Effect of myristoylation on GTP-dependent binding of ADP-ribosylation factor to Golgi

Kinetics of Arf1 inactivation regulates Golgi organisation and function in non-adherent fibroblasts

Arf1 belongs to the Arf family of small GTPases that localise at the Golgi and plasma membrane. Active Arf1 plays a crucial role in regulating Golgi organisation and function. In mouse fibroblasts, loss of adhesion triggers a consistent drop (∼50%) in Arf1 activation that causes the Golgi to disorganise but not fragment. In suspended cells, the trans-Golgi (GalTase) disperses more prominently than cis-Golgi (Man II), accompanied by increased active Arf1 (detected using GFP-ABD: ARHGAP10 Arf1 binding domain) associated with the cis-Golgi compartment. Re-adhesion restores Arf1 activation at the trans-Golgi as it reorganises. Arf1 activation at the Golgi is regulated by Arf1 Guanine nucleotide exchange factors (GEFs), GBF1, and BIG1/2. In non-adherent fibroblasts, the cis-medial Golgi provides a unique setting to test and understand the role GEF-mediated Arf1 activation has in regulating Golgi organisation. Labelled with Man II-GFP, non-adherent fibroblasts treated with increasing concentrations of Brefeldin-A (BFA) (which inhibits BIG1/2 and GBF1) or Golgicide A (GCA) (which inhibits GBF1 only) comparably decrease active Arf1 levels. They, however, cause a concentration-dependent increase in cis-medial Golgi fragmentation and fusion with the endoplasmic reticulum (ER). Using selected BFA and GCA concentrations, we find a change in the kinetics of Arf1 inactivation could mediate this by regulating cis-medial Golgi localisation of GBF1. On loss of adhesion, a ∼50% drop in Arf1 activation over 120 min causes the Golgi to disorganise. The kinetics of this drop, when altered by BFA or GCA treatment causes a similar decline in Arf1 activation but over 10 min. This causes the Golgi to now fragment which affects cell surface glycosylation and re-adherent cell spreading. Using non-adherent fibroblasts this study reveals the kinetics of Arf1 inactivation, with active Arf1 levels, to be vital for Golgi organisation and function.

Structural basis for activation of Arf1 at the Golgi complex

The Golgi complex is the central sorting station of the eukaryotic secretory pathway. Traffic through the Golgi requires activation of Arf guanosine triphosphatases that orchestrate cargo sorting and vesicle formation by recruiting an array of effector proteins. Arf activation and Golgi membrane association is controlled by large guanine nucleotide exchange factors (GEFs) possessing multiple conserved regulatory domains. Here we present cryoelectron microscopy (cryoEM) structures of full-length Gea2, the yeast paralog of the human Arf-GEF GBF1, that reveal the organization of these regulatory domains and explain how Gea2 binds to the Golgi membrane surface. We find that the GEF domain adopts two different conformations compatible with different stages of the Arf activation reaction. The structure of a Gea2-Arf1 activation intermediate suggests that the movement of the GEF domain primes Arf1 for membrane insertion upon guanosine triphosphate binding. We propose that conformational switching of Gea2 during the nucleotide exchange reaction promotes membrane insertion of Arf1.

Arf1 is a GTPase that regulates Golgi trafficking by recruiting many effector proteins. Muccini et al. report cryoEM structures of the Arf1 activator Gea2, capturing Gea2 in multiple conformational states including a Gea2-Arf1 activation intermediate. The structures help explain how Gea2 activates Arf1 on the Golgi membrane surface.

Targeting N-myristoylation for therapy of B-cell lymphomas

Myristoylation, the N-terminal modification of proteins with the fatty acid myristate, is critical for membrane targeting and cell signaling. Because cancer cells often have increased N-myristoyltransferase (NMT) expression, NMTs were proposed as anti-cancer targets. To systematically investigate this, we performed robotic cancer cell line screens and discovered a marked sensitivity of hematological cancer cell lines, including B-cell lymphomas, to the potent pan-NMT inhibitor PCLX-001. PCLX-001 treatment impacts the global myristoylation of lymphoma cell proteins and inhibits early B-cell receptor (BCR) signaling events critical for survival. In addition to abrogating myristoylation of Src family kinases, PCLX-001 also promotes their degradation and, unexpectedly, that of numerous non-myristoylated BCR effectors including c-Myc, NFκB and P-ERK, leading to cancer cell death in vitro and in xenograft models. Because some treated lymphoma patients experience relapse and die, targeting B-cell lymphomas with a NMT inhibitor potentially provides an additional much needed treatment option for lymphoma.

Invited review: Small GTPases and their GAPs

Widespread utilization of small GTPases as major regulatory hubs in many different biological systems derives from a conserved conformational switch mechanism that facilitates cycling between GTP-bound active and GDP-bound inactive states under control of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), which accelerate slow intrinsic rates of activation by nucleotide exchange and deactivation by GTP hydrolysis, respectively. Here we review developments leading to current understanding of intrinsic and GAP catalyzed GTP hydrolytic reactions in small GTPases from structural, molecular and chemical mechanistic perspectives. Despite the apparent simplicity of the GTPase cycle, the structural bases underlying the hallmark hydrolytic reaction and catalytic acceleration by GAPs are considerably more diverse than originally anticipated. Even the most fundamental aspects of the reaction mechanism have been challenging to decipher. Through a combination of experimental and in silico approaches, the outlines of a consensus view have begun to emerge for the best studied paradigms. Nevertheless, recent observations indicate that there is still much to be learned. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 431-448, 2016.

ADP-ribosylation factor 6 acts as an allosteric activator for the folded but not disordered cholera toxin A1 polypeptide

The catalytic A1 subunit of cholera toxin (CTA1) has a disordered structure at 37°C. An interaction with host factors must therefore place CTA1 in a folded conformation for the modification of its Gsα target which resides in a lipid raft environment. Host ADP-ribosylation factors (ARFs) act as in vitro allosteric activators of CTA1, but the molecular events of this process are not fully characterized. Isotope-edited Fourier transform infrared spectroscopy monitored ARF6-induced structural changes to CTA1, which were correlated to changes in CTA1 activity. We found ARF6 prevents the thermal disordering of structured CTA1 and stimulates the activity of stabilized CTA1 over a range of temperatures. Yet ARF6 alone did not promote the refolding of disordered CTA1 to an active state. Instead, lipid rafts shifted disordered CTA1 to a folded conformation with a basal level of activity that could be further stimulated by ARF6. Thus, ARF alone is unable to activate disordered CTA1 at physiological temperature: additional host factors such as lipid rafts place CTA1 in the folded conformation required for its ARF-mediated activation. Interaction with ARF is required for in vivo toxin activity, as enzymatically active CTA1 mutants that cannot be further stimulated by ARF6 fail to intoxicate cultured cells.

