GTP but not GDP analogues promote association of ADP-ribosylation factors, 20-kDa protein activators of cholera toxin, with phospholipids and PC-12 cell membranes

Induction of membrane curvature by proteins involved in Golgi trafficking

The Golgi apparatus serves a key role in processing and sorting lipids and proteins for delivery to their final cellular destinations. Vesicle exit from the Golgi initiates with directional deformation of the lipid bilayer to produce a bulge. Several mechanisms have been described by which lipids and proteins can induce directional membrane curvature to promote vesicle budding. Here we review some of the mechanisms implicated in inducing membrane curvature at the Golgi to promote vesicular trafficking to various cellular destinations.

Molecular analysis of ARF1 expression profiles during development of physic nut (Jatropha curcas L.)

A cDNA clone designated arf1 was isolated from a physic nut (Jatropha curcas L.) endosperm cDNA library which encodes a small GTP-binding protein and has significant homology to ADP-ribosylation factors (ARF) in plants, animals and microbes. The cDNA contains an open reading frame that encodes a polypeptide of 181 amino acids with a calculated molecular mass of 20.7 kDa. The deduced amino acid sequence showed high homology to known ARFs from other organisms. The products of the arf1 obtained by overexpression in E. coli revealed the specific binding activity toward GTP. The expression of arf1 was observed in flowers, roots, stems and leaves as analyzed by RT-PCR, and its transcriptional level was highest in flowers. In particular, the accumulation of arf1 transcripts was different under various environmental stresses in seedlings. The results suggest that arf1 plays distinct physiological roles in Jatropha curcas cells.

Immunolocalization and high affinity interactions of acyl-CoAs with proteins: an original study with anti-acyl-CoA antibodies

Anti-acyl-Coenzyme A (acyl-CoA) antibodies were used to detect fatty acyl-CoAs in cultured rat hippocampal neurons, in which important lipid metabolism and transport occur. Hippocampus was chosen because of his involvement in many cerebral functions and diseases. Immunofluorescence experiments showed an intense labelling within neurites and cell bodies. Labelling seems to be associated with vesicles and membrane domains. We have shown by immunoblot experiments that the labelling corresponded to acyl-CoAs which were in strong interaction with proteins, without being covalently bound to them. Immunoprecipitation experiments, followed by proteomic analysis, showed that anti-acyl-CoA antibodies were also able to immunoprecipitate multiprotein complexes, principally related to vesicle trafficking and/or to membrane rafts.

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.

Inhibition of formyl-methionyl-leucyl-phenylalanine-stimulated phospholipase D activation in rat neutrophils by the synthetic isoquinoline DMDI

The expression of phospholipase D (PLD) isoenzymes in neutrophils was investigated using reverse transcription-polymerase chain reaction analysis. Amplification products of predicted size were obtained from rat neutrophils with nucleotide sequences corresponding to PLD1a and PLD2. 1-(3',4'-Dimethoxybenzyl)-6,7-dichloroisoquinoline (DMDI) inhibited the formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PLD activation in rat neutrophils. The underlying cellular signaling mechanism of DMDI inhibition was investigated. The fMLP-induced protein tyrosine phosphorylation and the membrane translocation of ADP-ribosylation factor (ARF) and Rho A in neutrophils was attenuated by DMDI in a concentration-dependent manner. However, neither the membrane association of protein kinase C-alpha and -beta isoenzymes in fMLP-stimulated cells nor the GTPgammaS- and phorbol 12-myristate 13-acetate-stimulated membrane translocation of ARF and Rho A in a cell-free system was affected significantly by DMDI. These results indicate that the expression of PLD1a and PLD2 mRNA in neutrophils. Attenuation of protein tyrosine phosphorylation and the membrane association of ARF and Rho A probably play a concerted role in the inhibition of PLD by DMDI in rat neutrophils in response to fMLP.

