Overexpression of the ARF1 exchange factor ARNO inhibits the early secretory pathway and causes the disassembly of the Golgi complex

Crucial Players for Inter-Organelle Communication: PI5P4Ks and Their Lipid Product PI-4,5-P2 Come to the Surface

While organelles are individual compartments with specialized functions, it is becoming clear that organellar communication is essential for maintaining cellular homeostasis. This cooperation is carried out by various interactions taking place on the membranes of organelles. The membranes themselves contain a multitude of proteins and lipids that mediate these connections and one such class of molecules facilitating these relations are the phospholipids. There are several phospholipids, but the focus of this perspective is on a minor group called the phosphoinositides and specifically, phosphatidylinositol 4,5-bisphosphate (PI-4,5-P₂). This phosphoinositide, on intracellular membranes, is largely generated by the non-canonical Type II PIPKs, namely, Phosphotidylinositol-5-phosphate-4-kinases (PI5P4Ks). These evolutionarily conserved enzymes are emerging as key stress response players in cells. Further, PI5P4Ks have been shown to modulate pathways by regulating organelle crosstalk, revealing roles in preserving metabolic homeostasis. Here we will attempt to summarize the functions of the PI5P4Ks and their product PI-4,5-P₂ in facilitating inter-organelle communication and how they impact cellular health as well as their relevance to human diseases.

Promiscuity of the catalytic Sec7 domain within the guanine nucleotide exchange factor GBF1 in ARF activation, Golgi homeostasis, and effector recruitment

The integrity of the Golgi and trans-Golgi network (TGN) is disrupted by brefeldin A (BFA), which inhibits the Golgi-localized BFA-sensitive factor (GBF1) and brefeldin A-inhibited guanine nucleotide-exchange factors (BIG1 and BIG2). Using a cellular replacement assay to assess GBF1 functionality without interference from the BIGs, we show that GBF1 alone maintains Golgi architecture; facilitates secretion; activates ADP-ribosylation factor (ARF)1, 3, 4, and 5; and recruits ARF effectors to Golgi membranes. Unexpectedly, GBF1 also supports TGN integrity and recruits numerous TGN-localized ARF effectors. The impact of the catalytic Sec7 domain (Sec7d) on GBF1 functionality was assessed by swapping it with the Sec7d from ARF nucleotide-binding site opener (ARNO)/cytohesin-2, a plasma membrane GEF reported to activate all ARFs. The resulting chimera (GBF1-ARNO-GBF1 [GARG]) targets like GBF1, supports Golgi/TGN architecture, and facilitates secretion. However, unlike GBF1, GARG activates all ARFs (including ARF6) at the Golgi/TGN and recruits additional ARF effectors to the Golgi/TGN. Our results have general implications: 1) GEF's targeting is independent of Sec7d, but Sec7d influence the GEF substrate specificity and downstream effector events; 2) all ARFs have access to all membranes, but are restricted in their distribution by the localization of their activating GEFs; and 3) effector association with membranes requires the coincidental presence of activated ARFs and specific membrane identifiers.

Structure and function of nucleotide sugar transporters: Current progress

The proteomes of eukaryotes, bacteria and archaea are highly diverse due, in part, to the complex post-translational modification of protein glycosylation. The diversity of glycosylation in eukaryotes is reliant on nucleotide sugar transporters to translocate specific nucleotide sugars that are synthesised in the cytosol and nucleus, into the endoplasmic reticulum and Golgi apparatus where glycosylation reactions occur. Thirty years of research utilising multidisciplinary approaches has contributed to our current understanding of NST function and structure. In this review, the structure and function, with reference to various disease states, of several NSTs including the UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, GDP-fucose, UDP-N-acetylglucosamine/UDP-glucose/GDP-mannose and CMP-sialic acid transporters will be described. Little is known regarding the exact structure of NSTs due to difficulties associated with crystallising membrane proteins. To date, no three-dimensional structure of any NST has been elucidated. What is known is based on computer predictions, mutagenesis experiments, epitope-tagging studies, in-vitro assays and phylogenetic analysis. In this regard the best-characterised NST to date is the CMP-sialic acid transporter (CST). Therefore in this review we will provide the current state-of-play with respect to the structure–function relationship of the (CST). In particular we have summarised work performed by a number groups detailing the affect of various mutations on CST transport activity, efficiency, and substrate specificity.

ARAP1 regulates endocytosis of EGFR

Signaling through the EGF receptor is regulated by endocytosis. ARAP1 is a protein with Arf guanosine triphosphatase-activating protein (GAP) and Rho GAP domains. We investigated the role of ARAP1 in EGF receptor endocytic trafficking. Following EGF treatment of cells, ARAP1 rapidly and transiently associated with the edge of the cell and punctate structures containing Rab5, rabaptin 5 and EGFR but not early embryonic antigen 1 (EEA1). EGF associated with the ARAP1-positive punctate structures prior to EEA1-positive early endosomes. Recruitment of ARAP1 to the punctate structures required active Rab5 and an additional signal from EGFR. Decreasing ARAP1 levels with small interfering RNA accelerated association of EGF with EEA1 endosomes and degradation of EGFR. Phosphorylation of extracellular-signal-regulated kinase (ERK) and c-Jun-amino-terminal kinase (JNK) was diminished and more transient in cells with reduced levels of ARAP1 than in controls. Based on these findings, we propose that ARAP1 regulates the endocytic traffic of EGFR and, consequently, the rate of EGFR signal attenuation.