Arf activation at the Golgi is modulated by feed-forward stimulation of the exchange factor GBF1

ADP-ribosylation factors (Arfs) play central roles in the regulation of vesicular trafficking through the Golgi. Arfs are activated at the Golgi membrane by guanine-nucleotide-exchange factors (GEFs) that are recruited from cytosol. Here, we describe a novel mechanism for the regulation of recruitment and activity of the ArfGEF Golgi-specific BFA resistance factor 1 (GBF1). Conditions that alter the cellular Arf-GDP:Arf-GTP ratio result in GBF1 recruitment. This recruitment of GBF1 occurs selectively on cis-Golgi membranes in direct response to increased Arf-GDP. GBF1 recruitment requires Arf-GDP myristoylation-dependent interactions suggesting regulation of a membrane-bound factor. Once recruited, GBF1 causes increased Arf-GTP production at the Golgi, consistent with a feed-forward self-limiting mechanism of Arf activation. This mechanism is proposed to maintain steady-state levels of Arf-GTP at the cis-Golgi during cycles of Arf-dependent trafficking events.

ARAP2 signals through Arf6 and Rac1 to control focal adhesion morphology

Focal adhesions (FAs) are dynamic structures that connect the actin cytoskeleton with the extracellular matrix. At least six ADP-ribosylation factor (Arf) GTPase-activating proteins (GAPs), including ARAP2 (an Arf6 GAP), are implicated in regulation of FAs but the mechanisms for most are not well defined. Although Rac1 has been reported to function downstream of Arf6 to control membrane ruffling and cell migration, this pathway has not been directly examined as a regulator of FAs. Here we test the hypothesis that ARAP2 promotes the growth of FAs by converting Arf6·GTP to Arf6·GDP thereby preventing the activation of the Rho family GTP-binding protein Rac1. Reduced expression of ARAP2 decreased the number and size of FAs in cells and increased cellular Arf6·GTP and Rac1·GTP levels. Overexpression of ARAP2 had the opposite effects. The effects of ARAP2 on FAs and Rac1 were dependent on a functional ArfGAP domain. Constitutively active Arf6 affected FAs in the same way as did reduced ARAP2 expression and dominant negative mutants of Arf6 and Rac1 reversed the effect of reduced ARAP2 expression. However, neither dominant negative Arf6 nor Rac1 had the same effect as ARAP2 overexpression. We conclude that changes in Arf6 and Rac1 activities are necessary but not sufficient for ARAP2 to promote the growth of FAs and we speculate that ARAP2 has additional functions that are effector in nature to promote or stabilize FAs.

AMPK functions as an adenylate charge-regulated protein kinase

The energy sensor AMP-activated protein kinase (AMPK) is activated by metabolic stress and restores ATP levels in cells by switching off anabolic and switching on catabolic pathways. Recent discoveries demonstrate that AMPK is activated primarily by rising ADP levels and not, as previously thought, by AMP. AMPK activation is dependent on ADP-controlled phosphorylation of Thr172 on its activation loop, a mechanism of protein regulation that represents an example of an allosterically regulated modification (ARM). AMPK embodies many features of an adenylate charge regulatory system envisaged by Atkinson, where anabolic and catabolic pathway regulation is modulated by adenine nucleotide ratios. Here we discuss the current state of AMPK regulation by adenine nucleotides and we propose that AMPK functions as an adenylate charge-regulated protein kinase.

Dysregulated Arl1, a regulator of post-Golgi vesicle tethering, can inhibit endosomal transport and cell proliferation in yeast

Small monomeric G proteins regulated in part by GTPase-activating proteins (GAPs) are molecular switches for several aspects of vesicular transport. The yeast Gcs1 protein is a dual-specificity GAP for ADP-ribosylation factor (Arf) and Arf-like (Arl)1 G proteins, and also has GAP-independent activities. The absence of Gcs1 imposes cold sensitivity for growth and endosomal transport; here we present evidence that dysregulated Arl1 may cause these impairments. We show that gene deletions affecting the Arl1 or Ypt6 vesicle-tethering pathways prevent Arl1 activation and membrane localization, and restore growth and trafficking in the absence of Gcs1. A mutant version of Gcs1 deficient for both ArfGAP and Arl1GAP activity in vitro still allows growth and endosomal transport, suggesting that the function of Gcs1 that is required for these processes is independent of GAP activity. We propose that, in the absence of this GAP-independent regulation by Gcs1, the resulting dysregulated Arl1 prevents growth and impairs endosomal transport at low temperatures. In cells with dysregulated Arl1, an increased abundance of the Arl1 effector Imh1 restores growth and trafficking, and does so through Arl1 binding. Protein sequestration at the trans-Golgi membrane by dysregulated, active Arl1 may therefore be the mechanism of inhibition.

The yeast Arf GTPase-activating protein Age1 is regulated by phospholipase D for post-Golgi vesicular transport

Vesicular transport shuttles cargo among intracellular compartments. Several stages of vesicular transport are mediated by the small GTPase Arf, which is controlled in a cycle of GTP binding and hydrolysis by Arf guanine-nucleotide exchange factors and Arf GTPase-activating proteins (ArfGAPs), respectively. In budding yeast the Age2 + Gcs1 ArfGAP pair facilitates post-Golgi transport. We have found the AGE1 gene, encoding another ArfGAP, can in high gene-copy number alleviate the temperature sensitivity of cells carrying mutations affecting the Age2 + Gcs1 ArfGAP pair. Moreover, increased AGE1 gene dosage compensates for the complete absence of the otherwise essential Age2 + Gcs1 ArfGAP pair. Increased dosage of SFH2, encoding a phosphatidylinositol transfer protein, also allows cell growth in the absence of the Age2 + Gcs1 pair, but good growth in this situation requires Age1. The ability of Age1 to overcome the need for Age2 + Gcs1 depends on phospholipase D activity that regulates lipid composition. We show by direct assessment of Age1 ArfGAP activity that Age1 is regulated by lipid composition and can provide ArfGAP function for post-Golgi transport.

ACAP-A/B are ArfGAP homologs in dictyostelium involved in sporulation but not in chemotaxis

Arfs and Arf GTPase-activating proteins (ArfGAPs) are regulators of membrane trafficking and actin dynamics in mammalian cells. In this study, we identified a primordial Arf, ArfA, and two ArfGAPs (ACAP-A/B) containing BAR, PH, ArfGAP and Ankyrin repeat domains in the eukaryote Dictyostelium discoideum. In vitro, ArfA has similar nucleotide binding properties as mammalian Arfs and, with GTP bound, is a substrate for ACAP-A and B. We also investigated the physiological functions of ACAP-A/B by characterizing cells lacking both ACAP-A and B. Although ACAP-A/B knockout cells showed no defects in cell growth, migration or chemotaxis, they exhibited abnormal actin protrusions and ∼50% reduction in spore yield. We conclude that while ACAP-A/B have a conserved biochemical mechanism and effect on actin organization, their role in migration is not conserved. The absence of an effect on Dictyostelium migration may be due to a specific requirement for ACAPs in mesenchymal migration, which is observed in epithelial cancer cells where most studies of mammalian ArfGAPs were performed.