Cloning and expression analyses of cDNA encoding an ADP-ribosylation factor from wheat: tissue-specific expression of wheat ARF

We isolated and characterized a cDNA clone encoding a small GTP-binding protein, which has a high similarity to mammalian ADP-ribosylation factor (ARF), from a cDNA library prepared from immature spikes of wheat (Triticum aestivum L.). The cDNA contained an open reading frame that encodes a polypeptide of 181 amino acids with a calculated molecular mass of 20.7 kDa. The deduced amino acid sequence showed a quite high homology to known ARFs from other organisms. In particular, the wheat ARF was completely identical to the rice ARF1. Genomic Southern hybridization suggested that wheat ARF is encoded by at least two or three copies of ARF genes. Northern analyses showed that the accumulation of the ARF transcripts was nearly constant throughout various environmental stresses in both shoots and roots. However the RNA transcript was preferentially expressed in roots rather than in shoots. A similar expression pattern was also observed at the protein level by Western analysis. The relative abundance of the ARF proteins in root and flower tissues may indicate a high level of vesicular transporting activity in the roots and flowers of wheat plants.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Inhibition of noradrenaline release from PC12 cells by the long-term treatment with cholera toxin

Guanine nucleotide-binding (G) proteins are required for intracellular vesicular transport and endocytosis. In this study, we investigated the effects of short-term (2 h) and long-term (24 h) treatment with cholera toxin (CTX), which ADP-ribosylates proteins having arginine residues such as the alpha subunit of Gs (G(s alpha)), on exocytosis from the neurosecretory rat pheochromocytoma PC 12 cell line. Short-term treatment with CTX stimulated the accumulation of cyclic AMP, and synergistically enhanced both extracellular Ca2+-dependent [3H]noradrenaline (NA) releases (induced by high K+ and ATP) and Ca2+-independent release (induced by mastoparan, a peptide in wasp venom). Long-term treatment with CTX for 24h inhibited Ca2+-dependent and -independent stimulated [3H]NA release. The inhibitory effect of long-term CTX treatment was not derived from a cyclic AMP-dependent system, because (1) H-89, an inhibitor of protein kinase A, had no effect on the inhibition induced by CTX, (2) the long-term treatment with forskolin did not show an inhibitory effect. [32P]ADP-ribosylation of G(s alpha) and its immunoreactivity with anti-G(s alpha) antiserum in the crude membrane fraction was inhibited in the long-term CTX-treated cells, but not in the long-term forskolin-treated cells. [32P]ADP-ribosylation of G(s alpha) in the membrane fraction of short-term CTX-treated cells was approximately 90% of the level in the control cells. These findings suggest that CTX stimulates [3H]NA release via a cyclic AMP-dependent system in the short-term, and that long-term CTX treatment inhibited its release, maybe via ADP-ribosylation of CTX-sensitive proteins such as G(s alpha).

Exocytotic stimulation promotes association of the ADP-ribosylation factor with PC12 cell membranes

ADP-ribosylation factors (ARFs) are a family of small molecular, monomeric GTP-binding (G) proteins, initially identified by their ability to enhance cholera toxin (CTX) ADP-ribosyltransferase activity. ARFs have been implicated in protein transport and vesicle and endosome fusion. Although several reports show that synthetic peptides of the N-terminus of ARF inhibited Ca(2+)-dependent exocytosis in permeabilized adrenal chromaffin cells, the role of ARFs in exocytosis has not been established. In this study, we investigated the translocation of ARFs to the membrane fraction from the cytosol fraction in PC12 cells after exocytotic stimulation by measuring the immunoreactivity of ARFs (with anti-ARF anti-serum and with anti-ARF3 antibodies) and enzymatic ARF activity, which enhances the CTX effect. Both the immunoreactivity and the enzymatic activity of ARF in the membrane fraction increased about twofold, significantly, after exocytotic stimulation with ATP and KCl. The translocation of ARF and noradrenaline release was observed in the presence of extracellular CaCl2, but not in the absence of CaCl2. The ARF translocated to the membrane fraction after stimulation in intact cells seemed to be an inactive, perhaps is the GDP form, because ARF did not activate CTX in the absence of guanosine 5'-O-(thiotriphosphate) (GTP gamma S). As previously reported, ARF in the active, GTP gamma S-bound state bound to the membrane fractions. Thus ARF may have been active during translocation and inactivated later. The immunoreactivity of Gs alpha, one of the trimeric G proteins, was not changed before or after stimulation. These findings suggest that ARFs translocate to membranes from the cytosolic fraction after exocytotic stimulation in PC12 cells, and raise the possibility that ARFs regulate exocytosis.

