A human exchange factor for ARF contains Sec7- and pleckstrin-homology domains

Denervation and high-fat diet reduce insulin signaling in T-tubules in skeletal muscle of living mice

OBJECTIVE

Insulin stimulates muscle glucose transport by translocation of GLUT4 to sarcolemma and T-tubules. Despite muscle glucose uptake playing a major role in insulin resistance and type 2 diabetes, the temporal and spatial changes in insulin signaling and GLUT4 translocation during these conditions are not well described.

RESEARCH DESIGN AND METHODS

We used time-lapse confocal imaging of green fluorescent protein (GFP) ADP-ribosylation factor nucleotide-binding site opener (ARNO) (evaluation of phosphatidylinositide 3-kinase activation) and GLUT4-GFP-transfected quadriceps muscle in living, anesthetized mice either muscle denervated or high-fat fed. T-tubules were visualized with sulforhodamine B dye. In incubated muscle, glucose transport was measured by 2-deoxy-D-[(3)H]-glucose uptake, and functional detubulation was carried out by osmotic shock. Muscle fibers were immunostained for insulin receptors.

RESULTS

Denervation and high-fat diet reduced insulin-mediated glucose transport. In denervated muscle, insulin-stimulated phosphatidylinositol 3,4,5 P(3) (PIP3) production was abolished in T-tubules, while PIP3 production at sarcolemma was increased 2.6-fold. Correspondingly, GLUT4-GFP translocation to T-tubules was abolished, while translocation to sarcolemma was increased 2.3-fold. In high fat-fed mice, a approximately 65% reduction in both insulin-induced T-tubular PIP3 production and GLUT4-GFP translocation was seen. Sarcolemma was less affected, with reductions of approximately 40% in PIP3 production and approximately 15% in GLUT4-GFP translocation. Access to T-tubules was not compromised, and insulin receptor distribution in sarcolemma and T-tubules was unaffected by denervation or high-fat feeding. Detubulation of normal muscle reduced basal and abolished insulin-induced glucose transport.

CONCLUSIONS

Our findings demonstrate, for the first time, that impaired insulin signaling and GLUT4 translocation is compartmentalized in muscle and primarily localized to T-tubules and not sarcolemma during insulin resistance.

The small G proteins of the Arf family and their regulators

Small G proteins play a central role in the organization of the secretory and endocytic pathways. The majority of such small G proteins are members of the Rab family, which are anchored to the bilayer by C-terminal prenyl groups. However, the recruitment of some effectors, including vesicle coat proteins, is mediated by a second class of small G proteins that is unique in having an N-terminal amphipathic helix that becomes available for membrane insertion upon GTP binding. Sar1, Arf1, and Arf6 are the best-characterized members of this ADP-ribosylation factor (Arf) family. In addition, all eukaryotes contain additional distantly related G proteins, often called Arf like, or Arls. The complete Arf family in humans has 29 members. The roles of these related G proteins are poorly understood, but recent work has shown that some are involved in membrane traffic or organizing the cytoskeleton. Here we review what is known about all the members of the Arf family, along with the known regulatory molecules that convert them between GDP- and GTP-bound states.

ARF1 is directly involved in dynamin-independent endocytosis

Endocytosis of glycosylphosphatidyl inositol (GPI)-anchored proteins (GPI-APs) and the fluid phase takes place primarily through a dynamin- and clathrin-independent, Cdc42-regulated pinocytic mechanism. This mechanism is mediated by primary carriers called clathrin-independent carriers (CLICs), which fuse to form tubular early endocytic compartments called GPI-AP enriched endosomal compartments (GEECs). Here, we show that reduction in activity or levels of ARF1 specifically inhibits GPI-AP and fluid-phase endocytosis without affecting other clathrin-dependent or independent endocytic pathways. ARF1 is activated at distinct sites on the plasma membrane, and by the recruitment of RhoGAP domain-containing protein, ARHGAP10, to the plasma membrane, modulates cell-surface Cdc42 dynamics. This results in the coupling of ARF1 and Cdc42 activity to regulate endocytosis at the plasma membrane. These findings provide a molecular basis for a crosstalk of endocytosis with secretion by the sharing of a key regulator of secretory traffic, ARF1.

A Role for ARF6 and ARNO in the regulation of endosomal dynamics in neurons

During development, neuronal processes extend, branch and navigate to ultimately synapse with target tissue. We have shown a regulatory role for ARNO and ARF6 in dendritic branching and axonal elongation and branching during neuritogenesis, particularly with respect to cytoskeletal dynamics. Here, we have examined the role of ARF6 and the ARF GEF ARNO in endosomal dynamics during neurite elongation in hippocampal neurons. Axonal and dendritic endosomes were labeled by expression of the endosomal marker, endotubin. Expression of endotubin-green fluorescent protein resulted in targeting to tubular-vesicular structures throughout the somatodendritic and axonal domains. These endosomal structures did not colocalize with conventional early or late endosomal markers or with the synaptic vesicle marker, SV2. However, they did label with internalized lectin, indicating that they are endosomal structures. Expression of catalytically inactive ARNO (ARNO-E156K) or inactive ARF6 (ARF6-T27N) caused a redistribution of endotubin to the cell surface of the axons and dendrites. In contrast, expression of these constructs had no effect upon the distribution of SV2-positive structures. Furthermore, expression of inactive ARF1 (ARF1-T31N) did not change endotubin distribution. These results suggest that endotubin labels a distinct endosomal structure in neurons and that ARNO and ARF6 mediate neurite extension through the regulation of this compartment.

Guanine nucleotide exchange factors of the cytohesin family and their roles in signal transduction

Members of the cytohesin protein family, a group of guanine nucleotide exchange factors for adenosine diphosphate ribosylation factor (ARF) guanosine triphosphatases, have recently emerged as important regulators of signal transduction in vertebrate and invertebrate biology. These proteins share a modular domain structure, comprising carboxy-terminal membrane recruitment elements, a Sec7 homology effector domain, and an amino-terminal coiled-coil domain that serve as a platform for their integration into larger signaling complexes. Although these proteins have a highly similar overall build, their individual biological functions appear to be at least partly specific. Cytohesin-1 had been identified as a regulator of beta2 integrin inside-out regulation in immune cells and was subsequently shown to be involved in mitogen-associated protein kinase signaling in tumor cell proliferation as well as in T-helper cell activation and differentiation. Cytohesin-3, which had been discovered to be strongly associated with T-cell anergy, was very recently described as an essential component of insulin signal transduction in Drosophila and in human and murine liver cells. Future work will aim to dissect the mechanistic details of the modes of action of the cytohesins as well as to define the precise roles of these versatile proteins in vertebrates at the genetic level.