N-myristoylated proteins, key components in intracellular signal transduction systems enabling rapid and flexible cell responses

N-myristoylation, one of the co- or post-translational modifications of proteins, has so far been regarded as necessary for anchoring of proteins to membranes. Recently, we have revealed that N(alpha)-myristoylation of several brain proteins unambiguously regulates certain protein-protein interactions that may affect signaling pathways in brain. Comparison of the amino acid sequences of myristoylated proteins including those in other organs suggests that this regulation is involved in signaling pathways not only in brain but also in other organs. Thus, it has been shown that myristoylated proteins in cells regulate the signal transduction between membranes and cytoplasmic fractions. An algorithm we have developed to identify myristoylated proteins in cells predicts the presence of hundreds of myristoylated proteins. Interestingly, a large portion of the myristoylated proteins thought to take part in signal transduction between membranes and cytoplasmic fractions are included in the predicted myristoylated proteins. If the proteins functionally regulated by myristoylation, a posttranslational protein modification, were understood as cross-talk points within the intracellular signal transduction system, known signaling pathways could thus be linked to each other, and a novel map of this intracellular network could be constructed. On the basis of our recent results, this review will highlight the multifunctional aspects of protein N-myristoylation in brain.

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

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

ARF6 regulates angiotensin II type 1 receptor endocytosis by controlling the recruitment of AP-2 and clathrin

We have previously shown that the ADP-ribosylation factor 6 (ARF6), a small GTP-binding protein, is important for the internalization of several G protein-coupled receptors. Here, we propose to elucidate the molecular steps controlled by ARF6 in the endocytic process of the angiotensin II type 1 receptor (ATR), a model receptor being internalized via the clathrin-coated vesicle pathway. In HEK 293 cells, angiotensin II stimulation leads to the formation of a complex including ARF6, the beta-subunit of AP-2 and the heavy chain of clathrin. In vitro experiments indicate that the interactions between ARF6 and the beta-subunit of AP-2 as well as with the heavy chain of clathrin are direct, and dependent upon the nature of the nucleotide bound to ARF6. beta2-adaptin binds to ARF6-GDP while clathrin preferentially interacts with ARF6 when loaded with GTP. These interactions have an important physiological consequence. Indeed, depletion of ARF6 prevents the agonist-dependent recruitment of beta2-adaptin and clathrin to the activated ATR. Interestingly, in these cells, the plasma membrane redistribution of either beta2-adaptin-GFP or betaarrestin 2-GFP, following Ang II stimulation, is altered. Both proteins are defective in clustering into large punctated structure at the plasma membrane compared to control conditions. Taken together, these results suggest that the cycling of ARF6 between its GDP-and GTP-bound states coordinates the recruitment of AP-2 and clathrin to activated receptors during the endocytic process.

NMR structural studies of the myristoylated N-terminus of ADP ribosylation factor 6 (Arf6)

Arf proteins are guanine nucleotide binding proteins that are implicated in endocytotic pathways and vesicle trafficking. The two widely studied isoforms of Arf proteins (Arf1 and Arf6) have different cellular functions and localizations but similar structures. Arf proteins have an N-terminal helix with a covalently bound myristoyl group. Except structural models, there are no three dimensional structures of the myristoylated N-terminal peptide or the intact myristoylated Arf proteins. However, understanding the role of both the myristoyl group and the N-terminal helix based on the details of their molecular structures is of great interest. In the solution structure of myristoylated N-terminal peptide of Arf6 described here, the myristoyl group folds toward the N-terminus to interact with the hydrophobic residues in particular, the phenyl ring. Also, the structure of the dodecylphosphocholine (DPC) micelle-bound of the peptide together with paramagnetic studies showed that the myristoyl group is inserted into the micelle while residues V4-G10 interact with the surface of the micelle. The structural differences between the unbound and micelle-bound myristoylated N-terminal peptide of Arf6 involves the myristoyl group and the side chains of the hydrophobic residues.

The Arf1p GTPase-activating protein Glo3p executes its regulatory function through a conserved repeat motif at its C-terminus

ADP-ribosylation factors (Arfs), key regulators of intracellular membrane traffic, are known to exert multiple roles in vesicular transport. We previously isolated eight temperature-sensitive (ts) mutants of the yeast ARF1 gene, which showed allele-specific defects in protein transport, and classified them into three groups of intragenic complementation. In this study, we show that the overexpression of Glo3p, one of the GTPase-activating proteins of Arf1p (ArfGAP), suppresses the ts growth of a particular group of the arf1 mutants (arf1-16 and arf1-17). Other ArfGAPs do not show such a suppression activity. All these ArfGAPs show sequence similarity in the ArfGAP catalytic domain, but are divergent in the rest of molecules. By domain swapping analysis of Glo3p and another ArfGAP, Gcs1p, we have shown that the non-catalytic C-terminal region of Glo3p is required for the suppression of the growth defect in the arf1 ts mutants. Interestingly, Glo3p and its homologues from other eukaryotes harbor a well-conserved repeated ISSxxxFG sequence near the C-terminus, which is not found in Gcs1p and its homologues. We name this region the Glo3 motif and present evidence that the motif is required for the function of Glo3p in vivo.

The Gcs1 Arf-GAP mediates Snc1,2 v-SNARE retrieval to the Golgi in yeast

Gcs1 is an Arf GTPase-activating protein (Arf-GAP) that mediates Golgi-ER and post-Golgi vesicle transport in yeast. Here we show that the Snc1,2 v-SNAREs, which mediate endocytosis and exocytosis, interact physically and genetically with Gcs1. Moreover, Gcs1 and the Snc v-SNAREs colocalize to subcellular structures that correspond to the trans-Golgi and endosomal compartments. Studies performed in vitro demonstrate that the Snc-Gcs1 interaction results in the efficient binding of recombinant Arf1Delta17N-Q71L to the v-SNARE and the recruitment of purified coatomer. In contrast, the presence of Snc had no effect on Gcs1 Arf-GAP activity in vitro, suggesting that v-SNARE binding does not attenuate Arf1 function. Disruption of both the SNC and GCS1 genes results in synthetic lethality, whereas overexpression of either SNC gene inhibits the growth of a distinct subset of COPI mutants. We show that GFP-Snc1 recycling to the trans-Golgi is impaired in gcs1Delta cells and these COPI mutants. Together, these results suggest that Gcs1 facilitates the incorporation of the Snc v-SNAREs into COPI recycling vesicles and subsequent endosome-Golgi sorting in yeast.

