of Arno might provide an explanation for its function in signaling pathways that, once triggered, must move forward decisively.

An insight into the sialotranscriptome of the brown dog tick, Rhipicephalus sanguineus

BACKGROUND

Rhipicephalus sanguineus, known as the brown dog tick, is a common ectoparasite of domestic dogs and can be found worldwide. R.sanguineus is recognized as the primary vector of the etiological agent of canine monocytic ehrlichiosis and canine babesiosis. Here we present the first description of a R. sanguineus salivary gland transcriptome by the production and analysis of 2,034 expressed sequence tags (EST) from two cDNA libraries, one consctructed using mRNA from dissected salivary glands from female ticks fed for 3-5 days (early to mid library, RsSGL1) and the another from ticks fed for 5 days (mid library, RsSGL2), identifying 1,024 clusters of related sequences.

RESULTS

Based on sequence similarities to nine different databases, we identified transcripts of genes that were further categorized according to function. The category of putative housekeeping genes contained approximately 56% of the sequences and had on average 2.49 ESTs per cluster, the secreted protein category contained 26.6% of the ESTs and had 2.47 EST's/clusters, while 15.3% of the ESTs, mostly singletons, were not classifiable, and were annotated as "unknown function". The secreted category included genes that coded for lipocalins, proteases inhibitors, disintegrins, metalloproteases, immunomodulatory and antiinflammatory proteins, as Evasins and Da-p36, as well as basic-tail and 18.3 kDa proteins, cement proteins, mucins, defensins and antimicrobial peptides. Comparison of the abundance of ESTs from similar contigs of the two salivary gland cDNA libraries allowed the identification of differentially expressed genes, such as genes coding for Evasins and a thrombin inhibitor, which were over expressed in the RsSGL1 (early to mid library) versus RsSGL2 (mid library), indicating their role in inhibition of inflammation at the tick feeding site from the very beginning of the blood meal. Conversely, sequences related to cement (64P), which function has been correlated with tick attachment, was largely expressed in the mid library.

CONCLUSIONS

Our survey provided an insight into the R. sanguineus sialotranscriptome, which can assist the discovery of new targets for anti-tick vaccines, as well as help to identify pharmacologically active proteins.

Cytohesin-1 regulates the Arf6-phospholipase D signaling axis in human neutrophils: impact on superoxide anion production and secretion

Polymorphonuclear neutrophil (PMN) stimulation with fMLP stimulates small G proteins such as ADP-ribosylation factors (Arfs) Arf1 and Arf6, leading to phospholipase D (PLD) activation and functions such as degranulation and the oxidative burst. However, the molecular links between fMLF receptors and PLD remain unclear. PMNs express cytohesin-1, an Arf-guanine exchange factor that activates Arfs, and its expression is strongly induced during the acquisition of the neutrophilic phenotype by neutrophil-like cells. The role of cytohesin-1 in the activation of the fMLF-Arf-PLD signaling axis, and the accomplishment of superoxide anion production, and degranulation was investigated in PMNs using the selective inhibitor of cytohesin, Sec 7 inhibitor H3 (secinH3). Cytohesin-1 inhibition with secinH3 leads to Arf6 but not Arf1 inhibition, demonstrating the specificity for Arf6, and fMLF-mediated activation of PLD and of the oxidative burst as well. We observed a decrease in fMLF-mediated protein secretion and expression of cell surface markers corresponding to primary (CD63/myeloperoxidase), secondary (CD66/lactoferrin), and tertiary (matrix metalloproteinase-9) granules in PMNs incubated with secinH3. Similarly, silencing cytohesin-1 or Arf6 in PLB-985 cells negatively affected fMLF-induced activation of PLD, superoxide production, and expression of granule markers on the cell surface. In contrast, stable overexpression of cytohesin-1 in PLB-985 cells enhanced fMLF-induced activation of Arf6, PLD, and NADPH oxidase. The results of this study provide evidence for an involvement of cytohesin-1 in the regulation of the functional responses of human PMNs and link these events, in part at least, to the activation of Arf6.

