Distribution of ARF6 between membrane and cytosol is regulated by its GTPase cycle

Rab35 Is Required for Embryonic Development and Kidney and Ureter Homeostasis through Regulation of Epithelial Cell Junctions

Key Points

Loss of Rab35 leads to nonobstructive hydronephrosis because of loss of ureter epithelium. Rab35 regulates kidney and ureter epithelial cell adhesion and polarity. Rab35 is required for embryonic development.

Background

Rab35 is a member of a GTPase family of endocytic trafficking proteins. Studies in cell lines have indicated that Rab35 participates in cell adhesion, polarity, cytokinesis, and primary cilia length and composition. In addition, sea urchin Rab35 regulates actin organization and is required for gastrulation. In mice, loss of Rab35 in the central nervous system disrupts hippocampal development and neuronal organization. Outside of the central nervous system, the functions of mammalian Rab35 in vivo are unknown.

Methods

We generated and analyzed the consequences of both congenital and conditional null Rab35 mutations in mice. Using a LacZ reporter allele, we assessed Rab35 expression during development and postnatally. We assessed Rab35 loss in the kidney and ureter using histology, immunofluorescence microscopy, and western blotting.

Results

Congenital Rab35 loss of function caused embryonic lethality: homozygous mutants arrested at E7.5 with cardiac edema. Conditional loss of Rab35, either during gestation or postnatally, caused hydronephrosis. The kidney and ureter phenotype were associated with disrupted actin cytoskeletal architecture, altered Arf6 epithelial polarity, reduced adherens junctions, loss of tight junction formation, defects in epithelial growth factor receptor expression and localization, disrupted cell differentiation, and shortened primary cilia.

Conclusions

Rab35 may be essential for mammalian development and the maintenance of kidney and ureter architecture. Loss of Rab35 leads to nonobstructive hydronephrosis, making the Rab35 mutant mouse a novel mammalian model to study mechanisms underlying this disease.

Rab35 is required for embryonic development and kidney and ureter homeostasis through regulation of epithelial cell junctions

Background

Rab35 is a member of a GTPase family of endocytic trafficking proteins. Studies in cell lines have indicated that Rab35 participates in cell adhesion, polarity, cytokinesis, and primary cilia length and composition. Additionally, sea urchin Rab35 regulates actin organization and is required for gastrulation. In mice, loss of Rab35 in the CNS disrupts hippocampal development and neuronal organization. Outside of the CNS, the functions of mammalian Rab35 in vivo are unknown.

Methods

We generated and analyzed the consequences of both congenital and conditional null Rab35 mutations in mice. Using a LacZ reporter allele, we assessed Rab35 expression during development and postnatally. We assessed Rab35 loss in the kidney and ureter using histology, immunofluorescence microscopy, and western blotting.

Results

Congenital Rab35 loss of function caused embryonic lethality: homozygous mutants arrested at E7.5 with cardiac edema. Conditional loss of Rab35, either during gestation or postnatally, caused hydronephrosis. The kidney and ureter phenotype were associated with disrupted actin cytoskeletal architecture, altered Arf6 epithelial polarity, reduced adherens junctions, loss of tight junction formation, defects in EGFR expression and localization, disrupted cell differentiation, and shortened primary cilia.

Conclusion

Rab35 is essential for mammalian development and the maintenance of kidney and ureter architecture. Loss of Rab35 leads to non-obstructive hydronephrosis, making the Rab35 mutant mouse a novel mammalian model to study mechanisms underlying this disease.

Emerging Roles of Small GTPases in Islet β-Cell Function

Several small guanosine triphosphatases (GTPases) from the Ras protein superfamily regulate glucose-stimulated insulin secretion in the pancreatic islet β-cell. The Rho family GTPases Cdc42 and Rac1 are primarily involved in relaying key signals in several cellular functions, including vesicle trafficking, plasma membrane homeostasis, and cytoskeletal dynamics. They orchestrate specific changes at each spatiotemporal region within the β-cell by coordinating with signal transducers, guanine nucleotide exchange factors (GEFs), GTPase-activating factors (GAPs), and their effectors. The Arf family of small GTPases is involved in vesicular trafficking (exocytosis and endocytosis) and actin cytoskeletal dynamics. Rab-GTPases regulate pre-exocytotic and late endocytic membrane trafficking events in β-cells. Several additional functions for small GTPases include regulating transcription factor activity and mitochondrial dynamics. Importantly, defects in several of these GTPases have been found associated with type 2 diabetes (T2D) etiology. The purpose of this review is to systematically denote the identities and molecular mechanistic steps in the glucose-stimulated insulin secretion pathway that leads to the normal release of insulin. We will also note newly identified defects in these GTPases and their corresponding regulatory factors (e.g., GDP dissociation inhibitors (GDIs), GEFs, and GAPs) in the pancreatic β-cells, which contribute to the dysregulation of metabolism and the development of T2D.

Arf6 Can Trigger Wave Regulatory Complex-Dependent Actin Assembly Independent of Arno

The small GTPase ADP-ribosylation factor 6 (Arf6) anchors at the plasma membrane to orchestrate key functions, such as membrane trafficking and regulating cortical actin cytoskeleton rearrangement. A number of studies have identified key players that interact with Arf6 to regulate actin dynamics in diverse cell processes, yet it is still unknown whether Arf6 can directly signal to the wave regulatory complex to mediate actin assembly. By reconstituting actin dynamics on supported lipid bilayers, we found that Arf6 in co-ordination with Rac1(Ras-related C3 botulinum toxin substrate 1) can directly trigger actin polymerization by recruiting wave regulatory complex components. Interestingly, we demonstrated that Arf6 triggers actin assembly at the membrane directly without recruiting the Arf guanine nucleotide exchange factor (GEF) ARNO (ARF nucleotide-binding site opener), which is able to activate Arf1 to enable WRC-dependent actin assembly. Furthermore, using labelled E. coli, we demonstrated that actin assembly by Arf6 also contributes towards efficient phagocytosis in THP-1 macrophages. Taken together, this study reveals a mechanism for Arf6-driven actin polymerization.

The Small GTPase Arf6: An Overview of Its Mechanisms of Action and of Its Role in Host⁻Pathogen Interactions and Innate Immunity

The small GTase Arf6 has several important functions in intracellular vesicular trafficking and regulates the recycling of different types of cargo internalized via clathrin-dependent or -independent endocytosis. It activates the lipid modifying enzymes PIP 5-kinase and phospholipase D, promotes actin polymerization, and affects several functionally distinct processes in the cell. Arf6 is used for the phagocytosis of pathogens and can be directly or indirectly targeted by various pathogens to block phagocytosis or induce the uptake of intracellular pathogens. Arf6 is also used in the signaling of Toll-like receptors and in the activation of NADPH oxidases. In this review, we first give an overview of the different roles and mechanisms of action of Arf6 and then focus on its role in innate immunity and host–pathogen interactions.