ADP-ribosylation factor and Rho proteins mediate fMLP-dependent activation of phospholipase D in human neutrophils

Activation of intact human neutrophils by fMLP stimulates phospholipase D (PLD) by an unknown signaling pathway. The small GTPase, ADP-ribosylation factor (ARF), and Rho proteins regulate the activity of PLD1 directly. Cell permeabilization with streptolysin O leads to loss of cytosolic proteins including ARF but not Rho proteins from the human neutrophils. PLD activation by fMLP is refractory in these cytosol-depleted cells. Readdition of myr-ARF1 but not non-myr-ARF1 restores fMLP-stimulated PLD activity. C3 toxin, which inactivates Rho proteins, reduces the ARF-reconstituted PLD activity, illustrating that although Rho alone does not stimulate PLD activity, it synergizes with ARF. To identify the signaling pathway to ARF and Rho activation by fMLP, we used pertussis toxin and wortmannin to examine the requirement for heterotrimeric G proteins of the Gi family and for phosphoinositide 3-kinase, respectively. PLD activity in both intact cells and the ARF-restored response in cytosol-depleted cells is inhibited by pertussis toxin, indicating a requirement for Gi2/Gi3 protein. In contrast, wortmannin inhibited only fMLP-stimulated PLD activity in intact neutrophils, but it has no effect on myr-ARF1-reconstituted activity. fMLP-stimulated translocation of ARF and Rho proteins to membranes is not inhibited by wortmannin. It is concluded that activation of Gi proteins is obligatory for ARF/Rho activation by fMLP, but activation of phosphoinositide 3-kinase is not required.

ADP ribosylation factor 1 is required for synaptic vesicle budding in PC12 cells

Carrier vesicle generation from donor membranes typically progresses through a GTP-dependent recruitment of coats to membranes. Here we explore the role of ADP ribosylation factor (ARF) 1, one of the GTP-binding proteins that recruit coats, in the production of neuroendocrine synaptic vesicles (SVs) from PC12 cell membranes. Brefeldin A (BFA) strongly and reversibly inhibited SV formation in vivo in three different PC12 cell lines expressing vesicle-associated membrane protein-T Antigen derivatives. Other membrane traffic events remained unaffected by the drug, and the BFA effects were not mimicked by drugs known to interfere with formation of other classes of vesicles. The involvement of ARF proteins in the budding of SVs was addressed in a cell-free reconstitution system (Desnos, C., L. Clift-O'Grady, and R.B. Kelly. 1995. J. Cell Biol. 130:1041-1049). A peptide spanning the effector domain of human ARF1 (2-17) and recombinant ARF1 mutated in its GTPase activity, both inhibited the formation of SVs of the correct size. During in vitro incubation in the presence of the mutant ARFs, the labeled precursor membranes acquired different densities, suggesting that the two ARF mutations block at different biosynthetic steps. Cell-free SV formation in the presence of a high molecular weight, ARF-depleted fraction from brain cytosol was significantly enhanced by the addition of recombinant myristoylated native ARF1. Thus, the generation of SVs from PC12 cell membranes requires ARF and uses its GTPase activity, probably to regulate coating phenomena.

Pertussis toxin-insensitive effects of mastoparan, a wasp venom peptide, in PC12 cells