Kinetics of the interaction of myo1c with phosphoinositides

myo1c is a single-headed myosin that dynamically links membranes to the actin cytoskeleton. A putative pleckstrin homology domain has been identified in the myo1c tail that binds phosphoinositides and soluble inositol phosphates with high affinity. However, the kinetics of association and dissociation and the influence of phospholipid composition on the kinetics have not been determined. Stopped-flow spectroscopy was used to measure the binding and dissociation of a recombinant myo1c construct containing the tail and regulatory domains (myo1c(IQ-tail)) to and from 100-nm diameter large unilamellar vesicles (LUVs). We found the time course of association of myo1c(IQ-tail) with LUVs containing 2% phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) followed a two-exponential time course, and the rate of the predominant fast phase depended linearly upon the total lipid concentration. The apparent second-order rate constant was approximately diffusion-limited. Increasing the molar ratio of anionic phospholipid by adding phosphatidylserine, additional PtdIns(4,5)P(2), or by situating PtdIns(4,5)P(2) in a more physiologically relevant lipid background increased the apparent association rate constant less than 2-fold. myo1c(IQ-tail) dissociated from PtdIns(4,5)P(2) at a slower rate (2.0 s(-1)) than the pleckstrin homology domain of phospholipase C-delta (13 s(-1)). The presence of additional anionic phospholipid reduced the myo1c(IQ-tail) dissociation rate constant >50-fold but marginally changed the dissociation rate of phospholipase C-delta, suggesting that additional electrostatic interactions in myo1c(IQ-tail) help to stabilize binding. Remarkably, high concentrations of soluble inositol phosphates induce dissociation of myo1c(IQ-tail) from LUVs, suggesting that phosphoinositides are able to bind to and dissociate from myo1c(IQ-tail) as it remains bound to the membrane.

The pleckstrin homology domain of the Arf6-specific exchange factor EFA6 localizes to the plasma membrane by interacting with phosphatidylinositol 4,5-bisphosphate and F-actin

The Arf6-specific exchange factor EFA6 coordinates membrane trafficking with actin cytoskeleton remodeling. It localizes to the plasma membrane where it catalyzes Arf6 activation and induces the formation of actin-based membrane ruffles. We have shown previously that the pleckstrin homology (PH) domain of EFA6 was responsible for its membrane localization. In this study we looked for the partners of the PH domain at the plasma membrane. Mutations of the conserved basic residues suspected to be involved in the binding to phosphoinositides redistribute EFA6-PH to the cytosol. In addition, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) breakdown also leads to the solubilization of EFA6-PH. Direct binding measured by surface plasmon resonance gives an apparent affinity of approximately 0.5 microm EFA6-PH for PI(4,5)P2. Moreover, we observed in vitro that the catalytic activity of EFA6 is strongly increased by PI(4,5)P2. These results indicate that the plasma membrane localization of EFA6-PH is based on its interaction with PI(4,5)P2, and this interaction is necessary for an optimal catalytic activity of EFA6. Furthermore, we demonstrated by fluorescence recovery after photobleaching and Triton X-100 detergent solubility experiments that in addition to the phophoinositides, EFA6-PH is linked to the actin cytoskeleton. We observed both in vivo and in vitro that EFA6-PH interacts directly with F-actin. Finally, we demonstrated that EFA6 could bind simultaneously filamentous actin and phospholipids vesicles. Our results explain how the exchange factor EFA6 via its PH domain could coordinate at the plasma membrane actin cytoskeleton organization with membrane trafficking.

