The Arf6 GAP centaurin α-1 is a neuronal actin-binding protein which also functions via GAP-independent activity to regulate the actin cytoskeleton

PI3K-alpha translocation mediates nuclear PtdIns(3,4,5)P3 effector signaling in colorectal cancer

The canonical view of phosphatidylinositol 3-kinase alpha (PI3Kα) signaling describes PtdIns(3,4,5)P₃ generation and activation of downstream effectors at the plasma membrane or at microtubule-bound endosomes. Here, we show that colorectal cancer (CRC) cell lines exhibit a diverse plasma membrane-nuclear distribution of PI3Kα, controlling corresponding levels of subcellular PtdIns(3,4,5)P₃ pools. PI3Kα nuclear translocation was mediated by the importin β-dependent nuclear import pathway. By PtdIns(3,4,5)P₃ affinity capture mass spectrometry done in the presence of SDS on CRC cell lines with PI3Kα nuclear localization, we identified 867 potential nuclear PtdIns(3,4,5)P₃ effector proteins. Nuclear PtdIns(3,4,5)P₃ interactome proteins were characterized by non-canonical PtdIns(3,4,5)P₃ binding domains and showed overrepresentation for nuclear membrane, nucleolus and nuclear speckles. The nuclear PtdIns(3,4,5)P₃ interactome was enriched for proteins related to RNA metabolism, with splicing reporter assays and SC-35 foci staining suggesting a role of EGF-stimulated nuclear PI3Kα signaling in modulating pre-mRNA splicing. In patient tumors, nuclear p110α staining was associated with lower T stage and mucinous histology. These results indicate that PI3Kα translocation mediates nuclear PtdIns(3,4,5)P₃ effector signaling in human CRC, modulating signaling responses.

Functional Role of AGAP2/PIKE-A in Fcγ Receptor-Mediated Phagocytosis

In phagocytes, cytoskeletal and membrane remodeling is finely regulated at the phagocytic cup. Various smaFll G proteins, including those of the Arf family, control these dynamic processes. Human neutrophils express AGAP2, an Arf GTPase activating protein (ArfGAP) that regulates endosomal trafficking and focal adhesion remodeling. We first examined the impact of AGAP2 on phagocytosis in CHO cells stably expressing the FcγRIIA receptor (CHO-IIA). In unstimulated CHO-IIA cells, AGAP2 only partially co-localized with cytoskeletal elements and intracellular compartments. In CHO-IIA cells, AGAP2 transiently accumulated at actin-rich phagocytic cups and increased Fcγ receptor-mediated phagocytosis. Enhanced phagocytosis was not dependent on the N-terminal GTP-binding protein-like (GLD) domain of AGAP2. AGAP2 deleted of its GTPase-activating protein (GAP) domain was not recruited to phagocytic cups and did not enhance the engulfment of IgG-opsonized beads. However, the GAP-deficient [R618K]AGAP2 transiently localized at the phagocytic cups and enhanced phagocytosis. In PLB-985 cells differentiated towards a neutrophil-like phenotype, silencing of AGAP2 reduced phagocytosis of opsonized zymosan. In human neutrophils, opsonized zymosan or monosodium urate crystals induced AGAP2 phosphorylation. The data indicate that particulate agonists induce AGAP2 phosphorylation in neutrophils. This study highlights the role of AGAP2 and its GAP domain but not GAP activity in FcγR-dependent uptake of opsonized particles.