Active Arf6 recruits ARNO/cytohesin GEFs to the PM by binding their PH domains

ARNO is a soluble guanine nucleotide exchange factor (GEF) for the Arf family of GTPases. Although in biochemical assays ARNO prefers Arf1 over Arf6 as a substrate, its localization in cells at the plasma membrane (PM) suggests an interaction with Arf6. In this study, we found that ARNO activated Arf1 in HeLa and COS-7 cells resulting in the recruitment of Arf1 on to dynamic PM ruffles. By contrast, Arf6 was activated less by ARNO than EFA6, a canonical Arf6 GEF. Remarkably, Arf6 in its GTP-bound form recruited ARNO to the PM and the two proteins could be immunoprecipitated. ARNO binding to Arf6 was not mediated through the catalytic Sec7 domain, but via the pleckstrin homology (PH) domain. Active Arf6 also bound the PH domain of Grp1, another ARNO family member. This interaction was direct and required both inositol phospholipids and GTP. We propose a model of sequential Arf activation at the PM whereby Arf6-GTP recruits ARNO family GEFs for further activation of other Arf isoforms.

Analysis of the interaction between cytohesin 2 and IPCEF1

IPCEF1 has been reported to interact with ADP-ribosylation factor GTP exchange factors of the cytohesin family and function by modulating the cytohesin 2 activity. This article describes methods used to study the interaction and activation of cytohesin GEFs by IPCEF1. The experimental approaches described here include physical and functional interaction assays by which the association of IPCEF1 with cytohesin 2 is explored both in vitro and in vivo. The methods used to analyze the physical association include GST-pull down and coimmunoprecipitation approaches. We also used yeast two-hybrid and colocalization assays to study the interaction between IPCEF1 and cytohesins. The functional relationship between IPCEF1 and cytohesin 2 was assessed by studying the effect of IPCEF1 on the in vitro and in vivo stimulation of ADP-ribosylation factor 6 GTP formation by cytohesin 2.

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.

The Structure of RalF, an ADP-ribosylation Factor Guanine Nucleotide Exchange Factor from Legionella pneumophila, Reveals the Presence of a Cap over the Active Site

The Legionella pneumophila protein RalF is secreted into host cytosol via the Dot/Icm type IV transporter where it acts to recruit ADP-ribosylation factor (Arf) to pathogen-containing phagosomes in the establishment of a replicative organelle. The presence in RalF of the Sec7 domain, present in all Arf guanine nucleotide exchange factors, has suggested that recruitment of Arf is an early step in pathogenesis. We have determined the crystal structure of RalF and of the isolated Sec7 domain and found that RalF is made up of two domains. The Sec7 domain is homologous to mammalian Sec7 domains. The C-terminal domain forms a cap over the active site in the Sec7 domain and contains a conserved folding motif, previously observed in adaptor subunits of vesicle coat complexes. The importance of the capping domain and of the glutamate in the "glutamic finger," conserved in all Sec7 domains, to RalF functions was examined using three different assays. These data highlight the functional importance of domains other than Sec7 in Arf guanine nucleotide exchange factors to biological activities and suggest novel mechanisms of regulation of those activities.

Interaction protein for cytohesin exchange factors 1 (IPCEF1) binds cytohesin 2 and modifies its activity

The ADP-ribosylation factor 6 (ARF6) small GTPase functions as a GDP/GTP-regulated switch in the pathways that stimulate actin reorganization and membrane ruffling. The formation of active ARF6GTP is stimulated by guanine nucleotide exchange factors (GEFs) such as cytohesins, which translocate to the plasma membrane in agonist-stimulated cells by binding the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate through the pleckstrin homology domain with subsequent ARF6 activation. Using cytohesin 2 as bait in yeast two-hybrid screening, we have isolated a cDNA encoding a protein termed interaction protein for cytohesin exchange factors 1 (IPCEF1). Using yeast two-hybrid and glutathione S-transferase pull-down assays coupled with deletion mutational analysis, the specific domains required for the cytohesin 2-IPCEF1 interaction were mapped to the coiled-coil domain of cytohesin 2 and the C-terminal 121 amino acids of IPCEF1. IPCEF1 also interacts with the other members of the cytohesin family of ARF GEFs, suggesting that the interaction with IPCEF1 is highly conserved among the cytohesin family of ARF GEFs. The interaction of cytohesin 2 and IPCEF1 in mammalian cells was demonstrated by immunoprecipitation. Immunofluorescence analysis revealed that IPCEF1 co-localizes with cytohesin 2 to the cytosol in unstimulated cells and translocates to the plasma membrane via binding to cytohesin 2 in epidermal growth factor-stimulated cells. However, a deletion mutant of IPCEF1 that lacks the cytohesin 2 binding site failed to co-migrate with cytohesin 2 to the membrane in stimulated cells. The functional significance of the IPCEF1-cytohesin 2 interaction is demonstrated by showing that IPCEF1 increases the in vitro and in vivo stimulation of ARFGTP formation by cytohesin 2.