Structural basis and mechanism of autoregulation in 3-phosphoinositide-dependent Grp1 family Arf GTPase exchange factors

Arf GTPases regulate membrane trafficking and actin dynamics. Grp1, ARNO, and Cytohesin-1 comprise a family of phosphoinositide-dependent Arf GTPase exchange factors with a Sec7-pleckstrin homology (PH) domain tandem. Here, we report that the exchange activity of the Sec7 domain is potently autoinhibited by conserved elements proximal to the PH domain. The crystal structure of the Grp1 Sec7-PH tandem reveals a pseudosubstrate mechanism of autoinhibition in which the linker region between domains and a C-terminal amphipathic helix physically block the docking sites for the switch regions of Arf GTPases. Mutations within either element result in partial or complete activation. Critical determinants of autoinhibition also contribute to insulin-stimulated plasma membrane recruitment. Autoinhibition can be largely reversed by binding of active Arf6 to Grp1 and by phosphorylation of tandem PKC sites in Cytohesin-1. These observations suggest that Grp1 family GEFs are autoregulated by mechanisms that depend on plasma membrane recruitment for activation.

Interrogating Yeast Surface-displayed Human Proteome to Identify Small Molecule-binding Proteins

Identifying proteins that interact with small molecules is often a challenging step in understanding cellular signaling pathways or molecular mechanisms of drug action. In this report, we describe the construction of libraries displaying human protein fragments on the surface of yeast cells and demonstrate the utility of these libraries for the study of small molecule/protein interactions. The libraries were used to select protein fragments with affinity for the phosphatidylinositides phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3). We recovered cDNA inserts encoding pleckstrin homology domains, a phosphotyrosine-binding domain, and a fragment of apolipoprotein H. The pleckstrin homology and phosphotyrosine-binding domains are known phosphatidylinositide-binding domains, demonstrating the effectiveness of our approach. Binding of apolipoprotein H to PtdIns(4,5)P2 and PtdIns(3,4,5)P3 has not been reported previously and thus represents novel interactions. We expect that this method will be generally applicable to the study of small molecule/protein interactions and may facilitate the study of cellular signaling pathways and mechanisms of drug action or toxicity.

ARL4D recruits cytohesin-2/ARNO to modulate actin remodeling

ARL4D is a developmentally regulated member of the ADP-ribosylation factor/ARF-like protein (ARF/ARL) family of Ras-related GTPases. Although the primary structure of ARL4D is very similar to that of other ARF/ARL molecules, its function remains unclear. Cytohesin-2/ARF nucleotide-binding-site opener (ARNO) is a guanine nucleotide-exchange factor (GEF) for ARF, and, at the plasma membrane, it can activate ARF6 to regulate actin reorganization and membrane ruffling. We show here that ARL4D interacts with the C-terminal pleckstrin homology (PH) and polybasic c domains of cytohesin-2/ARNO in a GTP-dependent manner. Localization of ARL4D at the plasma membrane is GTP- and N-terminal myristoylation-dependent. ARL4D(Q80L), a putative active form of ARL4D, induced accumulation of cytohesin-2/ARNO at the plasma membrane. Consistent with a known action of cytohesin-2/ARNO, ARL4D(Q80L) increased GTP-bound ARF6 and induced disassembly of actin stress fibers. Expression of inactive cytohesin-2/ARNO(E156K) or small interfering RNA knockdown of cytohesin-2/ARNO blocked ARL4D-mediated disassembly of actin stress fibers. Similar to the results with cytohesin-2/ARNO or ARF6, reduction of ARL4D suppressed cell migration activity. Furthermore, ARL4D-induced translocation of cytohesin-2/ARNO did not require phosphoinositide 3-kinase activation. Together, these data demonstrate that ARL4D acts as a novel upstream regulator of cytohesin-2/ARNO to promote ARF6 activation and modulate actin remodeling.