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

Cytohesins are guanine-nucleotide exchange factors (GEF) for the Arf family of GTPases. One member of the Arf family, ARF6, plays an active role in the intracellular trafficking of G protein-coupled receptors. We have previously reported that Galphaq signaling leads to the activation of ARF6, possibly through a direct interaction with cytohesin-2/ARNO. Here, we report that Galphaq can directly interact with cytohesin-1, another Arf-GEF of the ARNO/cytohesin family. Cytohesin-1 preferentially associated with a constitutively active mutant of Galphaq (Galphaq-Q209L) compared to wild-type Galphaq in HEK293 cells. Stimulation of TPbeta, a Galphaq-coupled receptor, to activate Galphaq resulted in the promotion of a protein complex between Galphaq and cytohesin-1. Confocal immunofluorescence microscopy revealed that wild-type Galphaq and cytohesin-1 co-localized in intracellular compartments and at or near the plasma membrane. In contrast, expression of Galphaq-Q209L induced a drastic increase in the localization of cytohesin-1 at the plasma membrane. Expression of a dominant-negative mutant of cytohesin-1 reduced by 40% the agonist-induced internalization of TPbeta, a process that we previously demonstrated to be dependent on Galphaq-mediated signaling and Arf6 activation. Using deletion mutants, we show that cytohesin-1 interacts with Galphaq through its N-terminal coiled-coil domain. Cytohesin-1 and cytohesin-2/ARNO mutants lacking the coiled-coil domain were unable to relay Galphaq-mediated activation of Arf6. This is the first report of an interaction between the coiled-coil domain of the cytohesin/ARNO family of Arf-GEFs and a member of the heterotrimeric G proteins.

Identification of a guanine nucleotide exchange factor for Arf3, the yeast orthologue of mammalian Arf6

Small G proteins of the Arf and Rab families are fundamental to the organisation and activity of intracellular membranes. One of the most well characterised of these G proteins is mammalian Arf6, a protein that participates in many cellular processes including endocytosis, actin remodelling and cell adhesion. Exchange of GDP for GTP on Arf6 is performed by a variety of guanine nucleotide exchange factors (GEFs), principally of the cytohesin (PSCD) and EFA6 (PSD) families. In this paper we describe the characterisation of a GEF for the yeast orthologue of Arf6, Arf3, which we have named Yel1 (yeast EFA6-like-1) using yeast genetics, fluorescence microscopy and in vitro nucleotide exchange assays. Yel1 appears structurally related to the EFA6 family of GEFs, having an N-terminal Sec7 domain and C-terminal PH and coiled-coil domains. We find that Yel1 is constitutively targeted to regions of polarised growth in yeast, where it co-localises with Arf3. Moreover the Sec7 domain of Yel1 is required for its membrane targeting and for that of Arf3. Finally we show that the isolated Yel1 Sec7 domain strongly stimulates nucleotide exchange activity specifically on Arf3 in vitro.

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.

Structure-based discovery of an inhibitor of Arf activation by Sec7 domains through targeting of protein-protein complexes

Small molecules that produce nonfunctional protein-protein complexes are an alternative to competitive inhibitors for the inhibition of protein functions. Here we target the activation of the small GTP-binding protein Arf1, a major regulator of membrane traffic, by the Sec7 catalytic domain of its guanine nucleotide exchange factor ARNO. The crystal structure of the Arf1-GDP/ARNO complex, which initiates the exchange reaction, was used to discover an inhibitor, LM11, using in silico screening of a flexible pocket near the Arf1/ARNO interface. Using fluorescence kinetics and anisotropy, NMR spectroscopy and mutagenesis, we show that LM11 acts following a noncompetitive mechanism in which the inhibitor targets both Arf1-GDP and the Arf1-GDP/ARNO complex and produces a nonfunctional Arf-GDP/ARNO complex whose affinity is similar to that of the native complex. In addition, LM11 recognizes features of both Arf and ARNO near the Arf/Sec7 interface, a characteristic reminiscent of the paradigm interfacial inhibitor Brefeldin A. We then show that LM11 is a cell-active inhibitor that impairs Arf-dependent trafficking structures at the Golgi. Furthermore, LM11 inhibits ARNO-dependent migration of Madin-Darby canine kidney (MDCK) cells, demonstrating that ARNO is a target of LM11 in cells. Remarkably, LM11 inhibits the activation of Arf1 but not Arf6 in vitro, pointing to a possible synergy between Arf1 and Arf6 activation by ARNO in cell migration. Our design method shows that flexible regions in protein-protein complexes provide drugable sites with the potential to develop novel tools for investigating and inhibiting signaling pathways.

Molecular determinants of the interaction between coxsackievirus protein 3A and guanine nucleotide exchange factor GBF1

The 3A protein of coxsackievirus B3 (CVB3), a small membrane protein that forms homodimers, inhibits endoplasmic reticulum-to-Golgi complex transport. Recently, we described the underlying mechanism by showing that the CVB3 3A protein binds to and inhibits the function of GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factor 1 (Arf1), thereby interfering with Arf1-mediated COP-I recruitment. This study was undertaken to gain more insight into the molecular determinants underlying the interaction between 3A and GBF1. Here we show that 3A mutants that have lost the ability to dimerize are no longer able to bind to GBF1 and trap it on membranes. Moreover, we identify a conserved region in the N terminus of 3A that is crucial for GBF1 binding but not for 3A dimerization. Analysis of the binding domain in GBF1 showed that the extreme N terminus, the dimerization/cyclophilin binding domain, and the homology upstream of Sec7 domain are required for the interaction with 3A. In contrast to that of full-length GBF1, overexpression of a GBF1 mutant lacking its extreme N terminus failed to rescue the effects of 3A. Together, these data provide insight into the molecular requirements of the interaction between 3A and GBF1.

The Arl4 Family of Small G Proteins Can Recruit the Cytohesin Arf6 Exchange Factors to the Plasma Membrane

The small GTPase Arf6 regulates endocytosis, actin dynamics, and cell adhesion, and one of its major activators is the exchange factor Arf nucleotide-binding site opener (ARNO), also called cytohesin-2 [1, 2]. ARNO must be recruited from the cytosol to the plasma membrane in order to activate Arf6, and in addition to a Sec7 nucleotide-exchange domain it contains a C-terminal pleckstrin homology (PH) domain that binds phosphoinositides [3, 4]. ARNO and its three relatives, cytohesin-1, Grp1/cytohesin-3, and cytohesin-4, are expressed as two splice variants, with either two or three glycines in a loop in the phosphoinositide-binding pocket of the PH domain [5, 6]. The diglycine form binds PtdIns(3,4,5)P(3) with high affinity and mediates recruitment of cytohesins to the plasma membrane in response to insulin and growth factors [7, 8]. However, the triglycine form has only micromolar affinity for both PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2), affinities that are insufficient to confer membrane recruitment, raising the question of how the triglycine forms of cytohesins are regulated [5, 9]. Here we show that three related Arf-like GTPases of unknown function, Arl4a, Arl4c, and Arl4d, are able to recruit ARNO and other cytohesins to the plasma membrane by binding to their PH domains irrespective of whether they are in the diglycine or triglycine form. The Arl4 family thus defines a signal-transduction pathway that can mediate the plasma-membrane recruitment of cytohesins independently of a requirement for the generation of PtdIns(3,4,5)P(3).