DDEFL1 correlated with Rho GTPases activity in breast cancer

DDEFL1 is related to maintaining a limiting amount of ARF6 in GTP-loaded form by accelerating its GTP hydrolysis activity, which has been implicated in hepatocellular cancer pathogenesis and lung cancer development. We investigated DDEFL1 expression in breast cancer and paired normal breast tissues by immunohistochemistry and found that DDEFL1 expression was significantly associated with tumor size, lymph node metastasis, high content of elastosis and TNM stage but not with menopausal status or age. We detected the mRNA and protein expression of DDEFL1 in breast cancer cell lines by Western blotting and quantitative real-time PCR (qRT-PCR). DDEFL1 was obvious in MDA-MB-435s and MDA-MB-231 but very weak in ZR-75-1. Further experiments were conducted to evaluate the effect of DDEFL1 small interfering RNA (siRNA) transfection on the biological behavior of MDA-MB-231. After transfection, the effects of DDEFL1 inhibition on expression of mRNA and protein were also analyzed by Western blotting and qRT-PCR. Increased apoptosis and invasive ability, decreased cellular proliferation was found in MDA-MB-231 with successful DDEFL1 siRNA transient transfection (p < 0.05). Western blotting and qRT-PCR results showed that the DDEFL1 inhibition up-regulated Caspase-3, Apaf-1, cytochrome c, and Bax expression and down-regulated Bcl-2 expression. The DDEFL1 inhibition also down-regulated the mRNA and protein expression of Rho, CDC42 and Rac1. Our study provided a functional linkage through DDEFL1 with breast cancer biological behaviours by Rho GTPases. Possible implication of our main finding for the DDEFL1 role in breast cancer and the downstream signaling pathways for the treatment of breast cancer.

Cellular and subcellular localization of ADP-ribosylation factor 6 in mouse peripheral tissues

ADP-ribosylation factor 6 (Arf6) is a small GTPase that regulates endosomal trafficking and actin cytoskeleton remodeling. In the present study, we comprehensively examined the cellular and subcellular localization of Arf6 in adult mouse peripheral tissues by immunofluorescence and immunoelectron microscopy using the heat-induced antigen retrieval method with Tris-EDTA buffer (pH 9.0). Marked immunolabeling of Arf6 was observed particularly in epithelial cells of several tissues including the esophagus, stomach, small and large intestines, trachea, kidney, epididymis, oviduct, and uterus. In most epithelial cells of simple or pseudostratified epithelia, Arf6 exhibited predominant localization to the basolateral membrane and a subpopulation of endosomes. At an electron microscopic level, Arf6 was localized along the basolateral membrane, with dense accumulation at interdigitating processes and infoldings. Arf6 was present in a ring-like appearance at intercellular bridges in spermatogonia and spermatocytes in the testis and at the Flemming body of cytokinetic somatic cells in the ovarian follicle, thymus, and spleen. The present study provides anatomical clues to help understand the physiological roles of Arf6 at the whole animal level.

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

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

IMPORTANCE

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

Arf6 regulation of Gyrating-clathrin

'Gyrating-' or 'G'-clathrin are coated endocytic structures located near peripheral sorting endosomes (SEs), which exhibit highly dynamic but localized movements when visualized by live-cell microscopy. They have been implicated in recycling of transferrin from the sorting endosome directly to the cell surface, but there is no information about their formation or regulation. We show here that G-clathrin comprise a minority of clathrin-coated structures in the cell periphery and are brefeldin A (BFA)-resistant. Arf6-GTP substantially increases G-clathrin levels, probably by lengthening coated bud lifetimes as suggested by photobleaching and photoactivation results, and an Arf6(Q67L)-GTP mutant bearing an internal GFP tag can be directly visualized in G-clathrin structures in live cells. Upon siRNA-mediated depletion of Arf6 or expression of Arf6(T27N), G-clathrin levels rise and are primarily Arf1-dependent, yet still BFA-resistant. However, BFA-sensitive increased G-clathrin levels are observed upon acute incubation with cytohesin inhibitor SecinH3, indicating a shift in GEF usage. Depletion of both Arf6 and Arf1 abolishes G-clathrin, and results in partial inhibition of fast transferrin recycling consistent with the latter's participation in this pathway. Collectively, these results demonstrate that the dynamics of G-clathrin primarily requires completion of the Arf6 guanine nucleotide cycle, but can be regulated by multiple Arf and GEF proteins, reflecting both overlapping mechanisms operative in their regulation and the complexity of processes involved in endosomal sorting.

Several ADP-ribosylation factor (Arf) isoforms support COPI vesicle formation

Newly synthesized proteins and lipids are transported in vesicular carriers along the secretory pathway. Arfs (ADP-ribosylation factors), a family of highly conserved GTPases within the Ras superfamily, control recruitment of molecular coats to membranes, the initial step of coated vesicle biogenesis. Arf1 and coatomer constitute the minimal cytosolic machinery leading to COPI vesicle formation from Golgi membranes. Although some functional redundancies have been suggested, other Arf isoforms have been poorly analyzed in this context. In this study, we found that Arf1, Arf4, and Arf5, but not Arf3 and Arf6, associate with COPI vesicles generated in vitro from Golgi membranes and purified cytosol. Using recombinant myristoylated proteins, we show that Arf1, Arf4, and Arf5 each support COPI vesicle formation individually. Unexpectedly, we found that Arf3 could also mediate vesicle biogenesis. However, Arf3 was excluded from the vesicle fraction in the presence of the other isoforms, highlighting a functional competition between the different Arf members.

Adenosine diphosphate-ribosylation factor 6 is required for epidermal growth factor-induced glioblastoma cell proliferation

BACKGROUND

: Epidermal growth factor (EGF) signaling plays a pivotal role in gliomagenesis. The authors previously demonstrated that adenosine diphospate-ribosylation factor 6 (ARF6), a member of the Ras-related small guanosine-5'-triphospate-binding protein family, is required for EFA6A-induced glioma cell migration and invasion. However, the role of ARF6 in EGF signaling is unknown.