Recent studies have shown that mastoparan, an amphiphilic peptide derived from wasp venom, modifies the secretion of neurotransmitters and hormones from a variety of cell types. Mastoparan interacts with heterotrimeric guanine nucleotide-binding proteins (G proteins) such as Gi and G(o), which are ADP-ribosylated by pertussis toxin (PTX) and thereby uncoupled from receptors. Previously, some of the effects of mastoparan including secretion were reported to be modified selectively by PTX but not by cholera toxin (CTX). In the present study, we examined the influence of bacterial toxins on the effects of mastoparan in PC12 cells. Mastoparan stimulated [3H]noradrenaline (NA) release from prelabeled PC12 cells in the absence of CaCl2, although high K+ or ATP-stimulated the release in a Ca(2+)-dependent manner. Pretreatment with CTX, not PTX, for 24 h inhibited mastoparan-stimulated [3H]NA release. Mastoparan inhibited forskolin-stimulated cyclic AMP accumulation in a dose-dependent manner, although mastoparan had no effect by itself. Pretreatment with PTX completely abolished the inhibitory effect of carbachol via Gi on cyclic AMP accumulation and partially reduced the effect of mastoparan. However, the inhibitory effect of 20 microM mastoparan was not modified by pretreatment with PTX. Thus, we investigated the effect of mastoparan on CTX-catalyzed [32P]ADP-ribosylation of proteins in PC12 cells. A subunit of CTX (CTX-A) catalyzed [32P]ADP-ribosylation of many proteins in the cytosolic fraction of PC12 cells. One of these was a 20 kDa protein, named ADP-ribosylating factor (ARF). The addition of mastoparan to assay mixtures inhibited ADP-ribosylation of many proteins including ARF and CTX-A in the presence of the cytosolic fraction. In the absence of the cytosolic fraction, however, mastoparan slightly enhanced ADP-ribosylation of bovine serum albumin and auto-ADP-ribosylation by CTX-A. Mastoparan did not inhibit ADP-ribosylation of the alpha subunit of Gs in the membrane fraction. These findings suggest that 1) mastoparan interacts with PTX-insensitive and CTX-sensitive factor(s) to stimulate NA release, and 2) mastoparan interacts with ARF inhibiting its activity to enhance the ADP-ribosylation reaction by CTX. ARF may be an exocytosis-linked G protein.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Novel aspects of the regulation of a cDNA (Arf1) from Chlamydomonas with high sequence identity to animal ADP-ribosylation factor 1

ADP-ribosylation factor (ARF) is a highly conserved, low molecular mass (ca. 21 kDa) GTP-binding protein that has been implicated in vesicle trafficking and signal transduction in yeast and mammalian cells. However, little is known of ARF in plant systems. A putative ARF polypeptide was identified in subcellular fractions of the green alga Chlamydomonas reinhardtii, based on [32P]GTP binding and immunoblot assays. A cDNA clone was isolated from Chlamydomonas (Arf1), which encodes a 20.7 kDa protein with 90% identity to human ARF1. Northern blot analyses showed that levels of Arf1 mRNA are highly regulated during 12 h/12 h light/dark (LD) cycles. A biphasic pattern of expression was observed: a transient peak of Arf1 mRNA occurred at the onset of the light period, which was followed ca. 12 h later by a more prominent peak in the early to mid-dark period. When LD-synchronized cells were shifted to continuous darkness, the dark-specific peak of Arf1 mRNA persisted, indicative of a circadian rhythm. The increase in Arf1 mRNA at the beginning of the light period, however, was shown to be light-dependent, and, moreover, dependent on photosynthesis, since it was prevented by DCMU. We conclude that the biphasic pattern of Arf1 mRNA accumulation during LD cycles is due to regulation by two different factors, light (which requires photosynthesis) and the circadian clock. Thus, these studies identify a novel pattern of expression for a GTP-binding protein gene.

Cytosolic ADP-ribosylation factors are not required for endosome-endosome fusion but are necessary for GTP gamma S inhibition of fusion