Analysis of bronchoalveolar lavage fluid proteome from systemic sclerosis patients with or without functional, clinical and radiological signs of lung fibrosis

Lung fibrosis is a major cause of mortality and morbidity in systemic sclerosis (SSc). However, its pathogenesis still needs to be elucidated. We examined whether the alteration of certain proteins in bronchoalveolar lavage fluid (BALF) might have a protective or a causative role in the lung fibrogenesis process. For this purpose we compared the BALF protein profile obtained from nine SSc patients with lung fibrosis (SSc_Fib+) with that obtained from six SSc patients without pulmonary fibrosis (SSc_Fib-) by two-dimensional gel electrophoresis (2-DE). Only spots and spot-trains that were consistently expressed in a different way in the two study groups were taken into consideration. In total, 47 spots and spot-trains, corresponding to 30 previously identified proteins in human BALF, showed no significant variation between SSc_Fib+ patients and SSc_Fib- patients, whereas 24 spots showed a reproducible significant variation in the two study groups. These latter spots corresponded to 11 proteins or protein fragments, including serum albumin fragments (13 spots), 5 previously recognized proteins (7 spots), and 4 proteins (3 spots) that had not been previously described in human BALF maps, namely calumenin, cytohesin-2, cystatin SN, and mitochondrial DNA topoisomerase 1 (mtDNA TOP1). Mass analysis did not determine one protein-spot. The two study groups revealed a significant difference in BALF protein composition. Whereas levels of glutathione S-transferase P (GSTP), Cu–Zn superoxide dismutase (SOD) and cystatin SN were downregulated in SSc_Fib+ patients compared with SSc_Fib- patients, we observed a significant upregulation of α1-acid glycoprotein, haptoglobin-α chain, calgranulin (Cal) B, cytohesin-2, calumenin, and mtDNA TOP1 in SSc_Fib+ patients. Some of these proteins (GSTP, Cu–Zn SOD, and cystatin SN) seem to be involved in mechanisms that protect lungs against injury or inflammation, whereas others (Cal B, cytohesin-2, and calumenin) seem to be involved in mechanisms that drive lung fibrogenesis. Even if the 2-DE analysis of BALF did not provide an exhaustive identification of all BALF proteins, especially those of low molecular mass, it allows the identification of proteins that might have a role in lung fibrogenesis. Further longitudinal studies on larger cohorts of patients will be necessary to assess their usefulness as predictive markers of disease.

Regulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) and BIG2 activity via PKA and protein phosphatase 1gamma

Brefeldin A-inhibited guanine nucleotide-exchange proteins (GEPs) BIG1 and BIG2 activate ADP-ribosylation factor (ARF) GTPases, which are required for vesicular trafficking. Both molecules contain one or more sites for binding protein kinase A, i.e., A kinase-anchoring protein (AKAP) sequences. Elevation of cell cAMP caused PKA-catalyzed phosphorylation and nuclear accumulation of BIG1 but not BIG2. We then asked whether BIG1 phosphorylation altered its GEP activity. Incubation of BIG1 or BIG2 with PKA catalytic subunits and ATP resulted in retardation of their electrophoretic migration, consistent with PKA phosphorylation. Okadaic acid inhibits many protein phosphatases, including protein phosphatase 1 (PP1) and PP2A, that can reverse PKA-catalyzed phosphorylation. Incubation of HepG2 cells with okadaic acid caused concentration-dependent accumulation of presumably phosphorylated BIG1 and BIG2 with decreased mobility, which was increased by subsequent incubation in vitro with specific recombinant phosphatases, PP1gamma > PP2A >> PP1alpha. For assays of GEP activity, BIG1 and BIG2 were immunoprecipitated from cells that had been depleted, respectively, of BIG2 and BIG1 by using specific siRNA. GEP activity of each was significantly decreased after incubation with recombinant PKA plus ATP and restored by incubation with PP1gamma. In agreement with a role for PP1gamma in regulation of BIG, endogenous PP1gamma, but not PP1alpha or beta, was immunoprecipitated with BIG1 or BIG2 from microsomal fractions. All observations are consistent with the effects of BIG1 and BIG2 phosphorylation on vesicular trafficking, via alterations in ARF activation and regulatory roles for cAMP, PKA, and PP1gamma in ARF activation by BIG1 and BIG2.

New Insights into Membrane Trafficking and Protein Sorting

Protein transport in the secretory and endocytic pathways is a multistep process involving the generation of transport carriers loaded with defined sets of cargo, the shipment of the cargo-loaded transport carriers between compartments, and the specific fusion of these transport carriers with a target membrane. The regulation of these membrane-mediated processes involves a complex array of protein and lipid interactions. As the machinery and regulatory processes of membrane trafficking have been defined, it is increasingly apparent that membrane transport is intimately connected with a number of other cellular processes, such as quality control in the endoplasmic reticulum (ER), cytoskeletal dynamics, receptor signaling, and mitosis. The fidelity of membrane trafficking relies on the correct assembly of components on organelles. Recruitment of peripheral proteins plays a critical role in defining organelle identity and the establishment of membrane subdomains, essential for the regulation of vesicle transport. The molecular mechanisms for the biogenesis of membrane subdomains are also central to understanding how cargo is sorted and segregated and how different populations of transport carriers are generated. In this review we will focus on the emerging themes of organelle identity, membrane subdomains, regulation of Golgi trafficking, and advances in dissecting pathways in physiological systems.

RLIP76 (RalBP1) is an R-Ras effector that mediates adhesion-dependent Rac activation and cell migration

The Ras family of small GTPases regulates cell proliferation, spreading, migration and apoptosis, and malignant transformation by binding to several protein effectors. One such GTPase, R-Ras, plays distinct roles in each of these processes, but to date, identified R-Ras effectors were shared with other Ras family members (e.g., H-Ras). We utilized a new database of Ras-interacting proteins to identify RLIP76 (RalBP1) as a novel R-Ras effector. RLIP76 binds directly to R-Ras in a GTP-dependent manner, but does not physically associate with the closely related paralogues H-Ras and Rap1A. RLIP76 is required for adhesion-induced Rac activation and the resulting cell spreading and migration, as well as for the ability of R-Ras to enhance these functions. RLIP76 regulates Rac activity through the adhesion-induced activation of Arf6 GTPase and activation of Arf6 bypasses the requirement for RLIP76 in Rac activation and cell spreading. Thus, we identify a novel R-Ras effector, RLIP76, which links R-Ras to adhesion-induced Rac activation through a GTPase cascade that mediates cell spreading and migration.

COPI-mediated Transport

COPI-coated vesicles are protein and liquid carriers that mediate transport within the early secretory pathway. In this Topical Review, we present their main protein components and discuss current models for cargo sorting. Finally, we describe the striking similarities that exist between the COPI system and the two other characterized types of vesicular carriers: COPII- and clathrin-coated vesicles.