Supervised membrane swimming: small G-protein lifeguards regulate PIPK signalling and monitor intracellular PtdIns(4,5)P2 pools

Regulation of PIPK (phosphatidylinositol phosphate kinase) and PtdIns(4,5)P2 signalling by small G-proteins and their effectors is key to many biological functions. Through selective recruitment and activation of different PIPK isoforms, small G-proteins such as Rho, Rac and Cdc42 modulate actin dynamics and cytoskeleton-dependent cellular events in response to extracellular signalling. These activities affect a number of processes, including endocytosis, bacterial penetration into host cells and cytolytic granule-mediated targeted cell killing. Small G-proteins and their modulators are also regulated by phosphoinositides through translocation and conformational changes. Arf family small G-proteins act at multiple sites as regulators of membrane trafficking and actin cytoskeletal remodelling, and regulate a feedback loop comprising phospholipase D, phosphatidic acid, PIPKs and PtdIns(4,5)P2, contributing to enhancement of PtdIns(4,5)P2-mediated cellular events and receptor signalling. Na+, Kir (inwardly rectifying K+), Ca2+ and TRP (transient receptor potential) ion channels are regulated by small G-proteins and membrane pools of PtdIns(4,5)P2. Yeast phosphatidylinositol 4-phosphate 5-kinases Mss4 and Its3 are involved in resistance against disturbance of sphingolipid biosynthesis and maintenance of cell integrity through the synthesis of PtdIns(4,5)P2 and downstream signalling through the Rom2/Rho2 and Rgf1/Rho pathways. Here, we review models for regulated intracellular targeting of PIPKs by small G-proteins and other modulators in response to extracellular signalling. We also describe the spatial and temporal cross-regulation of PIPKs and small G-proteins that is critical for a number of cellular functions.

Role of the Arf6 GDP/GTP cycle and Arf6 GTPase-activating proteins in actin remodeling and intracellular transport

We have analyzed both biochemically and functionally a series of Arf6 mutants, providing new insights into the molecular mode of action of the small G protein Arf6. First, by comparing a fast-cycling mutant (Arf6(T157N)) and a GTPase-deficient mutant (Arf6(Q67L)), we established the necessity for completion of the Arf6 GDP/GTP cycle for recycling of major histocompatibility complex molecules to the plasma membrane. Second, we found that aluminum fluoride (AlF), known for inducing membrane protrusion in cells expressing exogenous wild-type Arf6, stabilized a functional wild-type Arf6.AlF(x) . GTPase-activating protein (GAP) complex in vitro and in vivo. We also found that the tandem mutation Q37E/S38I prevented the binding of two Arf GAPs, but not the effector ARHGAP10, and blocked the formation of membrane protrusion and actin reorganization. Together, our results with AlF(x) and Arf6(Q37E/S38I) demonstrate the critical role of the Arf6 GAPs as effectors for Arf6-regulated actin cytoskeleton remodeling. Finally, competition experiments conducted in vivo suggest the existence of a membrane receptor for GDP-bound Arf6.

ARNO through its coiled-coil domain regulates endocytosis at the apical surface of polarized epithelial cells

ARNO is a guanine-nucleotide exchange protein for the ARF family of GTPases. Here we show that in polarized epithelial cells, ARNO is localized exclusively to the apical plasma membrane, where it regulates endocytosis. Expression of ARNO stimulates apical endocytosis of the polymeric immunoglobulin receptor, and coexpression of ARF6 with ARNO leads to a synergistic stimulation of apical endocytosis. Expression of a dominant negative ARF6 mutant, ARF6-T27N, antagonizes this stimulatory effect. Deletion of the N-terminal coiled-coil (CC) domain of ARNO causes the mutant ARNO to localize to both the apical and basolateral plasma membranes. Expression of the CC domain alone abolishes ARNO-induced apical endocytosis as well as co-localization of IgA-receptor complexes with ARNO and clathrin. These results suggest that the CC domain contributes to the specificity of apical localization of ARNO through association with components of the apical plasma membrane. We conclude that ARNO acts together with ARF6 to regulate apical endocytosis.