ADAP1 promotes latent HIV-1 reactivation by selectively tuning KRAS-ERK-AP-1 T cell signaling-transcriptional axis

Immune stimulation fuels cell signaling-transcriptional programs inducing biological responses to eliminate virus-infected cells. Yet, retroviruses that integrate into host cell chromatin, such as HIV-1, co-opt these programs to switch between latent and reactivated states; however, the regulatory mechanisms are still unfolding. Here, we implemented a functional screen leveraging HIV-1's dependence on CD4⁺ T cell signaling-transcriptional programs and discovered ADAP1 is an undescribed modulator of HIV-1 proviral fate. Specifically, we report ADAP1 (ArfGAP with dual PH domain-containing protein 1), a previously thought neuronal-restricted factor, is an amplifier of select T cell signaling programs. Using complementary biochemical and cellular assays, we demonstrate ADAP1 inducibly interacts with the immune signalosome to directly stimulate KRAS GTPase activity thereby augmenting T cell signaling through targeted activation of the ERK-AP-1 axis. Single cell transcriptomics analysis revealed loss of ADAP1 function blunts gene programs upon T cell stimulation consequently dampening latent HIV-1 reactivation. Our combined experimental approach defines ADAP1 as an unexpected tuner of T cell programs facilitating HIV-1 latency escape.

ADAP1/Centaurin-α1 Negatively Regulates Dendritic Spine Function and Memory Formation in the Hippocampus

ADAP1/Centaurin-α1 (CentA1) functions as an Arf6 GTPase-activating protein highly enriched in the brain. Previous studies demonstrated the involvement of CentA1 in brain function as a regulator of dendritic differentiation and a potential mediator of Alzheimer’s disease (AD) pathogenesis. To better understand the neurobiological functions of CentA1 signaling in the brain, we developed Centa1 knock-out (KO) mice. The KO animals showed neither brain development nor synaptic ultrastructure deficits in the hippocampus. However, they exhibited significantly higher density and enhanced structural plasticity of dendritic spines in the CA1 region of the hippocampus compared with non-transgenic (NTG) littermates. Moreover, the deletion of Centa1 improved performance in the object-in-place (OIP) spatial memory task. These results suggest that CentA1 functions as a negative regulator of spine density and plasticity, and of hippocampus-dependent memory formation. Thus, CentA1 and its downstream signaling may serve as a potential therapeutic target to prevent memory decline associated with aging and brain disorders.

In vitro reconstitution reveals phosphoinositides as cargo-release factors and activators of the ARF6 GAP ADAP1

The differentiation of cells depends on a precise control of their internal organization, which is the result of a complex dynamic interplay between the cytoskeleton, molecular motors, signaling molecules, and membranes. For example, in the developing neuron, the protein ADAP1 (ADP-ribosylation factor GTPase-activating protein [ArfGAP] with dual pleckstrin homology [PH] domains 1) has been suggested to control dendrite branching by regulating the small GTPase ARF6. Together with the motor protein KIF13B, ADAP1 is also thought to mediate delivery of the second messenger phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) to the axon tip, thus contributing to PIP₃ polarity. However, what defines the function of ADAP1 and how its different roles are coordinated are still not clear. Here, we studied ADAP1's functions using in vitro reconstitutions. We found that KIF13B transports ADAP1 along microtubules, but that PIP₃ as well as PI(3,4)P₂ act as stop signals for this transport instead of being transported. We also demonstrate that these phosphoinositides activate ADAP1's enzymatic activity to catalyze GTP hydrolysis by ARF6. Together, our results support a model for the cellular function of ADAP1, where KIF13B transports ADAP1 until it encounters high PIP₃/PI(3,4)P₂ concentrations in the plasma membrane. Here, ADAP1 disassociates from the motor to inactivate ARF6, promoting dendrite branching.

ADAP1 promotes invasive squamous cell carcinoma progression and predicts patient survival

ADAP1, a GTPase-activating protein (GAP) for the small GTPase ARF6, is a strong predictor of poor survival in early-stage squamous cell carcinoma patients and a critical mediator of TGF-β-induced invasive cell migration by facilitating basement membrane breakdown.