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

GBF1, a cis-Golgi and VTCs-localized ARF-GEF, is implicated in ER-to-Golgi protein traffic

The formation and maturation of membrane carriers that transport cargo from the ER to the Golgi complex involves the sequential action of the coat protein complexes COPII and COPI. Recruitment of COPI to nascent carriers requires activation of ADP-ribosylation factors by a BrefeldinA-sensitive guanine nucleotide exchange factor. Using new antisera and a GFP-tagged protein, we demonstrate that the exchange factor GBF1 localized to both Golgi membranes and peripheral puncta, near but separate from ER exit sites. Live cell imaging revealed that GFP-GBF1 associates dynamically with both membranes through rapid exchange with a large cytosolic pool. Treatment with BrefeldinA dramatically altered this rapid exchange, causing accumulation of GBF1 on both Golgi and peripheral puncta before eventual redistribution to the ER in a microtubule-dependent manner. Measurement of diffusion coefficients and subcellular fractionation confirmed this shift in GBF1 from cytosolic to membrane bound. BrefeldinA-induced accumulation of GBF1 coincided with loss of COPI from peripheral puncta. Furthermore, recruitment of GBF1 to cargo-containing peripheral puncta coincided with recruitment of COPI, but not COPII. Strikingly, microinjection of anti-GBF1 antibodies specifically caused dissociation of COPI from membranes. These observations strongly suggest that GBF1 regulates COPI membrane recruitment in the early secretory pathway.

Involvement of a guanine nucleotide-exchange protein, ARF-GEP100/BRAG2a, in the apoptotic cell death of monocytic phagocytes

We previous identified adenosine 5'-diphosphate-ribosylation factor (ARF)-guanine nucleotide-exchange protein, 100 kDa (GEP(100)), as a novel GEP with a molecular size of approximately 100 kDa, which preferentially activates ARF6. In this study, we examined the effect of ARF-GEP(100) on monocytic cell apoptosis. Overexpression of ARF-GEP(100) in PMA-differentiated human monocyte-macrophage-like U937 cells and mouse macrophage RAW264.7 cells induced apoptotic cell death, which was detected by morphological changes (chromatin condensation, nucleus fragmentation, and shrinking of cytoplasm), annexin V-staining, and TUNEL assay. It is interesting that a mutant lacking the Sec7 domain, which is responsible for ARF activation, was able to induce apoptosis of the target cells to the level of that of a wild-type ARF-GEP(100). Furthermore, ARF-GEP(100)-silencing experiments indicated that the TNF-alpha-induced apoptosis was significantly suppressed among ARF-GEP(100)-depressed cells. These observations apparently suggest that ARF-GEP(100) is involved in the induction of apoptosis in monocytic phagocytes, possibly independent of ARF activation.

GEP100/BRAG2: activator of ADP-ribosylation factor 6 for regulation of cell adhesion and actin cytoskeleton via E-cadherin and alpha-catenin

GEP(100) (p100) was identified as an ADP-ribosylation factor (ARF) guanine nucleotide-exchange protein (GEP) that partially colocalized with ARF6 in the cell periphery. p100 preferentially accelerated guanosine 5[gamma-thio]triphosphate (GTPgammaS) binding by ARF6, which participates in protein trafficking near the plasma membrane, including receptor recycling, cell adhesion, and cell migration. Here we report that yeast two-hybrid screening of a human fetal brain cDNA library using p100 as bait revealed specific interaction with alpha-catenin, which is known as a regulator of adherens junctions and actin cytoskeleton remodeling. Interaction of p100 with alpha-catenin was confirmed by coimmunoprecipitation of the endogenous proteins from human HepG2 or CaSki cells, although colocalization was difficult to demonstrate microscopically. alpha-Catenin enhanced GTPgammaS binding by ARF6 in vitro in the presence of p100. Depletion of p100 by small interfering RNA (siRNA) treatment in HepG2 cells resulted in E-cadherin content 3-fold that in control cells and blocked hepatocyte growth factor-induced redistribution of E-cadherin, consistent with a known role of ARF6 in this process. F-actin was markedly decreased in normal rat kidney (NRK) cells overexpressing wild-type p100, but not its GEP-inactive mutants, also consistent with the conclusion that p100 has an important role in the activation of ARF6 for its functions in both E-cadherin recycling and actin remodeling.

ARF proteins: roles in membrane traffic and beyond

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

Imaging of insulin signaling in skeletal muscle of living mice shows major role of T-tubules

Insulin stimulates glucose transport in skeletal muscle by glucose transporter GLUT4 translocation to sarcolemma and membrane invaginations, the t-tubules. Although muscle glucose uptake plays a key role in insulin resistance and type 2 diabetes, the dynamics of GLUT4 translocation and the signaling involved are not well described. We have now developed a confocal imaging technique to follow trafficking of green fluorescent protein-labeled proteins in living muscle fibers in situ in anesthetized mice. Using this technique, by imaging the dynamics of GLUT4 translocation and phosphatidylinositol 3,4,5 P(3) (PIP(3)) production in response to insulin, here, for the first time, we delineate the temporal and spatial distribution of these processes in a living animal. We find a 10-min delay of maximal GLUT4 recruitment and translocation to t-tubules compared with sarcolemma. Time-lapse imaging of a fluorescent dye after intravenous injection shows that this delay is similar to the time needed for insulin diffusion into the t-tubule system. Correspondingly, immunostaining of muscle fibers shows that insulin receptors are present throughout the t-tubule system. Finally, PIP(3) production, an early event in insulin signaling, progresses slowly along the t-tubules with a 10-min delay between maximal PIP(3) production at sarcolemma compared with deep t-tubules following the appearance of dye-labeled insulin. Our findings in living mice indicate a major role of the t-tubules in insulin signaling in skeletal muscle and show a diffusion-associated delay in insulin action between sarcolemma and inner t-tubules.

Effect of protein kinase A on accumulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) in HepG2 cell nuclei

Brefeldin A-inhibited guanine nucleotide-exchange proteins, BIG1 and BIG2, are activators of ADP-ribosylation factor GTPases that are essential for regulating vesicular traffic among intracellular organelles. Biochemical analyses and immunofluorescence microscopy demonstrated BIG1 in nuclei as well as membranes and cytosol of serum-starved HepG2 cells. Within 20 min after addition of 8-Br-cAMP, BIG1 accumulated in nuclei, and this effect was blocked by protein kinase A (PKA) inhibitors H-89 and PKI, suggesting a dependence on PKA-catalyzed phosphorylation. BIG2 localization was not altered by cAMP, nor did BIG2 small interfering RNA influence nuclear accumulation of BIG1 induced by cAMP. Mutant BIG1 (S883A) in which Ala replaced Ser-883, a putative PKA phosphorylation site, did not move to the nucleus with cAMP addition, whereas replacement with Asp (S883D) resulted in nuclear accumulation of BIG1 without or with cAMP exposure, consistent with the mechanistic importance of a negative charge at that site. Mutation (712KPK714) of the nuclear localization signal inhibited BIG1 accumulation in nuclei, and PKA-catalyzed phosphorylation of S883, although necessary, was not sufficient for nuclear accumulation, as shown by the double mutation S883D/nuclear localization signal. A role for microtubules in cAMP-induced translocation of BIG1 is inferred from its inhibition by nocodazole. Thus, two more critical elements of BIG1 molecular structure were identified, as well as the potential function of microtubules in a novel PKA effect on BIG1 translocation.

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.