AlFx affects the formation of focal complexes by stabilizing the Arf-GAP ASAP1 in a complex with Arf1

Aluminum fluoride (AlFx) is known to activate directly the alpha subunit of G-proteins but not the homologous small GTP-binding proteins. However, AlFx can stabilize complexes formed between Ras, RhoA or Cdc42 and their corresponding GTPase-activating proteins (GAPs). Here, we demonstrate that Arf1GDP can be converted into an active conformation by AlFx to form a complex with the Arf-GAP ASAP1 in vitro and in vivo. Within this complex ASAP1, which GAP activity is inoperative, can still alter the recruitment of paxillin to the focal complexes, thus indicating that ASAP1 interferes with focal complexes independently of its GAP activity.

Docosahexaenoic acid: a positive modulator of Akt signaling in neuronal survival

Phosphatidylinositol 3-kinase [PI (3)K]/Akt signaling is a critical pathway in cell survival. Here, we demonstrate a mechanism where membrane alteration by the n-3 fatty acid status affects Akt signaling, impacting neuronal survival. Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid highly enriched in neuronal membranes, promotes neuronal survival by facilitating membrane translocation/activation of Akt through its capacity to increase phosphatidylserine (PS), the major acidic phospholipid in cell membranes. The activation of PI (3)K and phosphatidylsinositol triphosphate formation were not affected by DHA, indicating that membrane interaction of Akt is the event responsible for the DHA effect. Docosapentaenoic acid, which replaces DHA during n-3 fatty acid deficiency, was less effective in accumulating PS and translocating Akt and thus less effective in preventing apoptosis. Consistently, in vivo reduction of DHA by dietary depletion of n-3 fatty acids decreased hippocampal PS and increased neuronal susceptibility to apoptosis in cultures. This mechanism may contribute to neurological deficits associated with n-3 fatty acid deficiency and support protective effects of DHA in pathological models such as brain ischemia or Alzheimer's disease.

The small G-protein Arf6GTP recruits the AP-2 adaptor complex to membranes

The small GTP-binding protein ADP-ribosylation factor 6 (Arf6) is involved in plasma membrane/endosomes trafficking. However, precisely how the activation of Arf6 regulates vesicular transport is still unclear. Here, we show that, in vitro, recombinant Arf6GTP recruits purified clathrin-adaptor complex AP-2 (but not AP-1) onto phospholipid liposomes in the absence of phosphoinositides. We also show that phosphoinositides and Arf6 tightly cooperate to translocate AP-2 to the membrane. In vivo, Arf6GTP (but not Arf6GDP) was found associated to AP-2. The expression of the GTP-locked mutant of Arf6 leads to the plasma membrane redistribution of AP-2 in Arf6GTP-enriched areas. Finally, we demonstrated that the expression of the GTP-locked mutant of Arf6 inhibits transferrin receptor internalization without affecting its recycling. Altogether, our results demonstrated that Arf6GTP interacts specifically with AP-2 and promotes its membrane recruitment. These findings strongly suggest that Arf6 plays a major role in clathrin-mediated endocytosis by directly controlling the assembly of the AP-2/clathrin coat.

GRP1 pleckstrin homology domain: activation parameters and novel search mechanism for rare target lipid