Invasive squamous cell carcinoma (SCC) is aggressive cancer with a high risk of recurrence and metastasis, but the critical determinants of its progression remain elusive. Here, we identify ADAP1, a GTPase-activating protein (GAP) for ARF6 up-regulated in TGF-β-responding invasive tumor cells, as a strong predictor of poor survival in early-stage SCC patients. Using a mouse model of SCC, we show that ADAP1 overexpression promotes invasive tumor progression by facilitating cell migration and breakdown of the basement membrane. We found that ADAP1-rich, TGF-β-responding tumor cells exhibit cytoplasmic laminin localization, which correlated with the absence of laminin and type IV collagen from the pericellular basement membrane. Interestingly, although tumors overexpressing a GAP activity-deficient mutant of ADAP1 resulted in morphologically complex tumors, those tumor cells failed to breach the basement membrane. Moreover, Adap1 deletion in tumor cells ameliorated the basement membrane breakdown and had less invading cells in the stroma. Our study demonstrates that ADAP1 is a critical mediator of TGF-β-induced cancer invasion and might be exploited for the treatment of high-risk SCC.

ADAP1 limits neonatal cardiomyocyte hypertrophy by reducing integrin cell surface expression

The ArfGAP with dual PH domains 1 (ADAP1) regulates the activation of the hypertrophic mitogen-activated protein kinase ERK1/2 pathway in non-cardiomyocytes. However, its role in cardiomyocytes is unknown. Our aim was to characterize the role of ADAP1 in the hypertrophic process of cardiomyocytes. We assessed the expression of ADAP1 in the hearts of adult and neonatal rats by RT-qPCR and Western blotting and showed that it is preferentially expressed in cardiomyocytes. Adenoviral-mediated ADAP1 overexpression in cultured rat neonatal ventricular cardiomyocytes limited their serum-induced hypertrophic response as measured by immunofluorescence microscopy. Furthermore, ADAP1 overexpression completely blocked phenylephrine- and Mek1 constitutively active (Mek1ca) mutant-induced hypertrophy in these cells. The anti-hypertrophic effect of ADAP1 was not caused by a reduction in protein synthesis, interference with the Erk1/2 pathway, or disruption of the fetal gene program activation, as assessed by nascent protein labeling, Western blotting, and RT-qPCR, respectively. An analysis of cultured cardiomyocytes by confocal microscopy revealed that ADAP1 partially re-organizes α-actinin into dense puncta, a phenomenon that is synergized by Mek1ca overexpression. Biotin labeling of cell surface proteins from cardiomyocytes overexpressing ADAP1 revealed that it reduces the surface expression of β1-integrin, an effect that is strongly potentiated by Mek1ca overexpression. Our findings provide insights into the anti-hypertrophic function of ADAP1 in cardiomyocytes.

Regulators and Effectors of Arf GTPases in Neutrophils

Polymorphonuclear neutrophils (PMNs) are key innate immune cells that represent the first line of defence against infection. They are the first leukocytes to migrate from the blood to injured or infected sites. This process involves molecular mechanisms that coordinate cell polarization, delivery of receptors, and activation of integrins at the leading edge of migrating PMNs. These phagocytes actively engulf microorganisms or form neutrophil extracellular traps (NETs) to trap and kill pathogens with bactericidal compounds. Association of the NADPH oxidase complex at the phagosomal membrane for production of reactive oxygen species (ROS) and delivery of proteolytic enzymes into the phagosome initiate pathogen killing and removal. G protein-dependent signalling pathways tightly control PMN functions. In this review, we will focus on the small monomeric GTPases of the Arf family and their guanine exchange factors (GEFs) and GTPase activating proteins (GAPs) as components of signalling cascades regulating PMN responses. GEFs and GAPs are multidomain proteins that control cellular events in time and space through interaction with other proteins and lipids inside the cells. The number of Arf GAPs identified in PMNs is expanding, and dissecting their functions will provide important insights into the role of these proteins in PMN physiology.