Dual specificity of the interfacial inhibitor brefeldin a for arf proteins and sec7 domains

Guanine nucleotide exchange factors (GEFs), which activate small GTP-binding proteins (SMG) by stimulating their GDP/GTP exchange, are emerging as candidate targets for the inhibition of cellular pathways involved in diseases. However, their specific inhibition by competitive inhibitors is challenging, because GEF and SMG families comprise highly similar members. Nature shows us an alternative strategy called interfacial inhibition, exemplified by Brefeldin A (BFA). BFA inhibits the activation of Arf1 by its GEFs in vivo by stabilizing an abortive complex between Arf-GDP and the catalytic Sec7 domain of some of its GEFs. Here we characterize the specificity of BFA toward wild-type (ARNO and BIG1) and mutant Sec7 domains and toward class I, II, and III Arfs. We find that BFA sensitivity of the exchange reaction depends on the nature of both the Sec7 domain and the Arf protein. A single Phe/Tyr substitution is sufficient to achieve BFA sensitivity of the Sec7 domain, which is supported by our characterization of brefeldin C (BFC), a BFA analog that cannot interact with the Tyr residue, and by free energy computations. We further show that Arf1 and Arf5, but not Arf6, are BFA-sensitive, despite their having every BFA-interacting residue in common. Analysis of Arf6 mutants points to the dynamics of the interswitch, which is involved in membrane-to-nucleotide signal propagation, as contributing to, although not sufficient for, BFA sensitivity. Altogether, our results reveal the Tyr/Phe substitution as a novel tool for monitoring BFA sensitivity of cellular ArfGEFs and document the exquisite and dual specificity that can be achieved by an interfacial inhibitor.

Yeast Mon2p is a highly conserved protein that functions in the cytoplasm-to-vacuole transport pathway and is required for Golgi homeostasis

Although the small Arf-like GTPases Arl1-3 are highly conserved eukaryotic proteins, they remain relatively poorly characterized. The yeast and mammalian Arl1 proteins bind to the Golgi complex, where they recruit specific structural proteins such as Golgins. Yeast Arl1p directly interacts with Mon2p/Ysl2p, a protein that displays some sequence homology to the large Sec7 guanine exchange factors (GEFs) of Arf1. Mon2p also binds the putative aminophospholipid translocase (APT) Neo1p, which performs essential function(s) in membrane trafficking. Our detailed analysis reveals that Mon2p contains six distinct amino acid regions (A to F) that are conserved in several other uncharacterized homologs in higher eukaryotes. As the conserved A, E and F domains are unique to these homologues, they represent the signature of a new protein family. To investigate the role of these domains, we made a series of N- and C-terminal deletions of Mon2p. Although fluorescence and biochemical studies showed that the B and C domains (also present in the large Sec7 GEFs) predominantly mediate interaction with Golgi/endosomal membranes, growth complementation studies revealed that the C-terminal F domain is essential for the activity of Mon2p, indicating that Mon2p might also function independently of Arl1p. We provide evidence that Mon2p is required for efficient recycling from endosomes to the late Golgi. Intriguingly, although transport of CPY to the vacuole was nearly normal in the Deltamon2 strain, we found the constitutive delivery of Aminopeptidase 1 from the cytosol to the vacuole to be almost completely blocked. Finally, we show that Mon2p exhibits genetic and physical interactions with Dop1p, a protein with a putative function in cell polarity. We propose that Mon2p is a scaffold protein with novel conserved domains, and is involved in multiple aspects of endomembrane trafficking.

V-ATPase interacts with ARNO and Arf6 in early endosomes and regulates the protein degradative pathway

The recruitment of the small GTPase Arf6 and ARNO from cytosol to endosomal membranes is driven by V-ATPase-dependent intra-endosomal acidification. The molecular mechanism that mediates this pH-sensitive recruitment and its role are unknown. Here, we demonstrate that Arf6 interacts with the c-subunit, and ARNO with the a2-isoform of V-ATPase. The a2-isoform is targeted to early endosomes, interacts with ARNO in an intra-endosomal acidification-dependent manner, and disruption of this interaction results in reversible inhibition of endocytosis. Inhibition of endosomal acidification abrogates protein trafficking between early and late endosomal compartments. These data demonstrate the crucial role of early endosomal acidification and V-ATPase/ARNO/Arf6 interactions in the regulation of the endocytic degradative pathway. They also indicate that V-ATPase could modulate membrane trafficking by recruiting and interacting with ARNO and Arf6; characteristics that are consistent with the role of V-ATPase as an essential component of the endosomal pH-sensing machinery.

Membrane-proximal signaling events in beta-2 integrin activation

In the immune system, integrins have essential roles in leukocyte trafficking and function. These include immune cell attachment to endothelial and antigen-presenting cells, cytotoxicity, and extravasation into tissues. The integrin leukocyte function-associated antigen-1 (LFA-1), which is exclusively expressed on hematopoietic cells, has been intensely studied since this receptor is important for many functions of the immune system. LFA-1 is involved in a) the interaction between T-cells and antigen presenting cells, b) the adhesion of cells to post-capillary high endothelial venules or to activated endothelium at sites of inflammation (extravasation), c) the control of cell differentiation and proliferation, and d) the regulation of T-cell effector functions. Therefore, a precise understanding of the spatial and temporal control of LFA-1 interaction with its cellular counter-receptors, the intercellular adhesion molecules (ICAM) -1, -2 and -3, in the various contexts, is of high interest. LFA-1 mediated adhesion is induced by several extracellular stimuli in different cell types. In T-cells, LFA-1 becomes activated upon signaling from the T-cell receptor (TCR), and upon cytokine and chemokine sensing. Adhesion of monocytes to ICAM-1 is induced by lipopolysaccharide (LPS), a component of the bacterial cell wall. To investigate the regulation of LFA-1 adhesiveness, research has focused on the identification of interaction partners of the intracellular portions of the integrin alpha and beta subunits. This review will highlight recent developments on transmembrane and intracellular signaling proteins, which have been implicated in beta-2 integrin activation.

On the action of Brefeldin A on Sec7-stimulated membrane-recruitment and GDP/GTP exchange of Arf proteins

Arf (ADP-ribosylation factor) proteins form a special class of small GTP-binding proteins in that their activation by GDP/GTP exchange is coupled to their recruitment to membranes using a built-in structural mechanism. These coupled processes are stimulated by GEFs (guanine nucleotide-exchange factors) that carry a catalytic Sec7 domain, whose basic mechanism has been uncovered by biochemical and structural studies. Crystal structures of intermediates of the GDP/GTP exchange reaction, from which GDP has not dissociated, notably allowed a movie of the exchange reaction to be reconstituted. They showed that Sec7 domains secure Arf-GDP to membranes before they proceed to nucleotide dissociation, and thus are active participants to the coupling of membrane-recruitment to nucleotide exchange. The drug BFA (Brefeldin A) was used to trap the complex that initiates the exchange reaction, providing a structural basis for its inhibition of Arf and its action on the membrane-recruitment of isolated Sec7 domains. Based on the dissection of this basic mechanism, the survey of reported BFA effects in cells on large multidomain ArfGEFs of the BIG1/2 and GBF1 families shows that the levels and compartmental distribution of BFA-induced recruitment of ArfGEFs to membranes cannot be explained from isolated Sec7 domains acting as independent domains. This leads to the hypothesis that Sec7 activity is inhibited in these ArfGEFs by an intramolecular interaction, which would be released by interaction with a compartment-specific receptor.