METHODS

: The authors analyzed messenger RNA (mRNA) levels of ARF6 and EGF receptor (EGFR) in 16 high-grade glioma samples and in 6 low-grade glioma samples by reverse transcriptase-polymerase chain reaction analysis. To determine whether EGF induces ARF6 expression in human glioblastoma U87 cells through transcriptional regulation and EGFR activation, the levels of ARF6 were assayed in EGF-treated U87 cells that were preincubated with a transcriptional inhibitor (actinomycin D) and an EGFR tyrosine kinase inhibitor (PD153035), respectively. The downstream signaling of EGFR-mediated ARF6 up-regulation also was investigated using specific inhibitors of mitogen-activated protein kinase (MEK), phosphatidylinositol 3' kinase (PI3K), and Janus kinase 2. The involvement of SP1 in the downstream signaling was studied by using an SP1 inhibitor (mithramycin A). Small-interfering RNAs (siRNAs) targeting ARF6 were used to investigate the effects of ARF6 on EGF-mediated glioma cell proliferation.

RESULTS

: The results demonstrated that ARF6 and EGFR mRNA levels were elevated in glioma tissues. Furthermore, EGF stimulated ARF6 expression in U87 cells in a dose-dependent and time-dependant manner. This stimulation was caused by increased transcription of ARF6 and by activation of the MEK/extracellular signal-regulated kinase 1 and 2 (ERK1/2) and PI3K signaling pathways. It is noteworthy that SP1 was essential for EGF-induced ARF6 up-regulation. Finally, EGF-induced glioblastoma cell proliferation depended on ARF6, because the suppression of ARF6 by siRNA or by a dominant-negative mutant significantly inhibited EGF-induced cell proliferation.

CONCLUSIONS

: The results of the current study suggested that EGF-induced ARF6 expression plays a significant role in glioma cell proliferation. Cancer 2009. (c) 2009 American Cancer Society.

Regulation of c-Src nonreceptor tyrosine kinase activity by bengamide A through inhibition of methionine aminopeptidases

Methionine aminopeptidases (MetAPs) remove the N-terminal initiator methionine during protein synthesis, a prerequisite step for N-terminal myristoylation. N-myristoylation of proto-oncogene c-Src is essential for its membrane association and proper signal transduction. We used bengamides, a family of general MetAP inhibitors, to understand the downstream physiological functions of MetAPs. c-Src from bengamide A-treated cells retained its N-terminal methionine and suffered a decrease in N-terminal myristoylation, which was accompanied by a shift of its subcellular distribution from the plasma membrane to the cytosol. Furthermore, bengamide A decreased the tyrosine kinase activities of c-Src both in vitro and in vivo and eventually delayed cell-cycle progression through G(2)/M. Thus, c-Src is a physiologically relevant substrate for MetAPs whose dysfunction is likely to account for the cell-cycle effects of MetAP inhibitors including bengamide A.

VEGF signaling through NADPH oxidase-derived ROS

Angiogenesis is a key process involved in normal development and wound repair, as well as ischemic heart and limb diseases, and atherosclerosis. Vascular endothelial growth factor (VEGF), a potent angiogenesis factor, stimulates proliferation, migration, and tube formation of endothelial cells (ECs), primarily through the VEGF receptor type2 (VEGFR2). Reactive oxygen species (ROS) function as signaling molecules to mediate biological responses. In ECs, NADPH oxidase is one of the major sources of ROS and consists of catalytic subunits (Nox1, Nox2, and Nox4), p22phox, p47phox, p67phox, and the small GTPase Rac1. VEGF stimulates ROS production via activation of gp91phox (Nox2)-based NADPH oxidase, and ROS are involved in VEGFR2-mediated signaling linked to EC migration and proliferation. Moreover, ROS derived from NADPH oxidase are involved in postnatal angiogenesis. Localizing NADPH oxidase and its regulators at the specific subcellular compartment is an important mechanism for activating specific redox signaling events. This review focuses on a role of NADPH oxidase-derived ROS in angiogenesis and critical regulators involved in generation of spatially and temporally restricted ROS-dependent VEGF signaling at leading edge, focal adhesions/complexes, caveolae/lipid rafts, and cell-cell junctions in ECs. Understanding these mechanisms should facilitate the development of new therapeutic strategies to modulate new blood vessel formation.

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

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

ACRP30 is secreted from 3T3-L1 adipocytes via a Rab11-dependent pathway

Adipocytes are now known to secrete a range of adipokines that exhibit distinct biological functions. Here, we sought to understand the secretory pathways utilised by ACRP30 to the surface of adipocytes. We find that ACRP30 overlaps with adipsin in intracellular compartments distinct from Glut4, but nonetheless exhibits insulin-stimulated secretion from cells. Both adipsin and ACRP30 overlap with transferrin receptor-positive membranes, implying that the pathway of secretion involves the transferrin receptor-positive endosomal system. Consistent with this, we show that ablation of endosomes significantly inhibited the secretion of ACRP30, as did treatment of cells with Brefeldin A. In order to further probe the role of recycling endosomes on the secretion of ACRP30, we over-expressed a mutant form of Rab11, Rab11-S25N, in 3T3-L1 adipocytes and found that expression of this mutant significantly reduced basal and insulin-stimulated secretion. We also demonstrate that Arf6 also plays a role in the secretion of ACRP30. Collectively, these data implicate both Arf6 and Rab11 as crucial mediators of constitutive and insulin-stimulated secretion of ACRP30 and further suggest that recycling endosomes may play a central role in this process.

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.

Localization and regulation of phospholipase D2 by ARF6

Phospholipase D (PLD) and ADP-ribosylation factor 6 (ARF6) have been implicated in vesicular trafficking and rearrangement of the actin cytoskeleton. We have explored the co-localization of rat PLD1b and rat PLD2 with wild type and mutant forms of ARF6 in HeLa cells and studied their activation by ARF6 and the role of the actin cytoskeleton. GFP-tagged PLD1 had a similar pattern to multivesicular and late endosomes and the trans-Golgi apparatus, but not to other organelles. When wild type or dominant negative ARF6 and PLD1 or PLD2 were co-expressed, they had a similar localization in cytosolic particles and at the cell periphery. In contrast, dominant active ARF6 caused cell shrinkage and had a similar localization with PLD1 and PLD2 in dense structures, containing the trans-Golgi apparatus and actin. Disruption of the actin cytoskeleton with cytochalasin D did not induce the formation of these structures. To determine, if ARF6 selectively activated PLD1 or PLD2, wild type and mutant forms of the ARF isoform were transfected together with PLD1 or PLD2. Wild type ARF6 did not affect either PLD isozyme, but dominant active ARF6 selectively activated PLD2 and dominant negative ARF6 selectively inhibited PLD2. In contrast, dominant active ARF1 or Rac1 stimulated both PLD isozymes but the ARF1 effect on PLD2 was very small. Cytochalasin D did not affect the activation of PLD by phorbol ester. The localizations of PLD and ARF6 were also analyzed by fractionation after methyl-beta-cyclodextrin extraction to deplete cholesterol. The results showed that all PLD isoforms and ARF6 mutants existed in the membrane fraction, but only wild type ARF6 was dependent on the presence of cholesterol. These experiments showed that wild type ARF6 had a similar location with PLD isoforms on cell staining, but it did not colocalize with PLD isoforms in fractionation experiments. It is proposed that activated ARF6 translocates to the cholesterol independent microdomain and then activates PLD2 there. It is further concluded that PLD2 is selectively activated by ARF6 in vivo and that disruption of the actin cytoskeleton does not affect this activation.