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

Tubulin potentiates the interaction of the metalloendopeptidase nardilysin with the neuronal scaffold protein p42IP4/centaurin-α1 (ADAP1)

We found colocalization of the neuronal protein p42(IP4) (centaurin-α1; ArfGAP with dual pleckstrin homology domain [ADAP1]), the metalloendopeptidase nardilysin (NRD; involved in axonal maturation and myelination) and tubulin in the cytosol and at the plasma membrane of SH-SY5Y neuroblastoma cells. To examine the importance of tubulin for the interaction of NRD with p42(IP4), we treated cells with nocodazole, which interferes with tubulin polymerization. Nocodazole did not affect the colocalization of p42(IP4) and tubulin but caused a clear redistribution of the proteins in cells, so that the colocalization of p42(IP4), tubulin and NRD was visible exclusively in multiple foci. To reveal the mechanism of the interaction between NRD, p42(IP4) and tubulin observed in neuronal cells, we performed Far-Western blotting, a technique that directly detects protein-protein interactions on Western blots. This technique demonstrated that tubulin enhanced the binding of NRD to functionally renatured p42(IP4). The mutation of a highly conserved cysteine residue in NRD to alanine abolished the potentiation by tubulin. NRD lacking the characteristic acidic domain was able to bind p42(IP4) but addition of tubulin did not significantly potentiate the binding of this deletion mutant to p42(IP4). A function-abolishing mutation of the Zn(2+)-binding motif of NRD did not affect the potentiation by tubulin. Thus, the capacity of tubulin to enhance the interaction between p42(IP4) and NRD together with the known interaction of p42(IP4) with F-actin support the novel notion that p42(IP4) plays a possible role as a linker between the two networks, actin and tubulin, in neural cells.

High-Throughput Fluorescence Polarization Assay for the Enzymatic Activity of GTPase-Activating Protein of ADP-Ribosylation Factor (ARFGAP)

GTPase-activating proteins of ADP-ribosylation factors (ARFGAPs) play key cellular roles in vesicle production and trafficking, adhesion, migration, and development. Dysfunctional regulation of ARFGAPs has been implicated in various diseases, including cancer, Alzheimer disease, and autism. Unfortunately, there are few mechanistic details describing how ARFGAPs contribute to disease states. In this regard, it would be extremely helpful to have a set of small molecules that selectively and directly modulate specific ARFGAPs as probes to dissect ARFGAP-regulated cell signaling under various conditions. Currently, such probes are lacking, and their identification is hampered by the lack of a suitable high-throughput assay to monitor ARFGAP activity. Here, the authors describe and validate a robust high-throughput assay using fluorescence polarization to monitor the ability of diverse ARFGAPs to enhance the capacity of ARF1 to hydrolyze guanosine triphosphate.

CAPRI targets T29-T42: proving ground for new docking procedures

The critical assessment of protein interactions (CAPRI) experiment provides a unique opportunity for unbiased assessment of docking procedures. The recent CAPRI targets T29-T42 entailed docking of bound, unbound, and modeled structures, presenting a wide range of prediction difficulty. We submitted accurate predictions for targets T40, T41, and T42, a good prediction for T32 and acceptable predictions for T29 and T34. The accuracy of our docking results generally matched the prediction difficulty; hence, docking of modeled proteins produced less accurate results. However, there were interesting exceptions: an accurate prediction was submitted for the dimer of modeled tetratricopeptide repeat (T42) and only an acceptable prediction for the bound/unbound case T29. The ensembles of docking models produced in the scans included an acceptable or better prediction for every target. We show here that our recently developed postscan reevaluation procedure, which tests propensity and solvation measures of the whole interface and the interface core, successfully distinguished these predictions from false docking models. For enzyme-inhibitor targets, we show that the distance of the interface from the enzyme's centroid ranked high native like docking models. Also, for one case we demonstrate that docking of an ensemble of conformers produced by normal modes analysis can improve the accuracy of the prediction.