A specific role for ADP-ribosylation factors (ARFs) in in vitro endosome-endosome fusion has been proposed (Lenhard, J. M., Kahn, R. A., and Stahl, P. D. (1992) J. Biol. Chem. 267, 13047-13052). However, in vivo studies have failed to support a function for ARFs in the endocytic pathway, since an antagonist of ARF activities, brefeldin A, does not interfere with receptor internalization (Schonhorn, J. E., and Wessling-Resnick, M. (1994) Mol. Cell. Biochem. 135, 159-164). This controversy surrounding the exact function of ARF in endocytic vesicle traffic prompted us to critically re-examine the involvement of ARFs in cell-free endosome fusion. Cytosol depleted of ARF activity was capable of supporting in vitro endocytic vesicle fusion but failed to support inhibition of this reaction in the presence of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S). Addition of purified ARF1 restored the ability of the ARF-depleted cytosol to inhibit endosome fusion when incubated with GTP gamma S. Both endocytic vesicle fusion and the GTP gamma S-mediated inhibition of vesicle fusion were unaffected by brefeldin A. Moreover, the ATP requirement and kinetics of cell-free fusion are not altered by brefeldin A or depletion of cytosolic ARFs. These results suggest that cytosolic ARFs are not necessary for endosomal vesicle fusion in vitro but are responsible for inhibition of fusion in the presence of GTP gamma S and cytosolic factors in a brefeldin A-resistant manner.

Myristoylation of ADP-ribosylation factor 1 facilitates nucleotide exchange at physiological Mg2+ levels

Recombinant N-myristoylated bovine ADP-ribosylation factor 1 (myr-rARF1) has been expressed in bacteria and purified to near homogeneity with a high (85%) myristoylation efficiency. Myr-rARF1 and nonmyristoylated rARF1 have been compared with respect to their kinetics of guanine nucleotide exchange and their interactions with phospholipids. Myristoylation is shown to allow the release of bound GDP at physiological (mM) concentrations of Mg2+. GDP dissociation is slow in the absence of phospholipids but is accelerated 2-fold in the presence of phospholipid vesicles. On the contrary, myristoylation decreases 10-fold the rate of dissociation of GTP or guanosine 5'-O-(thiotriphosphate) (GTP gamma S) in the presence of phospholipids. As a result, myr-ARF1 can be spontaneously activated by GTP or GTP gamma S (t1/2 approximately 30 min at 37 degrees C) at 1 mM Mg2+, in the sole presence of phospholipid membranes without the need for a nucleotide exchange factor. In contrast to the nonacylated protein, the GDP-bound form of myr-ARF1 interacts with phospholipids, as demonstrated by its cosedimentation with phospholipid vesicles and its comigration with phospholipid/cholate micelles on gel filtration. The interaction is, however, weaker than for the GTP-bound form, suggesting that only the myristate in myr-ARF1GDP interacts with phospholipids, whereas both the myristate and the amino-terminal hydrophobic residues in myr-ARF1GTP bind to phospholipids.

ADP-ribosylation factors: a family of approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin

ADP-ribosylation factors (ARFs) comprise a family of approximately 20 kDa guanine nucleotide-binding proteins that were discovered as one of several cofactors required in cholera toxin-catalyzed ADP-ribosylation of Gs alpha, the guanine nucleotide-binding protein responsible for stimulation of adenylyl cyclase, and was subsequently found to enhance all cholera toxin-catalyzed reactions and to directly interact with, and activate the toxin. ARF is dependent on GTP or its analogues for activity, binds GTP with high affinity in the presence of dimyristoylphosphatidylcholine/cholate and contains consensus sequences for GTP-binding and hydrolysis. Six mammalian family members have been identified which have been classified into three groups (Class I, II, and III) based on size, deduced amino acid sequence identity, phylogenetic analysis and gene structure. ARFs are ubiquitous among eukaryotes, with a deduced amino acid sequence that is highly conserved across diverse species. They have recently been shown to associate with phospholipid and Golgi membranes in a GTP-dependent manner and are involved in regulating vesicular transport.

Stepwise assembly of functionally active transport vesicles

Budding of COP-coated vesicles (the likely carriers of newly synthesized proteins from the endoplasmic reticulum through the Golgi stack) from Golgi cisternae requires ADP-ribosylation factor (ARF), coatomer proteins from the cytosol, GTP, and fatty acyl-coenzyme A (CoA). The assembly of coated buds on the membranes requires coatomer, ARF, and GTP. When palmitoyl-CoA is added, membrane fission occurs at the coated bud, releasing coated vesicles. We show that COP-coated vesicles can be generated stepwise in vitro and isolated in a functionally active form, demonstrating that the minimal set of cytosolic components required for their formation as well as principal steps in their assembly have been identified.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.