Growth Factors Mobilize Multiple Pools ofKCaChannels in Developing Parasympathetic Neurons: Role of ADP-Ribosylation Factors and Related Proteins

In developing ciliary ganglion (CG) neurons, movement of functional large-conductance (BK type) Ca2+-activated K+( KCa) channels to the cell surface is stimulated by the endogenous growth factors TGFβ1 and β-neuregulin-1 (NRG1). Here we show that a brief NRG1 treatment (0.5–1.5 h) mobilizes KCachannels in a post-Golgi compartment, but longer treatments (>3.5 h) mobilize KCachannels located in the endoplasmic reticulum or Golgi apparatus. Specifically, the effects of 3.5 h NRG1 treatment were completely blocked by treatments that disrupt Golgi apparatus function. These include inhibition of microtubules, or inhibition of the ADP-ribosylation factor-1 (ARF1) system by brefeldin A, by over-expression of dominant-negative ARF1, or over-expression of an ARF1 GTPase-activating protein that blocks ARF1 cycling between GTP- and GDP-bound states. These treatments had no effect on stimulation of KCaevoked by 1.5 h treatment with NRG1, indicating that short-term responses to NRG1 do not require an intact Golgi apparatus. By contrast, both the acute and sustained effects of NRG1 were inhibited by treatments that block trafficking processes that occur close to the plasma membrane. Thus mobilization of KCawas blocked by treatments than inhibit ADP-ribosylation factor-6 (ARF6) signaling, including overexpression of dominant-negative ARF6, dominant-negative ARNO, or dominant-negative phospholipase D1. TGFβ1, the effects of which on KCaare much slower in onset, is unable to selectively mobilize channels in the post-Golgi pool, and its effects on KCaare completely blocked by inhibition of microtubules, Golgi function and also by plasma membrane ARF6 and phospholipase D1 signaling.

Novel role of ARF6 in vascular endothelial growth factor-induced signaling and angiogenesis

Vascular endothelial growth factor (VEGF) stimulates endothelial cell (EC) migration and proliferation primarily through the VEGF receptor-2 (VEGFR2). We have shown that VEGF stimulates a Rac1-dependent NAD(P)H oxidase to produce reactive oxygen species (ROS) that are involved in VEGFR2 autophosphorylation and angiogenic-related responses in ECs. The small GTPase ARF6 is involved in membrane trafficking and cell motility; however, its roles in VEGF signaling and physiological responses in ECs are unknown. In this study, we show that overexpression of dominant-negative ARF6 [ARF6(T27N)] almost completely inhibits VEGF-induced Rac1 activation, ROS production, and VEGFR2 autophosphorylation in ECs. Fractionation of caveolae/lipid raft membranes demonstrates that ARF6, Rac1, and VEGFR2 are localized in caveolin-enriched fractions basally. VEGF stimulation results in the release of VEGFR2 from caveolae/lipid rafts and caveolin-1 without affecting localization of ARF6, Rac1, or caveolin-1 in these fractions. The egress of VEGFR2 from caveolae/lipid rafts is contemporaneous with the tyrosine phosphorylation of caveolin-1 (Tyr14) and VEGFR2 and with their association with each other. ARF6(T27N) significantly inhibits both VEGF-induced responses. Immunofluorescence studies show that activated VEGFR2 and phosphocaveolin colocalize at focal complexes/adhesions after VEGF stimulation. Both overexpression of ARF6(T27N) and mutant caveolin-1(Y14F), which cannot be phosphorylated, block VEGF-stimulated EC migration and proliferation. Moreover, ARF6 expression is markedly upregulated in association with an increase in capillary density in a mouse hindlimb ischemia model of angiogenesis. Thus, ARF6 is involved in the temporal-spatial organization of caveolae/lipid rafts- and ROS-dependent VEGF signaling in ECs as well as in angiogenesis in vivo.

Differential expression and targeting of endogenous Arf1 and Arf6 small GTPases in kidney epithelial cells in situ

ADP-ribosylation factors (Arfs) are small GTPases that regulate vesicular trafficking in exo- and endocytotic pathways. As a first step in understanding the role of Arfs in renal physiology, immunocytochemistry and Western blotting were performed to characterize the expression and targeting of Arf1 and Arf6 in epithelial cells in situ. Arf1 and Arf6 were associated with apical membranes and subapical vesicles in proximal tubules, where they colocalized with megalin. Arf1 was also apically expressed in the distal tubule, connecting segment, and collecting duct (CD). Arf1 was abundant in intercalated cells (IC) and colocalized with V-ATPase in A-IC (apical) and B-IC (apical and/or basolateral). In contrast, Arf6 was associated exclusively with basolateral membranes and vesicles in the CD. In the medulla, basolateral Arf6 was detectable mainly in A-IC. Expression in principal cells became weaker throughout the outer medulla, and Arf6 was not detectable in principal cells in the inner medulla. In some kidney epithelial cells Arf1 but not Arf6 was also targeted to a perinuclear patch, where it colocalized with TGN38, a marker of the trans-Golgi network. Quantitative Western blotting showed that expression of endogenous Arf1 was 26–180 times higher than Arf6. These data indicate that Arf GTPases are expressed and targeted in a cell- and membrane-specific pattern in kidney epithelial cells in situ. The results provide a framework on which to base and interpret future studies on the role of Arf GTPases in the multitude of cellular trafficking events that occur in renal tubular epithelial cells.

Phylogenetic analysis of Sec7-domain-containing Arf nucleotide exchangers

The eukaryotic family of ADP-ribosylation factor (Arf) GTPases plays a key role in the regulation of protein trafficking, and guanine-nucleotide exchange is crucial for Arf function. Exchange is stimulated by members of another family of proteins characterized by a 200-amino acid Sec7 domain, which alone is sufficient to catalyze exchange on Arf. Here, we analyzed the phylogeny of Sec7-domain-containing proteins in seven model organisms, representing fungi, plants, and animals. The phylogenetic tree has seven main groups, of which two include members from all seven model systems. Three groups are specific for animals, whereas two are specific for fungi. Based on this grouping, we propose a phylogenetically consistent set of names for members of the Sec7-domain family. Each group, except for one, contains proteins with known Arf exchange activity, implying that all members of this family have this activity. Contrary to the current convention, the sensitivity of Arf exchange activity to the inhibitor brefeldin A probably cannot be predicted by group membership. Multiple alignment reveals group-specific domains outside the Sec7 domain and a set of highly conserved amino acids within it. Determination of the importance of these conserved elements in Arf exchange activity and other cellular functions is now possible.