ARNO and ARF6 regulate axonal elongation and branching through downstream activation of phosphatidylinositol 4-phosphate 5-kinase alpha

In the developing nervous system, controlled neurite extension and branching are critical for the establishment of connections between neurons and their targets. Although much is known about the regulation of axonal development, many of the molecular events that regulate axonal extension remain unknown. ADP-ribosylation factor nucleotide-binding site opener (ARNO) and ADP-ribosylation factor (ARF)6 have important roles in the regulation of the cytoskeleton as well as membrane trafficking. To investigate the role of these molecules in axonogenesis, we expressed ARNO and ARF6 in cultured rat hippocampal neurons. Expression of catalytically inactive ARNO or dominant negative ARF6 resulted in enhanced axonal extension and branching and this effect was abrogated by coexpression of constitutively active ARF6. We sought to identify the downstream effectors of ARF6 during neurite extension by coexpressing phosphatidyl-inositol-4-phosphate 5-Kinase alpha [PI(4)P 5-Kinase alpha] with catalytically inactive ARNO and dominant negative ARF6. We found that PI(4)P 5-Kinase alpha plays a role in neurite extension and branching downstream of ARF6. Also, expression of inactive ARNO/ARF6 depleted the actin binding protein mammalian ena (Mena) from the growth cone leading edge, indicating that these effects on axonogenesis may be mediated by changes in cytoskeletal dynamics. These results suggest that ARNO and ARF6, through PI(4)P 5-Kinase alpha, regulate axonal elongation and branching during neuronal development.

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

Phosphoinositides and the golgi complex

Phosphoinositides act as precursors of second messengers and membrane ligands for protein modules. Specific lipid kinases and phosphatases are located and differentially regulated in cell organelles, generating a non-uniform distribution of phosphoinositides. Although it is not clear whether and how the phosphoinositide pools are integrated, it is certain that they locally control fundamental processes, including membrane trafficking. This applies to the Golgi complex, where a direct, central role of the phosphatidylinositol 4,5-bisphosphate precursor phosphatidylinositol 4-phosphate has recently been reported.

Characterization of novel Rab6-interacting proteins involved in endosome-to-TGN transport

Rab6 GTPase regulates intracellular transport at the level of the Golgi complex. Using the yeast two-hybrid screen, we have isolated two clones that specifically interact with the three isoforms of Rab6 present in mammalian cells (Rab6A, A' and B). The cDNAs encode two proteins of 976 and 1120 amino acids (calculated molecular mass of 112 and 128 kDa, respectively) that we named Rab6IP2A and Rab6IP2B (for Rab6 Interacting Protein 2). The two proteins likely correspond to spliced variants of the same gene. Rab6IP2s have no significant homology with other known proteins, including Rab effectors or partners. They are ubiquitously expressed, mostly cytosolic and found in high molecular mass complexes in brain cytosol. We show that Rab6IP2s can be recruited on Golgi membranes in a Rab6:GTP-dependent manner. The overexpression of any form of Rab6IP2 has no detectable effect on the secretory pathway. In contrast, the retrograde transport of the Shiga toxin B subunit between the plasma membrane and the Golgi complex is partly inhibited in cells overexpressing the Rab6-binding domain of Rab6IP2. Our data suggest that Rab6IP2s is involved in the pathway regulated by Rab6A'.

Fragmentation and re-assembly of the Golgi apparatus in vitro. A requirement for phosphatidic acid and phosphatidylinositol 4,5-bisphosphate synthesis

Recent work from our laboratory demonstrated that phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), are required to maintain the structural integrity of the Golgi apparatus. To investigate the role of these lipids in regulating Golgi structure and function, we developed a novel assay to follow the release of post-Golgi vesicles. Isolated rat liver Golgi membranes were incubated with [(3)H]CMP sialic acid to radiolabel endogenous soluble and membrane glycoproteins present in the late Golgi and trans-Golgi network. The release of post-Golgi secretory vesicles was determined by measuring incorporation of (3)H-labeled proteins into a medium speed supernatant. Vesicle budding was dependent on temperature, cytosol, energy and time. Electron microscopy of Golgi fractions prior to and after incubation demonstrated that the stacked Golgi cisternae generated a heterogeneous population of vesicles (50- to 350-nm diameter). Inhibition of phospholipase D-mediated PA synthesis, by incubation with 1-butanol, resulted in the complete fragmentation of the Golgi membranes in vitro into 50- to 100-nm vesicles; this correlated with diminished PtdIns(4,5)P(2) synthesis. Following alcohol washout, PA synthesis resumed and in the presence of cytosol PtdIns(4,5)P(2) synthesis was restored. Most significantly, under these conditions the fragmented Golgi elements reformed into flattened cisternae and the re-assembled Golgi supported vesicle release. These data demonstrate that inositol phospholipid synthesis is essential for the structure and function of the Golgi apparatus.