K-ras4B and prenylated proteins lacking "second signals" associate dynamically with cellular membranes

We have used fluorescence microscopy and the technique of rapamycin-regulated protein heterodimerization to examine the dynamics of the subcellular localizations of fluorescent proteins fused to lipid-modified protein sequences and to wild-type and mutated forms of full-length K-ras4B. Singly prenylated or myristoylated fluorescent protein derivatives lacking a "second signal" to direct them to specific subcellular destinations, but incorporating a rapamycin-dependent heterodimerization module, rapidly translocate to mitochondria upon rapamycin addition to bind to a mitochondrial outer membrane protein incorporating a complementary heterodimerization module. Under the same conditions analogous constructs anchored to the plasma membrane by multiply lipid-modified sequences, or by a transmembrane helix, show very slow or no transfer to mitochondria, respectively. Interestingly, however, fluorescent protein constructs incorporating either full-length K-ras4B or its plasma membrane-targeting sequence alone undergo rapamycin-induced transfer from the plasma membrane to mitochondria on a time scale of minutes, demonstrating the rapidly reversible nature of K-ras4B binding to the plasma membrane. The dynamic nature of the plasma membrane targeting of K-ras4B could contribute to K-ras4B function by facilitating redistribution of the protein between subcellular compartments under particular conditions.

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

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

Functional analysis of TbARL1, an N-myristoylated Golgi protein essential for viability in bloodstream trypanosomes

Myristoyl-CoA:protein N-myristoyltransferase (NMT), an essential protein in Trypanosoma brucei and Leishmania major, catalyses the covalent attachment of the fatty acid myristate to the N-terminus of a range of target proteins. In order to define the essential targets contributing to lethality in the absence of NMT activity, we have focused on the ADP-ribosylation factor (Arf) family of GTP-binding proteins, as growth arrest in Saccharomyces cerevisiae mutants with reduced NMT activity correlates with a decrease in N-myristoylated Arf proteins. We have identified nine Arf/Arls in the T. brucei and T. cruzi genomes and ten in L. major. Characterization of the T. brucei ARL1 homologue has revealed that the protein is localized in the Golgi apparatus and is expressed only in the mammalian bloodstream form of the parasite and not in the insect procyclic stage. This is the only reported example to date of a differentially expressed ARL1 homologue in any species. We have used RNA interference to demonstrate that ARL1 is essential for viability in T. brucei bloodstream parasites. Prior to cell death, depletion of ARL1 protein in bloodstream parasites results in abnormal morphology, including disintegration of the Golgi structure, multiple flagella and nuclei, and the presence of large numbers of vesicles. The cells have only a minor apparent defect in endocytosis but exocytosis of variant surface glycoprotein to the parasite surface is significantly delayed. RNA interference of ARL1 in procyclic cells has no effect on parasite growth or morphology. Our results suggest that there may be different pathways regulating Golgi structure and function in the two major life cycle stages of T. brucei.

G protein-coupled receptor endocytosis in ADP-ribosylation factor 6-depleted cells

The internalization of G protein-coupled receptors is regulated by several important proteins that act in concert to finely control this complex cellular process. Here, we have applied the RNA interference approach to demonstrate that ADP-ribosylation factor 6 (ARF6) is essential for the endocytosis of a broad variety of receptors. Reduction of endogenous expression of ARF6 in HEK 293 cells resulted in a correlated inhibition of the beta(2) -adrenergic receptor internalization previously characterized as being sequestered via the clathrin-coated vesicle pathway. Furthermore, other receptors internalizing via this endocytic route, namely the angiotensin type 1 receptor and the vasopressin type 2 receptor, were also impaired in their ability to be sequestered when levels of endogenous ARF6 in cells were reduced. Interestingly, endocytosis of the endothelin type B receptor, characterized as being internalized via the caveolae pathway, was also markedly inhibited in ARF6-depleted cells. In contrast, internalization of the vasoactive intestinal peptide receptor was unaffected by reduced levels of ARF6. Finally, internalization of the acetylcholine-muscarinic type 2 receptor via the non-clathrin-coated vesicle pathway was also inhibited in ARF6-depleted cells. Taken together, our results demonstrate that ARF6 proteins play an essential role in the internalization process of most G protein-coupled receptors regardless of the endocytic route being utilized. However, this phenomenon is not general. In some cases, another ARF isoform or other proteins may be essential to regulate the endocytic process.

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

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

Role for Arf3p in development of polarity, but not endocytosis, in Saccharomyces cerevisiae

ADP-ribosylation factors (ARFs) are ubiquitous regulators of virtually every step of vesicular membrane traffic. Yeast Arf3p, which is most similar to mammalian ARF6, is not essential for cell viability and not required for endoplasmic reticulum-to-Golgi protein transport. Although mammalian ARF6 has been implicated in the regulation of early endocytic transport, we found that Arf3p was not required for fluid-phase, membrane internalization, or mating-type receptor-mediated endocytosis. Arf3p was partially localized to the cell periphery, but was not detected on endocytic structures. The nucleotide-binding, N-terminal region, and N-terminal myristate of Arf3p are important for its proper localization. C-Terminally green fluorescent protein-tagged Arf3, expressed from the endogenous promoter, exhibited a polarized localization to the cell periphery and buds, in a cell cycle-dependent manner. Arf3-GFP achieved its proper localization during polarity growth through an actin-independent pathway. Both haploid and homologous diploid arf3 mutants exhibit a random budding defect, and the overexpression of the GTP-bound form Arf3p(Q71L) or GDP-binding defective Arf3p(T31N) mutant interfered with budding-site selection. We conclude that the GTPase cycle of Arf3p is likely to be important for the function of Arf3p in polarizing growth of the emerging bud and/or an unidentified vesicular trafficking pathway.

A developmentally regulated ARF-like 5 protein (ARL5), localized to nuclei and nucleoli, interacts with heterochromatin protein 1

ARF-like proteins (ARLs) are distinct group of members of the ARF family of Ras-related GTPases. Although ARLs are very similar in primary structure to ARFs, their functions remain unclear. We cloned mouse (m) and human (h) ARL5 cDNAs to characterize the protein products and their molecular properties. mARL5 mRNA was more abundant in liver than in other adult tissues tested. mARL5, similar to mARL4, was developmentally regulated and localized to nuclei. hARL5 interacted with importin-alpha through its C-terminal bipartite nuclear localization signal. When expressed in COS-7 cells, mutant hARL5(T35N), which is predicted to be GDP bound, was concentrated in nucleoli. The N-terminus of hARL5, like that of ARF, was myristoylated. Yeast two-hybrid screening and in vitro protein-interaction assays showed that hARL5(Q80L), predicted to be GTP bound, interacted with heterochromatin protein 1alpha (HP1alpha), which is known to be associated with telomeres as well as with heterochromatin, and acted as a transcriptional suppressor in mammalian cells. The interaction was reproduced in COS cells, where hARL5(Q80L) was co-immunoprecipitated with HP1alpha. hARL5 interaction with HP1alpha was dependent on the nucleotide bound, and required the MIR-like motif. Moreover, hARL5(Q80L), but not hARL5 lacking the MIR-like motif, was partly co-localized with overexpressed HP1alpha. Our findings suggest that developmentally regulated ARL5, with its distinctive nuclear/nucleolar localization and interaction with HP1alpha, may play a role(s) in nuclear dynamics and/or signaling cascades during embryonic development.