Stimulation-Dependent Recycling of Integrin β1 Regulated by ARF6 and Rab11

In comparison to the internalization pathways of endocytosis, the recycling pathways are less understood. Even less defined is the process of regulated recycling, as few examples exist and their underlying mechanisms remain to be clarified. In this study, we examine the endocytic recycling of integrin beta1, a process that has been suggested to play an important role during cell motility by mediating the redistribution of integrins to the migrating front. External stimulation regulates the endocytic itinerary of beta1, mainly at an internal compartment that is likely to be a subset of the recycling endosomes. This stimulation-dependent recycling is regulated by ARF6 and Rab11, and also requires the actin cytoskeleton in an ARF6-dependent manner. Consistent with these observations being relevant for cell motility, mutant forms of ARF6 that affect either actin rearrangement or recycling inhibit the motility of a breast cancer cell line.

Betacap73-ARF6 interactions modulate cell shape and motility after injury in vitro

To understand the role that ARF6 plays in regulating isoactin dynamics and cell motility, we transfected endothelial cells (EC) with HA-tagged ARF6: the wild-type form (WT), a constitutively-active form unable to hydrolyze GTP (Q67L), and two dominant-negative forms, which are either unable to release GDP (T27N) or fail to bind nucleotide (N122I). Motility was assessed by digital imaging microscopy before Western blot analysis, coimmunoprecipitation, or colocalization studies using ARF6, beta-actin, or beta-actin-binding protein-specific antibodies. EC expressing ARF6-Q67L spread and close in vitro wounds at twice the control rates. EC expressing dominant-negative ARF6 fail to develop a leading edge, are unable to ruffle their membranes (N122I), and possess arborized processes. Colocalization studies reveal that the Q67L and WT ARF6-HA are enriched at the leading edge with beta-actin; but T27N and N122I ARF6-HA are localized on endosomes together with the beta-actin capping protein, betacap73. Coimmunoprecipitation and Western blot analyses reveal the direct association of ARF6-HA with betacap73, defining a role for ARF6 in signaling cytoskeletal remodeling during motility. Knowledge of the role that ARF6 plays in orchestrating membrane and beta-actin dynamics will help to reveal molecular mechanisms regulating actin-based motility during development and disease.

Role of ADP-ribosylation factor 6 (ARF6) in gastric acid secretion

ADP-ribosylation factor (ARF) proteins are monomeric GTPases that are essential for membrane transport and exocytosis in a number of secretory cells. We investigated ARF6, the activation of which is insensitive to brefeldin A, to determine whether it regulates membrane traffic in gastric parietal cells. ARF6 translocated from cytosol to tubulovesicle in the presence of GTPgammaS, a potential inhibitor of acid secretion in permeabilized cells, whereas under the Mg2+-chelated condition where activity of ARF-GTPase activating protein is inhibited, ARF6 translocated to the apical secretory membrane. Immunohistochemical examination revealed that ARF6 mainly located in parietal cell within the gastric glands, and it translocated from the cytosol to the intracellular canaliculi when the glands were stimulated. These results indicated that the distribution of ARF6 between cytosol and the two different membranes was regulated by its GTPase activity. In cultured gastric glands infected with adenovirus expressing ARF6 Q67L, a mutant lacking GTP hydrolysis activity, gastric acid secretion was inhibited. These results suggest that ARF6 regulates gastric acid secretion in parietal cell and that the GTP hydrolysis cycle of ARF6 is essential for the activation pathway.

Differential localization and function of ADP-ribosylation factor-6 in anergic human T cells: a potential marker for their identification

Anergy is a state of immunologic tolerance in which T cells are viable but incapable of responding to antigenic stimulation. Recent data indicate that anergic cells have a distinct gene expression program that determines their unique function. In this study we show that anergic human T cells selectively express the small GTPase ADP-ribosylation factor-6 (ARF6), which is involved in membrane traffic and regulation of the cortical actin cytoskeleton. ARF6 was expressed in the GTP-bound form that localizes at the plasma membrane, resulting in a distinct morphologic appearance of anergic cells. Forced expression of ARF6-GTP in Jurkat T cells prevented TCR-mediated reorganization of cortical actin, extracellular signal-regulated kinase1/2 activation, and IL-2 transcription. Forced expression of ARF6-GTP in primary human T cells inhibited extracellular signal-regulated kinase1/2 activation and proliferative responses. Importantly, T cells with the distribution pattern of ARF6-GTP were detected in peripheral blood, suggesting that anergic T cells may constitutively exist in vivo.

ADP-ribosylation factor 6 and a functional PIX/p95-APP1 complex are required for Rac1B-mediated neurite outgrowth

The mechanisms coordinating adhesion, actin organization, and membrane traffic during growth cone migration are poorly understood. Neuritogenesis and branching from retinal neurons are regulated by the Rac1B/Rac3 GTPase. We have identified a functional connection between ADP-ribosylation factor (Arf) 6 and p95-APP1 during the regulation of Rac1B-mediated neuritogenesis. P95-APP1 is an ADP-ribosylation factor GTPase-activating protein (ArfGAP) of the GIT family expressed in the developing nervous system. We show that Arf6 has a predominant role in neurite extension compared with Arf1 and Arf5. Cotransfection experiments indicate a specific and cooperative potentiation of neurite extension by Arf6 and the carboxy-terminal portion of p95-APP1. Localization studies in neurons expressing different p95-derived constructs show a codistribution of p95-APP1 with Arf6, but not Arf1. Moreover, p95-APP1-derived proteins with a mutated or deleted ArfGAP domain prevent Rac1B-induced neuritogenesis, leading to PIX-mediated accumulation at large Rab11-positive endocytic vesicles. Our data support a role of p95-APP1 as a specific regulator of Arf6 in the control of membrane trafficking during neuritogenesis.

Regulation of phospholipase D

Structural studies of plant and bacterial members of the phospholipase D (PLD) superfamily are providing information about the role of the conserved HKD domains in the structure of the catalytic center and the catalytic mechanism of mammalian PLD isozymes (PLD1 and PLD2). Mutagenesis and sequence comparison studies have also defined the presence of pleckstrin homology and phox homology domains in the N-terminus and have demonstrated that a conserved sequence at the C-terminus is required for catalysis. The N- and C-terminal regions of PLD1 also contain interaction sites for protein kinase C, which can directly activate the enzyme through a non-phosphorylating mechanism. Small G proteins of the Rho and ADP-ribosylation factor families also directly regulate the enzyme, with RhoA binding to a sequence in the C-terminus. Certain tyrosine kinases and members of the Ras subfamily of small G proteins can activate the enzyme, but the mechanisms appear to be indirect. The mechanisms by which agonists activate PLD in vivo probably involve multiple pathways.