Specificities for the small G proteins ARF1 and ARF6 of the guanine nucleotide exchange factors ARNO and EFA6

ARF1 and ARF6 are distant members of the ADP-ribosylation factor (ARF) small G-protein subfamily. Their distinct cellular functions must result from specificity of interaction with different effectors and regulators, including guanine nucleotide exchange factors (GEFs). ARF nucleotide-binding site opener (ARNO), and EFA6 are analogous ARF-GEFs, both comprising a catalytic "Sec7" domain and a pleckstrin homology domain. In vivo ARNO, like ARF1, is mostly cytosolic, with minor localizations at the Golgi and plasma membrane; EFA6, like ARF6, is restricted to the plasma membrane. However, depending on conditions, ARNO appears active on ARF6 as well as on ARF1. Here we analyze the origin of these ARF-GEF selectivities. In vitro, in the presence of phospholipid membranes, ARNO activates ARF1 preferentially and ARF6 slightly, whereas EFA6 activates ARF6 exclusively; the stimulation efficiency of EFA6 on ARF6 is comparable with that of ARNO on ARF1. These selectivities are determined by the GEFs Sec7 domains alone, without the pleckstrin homology and N-terminal domains, and by the ARF core domains, without the myristoylated N-terminal helix; they are not modified upon permutation between ARF1 and ARF6 of the few amino acids that differ within the switch regions. Thus selectivity for ARF1 or ARF6 must depend on subtle folding differences between the ARFs switch regions that interact with the Sec7 domains.

Characterization of cytohesin-1 monoclonal antibodies: expression in neutrophils and during granulocytic maturation of HL-60 cells

ADP-ribosylation factors (Arf) are small GTP-binding proteins involved in vesicular transport and the activation of phospholipase D (PLD). The conversion of Arf-GDP to Arf-GTP is promoted in vivo by guanine nucleotide exchange factors such as ARNO or cytohesin-1. In order to examine the expression of ARNO and cytohesin-1 in human granulocytes, we generated specific polyclonal and monoclonal antibodies (mAbs). We also overexpressed GFP-ARNO and GFP-cytohesin-1 in RBL-2H3 cells to characterize the specificity and the ability of cytohesin-1 mAbs to immunoprecipitate cytohesin-1. Among the hybridomas secreting cytohesin-1 mAbs, only the clones 2E11, 1E4, 3C8, 6F5, 4C7, 7A3 and 8F7 were found to be specific for cytohesin-1. Furthermore, mAb 2E11 immunoprecipitated GFP-cytohesin-1 but not GFP-ARNO under native conditions. In contrast, mAbs 5D8, 4C3, 2G8, 6G11, 4C3, 6D4, 7B4 and 6F8 detected both cytohesin-1 and ARNO as monitored by immunoblotting. Although mAb 6G11 detected both proteins, this antibody immunoprecipitated GFP-ARNO but not GFP-cytohesin-1 under native conditions. Another antibody, mAb 10A12, also selectively immunoprecipitated GFP-ARNO under native conditions, but the epitope recognized by this mAb is unlikely to be linear as no signal was obtained by immunoblotting. Immunoprecipitation with a cytohesin-1 polyclonal antibody and blotting with cytohesin-1 specific mAbs revealed that cytohesin-1 is highly expressed in neutrophils. Cytohesin-1 can be detected in HL-60 cells but the endogenous protein levels were low in undifferentiated cells. Using the specific cytohesin-1 mAb 2E11 we observed a marked increase in levels of cytohesin-1 expression during dibutyryl-cyclic AMP-induced granulocytic differentiation of HL-60 cells. These data suggest that cytohesin-1, which may have important functions in neutrophil physiology, can be useful as a potential marker for granulocytic differentiation.

Functional characterization of novel human ARFGAP3

ADP ribosylation factors (ARFs) are critical in the vesicular trafficking pathway. ARF activity is controlled by GTPase-activating proteins (GAPs). We have identified recently a novel tentative ARF GAP derived from human fetal liver, ARFGAP3 (originally named as ARFGAP1). In the present study, we demonstrated that ARFGAP3 had GAP activity in vitro and remarked that the GAP activity of ARFGAP3 was regulated by phospholipids, i.e. phosphatidylinositol 4,5-diphosphate as agonist and phosphatidylcholine as antagonist. ARFGAP3 is a predominantly cytosolic protein, and concentrated in the perinuclear region. Its transient ectopic overexpression in cultured mammalian cells reduced the constitutive secretion of secreted alkaline phosphatase, indicating that ectopic overexpression of ARFGAP3 inhibits the early secretory pathway of proteins in vivo. These results demonstrated that ARFGAP3 is a novel GAP for ARF1 and might be involved in intracellular traffic of proteins and vesicular transport as predicted.