Arf, Sec7 and Brefeldin A: a model towards the therapeutic inhibition of guanine nucleotide-exchange factors

GEFs (guanine nucleotide-exchange factors), which stimulate GDP dissociation from small G-proteins, are pivotal regulators of signalling pathways activated by small G-proteins. In the case of Arf proteins, which are major regulators of membrane traffic in the cell and have recently been found to be involved in an increasing number of human diseases, GDP/GTP exchange is stimulated by GEFs that carry a catalytic Sec7 domain. Recent structural results captured snapshots of the exchange reaction, revealing that Sec7 domains secure Arf-GDP to membranes before nucleotide exchange takes place, taking advantage of a built-in structural device in Arf proteins that couples their affinity for membranes to the nature of the bound nucleotide. One of the Arf-Sec7 intermediates was trapped by BFA (Brefeldin A), an uncompetitive inhibitor of Arf activation that has been instrumental in deciphering the molecular principles of membrane traffic at the Golgi. BFA targets a low-affinity Arf-Sec7 intermediate of the exchange reaction. It binds at the Arf-GDP/Sec7 interface, thus freezing the complex in an abortive conformation that cannot proceed to nucleotide dissociation. In the cell, this results in the specific inhibition of Arf1 by a subset of its GEFs, and the efficient and reversible block of membrane traffic at the Golgi. The mechanism of BFA leads to the concept of 'interfacial inhibition', in which a protein-protein interaction of therapeutic interest is stabilized, rather than impaired, by a drug. Up-regulated activity of small G-proteins is involved in various human diseases, making their GEFs attractive candidates to interrupt specifically the corresponding signalling pathway. Interfacial inhibitors are proposed as an alternative to competitive inhibitors that may be explored for their inhibition.

The small GTPase ADP-ribosylation factor 6 negatively regulates dendritic spine formation

Actin cytoskeletal reorganization and membrane trafficking are important for spine morphogenesis. Here we investigated whether the small GTPase, ADP-ribosylation factor 6 (ARF6), which regulates actin dynamics and peripheral vesicular trafficking, is involved in the regulation of spine formation. The developmental expression pattern of ARF6 in mouse hippocampus was similar to that of the post-synaptic density protein-95, and these molecules colocalized in mouse hippocampal neurons. Overexpression of a constitutively active ARF6 mutant in cultured hippocampal neurons decreased the spine density, whereas a dominant-negative ARF6 mutant increased the density. These results demonstrate a novel function for ARF6 as a key regulator of spine formation.

Ypt32p regulates the translocation of Chs3p from an internal pool to the plasma membrane

The transport of the chitin synthase III, Chs3p, to the plasma membrane is temporally and spatially regulated. Chs3p is delivered to the plasma membrane at the beginning of the cell cycle, forming chitin rings, and at the end of the cell cycle, forming the primary septum. During the rest of the cell cycle, it is maintained in intracellular compartments, termed chitosomes that share characteristics with the late Golgi and the early endosomes. Chs5p and Chs6p are required for the cell cycle-dependent delivery of Chs3p to the cell surface, but the mechanisms underlying the temporal regulation are still unknown. The Rab proteins, Ypt31/32p, are required for exit of secretory vesicles from the late Golgi and for recycling of proteins between the late Golgi and early endosomes. Either gain of Ypt32p function, by overexpression, or loss-of-function mutations alter the localization of Chs3p-GFP. Moreover, cells overexpressing Ypt32p accumulate chitin at the cell surface independent of Chs5p. Overexpression of Ypt32p also disrupts the localization of the late Golgi protein Sec7. We propose that Ypt31/32p have a role in regulating the delivery of Chs3p to the plasma membrane and deposition of chitin at the cell surface.

Crossing the divide--transport between the endoplasmic reticulum and Golgi apparatus in plants

The transport of proteins between the endoplasmic reticulum (ER) and the Golgi apparatus in plants is an exciting and constantly expanding topic, which has attracted much attention in recent years. The study of protein transport within the secretory pathway is a relatively new field, dating back to the 1970s for mammalian cells and considerably later for plants. This may explain why COPI- and COPII-mediated transport between the ER and the Golgi in plants is only now becoming clear, while the existence of these pathways in other organisms is relatively well documented. We summarize current knowledge of these protein transport routes, as well as highlighting key differences between those of plant systems and those of mammals and yeast. These differences have necessitated the study of plant-specific aspects of protein transport in the early secretory pathway, and this review discusses recent developments in this area. Advances in live-cell-imaging technology have allowed the observation of protein movement in vivo, giving a new insight into many of the processes involved in vesicle formation and protein trafficking. The use of these new technologies has been combined with more traditional methods, such as protein biochemistry and electron microscopy, to increase our understanding of the transport routes in the cell.

Heterotrimeric G protein-independent signaling of a G protein-coupled receptor. Direct binding of ARNO/cytohesin-2 to the carboxyl terminus of the A2A adenosine receptor is necessary for sustained activation of the ERK/MAP kinase pathway

The A2A adenosine receptor is a prototypical G(s)-coupled receptor, but it also signals, e.g. to mitogen-activated protein (MAP) kinase, via a pathway that is independent of heterotrimeric G proteins. Truncation of the carboxyl terminus affects the strength of the signal through these alternative pathways. In a yeast two-hybrid interaction hunt, we screened a human brain library for proteins that bound to the juxtamembrane portion of the carboxyl terminus of the A2A receptor. This approach identified ARNO/cytohesin-2, a nucleotide exchange factor for the small (monomeric) G proteins of the Arf (ADP-ribosylation factor) family, as a potential interaction partner. We confirmed a direct interaction by mutual pull down (of fusion proteins expressed in bacteria) and by immunoprecipitation of the proteins expressed in mammalian cells. To circumvent the long term toxicity associated with overexpression of ARNO/cytohesin-2, we created stable cell lines that stably expressed the A2A receptor and where ARNO/cytohesin-2 or the dominant negative version E156K-ARNO/cytohesin-2 was inducible by mifepristone. Cyclic AMP accumulation induced by an A2A-specific agonist was neither altered by ARNO/cytohesin-2 nor by the dominant negative version. This was also true for agonist-induced desensitization. In contrast, expression of dominant negative E156K-ARNO/cytohesin-2 and of dominant negative T27N-Arf6 abrogated the sustained phase of MAP kinase stimulation induced by the A2A receptor. We therefore conclude that ARNO/cytohesin-2 is required to support the alternative, heterotrimeric G protein-independent, signaling pathway of A2A receptor, which is stimulation of MAP kinase.

Regulation of actin ring formation by rho GTPases in osteoclasts

Actin ring formation is a prerequisite for osteoclast bone resorption. Although gelsolin null osteoclasts failed to exhibit podosomes, actin ring was observed in these osteoclasts. Wiscott-Aldrich syndrome protein (WASP) was observed in the actin ring of gelsolin null osteoclast. Osteoclasts stimulated with osteopontin simulated the effects of Rho and Cdc42 in phosphatidylinositol 4,5-bisphosphate (PIP2) association with WASP as well as formation of podosomes, peripheral microfilopodia-like structures, and actin ring. To explore the potential functions of Rho and Cdc42, TAT-mediated delivery of Rho proteins into osteoclasts was performed. Although Rho and Cdc42 are required for actin ring formation, transduction of either one of the proteins alone is insufficient for this process. Addition of osteopontin to osteoclasts transduced with Cdc42Val12 or transduction of osteoclasts with both RhoVal14 and Cdc42Val12 augments the formation of WASP-Arp2/3 complex and actin ring. Neomycin, an antibiotic, blocked the effects of osteopontin or TAT-RhoVal14 on PIP2 interaction with WASP. WASP distribution was found to be cytosolic in these osteoclasts. Depletion of WASP by short interfering RNA-mediated gene silencing blocked actin polymerization as well as actin ring formation in osteoclasts. These results suggest that Rho-mediated PIP2 interaction with WASP may contribute to the activation and membrane targeting of WASP. Subsequent interaction of Cdc42 and Arp2/3 with WASP may enhance cortical actin polymerization in the process of actin ring formation in osteoclasts.