RNA interference-mediated knockdown of CD49e (α5 integrin chain) in human thymic epithelial cells modulates the expression of multiple genes and decreases thymocyte adhesion

BACKGROUND

The thymus is a central lymphoid organ, in which bone marrow-derived T cell precursors undergo a complex process of maturation. Developing thymocytes interact with thymic microenvironment in a defined spatial order. A component of thymic microenvironment, the thymic epithelial cells, is crucial for the maturation of T-lymphocytes through cell-cell contact, cell matrix interactions and secretory of cytokines/chemokines. There is evidence that extracellular matrix molecules play a fundamental role in guiding differentiating thymocytes in both cortical and medullary regions of the thymic lobules. The interaction between the integrin α5β1 (CD49e/CD29; VLA-5) and fibronectin is relevant for thymocyte adhesion and migration within the thymic tissue. Our previous results have shown that adhesion of thymocytes to cultured TEC line is enhanced in the presence of fibronectin, and can be blocked with anti-VLA-5 antibody.

RESULTS

Herein, we studied the role of CD49e expressed by the human thymic epithelium. For this purpose we knocked down the CD49e by means of RNA interference. This procedure resulted in the modulation of more than 100 genes, some of them coding for other proteins also involved in adhesion of thymocytes; others related to signaling pathways triggered after integrin activation, or even involved in the control of F-actin stress fiber formation. Functionally, we demonstrated that disruption of VLA-5 in human TEC by CD49e-siRNA-induced gene knockdown decreased the ability of TEC to promote thymocyte adhesion. Such a decrease comprised all CD4/CD8-defined thymocyte subsets.

CONCLUSION

Conceptually, our findings unravel the complexity of gene regulation, as regards key genes involved in the heterocellular cell adhesion between developing thymocytes and the major component of the thymic microenvironment, an interaction that is a mandatory event for proper intrathymic T cell differentiation.

The brain-specific protein, p42(IP4) (ADAP 1) is localized in mitochondria and involved in regulation of mitochondrial Ca2+

In brain, p42(IP4) (centaurin-alpha1; recently named ADAP 1, which signifies ADP ribosylation factor GTPase activating protein with dual PH domains 1, within the large family of Arf-GTPase activating proteins) is mainly expressed in neurons. p42(IP4) operates as a dual receptor recognising two second messengers, the soluble inositol(1,3,4,5)tetrakisphosphate and the lipid phosphatidylinositol(3,4,5)trisphosphate. We show here for the first time that p42(IP4) is localized in mitochondria, isolated from rat brain and from cells transfected with p42(IP4). In rat brain mitochondria we additionally found interaction of p42(IP4) with 2', 3'-cyclic nucleotide 3'-phosphodiesterase and alpha-tubulin by pull-down binding assay and by immunoprecipitation. In mitochondria from Chinese hamster ovary cells, p42(IP4) is predominantly associated with the intermembrane space and the inner membrane. This localization of p42(IP4) indicates that p42(IP4) might have a still unknown mitochondrial function. We studied whether p42(IP4) is involved in Ca(2+)-induced permeability transition pore opening, which is important in mitochondrial events leading to programmed cell death. We used mouse neuroblastoma cells as a model for the functional studies of p42(IP4) in mitochondria. In mitochondria isolated from p42(IP4)-transfected mouse neuroblastoma cells, over-expression of p42(IP4) significantly decreased Ca(2+) capacity and lag time for Ca(2+) retention. Thus, we suggest that p42(IP4) is involved in the regulation of Ca(2+) transport in mitochondria. We propose that p42(IP4) promotes Ca(2+)-induced permeability transition pore opening and thus destabilizes mitochondria.

Consensus nomenclature for the human ArfGAP domain-containing proteins

At the FASEB summer research conference on “Arf Family GTPases”, held in Il Ciocco, Italy in June, 2007, it became evident to researchers that our understanding of the family of Arf GTPase activating proteins (ArfGAPs) has grown exponentially in recent years. A common nomenclature for these genes and proteins will facilitate discovery of biological functions and possible connections to pathogenesis. Nearly 100 researchers were contacted to generate a consensus nomenclature for human ArfGAPs. This article describes the resulting consensus nomenclature and provides a brief description of each of the 10 subfamilies of 31 human genes encoding proteins containing the ArfGAP domain.