Cytohesins and centaurins: mediators of PI 3-kinase-regulated Arf signaling

Receptor-activated phosphoinositide (PI) 3-kinases produce PtdIns(3, 4,5)P(3) and its metabolite PtdIns(3,4)P(2) that function as second messengers in membrane recruitment and activation of target proteins. The cytohesin and centaurin protein families are potential targets for PtdIns(3,4,5)P(3) that also regulate and interact with Arf GTPases. Consequently, these families are poised to transduce PI 3-kinase activation into coordinated control of Arf-dependent pathways. Proposed downstream events in PI 3-kinase-regulated Arf cascades include modulation of vesicular trafficking and the actin cytoskeleton.

Specific functional interaction of human cytohesin-1 and ADP-ribosylation factor domain protein (ARD1)

Activation of ADP-ribosylation factors (ARFs) is mediated by guanine nucleotide-exchange proteins, which accelerate conversion of inactive ARF-GDP to active ARF-GTP. ARF domain protein (ARD1), a 64-kDa GTPase with a C-terminal ADP-ribosylation factor domain, is localized to lysosomes and the Golgi apparatus. When ARD1 was used as bait to screen a human liver cDNA library using the yeast two-hybrid system, a cDNA for cytohesin-1, a approximately 50-kDa protein with ARF guanine nucleotide-exchange protein activity, was isolated. In this system, ARD1-GDP interacted well with cytohesin-1 but very poorly with cytohesin-2. In agreement, cytohesin-1, but not cytohesin-2, markedly accelerated [(35)S]guanosine 5'-3-O-(thio)triphosphate binding to ARD1. The effector region of the ARF domain of ARD1 appeared to be critical for the specific interaction with cytohesin-1. Replacement of single amino acids in the Sec7 domains of cytohesin-1 and -2 showed that residue 30 is critical for specificity. In transfected COS-7 cells, overexpressed ARD1 and cytohesin-1 were partially colocalized, as determined by confocal fluorescence microscopy. It was concluded that cytohesin-1 is likely to be involved in ARD1 activation, consistent with a role for ARD1 in the regulation of vesicular trafficking.

Effect of protein kinase A activity on the association of ADP-ribosylation factor 1 to golgi membranes

The small GTP-binding protein ADP-ribosylation factor 1 (ARF1) is an essential component of the molecular machinery that catalyzes the formation of membrane-bound transport intermediates. By using an in vitro assay that reproduces recruitment of cytosolic proteins onto purified, high salt-washed Golgi membranes, we have analyzed the role of cAMP-dependent protein kinase A (PKA) on ARF1 incorporation. Addition to this assay of either pure catalytic subunits of PKA (C-PKA) or cAMP increased ARF1 binding. By contrast, ARF1 association was inhibited following C-PKA inactivation with either PKA inhibitory peptide or RIIalpha as well as after cytosol depletion of C-PKA. C-PKA also stimulated recruitment and activation of a recombinant form of human ARF1 in the absence of additional cytosolic components. The binding step could be dissociated from the activation reaction and found to be independent of guanine nucleotides and saturable. This step was stimulated by C-PKA in an ATP-dependent manner. Dephosphorylated Golgi membranes exhibited a decreased ability to recruit ARF1, and this effect was reverted by addition of C-PKA. Following an increase in the intracellular level of cAMP, ARF proteins redistributed from cytosol to the perinuclear Golgi region of intact cells. Collectively, the results show that PKA exerts a key regulatory role in the recruitment of ARF1 onto Golgi membranes. In contrast, PKA modulators did not affect recruitment of beta-COP onto Golgi membranes containing prebound ARF1.

Interaction of GRASP, a protein encoded by a novel retinoic acid-induced gene, with members of the cytohesin family of guanine nucleotide exchange factors

A novel, retinoic acid-induced gene, GRP1-associated scaffold protein (GRASP), was isolated from P19 embryonal carcinoma cells using a subtractive screening strategy. GRASP was found to be highly expressed in brain and exhibited lower levels of expression in lung, heart, embryo, kidney, and ovary. The predicted amino acid sequence of GRASP is characterized by several putative protein-protein interaction motifs, suggesting that GRASP may be a component of a larger protein complex in the cell. Although GRASP does not harbor a predicted membrane spanning domain(s), the protein was observed to be associated with the plasma membrane of transiently transfected mammalian cells. Yeast two-hybrid screening revealed that GRASP interacted strongly with the General Receptor for Phosphoinositides 1 (GRP1), a brefeldin A-insensitive guanine nucleotide exchange factor for the ADP-ribosylation factor family of proteins. GRASP. GRP1 interactions were also demonstrated in vitro and in mammalian cells in which GRASP was shown to enhance GRP1 association with the plasma membrane. Furthermore, GRASP colocalized with endogenous ADP-ribosylation factors at the plasma membrane in transfected cells, suggesting that GRASP may modulate signaling by this family of small GTPases.