Multiple activities for Arf1 at the Golgi complex

The Arf family of GTPases regulates membrane traffic and organelle structure. At the Golgi complex, Arf proteins facilitate membrane recruitment of many cytoplasmic coat proteins to allow sorting of membrane proteins for transport, stimulate the activity of enzymes that modulate the lipid composition of the Golgi, and assemble a cytoskeletal scaffold on the Golgi. Arf1 is the Arf family member most closely studied for its function at the Golgi complex. A number of regulators that activate and inactivate Arf1 on the Golgi have been described that localize to different regions of the organelle. This spatial distribution of Arf regulators may facilitate the recruitment of the coat proteins and other Arf effectors to different regions of the Golgi complex.

Requirement of phosphatidylinositol-4,5-bisphosphate for HERC1-mediated guanine nucleotide release from ARF proteins

HERC1 is a giant multidomain protein involved in membrane trafficking through its interaction with vesicle coat proteins such as clathrin and ARF. Previously, it has been shown that the RCC1-like domain 1 (RLD1) of HERC1 stimulates guanine nucleotide dissociation on ARF1 and Rab proteins. In this study, we have analyzed whether HERC1 may also regulate ARF6 activity. We show that HERC1, through its RLD1, stimulates GDP release from ARF6 but, unexpectedly, it inhibits GDP/GTP exchange on ARF6 under conditions where ARNO stimulates it. Furthermore, we demonstrate that the activity of HERC1 as a guanine nucleotide release factor requires the presence of PI(4,5)P(2) bound to HERC1's RLD1. In agreement with this, we find that purified HERC1 contains PI(4,5)P(2) bound to the RLD1.

The domain architecture of large guanine nucleotide exchange factors for the small GTP-binding protein Arf

Background

Small G proteins, which are essential regulators of multiple cellular functions, are activated by guanine nucleotide exchange factors (GEFs) that stimulate the exchange of the tightly bound GDP nucleotide by GTP. The catalytic domain responsible for nucleotide exchange is in general associated with non-catalytic domains that define the spatio-temporal conditions of activation. In the case of small G proteins of the Arf subfamily, which are major regulators of membrane trafficking, GEFs form a heterogeneous family whose only common characteristic is the well-characterized Sec7 catalytic domain. In contrast, the function of non-catalytic domains and how they regulate/cooperate with the catalytic domain is essentially unknown.

Results

Based on Sec7-containing sequences from fully-annotated eukaryotic genomes, including our annotation of these sequences from Paramecium, we have investigated the domain architecture of large ArfGEFs of the BIG and GBF subfamilies, which are involved in Golgi traffic. Multiple sequence alignments combined with the analysis of predicted secondary structures, non-structured regions and splicing patterns, identifies five novel non-catalytic structural domains which are common to both subfamilies, revealing that they share a conserved modular organization. We also report a novel ArfGEF subfamily with a domain organization so far unique to alveolates, which we name TBS (TBC-Sec7).

Conclusion

Our analysis unifies the BIG and GBF subfamilies into a higher order subfamily, which, together with their being the only subfamilies common to all eukaryotes, suggests that they descend from a common ancestor from which species-specific ArfGEFs have subsequently evolved. Our identification of a conserved modular architecture provides a background for future functional investigation of non-catalytic domains.

Double cross-over gene replacement within the sec 7 domain of a GDP-GTP exchange factor from Plasmodium falciparum allows the generation of a transgenic brefeldin A-resistant parasite line

High molecular weight ADP ribosylation factor GDP-GTP exchange factors (ARF-GEF) play an essential role in the formation of COP I coated transport vesicles and are characterized by a structurally and functionally conserved sec 7 domain. The genome of the malaria parasite Plasmodium falciparum encodes a single ARF-GEF that contains an unusual sec 7 domain. In comparison to the sec 7 domain of other eukaryotes, the plasmodial sec 7 domain is characterized by an insertion sequence of 146 amino acids that disrupt helices essential for the GDP-GTP exchange activity of the protein. In a previous study we have shown a correlation between a methionine to isoleucine exchange in helix H of the sec 7 domain and resistance to brefeldin A in a parasite line generated by drug selection. Here we have transfected brefeldin A sensitive parasites with plasmid constructs containing the sec 7 domain of the resistant line either with or without the insertion sequence. Transfection with sec 7 sequences including the insertion resulted in brefeldin A resistant parasites in which double cross-over recombination had replaced the endogenous sec 7 sequences with the transgenic sequences. Thus, the point mutation in helix H is sufficient to confer brefeldin A resistance in P. falciparum. Transfections using constructs lacking the insertion did not result in resistant parasites. Gene replacement by targeted double cross-over recombination is a rare event in P. falciparum. This approach has taken advantage of the fact that the successful integration of the transgene results in a drug selectable phenotype. We anticipate that the strategy described here will be useful for the identification of mutations within target genes that have the potential to confer increased drug resistance.

BIG1 and BIG2, brefeldin A-inhibited guanine nucleotide-exchange factors for ADP-ribosylation factors

BIG1 and BIG2 are large (approximately 200 kDa) guanine nucleotide-exchange proteins for ADP-ribosylation factors, or ARFs, that were isolated based on sensitivity of their guanine nucleotide-exchange activity to inhibition by brefeldin A. The intracellular distributions of BIG1 and BIG2 differ from those of other ARF guanine nucleotide-exchange proteins. In addition to its presence in Golgi membranes, BIG2 is seen in peripheral vesicular structures that most likely represent recycling endosomes, and BIG1 is found in nuclei of serum-starved HepG2 cells. Several binding partners for BIG1 and BIG2 that were identified via yeast two-hybrid screens include FKBP13 and myosin IXb for BIG1 and, for BIG2, the regulatory RIalpha subunit of protein kinase A, Exo70, and the GABA receptor beta subunit. Autosomal recessive periventricular heterotopia with microcephaly, a disorder of human embryonic development due to defective vesicular trafficking, has been attributed to mutations in BIG2. Methods for purification of BIG1 and BIG2 from HepG2 cells are presented here, along with a summary of information regarding their structure and function.

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.