ADP-ribosylation factor 1 of Arabidopsis plays a critical role in intracellular trafficking and maintenance of endoplasmic reticulum morphology in Arabidopsis

ADP-ribosylation factors (Arf), a family of small GTP-binding proteins, play important roles in intracellular trafficking in animal and yeast cells. Here, we investigated the roles of two Arf homologs, Arf1 and Arf3 of Arabidopsis, in intracellular trafficking in plant cells. We generated dominant negative mutant forms of Arf 1 and Arf3 and examined their effect on trafficking of reporter proteins in protoplasts. Arf1[T31N] inhibited trafficking of H(+)-ATPase:green fluorescent protein (GFP) and sialyltransferase (ST):GFP to the plasma membrane and the Golgi apparatus. In addition, Arf1[T31N] caused relocalization of the Golgi reporter protein ST:GFP to the endoplasmic reticulum (ER). In protoplasts expressing Arf1[T31N], ST:red fluorescent protein remained in the ER, whereas H(+)-ATPase:GFP was mistargeted to another organelle. Also, expression of Arf1[T31N] in protoplasts resulted in profound changes in the morphology of the ER. The treatment of protoplasts with brefeldin A had exactly the same effect as Arf1[T31N] on various intracellular trafficking pathways. In contrast, Arf3[T31N] did not affect trafficking of any of these reporter proteins. Inhibition experiments using mutants with various domains swapped between Arf1 and Arf3 revealed that the N-terminal domain is interchangeable for trafficking inhibition. However, in addition to the T31N mutation, motifs in domains II, III, and IV of Arf1 were necessary for inhibition of trafficking of H(+)-ATPase:GFP. Together, these results strongly suggest that Arf1 plays a role in the intracellular trafficking of cargo proteins in Arabidopsis, and that Arf1 functions through a brefeldin A-sensitive factor.

The Gcs1 and Age2 ArfGAP proteins provide overlapping essential function for transport from the yeast trans-Golgi network

Many intracellular vesicle transport pathways involve GTP hydrolysis by the ADP-ribosylation factor (ARF) type of monomeric G proteins, under the control of ArfGAP proteins. Here we show that the structurally related yeast proteins Gcs1 and Age2 form an essential ArfGAP pair that provides overlapping function for TGN transport. Mutant cells lacking the Age2 and Gcs1 proteins cease proliferation, accumulate membranous structures resembling Berkeley bodies, and are unable to properly process and localize the vacuolar hydrolase carboxypeptidase (CPY) and the vacuolar membrane protein alkaline phosphatase (ALP), which are transported from the TGN to the vacuole by distinct transport routes. Immunofluorescence studies localizing the proteins ALP, Kex2 (a TGN resident protein), and Vps10 (the CPY receptor for transport from the TGN to the vacuole) suggest that inadequate function of this ArfGAP pair leads to a fragmentation of TGN, with effects on secretion and endosomal transport. Our results demonstrate that the Gcs1 + Age2 ArfGAP pair provides overlapping function for transport from the TGN, and also indicate that multiple activities at the TGN can be maintained with the aid of a single ArfGAP.

beta-Arrestin-mediated ADP-ribosylation factor 6 activation and beta 2-adrenergic receptor endocytosis

beta-Arrestins are multifunctional adaptor proteins known to regulate internalization of agonist-stimulated G protein-coupled receptors by linking them to endocytic proteins such as clathrin and AP-2. Here we describe a previously unappreciated mechanism by which beta-arrestin orchestrates the process of receptor endocytosis through the activation of ADP-ribosylation factor 6 (ARF6), a small GTP-binding protein. Involvement of ARF6 in the endocytic process is demonstrated by the ability of GTP-binding defective and GTP hydrolysis-deficient mutants to inhibit internalization of the beta(2)-adrenergic receptor. The importance of regulation of ARF6 function is shown by the ability of the ARF GTPase-activating protein GIT1 to inhibit and of the ARF nucleotide exchange factor, ARNO, to enhance receptor endocytosis. Endogenous beta-arrestin is found in complex with ARNO. Upon agonist stimulation of the receptor, beta-arrestin also interacts with the GDP-liganded form of ARF6, thereby facilitating ARNO-promoted GTP loading and activation of the G protein. Thus, the agonist-driven formation of a complex including beta-arrestin, ARNO, and ARF6 provides a molecular mechanism that explains how the agonist-stimulated receptor recruits a small G protein necessary for the endocytic process and controls its activation.

ADP ribosylation factor 6 binding to phosphatidylinositol 4,5-bisphosphate-containing vesicles creates defects in the bilayer structure: an electron spin resonance study

The effects of binding of myristoylated ADP ribosylation factor 6 (myr-ARF6), an activator of phospholipase D (PLD), to a model membrane were investigated using an electron spin resonance (ESR) labeling technique. Initial studies were conducted in vesicles composed of 1-palmitoyl-2-oleoyl phosphatidylethanolamine, dipalmitoylphosphatidylcholine, phosphatidylinositol 4,5-biphosphate (PIP(2)), and cholesterol. Recombinant ARF6 binding significantly enhances defects in both the headgroup and acyl-chain regions of the membrane, which are revealed by the emergence of sharp components in the spectra from a headgroup label, 1,2-dipalmitoylphosphatidyl-2,2,6,6-tetramethyl-1-piperidinyloxy-choline (DPPTC), and a chain label, 10PC, after myr-ARF6 binding. Binding of non-myristoylated ARF6 (non-ARF6) shows markedly reduced effects. Interestingly, no change in spectra from DPPTC was observed upon myr-ARF6 binding when PIP(2) in the vesicles was replaced by other negatively charged lipids, including phosphatidylinositol, phosphatidylserine, and phosphatidylglycerol, even when normalized for charge. The production of the sharp peak appears to be a specific event, because another GTP binding protein, CDC42, which binds PIP(2) and activates PLD, fails to induce changes in vesicle structure. These results suggest a previously unappreciated role for ARF in mediating a protein/lipid interaction that produces defects in lipid bilayers. This function may serve as an initial event in destabilizing membrane structure for subsequent membrane fusion or biogenesis of vesicles.

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.