Mechanism of ADP ribosylation factor-stimulated phosphatidylinositol 4,5-bisphosphate synthesis in HL60 cells

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) is required both as a substrate for the generation of lipid-derived second messengers as well as an intact lipid for many aspects of cell signaling, endo- and exocytosis, and reorganization of the cytoskeleton. ADP ribosylation factor (ARF) proteins regulate PI(4,5)P(2) synthesis, and here we have examined whether this is due to direct activation of Type I phosphatidylinositol 4-phosphate (PIP) 5-kinase or indirectly by phosphatidate (PA) derived from phospholipase D (PLD) in HL60 cells. ARF1 and ARF6 are both expressed in HL60 cells and can be depleted from the cells by permeabilization. Both ARFs increased the levels of PIP(2) (PI(4,5)P(2), PI(3,5)P(2), or PI(3,4)P(2) isomers) at the expense of PIP when added back to permeabilized cells. The PIP(2) could be hydrolyzed by phospholipase C, identifying it as PI(4,5)P(2). However, the ARF1-stimulated pool of PI(4,5)P(2) was accessible to the phospholipase C more efficiently in the presence of phosphatidylinositol transfer protein-alpha. To examine the role of PLD in the regulation of PI(4,5)P(2) synthesis, we used butanol to diminish the PLD-derived PA. PI(4,5)P(2) synthesis stimulated by ARF1 was not blocked by 0.5% butanol but could be blocked by 1.5% butanol. Although 0.5% butanol was optimal for maximal transphosphatidylation, PA production was still detectable. In contrast, 1.5% butanol was found to inhibit the activation of PLD by ARF1 and also decrease PIP levels by 50%. Thus the toxicity of 1.5% butanol prevented us from concluding whether PA was an important factor in raising PI(4,5)P(2) levels. To circumvent the use of alcohols, an ARF1 point mutant was identified (N52R-ARF1) that could selectively activate PIP 5-kinase alpha activity but not PLD activity. N52R-ARF1 was still able to increase PI(4,5)P(2) levels but at reduced efficiency. We therefore conclude that both PA derived from the PLD pathway and ARF proteins, by directly activating PIP 5-kinase, contribute to the regulation of PI(4,5)P(2) synthesis at the plasma membrane in HL60 cells.

Molecular mechanisms regulating the subcellular localization of p95-APP1 between the endosomal recycling compartment and sites of actin organization at the cell surface

Cell migration requires coordination between adhesion, actin organization and membrane traffic. Rac and ARF6 have been shown to cooperate for the organization of actin at the cell surface. Recently, the GIT family of ARF-GAPs has been identified, which includes proteins that can functionally interact with both ARF and Rac GTPases. The p95-APP1 protein is a member of this family, isolated as part of a multi-molecular complex interacting with GTP-Rac. Our previous work has indicated that this protein may be part of the machinery redirecting membrane recycling towards sites of protrusion during locomotion. By analyzing the distribution and the effects of truncated forms of p95-APP1, we show here that the lack of the ARF-GAP domain of p95-APP1 dramatically shifts its localization to large vesicles. The use of several markers of the endocytic pathway has revealed that the truncated p95-APP1 localizes specifically to a Rab11-, transferrin receptor-positive compartment. Other markers are excluded from the p95-APP1-positive vesicles, while known components of the multi-molecular complex colocalize with truncated p95-APP1 in this compartment. Coexpression of a constitutively active form of Rac induces the redistribution of the truncated constructs and of the associated PIX, PAK, and paxillin to peripheral sites of Rac-mediated actin organization, and the disassembly of the large Rab11-positive vesicles. Together, the data presented indicate that p95-APP1 is part of a complex that shuttles between the plasma membrane and the endocytic recycling compartment, and suggest that the dynamic redistribution of the p95-APP1-containing complex is mediated both by the ARF-GAP domain, and by the recruitment of the complex at the cell surface at sites of Rac activation.

Characterization of coated vesicles that participate in endocytic recycling

While the recycling pathway of endocytosis has been shown to participate in many cellular functions, little is known regarding the transport carriers that mediate this pathway. In this study, we overexpressed a point mutant of ADP-ribosylation factor 6 (ARF6), that perturbs its GTPase cycle, to accumulate endosome-derived coated vesicles. Characterization by their purification revealed that, upon cell homogenization, these vesicles were mostly aggregated with larger noncoated membranes, and could be released with high-salt treatment. Equilibrium centrifugation revealed that these vesicles had buoyant density similar to the COP-coated vesicles. To purify the ARF6-regulated vesicles to homogeneity, enriched fractions from equilibrium centrifugation were subjected to immunoisolation through the hemagglutinin (HA) epitope of the mutant ARF6, by using a newly developed, high-affinity, anti-HA monoclonal antibody. Surface iodination of the purified vesicles revealed multiple prominent proteins. Immunoblotting with antibodies against subunits of the currently known coat proteins suggested that these vesicles have a novel coat complex. These vesicles are carriers for endocytic recycling, because they are enriched for transferrin receptor and also the v-SNARE cellubrevin that functions in transport from the recycling endosome to the plasma membrane. Thus, we have characterized transport vesicles that participate in endocytic recycling.

Arfophilin is a common target of both class II and class III ADP-ribosylation factors

Arfophilin was first identified as a target protein for GTP-ARF5. The N-terminus of ARF5 (amino acids 2-17), which is distinct from that of class I or class III ARFs, is essential for binding to the C-terminus of arfophilin (amino acids 612-756). This study using GST fusion proteins in pulldown experiments in CHO-K1 cell lysates showed that, unexpectedly, ARF6 also bound to full-length arfophilin or the C-terminus of arfophilin (amino acids 612-756) in a GTP-dependent manner. Studies with ARF1/ARF6 chimeras further showed that the amino acid sequence of residues 37-80 of ARF6, which is different from the corresponding sequences in class I and class II ARFs, was essential for binding to arfophilin. Both GTP-ARF5 and GTP-ARF6 bound to arfophilin in CHO-K1 cell lysates, while GTP-ARF1 did not bind. In contrast, all three forms of ARF bound to arfaptin 2, with ARF1 showing the strongest binding. Yeast two-hybrid studies with wild-type, dominant negative, and constitutively active forms of ARF1, -5, and -6 and with ARF1/ARF6 chimeras confirmed these results, except that constitutively active ARF6 was autoactivating. Our findings suggest that both class II and III ARFs may influence the same cellular pathways through arfophilin as a common downstream effector.