Pleckstrin homology (PH) domains play a central role in a wide array of signaling pathways by binding second messenger lipids of the phosphatidylinositol phosphate (PIP) lipid family. A given type of PIP lipid is formed in a specific cellular membrane where it is generally a minor component of the bulk lipid mixture. For example, the signaling lipid PI(3,4,5)P(3) (or PIP(3)) is generated primarily in the inner leaflet of the plasma membrane where it is believed to never exceed 0.02% of the bulk lipid. The present study focuses on the PH domain of the general receptor for phosphoinositides, isoform 1 (GRP1), which regulates the actin cytoskeleton in response to PIP(3) signals at the plasma membrane surface. The study systematically analyzes both the equilibrium and kinetic features of GRP1-PH domain binding to its PIP lipid target on a bilayer surface. Equilibrium binding measurements utilizing protein-to-membrane fluorescence resonance energy transfer (FRET) to detect GRP1-PH domain docking to membrane-bound PIP lipids confirm specific binding to PIP(3). A novel FRET competitive binding measurement developed to quantitate docking affinity yields a K(D) of 50 +/- 10 nM for GRP1-PH domain binding to membrane-bound PIP(3) in a physiological lipid mixture approximating the composition of the plasma membrane inner leaflet. This observed K(D) lies in a suitable range for regulation by physiological PIP(3) signals. Interestingly, the affinity of the interaction decreases at least 12-fold when the background anionic lipids phosphatidylserine (PS) and phosphatidylinositol (PI) are removed from the lipid mixture. Stopped-flow kinetic studies using protein-to-membrane FRET to monitor association and dissociation time courses reveal that this affinity decrease arises from a corresponding decrease in the on-rate for GRP1-PH domain docking with little or no change in the off-rate for domain dissociation from membrane-bound PIP(3). Overall, these findings indicate that the PH domain interacts not only with its target lipid, but also with other features of the membrane surface. The results are consistent with a previously undescribed type of two-step search mechanism for lipid binding domains in which weak, nonspecific electrostatic interactions between the PH domain and background anionic lipids facilitate searching of the membrane surface for PIP(3) headgroups, thereby speeding the high-affinity, specific docking of the domain to its rare target lipid.

The GDP-bound form of Arf6 is located at the plasma membrane

The function of Arf6 has been investigated largely by using the T27N and the Q67L mutants, which are thought to be blocked in GDP- and GTP-bound states, respectively. However, these mutants have been poorly characterized biochemically. Here, we found that Arf6(T27N) is not an appropriate marker of the inactive GDP-bound form because it has a high tendency to lose its nucleotide in vitro and to denature. As a consequence, most of the protein is aggregated in vivo and localizes to detergent-insoluble structures. However, a small proportion of Arf6(T27N) is able to form a stable complex with its exchange factor EFA6 at the plasma membrane, accounting for its dominant-negative phenotype. To define the cellular localization of Arf6-GDP, we designed a new mutant, Arf6(T44N). In vitro, this mutant has a 30-fold decreased affinity for GTP. In vivo, it is mostly GDP bound and, in contrast to the wild type, does not switch to the active conformation when expressed with EFA6. This GDP-locked mutant is found at the plasma membrane, where it localizes with EFA6 and Ezrin in actin- and phosphatidylinositol (4,5)-bisphosphate-enriched domains. From these results, we conclude that the Arf6 GDP-GTP cycle takes place at the plasma membrane.

C-terminal anchoring of mid1p to membranes stabilizes cytokinetic ring position in early mitosis in fission yeast

mid1p is a key factor for the central positioning of the cytokinetic ring in Schizosaccharomyces pombe. In interphase and early mitosis, mid1p forms a medial cortical band overlying the nucleus, which may represent a landmark for cytokinetic ring assembly. It compacts before anaphase into a tight ring with other cytokinetic ring components. We show here that mid1p binds to the medial cortex by at least two independent means. First, mid1p C-terminus association with the cortex requires a putative amphipathic helix adjacent to mid1p nuclear localization sequence (NLS), which is predicted to insert directly into the lipid bilayer. This association is stabilized by the polybasic NLS. mid1p mutated within the helix and the NLS forms abnormal filaments in early mitosis that are not properly anchored to the medial cortex. Misplaced rings assemble in late mitosis, indicating that mid1p C-terminus binding to membranes stabilizes cytokinetic ring position. Second, the N terminus of mid1p has the ability to associate faintly with the medial cortex and is sufficient to form tight rings. In addition, we show that mid1p oligomerizes. We propose that membrane-bound oligomers of mid1p assemble recruitment "platforms" for cytokinetic ring components at the medial cortex and stabilize the ring position during its compaction.