ARL4, an ARF-like protein that is developmentally regulated and localized to nuclei and nucleoli

ADP-ribosylation factors (ARFs) are highly conserved approximately 20-kDa guanine nucleotide-binding proteins that participate in both exocytic and endocytic vesicular transport pathways via mechanisms that are only partially understood. Although several ARF-like proteins (ARLs) are known, their biological functions remain unclear. To characterize its molecular properties, we cloned mouse and human ARL4 (mARL4 and hARL4) cDNA. The appearance of mouse ARL4 mRNA during embryonic development coincided temporally with the sequential formation of somites and the establishment of brain compartmentation. Using ARL4-specific antibody for immunofluorescence microscopy, we observed that endogenous mARL4 in cultured Sertoli and neuroblastoma cells was mainly concentrated in nuclei. When expressed in COS7 cells, ARL4-T34N mutant, predicted to exist with GDP bound, was concentrated in nucleoli. Yeast two-hybrid screening and in vitro protein-interaction assays showed that hARL4 interacted with importin-alpha through its C-terminal NLS region and that the interaction was not nucleotide-dependent. Like ARL2 and -3, recombinant hARL4 did not enhance cholera toxin-catalyzed auto-ADP-ribosylation. Its binding of guanosine 5'-O-(thiotriphosphate) was modified by phospholipid and detergent, and the N terminus of hARL4, like that of ARF, was myristoylated. Our findings suggest that ARL4, with its distinctive nuclear/nucleolar localization and pattern of developmental expression, may play a unique role(s) in neurogenesis and somitogenesis during embryonic development and in the early stages of spermatogenesis in adults.

The binding of myristoylated N-terminal nonapeptide from neuro-specific protein CAP-23/NAP-22 to calmodulin does not induce the globular structure observed for the calmodulin-nonmyristylated peptide complex

CAP-23/NAP-22, a neuron-specific protein kinase C substrate, is Nalpha-myristoylated and interacts with calmodulin (CaM) in the presence of Ca2+ ions. Takasaki et al. (1999, J Biol Chem 274:11848-11853) have recently found that the myristoylated N-terminal nonapeptide of CAP-23/NAP-22 (mC/N9) binds to Ca2+ -bound CaM (Ca2+/CaM). In the present study, small-angle X-ray scattering was used to investigate structural changes of Ca2+/CaM induced by its binding to mC/N9 in solution. The binding of one mC/N9 molecule induced an insignificant structural change in Ca2+/CaM. The 1:1 complex appeared to retain the extended conformation much like that of Ca2+/CaM in isolation. However, it could be seen that the binding of two mC/N9 molecules induced a drastic structural change in Ca2+/CaM, followed by a slight structural change by the binding of more than two but less than four mC/N9 molecules. Under the saturated condition (the molar ratio of 1:4), the radius of gyration (Rg) for the Ca2+/CaM-mC/N9 complex was 19.8 +/- 0.3 A. This value was significantly smaller than that of Ca2+/CaM (21.9 +/- 0.3 A), which adopted a dumbbell structure and was conversely 2-3 A larger than those of the complexes of Ca2+/CaM with the nonmyristoylated target peptides of myosin light chain kinase or CaM kinase II, which adopted a compact globular structure. The pair distance distribution function had no shoulder peak at around 40 A, which was mainly due to the dumbbell structure. These results suggest that Ca2+/CaM interacts with Nalpha-myristoylated CAP-23/NAP-22 differently than it does with other nonmyristoylated target proteins. The N-terminal amino acid sequence alignment of CAP-23/NAP-22 and other myristoylated proteins suggests that the protein myristoylation plays important roles not only in the binding of CAP-23/NAP-22 to Ca2+/CaM, but also in the protein-protein interactions related to other myristoylated proteins.

Characterization of Cop I coat proteins in plant cells

Membrane traffic in eukaryotic cells is mediated by COP (coat protein)-coated vesicles. Their existence in plant cells has not yet been unequivocally demonstrated, although coated vesicles (probably with a COP coat) can be seen by electron microscopy. At the gene level, plant cells seem to contain all the components necessary to form COP-coated vesicles. In this paper, we have used antibodies raised against mammalian COPI coat proteins to detect putative homologues in rice (Oryza sativa) cells. Using these antibodies, we have found that rice cells contain alpha-, beta-, beta'-, and gamma-COP, as well as ADP-ribosylation factor (ARF) 1 protein. In addition, we show that antibodies against mammalian beta'-COP can immunoprecipitate not only beta'-COP but also alpha-, beta-, and gamma-COP, suggesting that COPI components in rice cells exist as a complex (or coatomer) in the cytosol, as in mammalian cells. Finally, we show that COP binding to membranes is GTP-dependent, and that ARF1 also binds to membranes in a GTP-dependent manner.

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.

Similarities in function and gene structure of cytohesin-4 and cytohesin-1, guanine nucleotide-exchange proteins for ADP-ribosylation factors

Activation of ADP-ribosylation factors (ARFs), approximately 20-kDa GTPases that are inactive in the GDP-bound form, depends on guanine nucleotide-exchange proteins (GEPs) to accelerate GTP binding. A novel ARF GEP, designated cytohesin-4, was cloned from a human brain cDNA library. Deduced amino acid sequence of the 47-kDa protein contains the same structural components present in cytohesin -1, -2, and -3, including an approximately 200-amino acid Sec7 domain with an approximately 100-residue pleckstrin homology domain near the C terminus. The Sec7 domain sequence is 77% identical to those of other cytohesins. Structures of the cytohesin-4 and cytohesin-1 genes were remarkably similar, except for an extra 3-base pair (GAG) exon present in cytohesin-1. Two mRNAs with and without the 3-base pair sequence were found in brain in different ratios for cytohesin-1, -2, and -3 but not cytohesin-4. Recombinant cytohesin-4 stimulated guanosine 5'-3-O-(thio)triphosphate binding by human ARF1 and ARF5 but not ARF6. Like other cytohesins and unlike the approximately 200-kDa ARF GEPs, it was not inhibited by brefeldin A. A cytohesin-4 mRNA of approximately 3.7 kilobases, abundant in leukocytes, was not detected in most tissues. Among separated populations of blood cells, approximately 90% of CD33(+) (monocytes), 80% of CD2(+) (NK/T), and 10-20% of CD19(+) (B) cells contained cytohesin-4 mRNA by in situ hybridization. Thus, in gene structure and brefeldin A-insensitive GEP activity, cytohesin-4 resembles other cytohesins, but its tissue distribution differs considerably, consistent with a different specific function.

Fatty acylation of proteins: new insights into membrane targeting of myristoylated and palmitoylated proteins

Covalent attachment of myristate and/or palmitate occurs on a wide variety of viral and cellular proteins. This review will highlight the latest advances in our understanding of the enzymology of N-myristoylation and palmitoylation as well as the functional consequences of fatty acylation of key signaling proteins. The role of myristate and palmitate in promoting membrane binding as well as specific membrane targeting will be reviewed, with emphasis on the Src family of tyrosine protein kinases and alpha subunits of heterotrimeric G proteins. The use of myristoyl switches and regulated depalmitoylation as mechanisms for achieving reversible membrane binding and regulated signaling will also be explored.