The TBC (Tre-2/Bub2/Cdc16) domain protein TRE17 regulates plasma membrane-endosomal trafficking through activation of Arf6

TBC (Tre-2/Bub2/Cdc16) domains are predicted to encode GTPase-activating proteins (GAPs) for Rab family G proteins. While approximately 50 TBC proteins are predicted to exist in humans, little is known about their substrate specificity. Here we show that TRE17 (also called Tre-2 and USP6), a founding member of the TBC family, targets the Arf family GTPase Arf6, which regulates plasma membrane-endosome trafficking. Surprisingly, TRE17 does not function as a GAP for Arf6 but rather promotes its activation in vivo. TRE17 associates directly with Arf6 in its GDP- but not GTP-bound state. Mapping experiments pinpoint the site of interaction to the TBC domain of TRE17. Forced expression of TRE17 promotes the localization of Arf6 to the plasma membrane, leading to Arf6 activation, presumably due to facilitated access to membrane-associated guanine nucleotide exchange factors (GEFs). Furthermore, TRE17 cooperates with Arf6 GEFs to induce GTP loading of Arf6 in vivo. Finally, short interfering RNA-mediated loss of TRE17 leads to attenuated Arf6 activation. These studies identify TRE17 as a novel regulator of the Arf6-regulated plasma membrane recycling system and reveal an unexpected function for TBC domains.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Molecular interactions of yeast Neo1p, an essential member of the Drs2 family of aminophospholipid translocases, and its role in membrane trafficking within the endomembrane system

Neo1p is an essential yeast member of the highly conserved Drs2 family of P-type ATPases with proposed aminophospholipid translocase activity. Here we present evidence that Neo1p localizes to endosomes and Golgi elements. In agreement with that finding, the temperature-sensitive neo1-37 and neo1-69 mutants exhibit defects in receptor-mediated endocytosis, vacuole biogenesis, and vacuolar protein sorting. Furthermore, neo1 mutants accumulate aberrantly shaped membranous structures most likely derived from vacuoles and the endosomal/Golgi system. At permissive temperatures, HA-Neo1-69p, like wild-type Neo1p, is stable and associates with endosomes. In contrast, HA-Neo1-37p is rapidly degraded and is predominantly retained within the endoplasmic reticulum (ER). Thus, the two neo1 alleles affect the stability and localization of the mutant polypeptides in different ways. A C-terminally truncated and a C-terminally epitope-tagged version of Neo1p are nonfunctional and also mislocalize to the ER. In agreement with a role within the endomembrane system, Neo1p exhibits genetic and physical interactions with Ysl2p, a potential guanine nucleotide exchange factor for Arl1p. Interestingly, deletion of ARL1 rescues the temperature sensitivity of neo1-37 and neo1-69. We demonstrate that Arl1p in its myristoylated and GTP-bound form is responsible for the inhibitory effect. Thus, Neo1p, Ysl2p, and Arl1p represent three proteins that collaborate in membrane trafficking within the endosomal/Golgi system.

Gene expression in mononuclear cells from patients with inflammatory bowel disease

OBJECTIVES

Discovery of Nod2 as the inflammatory bowel disease 1 (IBD1) susceptibility gene has brought to light the significance of mononuclear cells in inflammatory bowel disease pathogenesis. The purpose of this study was to examine changes in gene expression in peripheral blood mononuclear cells in patients with untreated Crohn's disease (CD) and ulcerative colitis (UC) as compared to patients with other inflammatory gastrointestinal disorders and to healthy controls.

METHODS

We used a 2400 gene cDNA glass slide array (MICROMAX) to examine gene expression in peripheral blood mononuclear cells from seven patients with Crohn's disease, five patients with ulcerative colitis, 10 patients with other inflammatory gastrointestinal disorders, and 22 age- and sex-matched controls. Results. Novel categories of genes differentially expressed in Crohn's disease and ulcerative colitis patients included genes regulating hematopoietic cell differentiation and leukemogenesis, lipid raft-associated signaling, the actin cytoskeleton, and vesicular trafficking.

CONCLUSIONS

Altered gene expression in mononuclear cells may contribute to inflammatory bowel disease pathogenesis.

BIG2, a guanine nucleotide exchange factor for ADP-ribosylation factors: its localization to recycling endosomes and implication in the endosome integrity

Small GTPases of the ADP-ribosylation factor (ARF) family play a key role in membrane trafficking by regulating coated vesicle formation, and guanine nucleotide exchange is essential for the ARF function. Brefeldin A blocks the ARF-triggered coat assembly by inhibiting the guanine nucleotide exchange on ARFs and causes disintegration of the Golgi complex and tubulation of endosomal membranes. BIG2 is one of brefeldin A-inhibited guanine nucleotide exchange factors for the ARF GTPases and is associated mainly with the trans-Golgi network. In the present study, we have revealed that another population of BIG2 is associated with the recycling endosome and found that expression of a catalytically inactive BIG2 mutant, E738K, selectively induces membrane tubules from this compartment. We also have shown that BIG2 has an exchange activity toward class I ARFs (ARF1 and ARF3) in vivo and inactivation of either ARF exaggerates the BIG2(E738K)-induced tubulation of endosomal membranes. These observations together indicate that BIG2 is implicated in the structural integrity of the recycling endosome through activating class I ARFs.

The role of ADP-ribosylation factor and SAR1 in vesicular trafficking in plants

Ras-like small GTP binding proteins regulate a wide variety of intracellular signalling and vesicular trafficking pathways in eukaryotic cells including plant cells. They share a common structure that operates as a molecular switch by cycling between active GTP-bound and inactive GDP-bound conformational states. The active GTP-bound state is regulated by guanine nucleotide exchange factors (GEF), which promote the exchange of GDP for GTP. The inactive GDP-bound state is promoted by GTPase-activating proteins (GAPs) which accelerate GTP hydrolysis by orders of magnitude. Two types of small GTP-binding proteins, ADP-ribosylation factor (Arf) and secretion-associated and Ras-related (Sar), are major regulators of vesicle biogenesis in intracellular traffic and are founding members of a growing family that also includes Arf-related proteins (Arp) and Arf-like (Arl) proteins. The most widely involved small GTPase in vesicular trafficking is probably Arf1, which not only controls assembly of COPI- and AP1, AP3, and AP4/clathrin-coated vesicles but also recruits other proteins to membranes, including some that may be components of further coats. Recent molecular, structural and biochemical studies have provided a wealth of detail of the interactions between Arf and the proteins that regulate its activity as well as providing clues for the types of effector molecules which are controlled by Arf. Sar1 functions as a molecular switch to control the assembly of protein coats (COPII) that direct vesicle budding from ER. The crystallographic analysis of Sar1 reveals a number of structurally unique features that dictate its function in COPII vesicle formation. In this review, I will summarize the current knowledge of Arf and Sar regulation in vesicular trafficking in mammalian and yeast cells and will highlight recent advances in identifying the elements involved in vesicle formation in plant cells. Additionally, I will briefly discuss the similarities and dissimilarities of vesicle traffic in plant, mammalian and yeast cells.

Towards a molecular pathway for myoblast fusion in Drosophila

Intercellular fusion among myoblasts is required for the generation of multinucleated muscle fibers during skeletal muscle development. Recent studies in Drosophila have shed light on the molecular mechanisms that underlie this process, and a signaling pathway that relays fusion signals from the cell membrane to the cytoskeleton has emerged. In this article, we review these recent advances and discuss how Drosophila offers a powerful model system to study myoblast fusion in vivo.