Activation of the luteinizing hormone/choriogonadotropin hormone receptor promotes ADP ribosylation factor 6 activation in porcine ovarian follicular membranes

Previously we demonstrated in a cell-free ovarian follicular plasma membrane model that agonist-dependent desensitization of the luteinizing hormone/choriogonadotropin receptor (LH/CG R) is GTP-dependent, mimicked by the addition of ADP-ribosylation factor (ARF) nucleotide binding site opener, which acts as a guanine nucleotide exchange factor for ARFs 1 and 6, and selectively inhibited by synthetic N-terminal ARF6 peptides. We therefore sought direct evidence that activation of the LH/CG R promotes activation of ARF1 and/or ARF6. Using a classic ARF activation assay, the cholera toxin-catalyzed ADP-ribosylation of G alpha(s), results show that LH/CG R activation stimulates an ARF protein by a brefeldin A-independent mechanism. Synthetic N-terminal inhibitory ARF6 but not ARF1 peptide blocks LH/CG R-stimulated ARF activity. LH/CG R activation also promotes the binding of a photoaffinity GTP analog to a protein that migrates on one- and two-dimensional polyacrylamide gel electrophoresis with ARF6. These results suggest that ARF6 is the predominant ARF activated by the LH/CG R. To activate ARF6, the LH/CG R does not appear to signal through the C-terminal regions of G alpha(i) or G alpha(q) or through the second or third intracellular loops or the N terminus of the cytoplasmic tail of the LH/CG R. Although exogenous recombinant ARNO promotes only a small increase in ARF6 activation in the presence of activated LH/CG R, hCG-stimulated ARF6 activation is reduced to basal levels by catalytically inactive ARF nucleotide binding-site opener. These results provide direct evidence that LH/CG R activation leads to the activation of membrane-delimited ARF6.

Protein kinase C delta and eta isoenzymes control the shedding of the interleukin 6 receptor alpha in myeloma cells

The soluble interleukin 6 receptor alpha is an agonistic molecule of interleukin 6 (IL-6) and is important in the biology of multiple myeloma. More precisely, it potentiates the deleterious effects of IL-6 during tumour progression, facilitating angiogenesis and bone resorption. Because the mechanisms involved in the shedding of the interleukin 6 receptor alpha (IL-6Ralpha) in multiple myeloma are not known, we have investigated them in the XG-6 human myeloma cell line. Here we provide evidence that PMA-induced IL-6Ralpha shedding is controlled by a metalloproteinase and by protein kinase C (PKC) isoenzymes that do not require Ca(2+) for their activation. We show that XG-6 cells express PKC-delta, -eta and -zeta isoenzymes. However, after stimulation with PMA, only PKC-delta and PKC-eta are activated, as shown by their translocation to the membrane. Treatment with PMA induces an increase in PKC-delta phosphorylation in its active loop. In addition, by using rottlerin, a specific inhibitor of PKC-delta, we demonstrate that PKC-delta is involved in the PMA-induced shedding of IL-6Ralpha. With the use of UO126, a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway, we show that the PMA-induced IL-6Ralpha shedding is mediated in part by the MAPK pathway. Finally, whereas GF109203X, a general PKC inhibitor, inhibits the activation of ERK1/2 (extracellular signal-regulated protein kinase 1/2), rottlerin has no inhibitory effect, indicating that the Ras/MAPK activation is PKC-dependent but PKC-delta-independent. Taken together, these results suggest that the PMA-induced shedding of IL-6Ralpha is mediated by a PKC isoenzyme network.

Expression of ARF6 mutants in neuroendocrine cells suggests a role for ARF6 in synaptic vesicle biogenesis

ARF6 regulates membrane trafficking between the plasma membrane and endosomes. We investigated the role of ARF6 in synaptic vesicle biogenesis as this process occurs both at the plasma membrane and at endosomes. We used a synaptic vesicle marker protein, p-selectin-horseradish peroxidase (HRP), to follow the effects of ARF6 expression on synaptic vesicle biogenesis in PC12 neuroendocrine cells. Expression of a constitutively active ARF6 mutant increased, while expression of a nucleotide-free ARF6 mutant decreased, p-selectin-HRP levels in the synaptic vesicle peak. These results provide the first direct evidence for a role for ARF6 in synaptic vesicle biogenesis.

Intra-endosomal pH-sensitive recruitment of the Arf-nucleotide exchange factor ARNO and Arf6 from cytoplasm to proximal tubule endosomes

Kidney proximal tubule epithelial cells have an extensive apical endocytotic apparatus that is critical for the reabsorption and degradation of proteins that traverse the glomerular filtration barrier and that is also involved in the extensive recycling of functionally important apical plasma membrane transporters. We show here that an Arf-nucleotide exchange factor, ARNO (ADP-ribosylation factor nucleotide site opener) as well as Arf6 and Arf1 small GTPases are located in the kidney proximal tubule receptor-mediated endocytosis pathway, and that ARNO and Arf6 recruitment from cytosol to endosomes is pH-dependent. In proximal tubules in situ, ARNO and Arf6 partially co-localized with the V-ATPase in apical endosomes in proximal tubules. Arf1 was localized both at the apical pole of proximal tubule epithelial cells, but also in the Golgi. By Western blot analysis ARNO, Arf6, and Arf1 were detected both in purified endosomes and in proximal tubule cytosol. A translocation assay showed that ATP-driven endosomal acidification triggered the recruitment of ARNO and Arf6 from proximal tubule cytosol to endosomal membranes. The translocation of both ARNO and Arf6 was reversed by V-type ATPase inhibitors and by uncouplers of endosomal intralumenal pH, and was correlated with the magnitude of intra-endosomal acidification. Our data suggest that V-type ATPase-dependent acidification stimulates the selective recruitment of ARNO and Arf6 to proximal tubule early endosomes. This mechanism may play an important role in the pH-dependent regulation of receptor-mediated endocytosis in proximal tubules in situ.