The neuronal Arf GAP centaurin alpha1 modulates dendritic differentiation

Centaurin alpha1 is an Arf GTPase-activating protein (GAP) that is highly expressed in the nervous system. In the current study, we show that endogenous centaurin alpha1 protein is localized in the synaptosome fraction, with peak expression in early postnatal development. In cultured dissociated hippocampal neurons, centaurin alpha1 localizes to dendrites, dendritic spines and the postsynaptic region. siRNA-mediated knockdown of centaurin alpha1 levels or overexpression of a GAP-inactive mutant of centaurin alpha1 leads to inhibition of dendritic branching, dendritic filopodia and spine-like protrusions in dissociated hippocampal neurons. Overexpression of wild-type centaurin alpha1 in cultured hippocampal neurons in early development enhances dendritic branching, and increases dendritic filopodia and lamellipodia. Both filopodia and lamellipodia have been implicated in dendritic branching and spine formation. Following synaptogenesis in cultured neurons, wild-type centaurin alpha1 expression increases dendritic filopodia and spine-like protrusions. Expression of a GAP-inactive mutant diminishes spine density in CA1 pyramidal neurons within cultured organotypic hippocampal slice cultures. These data support the conclusion that centaurin alpha1 functions through GAP-dependent Arf regulation of dendritic branching and spines that underlie normal dendritic differentiation and development.

Interaction of the brain-specific protein p42IP4/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42IP4, PtdIns(3,4,5)P3 and Ins(1,3,4,5)P4, with stereospecificity

The brain-specific protein p42IP4, also called centaurin-alpha1, specifically binds phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Here, we investigate the interaction of p42IP4/centaurin-alpha1 with nardilysin (NRDc), a member of the M16 family of zinc metalloendopeptidases. Members of this peptidase family exhibit enzymatic activity and also act as receptors for other proteins. We found that p42IP4/centaurin-alpha1 binds specifically to NRDc from rat brain. We further detected that centaurin-alpha2, a protein that is highly homologous to p42IP4/centaurin-alpha1 and expressed ubiquitously, also binds to NRDc. In vivo interaction was demonstrated by co-immunoprecipitation of p42IP4/centaurin-alpha1 with NRDc from rat brain. The acidic domain of NRDc (NRDc-AD), which does not participate in catalysis, is sufficient for the protein interaction with p42IP4. Interestingly, preincubation of p42IP4 with its cognate ligands D-Ins(1,3,4,5)P4 and the lipid diC8PtdIns(3,4,5)P3 negatively modulates the interaction between the two proteins. D-Ins(1,3,4,5)P4 and diC8PtdIns(3,4,5)P3 suppress the interaction with virtually identical concentration dependencies. This inhibition is highly ligand specific. The enantiomer L-Ins(1,3,4,5)P4 is not effective. Similarly, the phosphoinositides diC8PtdIns(3,4)P2, diC8PtdIns(3,5)P2 and diC8PtdIns(4,5)P2 all have no influence on the interaction. Further experiments revealed that endogenous p42IP4 from rat brain binds to glutathione-S-transferase (GST)-NRDc-AD. The proteins dissociate from each other when incubated with D-Ins(1,3,4,5)P4, but not with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In summary, we demonstrate that p42IP4 binds to NRDc via the NRDc-AD, and that this interaction is controlled by the cognate cellular ligands of p42IP4/centaurin-alpha1. Thus, specific ligands of p42IP4 can modulate the recruitment of proteins, which are docked to p42IP4, to specific cellular compartments.