Structural determinants of phosphoinositide selectivity in splice variants of Grp1 family PH domains

The pleckstrin homology (PH) domains of the homologous proteins Grp1 (general receptor for phosphoinositides), ARNO (Arf nucleotide binding site opener), and Cytohesin-1 bind phosphatidylinositol (PtdIns) 3,4,5-trisphosphate with unusually high selectivity. Remarkably, splice variants that differ only by the insertion of a single glycine residue in the beta1/beta2 loop exhibit dual specificity for PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2). The structural basis for this dramatic specificity switch is not apparent from the known modes of phosphoinositide recognition. Here, we report crystal structures for dual specificity variants of the Grp1 and ARNO PH domains in either the unliganded form or in complex with the head groups of PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3). Loss of contacts with the beta1/beta2 loop with no significant change in head group orientation accounts for the significant decrease in PtdIns(3,4,5)P(3) affinity observed for the dual specificity variants. Conversely, a small increase rather than decrease in affinity for PtdIns(4,5)P(2) is explained by a novel binding mode, in which the glycine insertion alleviates unfavorable interactions with the beta1/beta2 loop. These observations are supported by a systematic mutational analysis of the determinants of phosphoinositide recognition.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

The structural requirements for ceramide activation of serine-threonine protein phosphatases

The protein phosphatases1 (PP1) and 2A (PP2A) serve as ceramide-activated protein phosphatases (CAPP). In this study, the structural requirements for interaction between ceramide and CAPP were determined. D-erythro-C(6) ceramide activated the catalytic subunit of PP2A (PP2Ac) approximately 3-fold in a stereospecific manner. In contrast, saturation of the 4-5 double bond, producing D-erythro-dihydro C(6) ceramide, inhibited PP2Ac (IC(50) = 8.5 microM). Furthermore, phyto C(6) ceramide, D-erythro-dehydro C(6) ceramide, and D-erythro-cis-C(6) ceramide had no effect on PP2Ac activity. Modification of the sphingoid chain also abolished the ability of ceramide to activate PP2Ac. Further studies demonstrated the requirement for the amide group, the primary hydroxyl group, and the secondary hydroxyl group of the sphingoid backbone for activation of PP2Ac through the synthesis and evaluation of D-erythro-urea C(6) ceramide, L-erythro-urea C(6) ceramide, D-erythro-N-methyl C(6) ceramide, D-erythro-L-O-methyl C(6) ceramide, D-erythro-3-O-methyl C(6) ceramide, and (2S) 3-keto C(6) ceramide. None of these compounds induced significant activation of PP2Ac. Liposome binding studies were also conducted using analogs of D-erythro-C C(6) ceramide, and the results showed that the ability of ceramide analogs to influence CAPP (activation or inhibition) was associated with the ability of the analogs to bind to CAPP. This study demonstrates strict structural requirements for interaction of ceramide with CAPP, and disclose ceramide as a very specific regulator of CAPP. The studies also begin to define features that transform ceramide analogs into inhibitors of CAPP.

Exploring the sialome of the blood-sucking bug Rhodnius prolixus

Rhodnius prolixus is a Hemiptera that feeds exclusively on vertebrate blood in all life stages. Its salivary glands produce potent pharmacological substances that counteract host hemostasis, including anti-clotting, anti-platelet, and vasodilatory substances. To obtain a further insight into the salivary biochemical and pharmacological complexity of this insect, a cDNA library was randomly sequenced, and salivary gland homogenates were fractionated by HPLC to obtain aminoterminal sequences of abundantly expressed proteins. Results indicate a remarkable expansion of the lipocalin family in Rhodnius salivary glands, among other protein sequences described. A summary of 31 new full length proteins deducted from their mRNA sequence is described, including several new members of the nitrophorin, triabin, and pallidipin families. The electronic version of the complete tables is available at http://www.ncbi.nlm.nih.gov/projects/vectors/rhodnius_prolixus.