N-terminal targeting of guanine nucleotide exchange factors (GEF) for ADP ribosylation factors (ARF) to the Golgi

B2-1 (cytohesin-1) is a member of a group of proteins (including ARNO and ARNO3) that are all of similar size and domain composition. The three proteins contain an N-terminal coiled-coil domain, followed by a Sec7 and a pleckstrin homology (PH) domain. While it is well established that the Sec7 domain functions as a guanine nucleotide exchange factor (GEF) for ADP-ribosylation factors (ARFs) and the PH domain anchors the proteins to membrane phosphoinositols, the function of the N-terminal domain is unknown. Here we show that the N terminus of B2-1 (residues 1–54) is necessary and sufficient to target the protein to the Golgi. The Sec7+PH domains of B2-1 (residues 55–398) are not sufficient for Golgi localization. Further deletion analysis and point mutagenesis indicate that the coiled-coil domain within the N terminus is responsible for Golgi targeting. Furthermore, ARNO and ARNO3 N termini also have the same capability of targeting to the Golgi. We conclude that the N-terminal, (α)-helical, coiled-coil domain is used to target this family of proteins to the Golgi complex.

Phosphoinositides determine specificity of the guanine-nucleotide exchange activity of cytohesin-1 for ADP-ribosylation factors derived from a mammalian expression system

ADP-ribosylation factors (ARFs) are small Ras-like GTPases which play important roles in intracellular vesicle transport and in the remodeling of the actin cytoskeleton. Guanine nucleotide exchange factors (GEFs) for ARFs have recently been identified. One of them, cytohesin-1, a 47-kDa cytoplasmic protein acts as an inside-out signaling molecule and regulates binding of the beta2 integrin leukocyte function antigen 1 (LFA-1) to its ligand intercellular adhesion molecule 1 (ICAM-1). In this study, we address the regulation of the GEF activity of cytohesin-1 by phosphoinositides, using mammalian expression of functional ARF-Ig chimeras. The fusion proteins, which can be quantitatively immunoprecipitated on protein A-Sepharose, target to the expected intracellular compartments, and they are readily induced to bind GTP in vitro. We show that both ARF1-Ig and ARF6-Ig chimeras are activated in vitro by cytohesin-1. However, GEF activity towards ARF6 is strongly suppressed by phosphatidylinositol-(3,4,5)-trisphosphate (PtdInsP3). In contrast, cytohesin-1-dependent GTP binding of ARF1 is significantly enhanced by PtdInsP3. We conclude that the membrane phospholipid PtdInsP3 determines the specificity of the GEF activity of cytohesin-1.

Identification of a plasma membrane-associated guanine nucleotide exchange factor for ARF6 in chromaffin cells. Possible role in the regulated exocytotic pathway

ADP-ribosylation factors (ARFs) constitute a family of structurally related proteins that forms a subset of the Ras superfamily of regulatory GTP-binding proteins. Like other GTPases, activation of ARFs is facilitated by specific guanine nucleotide exchange factors (GEFs). In chromaffin cells, ARF6 is associated with the membrane of secretory granules. Stimulation of intact cells or direct elevation of cytosolic calcium in permeabilized cells triggers the rapid translocation of ARF6 to the plasma membrane and the concomitant activation of phospholipase D (PLD) in the plasma membrane. Both calcium-evoked PLD activation and catecholamine secretion in permeabilized cells are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6, suggesting that the ARF6-dependent PLD activation near the exocytotic sites represents a key event in the exocytotic reaction in chromaffin cells. In the present study, we demonstrate the occurrence of a brefeldin A-insensitive ARF6-GEF activity in the plasma membrane and in the cytosol of chromaffin cells. Furthermore, reverse transcriptase-polymerase chain reaction and immunoreplica analysis indicate that ARNO, a member of the brefeldin A-insensitive ARF-GEF family, is expressed and predominantly localized in the cytosol and in the plasma membrane of chromaffin cells. Using permeabilized chromaffin cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits, in a dose-dependent manner, both PLD activation and catecholamine secretion in calcium-stimulated cells. Furthermore, co-expression in PC12 cells of a catalytically inactive ARNO mutant with human growth hormone as a marker of secretory granules in transfected cells resulted in a 50% inhibition of growth hormone secretion evoked by depolarization with high K(+). The possibility that the plasma membrane-associated ARNO participates in the exocytotic pathway by activating ARF6 and downstream PLD is discussed.