ARF-GEP(100), a guanine nucleotide-exchange protein for ADP-ribosylation factor 6

A human cDNA encoding an 841-aa guanine nucleotide-exchange protein (GEP) for ADP-ribosylation factors (ARFs), named ARF-GEP(100), which contains a Sec7 domain, a pleckstrin homology (PH)-like domain, and an incomplete IQ-motif, was identified. On Northern blot analysis of human tissues, a approximately 8-kb mRNA that hybridized with an ARF-GEP(100) cDNA was abundant in peripheral blood leukocytes, brain, and spleen. ARF-GEP(100) accelerated [(35)S]GTPgammaS binding to ARF1 (class I) and ARF5 (class II) 2- to 3-fold, and to ARF6 (class III) ca. 12-fold. The ARF-GEP(100) Sec7 domain contains Asp(543) and Met(555), corresponding to residues associated with sensitivity to the inhibitory effect of the fungal metabolite brefeldin A (BFA) in yeast Sec7, but also Phe(535) and Ala(536), associated with BFA-insensitivity. The PH-like domain differs greatly from those of other ARF GEPs in regions involved in phospholipid binding. Consistent with its structure, ARF-GEP(100) activity was not affected by BFA or phospholipids. After subcellular fractionation of cultured T98G human glioblastoma cells, ARF6 was almost entirely in the crude membrane fraction, whereas ARF-GEP(100), a 100-kDa protein detected with antipeptide antibodies, was cytosolic. On immunofluorescence microscopy, both proteins had a punctate pattern of distribution throughout the cells, with apparent colocalization only in peripheral areas. The coarse punctate distribution of EEA-1 in regions nearer the nucleus appeared to coincide with that of ARF-GEP(100) in those areas. No similar coincidence of ARF-GEP(100) with AP-1, AP-2, catenin, LAMP-1, or 58K was observed. The new human BFA-insensitive GEP may function with ARF6 in specific endocytic processes.

p95-APP1 links membrane transport to Rac-mediated reorganization of actin

Motility requires protrusive activity at the cellular edge, where Rho family members regulate actin dynamics. Here we show that p95-APP1 (ArfGAP-putative, Pix-interacting, paxillin-interacting protein 1), a member of the GIT1/PKL family, is part of a complex that interacts with Rac. Wild-type and truncated p95-APP1 induce actin-rich protrusions mediated by Rac and ADP-ribosylation factor 6 (Arf6). Distinct p95-APP1-derived polypeptides have different distributions, indicating that p95-APP1 cycles between the cell surface and endosomes. Our results show that p95-APP1 functionally interacts with Rac and localizes to endosomal compartments, thus identifying p95-APP1 as a molecular link between actin organization, adhesion, and membrane transport during cell motility.

Structure of Arf6-GDP suggests a basis for guanine nucleotide exchange factors specificity

Arf6 is an isoform of Arf that localizes at the periphery of the cell where it has an essential role in endocytotic pathways. Its function does not overlap with that of Arf1, although the two proteins share approximately 70% sequence identity and they have switch regions, whose conformation depends on the nature of the guanine nucleotide, with almost identical sequences. The crystal structure of Arf6-GDP at 2.3 A shows that it has a conformation similar to that of Arf1-GDP, which cannot bind membranes with high affinity. Significantly, the switch regions of Arf6 deviate by 2-5 A from those of Arf1. These differences are a consequence of the shorter N-terminal linker of Arf6 and of discrete sequence changes between Arf6 and Arf1. Mutational analysis shows that one of the positions which differs between Arf1 and Arf6 affects the configuration of the nucleotide binding site and thus the nucleotide binding properties of the Arf variant. Altogether, our results provide a structural basis for understanding how Arf1 and Arf6 can be distinguished by their guanine nucleotide exchange factors and suggest a model for the nucleotide/membrane cycle of Arf6.

The ADP ribosylation factor nucleotide exchange factor ARNO promotes beta-arrestin release necessary for luteinizing hormone/choriogonadotropin receptor desensitization

Desensitization of guanine nucleotide binding protein-coupled receptors is a ubiquitous phenomenon characterized by declining effector activity upon persistent agonist stimulation. The luteinizing hormone/choriogonadotropin receptor (LH/CGR) in ovarian follicles exhibits desensitization of effector adenylyl cyclase activity in response to the mid-cycle surge of LH. We have previously shown that uncoupling of the agonist-activated LH/CGR from the stimulatory G protein (G(s)) is dependent on GTP and attributable to binding of beta-arrestin present in adenylyl cyclase-rich follicular membrane fraction to the third intracellular (3i) loop of the receptor. Here, we report that LH/CGR-dependent desensitization is mimicked by ADP ribosylation factor nucleotide-binding site opener, a guanine nucleotide exchange factor of the small G proteins ADP ribosylation factors (Arfs) 1 and 6, and blocked by synthetic N-terminal Arf6 peptide, suggesting that the GTP-dependent step of LH/CGR desensitization is receptor-dependent Arf6 activation. Arf activation by GTP and ADP ribosylation factor nucelotide-binding site opener promotes the release of docked beta-arrestin from the membrane, making beta-arrestin available for LH/CGR; Arf6 but not Arf1 peptides block beta-arrestin release from the membrane. Thus, LH/CGR appears to activate two membrane delimited signaling cascades via two types of G proteins: heterotrimeric G(s) and small G protein Arf6. Arf6 activation releases docked beta-arrestin necessary for receptor desensitization, providing a feedback mechanism for receptor self-regulation.

Turning on ARF: the Sec7 family of guanine-nucleotide-exchange factors

ARF proteins are important regulators of membrane dynamics and protein transport within the eukaryotic cell. The Sec7 domain is approximately 200 amino acids in size and stimulates guanine-nucleotide exchange on members of the ARF class of small GTPases. The members of one subclass of Sec7-domain proteins are direct targets of the secretion-inhibiting drug brefeldin A, which blocks the exchange reaction by trapping a reaction intermediate in an inactive, abortive complex. A separate subclass of Sec7-domain proteins is involved in signal transduction and possess a domain that mediates membrane binding in response to extracellular signals.

Identification of arfophilin, a target protein for GTP-bound class II ADP-ribosylation factors

Yeast two-hybrid screening of a human kidney cDNA library using the GTP-bound form of a class II ADP-ribosylation factor (ARF5) identified a novel ARF5-binding protein with a calculated molecular mass of 82.4 kDa, which was named arfophilin. Northern hybridization analysis showed high level arfophilin mRNA expression in human heart and skeletal muscle. Arfophilin bound only to the active, GTP-bound form of ARF5 and did not bind to GTP-ARF3, which is a class I ARF. The N terminus of ARF5 (1-17 amino acids) was essential for binding to arfophilin. The GTP-bound form of ARF5 with amino acid residues in the N terminus mutated to those in ARF4 (another class II ARF) also bound to arfophilin, suggesting it is a target protein for GTP-bound forms of class II ARFs. The binding site for ARF on arfophilin was localized to the C terminus (residues 612-756), which contains putative coiled-coil structures. Recombinant arfophilin overexpressed in CHO-K1 cells was localized in the cytosol and translocated to a membrane fraction in association with GTP-bound ARF5. ARF5 containing the N terminus of ARF3 did not promote translocation indicating that class II ARFs are specific carriers for arfophilin.