Direct binding of SWAP-70 to non-muscle actin is required for membrane ruffling

Membrane ruffling induced by growth factor stimulation is caused by actin remodeling, which is mediated by various signaling molecules including Rac. We have shown that SWAP-70, which binds phosphatidylinositol trisphosphate, is one such molecule required for membrane ruffling in mouse kidney cells. Here, we show that SWAP-70 directly binds to F-actin. The bacterially expressed C-terminal region of SWAP-70 co-sedimented with non-muscle F-actin, suggesting direct binding of SWAP-70 to F-actin. The binding was much weaker in muscle F-actin. A truncated mutant of SWAP-70 containing only the C-terminal region readily colocalizes with F-actin, supporting this idea. Full-length SWAP-70 does not colocalize with F-actin unless cells are stimulated with growth factors, suggesting the presence of a stimuli-dependent regulatory mechanism for actin-binding activity in vivo. Overexpression of the mutant SWAP-70 lacking this binding domain inhibits the membrane ruffling induced by epidermal growth factor stimulation in COS7 cells. This dominant-negative effect is also observed in membrane ruffling induced by a dominant-active Rac, suggesting that SWAP-70 cooperates with Rac. These results suggest that the binding activity of SWAP-70 to non-muscle F-actin is required for membrane ruffling.

Centaurin-alpha1 is a phosphatidylinositol 3-kinase-dependent activator of ERK1/2 mitogen-activated protein kinases

Centaurin-alpha1 is known to be a phosphatidylinositol 3,4,5-triphosphate (PIP3)-binding protein that has two pleckstrin homology domains and a putative ADP ribosylation factor GTPase-activating protein domain. However, the physiological function of centaurin-alpha1 is still not understood. Here we have shown that transient expression of centaurin-alpha1 in COS-7 cells results in specific activation of ERK, and the activation is inhibited by co-expression of a dominant negative form of Ras. We have also found that a mutant form of centaurin-alpha1 that is unable to bind PIP3 fails to induce ERK activation and that a phosphatidylinositol 3-kinase inhibitor LY294002 inhibits centaurin-alpha1-dependent ERK activation. Furthermore, transient knockdown of centaurin-alpha1 by small interfering RNAs results in reduced ERK activation after epidermal growth factor stimulation in T-REx 293 cells. These results suggest that centaurin-alpha1 contributes to ERK activation in growth factor signaling, linking the PI3K pathway to the ERK mitogen-activated protein kinase pathway through its ability to interact with PIP3.

p42IP4/Centaurin α1, a Brain-specific PtdIns(3,4,5)P3/Ins(1,3,4,5)P4-binding Protein: Membrane Trafficking Induced by Epidermal Growth Factor is Inhibited by Stimulation of Phospholipase C-coupled Thrombin Receptor

The brain-specific 42-kDa protein, p42(IP4), contains a N-terminal zinc finger (ZF) motif and a tandem of two pleckstrin homology (PH) domains. p42(IP4) binds in vitro the second messengers phosphatidylinositol(3,4,5)trisphosphate (PtdIns(3,4,5)P3) and inositol(1,3,4,5)tetrakisphosphate (Ins(1,3,4,5)P4). We observed by confocal microscopy in live HEK 293 cells the GFP-p42(IP4), a chimera of human p42(IP4) and green fluorescence protein (GFP). There, we studied the influence of thrombin, which raises Ins(1,3,4,5)P4, on membrane translocation of GFP-p42(IP4), induced by epidermal growth factor (EGF). Thrombin in the presence of LiCl inhibited the EGF-induced membrane recruitment of GFP-p42(IP4). In the absence of LiCl, thrombin weakened the EGF-mediated membrane recruitment of GFP-p42(IP4). Furthermore, the participation of p42(IP4) protein domains on the EGF-mediated membrane translocation was analyzed. We used several p42(IP4) variants, in which one of the domains was deleted. Alternatively, single p42(IP4) domain-GFP fusion proteins were generated. Only the p42(IP4) variant lacking the ZF domain showed a very weak membrane translocation in response to EGF stimulation, but all the other p42(IP4) variants did not translocate. Thus, we conclude that the combination of both PH domains with ZF is required for membrane translocation of p42(IP4).