Type I phosphatidylinositol 4-phosphate 5-kinase directly interacts with ADP-ribosylation factor 1 and is responsible for phosphatidylinositol 4,5-bisphosphate synthesis in the golgi compartment

Phosphatidylinositol (PtdIns) 4,5-bisphosphate is involved in many aspects of membrane traffic, but the regulation of its synthesis is only partially understood. Golgi membranes contain PI 4-kinase activity and a pool of phosphatidylinositol phosphate (PIP), which is further increased by ADP-ribosylation factor 1 (ARF1). COS7 cells were transfected with alpha and beta forms of PI 4-kinase, and only membranes from COS7 cells transfected with PI 4-kinase beta increased their content of PIP when incubated with ARF1. PtdIns(4, 5)P(2) content in Golgi membranes was nonexistent but could be increased to a small extent upon adding either cytosol or Type I or Type II PIP kinases. However, when ARF1 was present, PtdIns(4,5)P(2) levels increased dramatically when membranes were incubated in the presence of cytosol or Type I, but not Type II, PIP kinase. To examine whether ARF1 could directly activate Type I PIP 5-kinase, we used an in vitro assay consisting of phosphatidycholine-containing liposomes, ARF1, and PIP 5-kinase. ARF1 increased Type I PIP 5-kinase activity in a guanine nucleotide-dependent manner, identifying this enzyme as a direct effector for ARF1.

B2-1, a Sec7- and pleckstrin homology domain-containing protein, localizes to the Golgi complex

B2-1 is a human protein that contains both a Sec7 and a pleckstrin homology domain. The yeast Sec7 protein was previously shown to be involved in vesicle formation in the Golgi and endoplasmic reticulum. Recently, several groups have shown that B2-1 and highly similar proteins (e.g., ARNO, ARNO3) have varied cellular functions and subcellular locations. One of these is an association of the B2-1 Sec7 domain with the plasma membrane, binding to the cytoplasmic portion of the integrin beta2 chain (CD18) and is postulated to be involved in inside-out signaling. Other groups have shown that B2-1 and these related proteins are guanine nucleotide-exchange factors that act upon ADP ribosylation factors (ARFs) and are localized to the Golgi or plasma membrane. Here we report the subcellular localization of B2-1 protein. Interestingly, B2-1 does not localize to the plasma membrane but rather associates with a distinct Golgi complex compartment. B2-1's distribution can be disrupted by brefeldin A, a drug that rapidly disrupts the Golgi apparatus by inhibiting ARF activity. Furthermore, transient transfection of GFP-tagged B2-1 shows Golgi complex targeting. Excessive overexpression of transfected B2-1 causes partial Golgi dispersion.

Inhibition of phosphatidic acid synthesis alters the structure of the Golgi apparatus and inhibits secretion in endocrine cells

In mammalian cells, activation of a Golgi-associated phospholipase D by ADP-ribosylation factor results in the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). This reaction stimulates the release of nascent secretory vesicles from the trans-Golgi network of endocrine cells. To understand the role of PA in mediating secretion, we have exploited the transphosphatidylation activity of phospholipase D. Rat anterior pituitary GH3 cells, which secrete growth hormone and prolactin, were treated with 1-butanol resulting in the synthesis of phosphatidylbutanol rather than PA. Under these conditions transport from the ER through the Golgi apparatus and secretion of polypeptide hormones were inhibited quantitatively. Furthermore, the in vitro synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) by Golgi membranes was inhibited quantitatively. Most significantly, in the presence of 1-butanol the architecture of the Golgi apparatus was disrupted, resulting in its disassembly and fragmentation. Removal of the alcohol resulted in the rapid restoration of Golgi structure and secretion of growth hormone and prolactin. Our results suggest that PA stimulation of PtdIns(4,5)P(2) synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

Characterization, chromosomal assignment, and tissue expression of a novel human gene belonging to the ARF GAP family

We have identified and characterized a novel human ADP-ribosylation factor GTPase-activating protein (ARFGAP1) gene that is related to other members of the ARF GAP family. The full-length cDNA for human ARFGAP1 was cloned following the identification of an EST obtained by large-scale cDNA library sequencing through a Blast search of public databases. Structurally, ARFGAP1 encodes a polypeptide of 516 amino acids, which contained a typical GATA-1-type zinc finger motif (CXXCX(16)CXXC) with the four cysteine residues that are highly conserved among other members of the ARF GAP family. The conserved ARF GAP domain may emphasize the biological importance of this gene. The ARFGAP1 gene, which contained 16 exons ranging from 0.5 to 9.3 kb, was mapped to human chromosome 22q13.2-q13.3 using radiation hybridization and in silico analyses. ARFGAP1 is strongly expressed in endocrine glands and testis. Interestingly, the expression of ARFGAP1 in testis is about sixfold higher than that in ovary, indicating a possible role of ARFGAP1 in the physiological function of sperm. Expression of ARFGAP1 in four human fetal tissues and seven cancer cell lines was also detected.