Analysis of recombinant human ADP-ribosylation factors by reversed-phase high-performance liquid chromatography and electrospray mass spectrometry

Two complementary approaches utilizing reverse-phase high-performance liquid chromatography and liquid chromatography/mass spectrometry were developed to analyze recombinantly produced Group I and Group II human ADP-ribosylation factors (ARFs). We observe that the NH2 termini of Group II ARFs (ARF4 and ARF5) are efficiently processed by removal of the initiating methionine. In contrast, the NH2 termini of Group I ARFs (ARF1 and ARF3), although fully deformylated, undergo only partial methionine cleavage. This result is unexpected as ARFs are canonical substrates for methionine processing in both bacterial and eukaryotic systems, but it may explain the difficulties encountered by many researchers attempting to produce myristoylated ARFs in Escherichia coli. Additionally, we observe that a significant fraction of purified ARF4 contains a modification which we demonstrate to be consistent with mono-glutathionation. Both methionine retention and glutathione modification may impact ARF function and the methods presented here should be employed to determine the quality of recombinant ARFs.

Guanine nucleotide exchange on ADP-ribosylation factors catalyzed by cytohesin-1 and its Sec7 domain

ADP-ribosylation factors (ARFs) are 20-kDa guanine nucleotide-binding proteins that require specific guanine nucleotide-exchange proteins (GEPs) to accelerate the conversion of inactive ARF-GDP to active ARF-GTP. Cytohesin-1, a 46-kDa ARF GEP, contains a central Sec7 domain of 188 amino acids similar in sequence to a region of the yeast Sec7 protein. Cytohesin-1 and its 22-kDa Sec7 domain (C-1 Sec7), synthesized in Escherichia coli, were assayed with recombinant non-myristoylated ARFs and related proteins to compare their GEP activities. Both were effective with native mammalian ARFs 1 and 3. Cytohesin-1 accelerated GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) binding to recombinant human ARF1 (rARF1), yeast ARF3, and ARD1 (a 64-kDa guanine nucleotide-binding protein containing a C-terminal ARF domain). In contrast, C-1 Sec7 enhanced GTPgammaS binding to recombinant human ARFs 1, 5, and 6; yeast ARFs 1, 2, and 3; ARD1; two ARD1 mutants that contain the ARF domain; and Delta13ARF1, which lacks the N-terminal alpha-helix. Neither C-1 Sec7 nor cytohesin-1 increased GTPgammaS binding to human ARF-like ARL proteins 1, 2, and 3. Thus, ARLs, initially differentiated from ARFs because of their inability to activate cholera toxin, differ also in their failure to interact functionally with C-1 Sec7 or cytohesin-1. As C-1 Sec7 was much less substrate-specific than cytohesin-1, it appears that structure outside of the Sec7 domain is important for ARF specificity. Data obtained with mutant ARF constructs are all consistent with the conclusion that the ARF N terminus is an important determinant of cytohesin-1 specificity.

Effects of arfaptin 1 on guanine nucleotide-dependent activation of phospholipase D and cholera toxin by ADP-ribosylation factor

Arfaptin 1, a approximately 39-kDa protein based on the deduced amino acid sequence, had been initially identified in a yeast two-hybrid screen using dominant active ARF3 (Q71L) as bait with an HL-60 cDNA library. It was suggested that arfaptin 1 may be involved in Golgi functions, since the FLAG-tagged protein was associated with Golgi membranes when expressed in COS-7 cells and could be bound to Golgi in vitro in an ADP-ribosylation factor (ARF)- and GTPgammaS-dependent, brefeldin A-inhibited fashion. Arfaptin 2, found in the same two-hybrid screen as arfaptin 1, is 60% identical in amino acid sequence and may or may not have an analogous function. We now report some effects of arfaptin 1 on ARF activation of phospholipase D and cholera toxin ADP-ribosyltransferase. Arfaptin 1 inhibited activation of both enzymes in a concentration-dependent manner and was without effect in the absence of ARF. Two ARF1 mutants that activated the toxin, one lacking 13 N-terminal amino acids and the other, in which 73 residues at the N terminus were replaced with the analogous sequence from ARL1, were not inhibited by arfaptin, consistent with the conclusion that arfaptin interaction requires the N terminus of ARF. This region has also been implicated in phospholipase D activation, but whether the two proteins interact with the same structural elements in ARF remains to be determined. Arfaptin inhibition of the action of ARF5 and ARF6 was less than that of ARF1 and ARF3; its effects were less on nonmyristoylated than myristoylated ARFs. Arfaptin effects on guanine nucleotide binding by ARFs were minimal whether or not a purified ARF guanine nucleotide-exchange protein was present. These findings indicate that arfaptin acts as an inhibitor of ARF actions in vitro, raising the possibility that it has a similar role in vivo.

Expression of an ADP-ribosylation factor like gene, ARF4L, is induced after transient forebrain ischemia in the gerbil

To elucidate the molecular mechanisms underlying post-ischemic phenomena including delayed neuronal death, we screened for genes which were induced in the hippocampus after transient global ischemia in the Mongolian gerbil by a differential display method, and cloned a gerbil homologue of human ADP-ribosylation factor 4L (ARF4L). Although the physiological roles of ARF4L are unknown, it is likely that ARF4L participates in vesicle transport between the endoplasmic reticulum (ER) and Golgi complex as it contains a GTP binding site, myristoylation site and coatmer binding motif (KKXX). In situ hybridization analysis indicated that the expression of ARF4L mRNA was elevated in neurons of the dentate gyrus (DG) and CA1 regions. In DG, the signals were detected 3 h after ischemia and peaked at 6 h with subsequent gradual reduction. On the other hand, in the CA1 region where cell death occurs in this model, ARF4L mRNA was slightly detected from 1 to 2 days after ischemia but was absent after 3 days. Other vesicle transport-related genes such as ARF1, ARL4 and beta-COP were also induced after 5-min ischemia, suggesting that vesicle transport was activated in hippocampal neurons after ischemic stress. To determine the cause of the induction of ARF4L gene expression after transient ischemia, we examined the changes in ARF4L mRNA expression in HEK 293 cells under hypoxic conditions compared with HSP70. The expression of ARF4L mRNA was elevated at 12 h after hypoxia exposure, similarly to HSP70. These results will help to elucidate the association of upregulation of vesicle transport systems including ARF4L and stress responses of neurons after transient ischemia.

Regulated exocytosis in chromaffin cells. Translocation of ARF6 stimulates a plasma membrane-associated phospholipase D

The ADP-ribosylation factor (ARF) GTP-binding proteins have been implicated in a wide range of vesicle transport and fusion steps along the secretory pathway. In chromaffin cells, ARF6 is specifically associated with the membrane of secretory chromaffin granules. Since ARF6 is an established regulator of phospholipase D (PLD), we have examined the intracellular distribution of ARF6 and PLD activity in resting and stimulated chromaffin cells. We found that stimulation of intact chromaffin cells or direct elevation of cytosolic calcium in permeabilized cells triggered the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. To probe the existence of an ARF6-dependent PLD in chromaffin cells, we measured the PLD activity in purified plasma membranes. PLD could be activated by a nonhydrolyzable analogue of GTP and by recombinant myristoylated ARF6 and inhibited by specific anti-ARF6 antibodies. Furthermore, a synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 inhibited both PLD activity and catecholamine secretion in calcium-stimulated chromaffin cells. The possibility that ARF6 participates in the exocytotic reaction by controlling a plasma membrane-bound PLD and thereby generating fusogenic lipids at the exocytotic sites is discussed.