Specific localization and timing in neuronal signal transduction mediated by protein-lipid interactions

A large number of signaling proteins translocate from the cytosol to the plasma membrane in response to receptor and electrical stimuli. The site of translocation to the plasma membrane and the "on" and "off" rates of the translocation process are critical for defining the specificity of the signaling response. In addition to targeting mechanisms based on protein-protein interactions, signaling proteins have evolved a large repertoire of covalent lipid modifications and lipid binding protein modules that regulate reversible membrane association. The time constants of these membrane interactions range from milliseconds to several hours. Here we discuss how diversity in lipid-based membrane anchoring and targeting motifs contributes to plasticity in neuronal signaling by providing local and regional control mechanisms as well as a means to transduce and integrate signals over a broad range of different time scales.

The N-terminal coiled coil domain of the cytohesin/ARNO family of guanine nucleotide exchange factors interacts with the scaffolding protein CASP

Cytohesin is a guanine nucleotide exchange factor that regulates members of the ADP-ribosylation factor (ARF) family of small GTPases. All of the members of the cytohesin family (including ARNO, ARNO3, and the newly characterized cytohesin-4) have a similar domain distribution consisting of a Sec7 homology domain, a pleckstrin homology domain, and an N-terminal coiled coil. In this study, we attempt to identify proteins that interact specifically with the coiled coil motif of cytohesin. Yeast two-hybrid screening of a B cell library using the cytohesin N terminus as bait, identified CASP, a scaffolding protein of previously unknown function, as a binding partner. CASP contains an internal coiled coil motif that is required for cytohesin binding both in vitro and in COS-1 cells. The specificity of the coiled coil of CASP is not restricted to cytohesin, however, because it is also capable of interacting with other members of the cytohesin/ARNO family, ARNO and ARNO3. In immunofluorescence experiments, CASP localizes to perinuclear tubulovesicular structures that are in close proximity to the Golgi. These structures remain relatively undisturbed when the cells are treated with brefeldin A. In epidermal growth factor-stimulated COS-1 cells overexpressing cytohesin and CASP, cytohesin recruits CASP to membrane ruffles, revealing a functional interaction between the two proteins. These observations collectively suggest that CASP is a scaffolding protein that facilitates the function of at least one member of the cytohesin/ARNO family in response to specific cellular stimuli.

Protective effects of docosahexaenoic acid in staurosporine-induced apoptosis: involvement of phosphatidylinositol-3 kinase pathway

Docosahexaenoic acid (22:6n-3, DHA) is highly enriched in neuronal membranes and is considered to be essential for proper brain function. We have previously demonstrated in Neuro 2A cells that DHA as a membrane component protects cells from apoptotic death induced by serum deprivation (Kim et al. 2000). In the present study we demonstrate that staurosporine (ST) induces apoptosis in Neuro 2A cells and DHA enrichment prior to the ST treatment significantly inhibits the apoptotic cell death, as evidenced by the reduction of caspase-3 activity, cleavage of pro-caspase-3 to active caspase-3, DNA strand-breaking and laddering. Enrichment of cells with other fatty acids such as oleic and arachidonic acids did not exert such an effect, indicating that the antiapoptotic effect was specific to DHA enrichment. Among the several protein kinase inhibitors, only phosphatidylinositol 3-kinase (PI3-K) inhibitors, wortmanin, and LY-294002 abolished the protective effect of DHA in ST-induced apoptosis. Concurrently, ST-treatment significantly decreased the phosphorylation status of Akt at Ser-473 and Thr-308 as well as Akt activity, and this reduction was partially prevented by DHA enrichment. The extent of the antiapoptotic effect of DHA correlated with a time-dependent increase in the phosphatidylserine (PS) content upon DHA enrichment. When cells were enriched with DHA in serine-free medium, the PS increase diminished and the DHA effect on caspase-3 activation as well as Akt phosphorylation in ST-induced apoptosis was no longer apparent, suggesting that DHA's role in accumulating membrane PS is an important component for the observed protection. In summary, DHA enrichment uniquely protects ST-induced apoptosis in a PS- and PI3-K-dependent manner. From these data, we suggest that the antiapoptotic effect of DHA is mediated at least in part through the PI3-K/Akt pathway, facilitated by DHA-induced PS accumulation.