The role of ARF and Rab GTPases in membrane transport

Two key events of intracellular transport and membrane trafficking in eukaryotic cells, the formation of transport vesicles and their specific delivery to target membranes, are controlled by small GTPases of the ADP-ribosylation factor (ARF) and Rab families, respectively. The past 18 months have seen the identification of proteins that regulate ARF and Rab GDP/GTP cycle, as well as the characterization of their effectors, shedding light on the molecular mechanisms of ARF and Rab function.

p200 ARF-GEP1: a Golgi-localized guanine nucleotide exchange protein whose Sec7 domain is targeted by the drug brefeldin A

The drug brefeldin A (BFA) disrupts protein traffic and Golgi morphology by blocking activation of ADP ribosylation factors (ARFs) through an unknown mechanism. Here, we investigated the cellular localization and BFA sensitivity of human p200 ARF-GEP1 (p200), a ubiquitously expressed guanine nucleotide exchange factor of the Sec7 domain family. Multiple tagged forms of the full-length polypeptide localized to tight ribbon-like perinuclear structures that overlapped with the Golgi marker mannosidase II and were distinct from the pattern observed with ERGIC53/58. Analysis of several truncated forms mapped the Golgi-localization signal to the N-terminal third of p200. BFA treatment of transiently or stably transfected cells resulted in the redistribution of Golgi markers and in loss of cell viability, thereby indicating that overproduction of p200 may not be sufficient to overcome the toxic effect. A 39-kDa fragment spanning the Sec7 domain catalyzed loading of guanosine 5'-[gamma-thio]triphosphate onto class I ARFs and displayed clear sensitivity to BFA. Kinetic analysis established that BFA did not compete with ARF for interaction with p200 but, rather, acted as an uncompetitive inhibitor that only targeted the p200-ARF complex with an inhibition constant of 7 microM. On the basis of these results, we propose that accumulation of an abortive p200-ARF complex in the presence of BFA likely leads to disruption of Golgi morphology. p200 mapped to chromosome 8q13, 3.56 centirays from WI-6151, and database searches revealed the presence of putative isoforms whose inhibition may account for the effects of BFA on various organelles.

Blockade of membrane transport and disassembly of the Golgi complex by expression of syntaxin 1A in neurosecretion-incompetent cells: prevention by rbSEC1

The t-SNAREs syntaxin1A and SNAP-25, i.e. the members of the complex involved in regulated exocytosis at synapses and neurosecretory cells, are delivered to their physiological site, the plasma membrane, when transfected into neurosecretion-competent cells, such as PC12 and AtT20. In contrast, when transfection is made into cells incompetent for neurosecretion, such as those of a defective PC12 clone and the NRK fibroblasts, which have no endogenous expression of these t-SNAREs, syntaxin1A (but neither two other syntaxin family members nor SNAP-25) remains stuck in the Golgi-TGN area with profound consequences to the cell: blockade of both membrane (SNAP-25, GAT-1) and secretory (chromogranin B) protein transport to the cell surface; progressive disassembly of the Golgi complex and TGN; ultimate disappearance of the latter structures, with intermixing of their markers (mannosidase II; TGN-38) with those of the endoplasmic reticulum (calreticulin) and with syntaxin1A itself. When, however, syntaxin 1A is transfected together with rbSec1, a protein known to participate in neurosecretory exocytosis via its dynamic interaction with the t-SNARE, neither the blockade nor the alterations of the Golgi complex take place. Our results demonstrate that syntaxin1A, in addition to its role in exocytosis at the cell surface, possesses a specific potential to interfere with intracellular membrane transport and that its interaction with rbSec1 is instrumental to its physiological function not only at the plasma membrane but also within the cell. At the latter site, the rbSec1-induced conversion of syntaxin1A into a form that can be transported and protects the cell from the development of severe structural and membrane traffic alterations.

Structural basis for the inhibitory effect of brefeldin A on guanine nucleotide-exchange proteins for ADP-ribosylation factors

Protein secretion through the endoplasmic reticulum and Golgi vesicular trafficking system is initiated by the binding of ADP-ribosylation factors (ARFs) to donor membranes, leading to recruitment of coatomer, bud formation, and eventual vesicle release. ARFs are approximately 20-kDa GTPases that are active with bound GTP and inactive with GDP bound. Conversion of ARF-GDP to ARF-GTP is regulated by guanine nucleotide-exchange proteins. All known ARF guanine nucleotide-exchange proteins contain a Sec7 domain of approximately 200 amino acids that includes the active site and fall into two classes that differ in molecular size and susceptibility to inhibition by the fungal metabolite brefeldin A (BFA). To determine the structural basis of BFA sensitivity, chimeric molecules were constructed by using sequences from the Sec7 domains of BFA-sensitive yeast Sec7 protein (ySec7d) and the insensitive human cytohesin-1 (C-1Sec7). Based on BFA inhibition of the activities of these molecules with recombinant yeast ARF2 as substrate, the Asp965-Met975 sequence in ySec7d was shown to be responsible for BFA sensitivity. A C-1Sec7 mutant in which Ser199, Asn204, and Pro209 were replaced with the corresponding ySec7d amino acids, Asp965, Gln970, and Met975, exhibited BFA sensitivity similar to that of recombinant ySec7d (rySec7d). Single replacement in C-1Sec7 of Ser199 or Pro209 resulted in partial inhibition by BFA, whereas replacement of Gln970 in ySec7d with Asn (as found in C-1Sec7) had no effect. As predicted, the double C-1Sec7 mutant with S199D and P209M was BFA-sensitive, demonstrating that Asp965 and Met975 in ySec7d are major molecular determinants of BFA sensitivity.