Characterization of an ADP-ribosylation factor-like 1 protein in Saccharomyces cerevisiae

ADP-ribosylation factors (ARFs) are highly conserved approximately 20-kDa guanine nucleotide-binding proteins that enhance the ADP-ribosyltransferase activity of cholera toxin and are believed to participate in vesicular transport in both exocytic and endocytic pathways. Several ARF-like proteins (ARLs) have been cloned from Drosophila, rat, and human; however, the biological functions of ARLs are unknown. We have identified a yeast gene (ARL1) encoding a protein that is structurally related (>60% identical) to human, rat, and Drosophila ARL1. Biochemical analyses of purified recombinant yeast ARL1 (yARL1) protein revealed properties similar to those ARF and ARL1 proteins, including the ability to bind and hydrolyze GTP. Like other ARLs, recombinant yARL1 protein did not stimulate cholera toxin-catalyzed auto-ADP-ribosylation. yARL1 was not recognized by antibodies against mammalian ARLs or yeast ARFs. Anti-yARL1 antibodies did not cross-react with yeast ARFs, but did react with human ARLs. On subcellular fractionation, yARL1, similar to yARF1, was localized to the soluble fraction. The amino terminus of yARL1, like that of ARF, was myristoylated. Unlike Drosophila Arl1, yeast ARL1 was not essential for cell viability. Like rat ARL1, yARL1 might be associated in part with the Golgi complex. However, yARL1 was not required for endoplasmic reticulum-to-Golgi protein transport, and it may offer an opportunity to define an ARL function in another kind of vesicular trafficking, such as the regulated secretory pathway.

Arfaptin 1, a putative cytosolic target protein of ADP-ribosylation factor, is recruited to Golgi membranes

ADP-ribosylation factors (ARFs) have been implicated in vesicle transport in the Golgi complex. Employing yeast two-hybrid screening of an HL60 cDNA library using a constitutively active mutant of ARF3 (ARF3.Q71L), as a probe, we have identified a cDNA encoding a novel protein with a calculated molecular mass of 38.6 kDa, which we have named arfaptin 1. The mRNA of arfaptin 1 was ubiquitously expressed, and recombinant arfaptin 1 bound preferentially to class I ARFs, especially ARF1, but only in the GTP-bound form. The interactions were independent of myristoylation of ARF. Arfaptin 1 in cytosol was recruited to Golgi membranes by ARF in a guanosine 5'-O-(3-thiotriphosphate)-dependent and brefeldin A-sensitive manner. When expressed in COS cells, arfaptin 1 was localized to the Golgi complex. The yeast two-hybrid system yielded another clone, which encoded a putative protein, which we have named arfaptin 2. This consisted of the same number of amino acids as arfaptin 1 and was 60% identical to it. Arfaptin 2 was also ubiquitously expressed and bound to the GTP-, but not GDP-liganded form of class I ARFs, especially ARF1. These results suggest that arfaptins 1 and 2 may be direct target proteins of class 1 ARFs. Arfaptin 1 may be involved in Golgi function along with ARF1.

Comparative activity of ADP-ribosylation factor family members in the early steps of coated vesicle formation on rat liver Golgi membranes

We have compared the abilities of mammalian ADP-ribosylation factors (ARFs) 1, 5, and 6 and Saccharomyces cerevisiae ARF2 to serve as substrates for the rat liver Golgi membrane guanine nucleotide exchange factor and to initiate the formation of clathrin- and coatomer protein (COP) I-coated vesicles on these membranes. While Golgi membranes stimulated the exchange of GTPgammaS for GDP on all of the ARFs tested, mammalian ARF1 was the best substrate, with an apparent Km of 5 microM. In all cases myristoylation of ARF was required for stimulation. Agents that inhibit the Golgi membrane guanine nucleotide exchange factor (the fungal metabolite brefeldin A and trypsin treatment) selectively inhibited the guanine nucleotide exchange on mammalian ARF1. Taken together, these data indicate that of the ARFs tested, only mammalian ARF1 is activated efficiently by the Golgi guanine nucleotide exchange factor. The other ARFs are activated mainly by another mechanism, possibly phospholipid-mediated. Once activated, all of the membrane-associated, myristoylated ARFs promoted the recruitment of coatomer to about the same extent. Mammalian ARFs 1 and 5 were the most effective in promoting the recruitment of the AP-1 adaptor complex, whereas yeast ARF2 was the least active. These data indicate that the specificity for ARF action on the Golgi membranes is primarily determined by the Golgi guanine nucleotide exchange factor, which has a strong preference for myristoylated mammalian ARF1.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Isolation of a brefeldin A-inhibited guanine nucleotide-exchange protein for ADP ribosylation factor (ARF) 1 and ARF3 that contains a Sec7-like domain

Brefeldin A (BFA) inhibited the exchange of ADP ribosylation factor (ARF)-bound GDP for GTP by a Golgi-associated guanine nucleotide-exchange protein (GEP) [Helms, J.B. & Rothman, J.E. (1992) Nature (London) 360, 352-354; Donaldson, J.G., Finazzi, D. & Klausner, R.D. (1992) Nature (London) 360, 350-352]. Cytosolic ARF GEP was also inhibited by BFA, but after purification from bovine brain and rat spleen, it was no longer BFA-sensitive [Tsai, S.-C., Adamik, R., Moss, J. & Vaughan, M. (1996) Proc. Natl. Acad. Sci. USA 93, 305-309]. We describe here purification from bovine brain cytosol of a BFA-inhibited GEP. After chromatography on DEAE-Sephacel, hydroxylapatite, and Mono Q and precipitation at pH 5.8, GEP was eluted from Superose 6 as a large molecular weight complex at the position of thyroglobulin (approximately 670 kDa). After SDS/PAGE of samples from column fractions, silver-stained protein bands of approximately 190 and 200 kDa correlated with activity. BFA-inhibited GEP activity of the 200-kDa protein was demonstrated following electroelution from the gel and renaturation by dialysis. Four tryptic peptides from the 200-kDa protein had amino acid sequences that were 47% identical to sequences in Sec7 from Saccharomyces cerevisiae (total of 51 amino acids), consistent with the view that the BFA-sensitive 200-kDa protein may be a mammalian counterpart of Sec7 that plays a similar role in cellular vesicular transport and Sec7 may be a GEP for one or more yeast ARFs.