The G protein-activating peptide, mastoparan, and the synthetic NH2-terminal ARF peptide, ARFp13, inhibit in vitro Golgi transport by irreversibly damaging membranes

Sar1 translocation onto the ER-membrane for vesicle budding has different pathways for promotion and suppression of ER-to-Golgi transport mediated through H89-sensitive kinase and ER-resident G protein

ER-to-Golgi protein transport involves transport vesicles of which formation is initiated by assembly of Sar1. The assembly of Sar1 is suppressed by protein kinase inhibitor H89, suggesting that ER-to-Golgi transport is regulated progressively by H89 sensitive kinase. ER-resident G(i2) protein suppresses vesicle formation with inhibition of Sar1 assembly. This study examined whether these promotion and suppression of vesicle transport share the same signal pathway, by examining the effects of G(i/o) protein activator mastoparan 7 (Mp-7) and H89 on Sar1 and Sec23 recruitment onto microsomes. In a cell-free system for Sar1 translocation assay, GTPγS addition induced the translocation of Sar1 onto microsomes. Mp-7 and H89 decreased the Sar1 translocation. Double treatment of Mp-7 and H89 strongly decreased Sar1 translocation. In single and double treatments, however, G(i/o) protein inactivator pertussis toxin (IAP) partially restored the suppressive effect of Mp-7, but had not any effect on H89-induced effect. Then, the assembly of Sec23 onto the microsome was also increased by the addition of GTPγS. Sec23 translocation was decreased by Mp-7 and/or H89 treatment and recovered by IAP pretreatment except for H89 single treatment, similarly to Sar1 translocation in each treatment. Inhibitory effects of H89 and Mp-7on ER-to-Golgi vesicle transport by H89 or Mp-7 were also confirmed in a cell culture system by BFA-dispersion and BFA-reconstruction experiments. These findings indicate that promotion and suppression of ER-to-Golgi vesicle transport are modulated through separate signal pathways.

ER-resident Gi2 protein controls sar1 translocation onto the ER during budding of transport vesicles

In our previous study, fluoride ([AlF(4) ](-) ) disturbed ER-to-Golgi transport through the activation of ER-resident heterotrimeric G protein (ER-G protein). Therefore, ER-G protein may be implicated in ER-to-Golgi transport at the early stage prior to coat protein assembly. Sar1 translocation onto the endoplasmic reticulum (ER) membrane is suppressed by non-selective protein kinase inhibitor H89, suggesting the participation of H89-sensitive kinase in this process. To investigate the involvement of ER-G protein in ER-to-Golgi transport, the effect of G(i) protein activator (mastoparan 7) was examined on Sar1 translocation onto the ER in a cell-free system consisting of microsome membrane and cytosol. Sar1 translocation onto the microsome membrane was induced by addition of GTPγS in the cell-free system. Translocation of Sar1 by GTPγS was suppressed significantly by both H89 and mastoparan 7. Mastoparan 7 suppressed the translocation of Sar1 onto the microsome membrane with dosage dependency, but mastoparan 17, the inactive analog of mastoparan 7, had no effect on Sar1 translocation. The suppressive effect of mastoparan 7 was recovered by treatment with pertussis toxin (IAP). Moreover, G(i2) protein was detected on the microsome membrane by western blotting for heterotrimeric G(i) proteins. These results indicate that ER-G(i2) protein modulated Sar1 translocation onto the ER, suggesting that ER-resident G(i2) protein is an important negative regulator of vesicular transport at the early stage of vesicle formation before coat protein assembly on the ER.

Arf-1 (ADP-ribosylation factor-1) is involved in the activation of a mammalian Na+-selective current

Stimulation of mammalian cells often results in an increase in the intracellular Na(+) concentration, brought about by Na(+) influx into the cell via Na(+)-permeable ion channels. In some cell types, particularly renal epithelia and mast cells, non-hydrolysable analogues of GTP, such as GTP[S] (guanosine 5'-[gamma-thio]triphosphate), activate a non-voltage-activated Na(+)-selective current. We have carried out whole-cell patch-clamp experiments to examine how GTP[S] activates the Na(+) current in a rat mast cell line. The ability of GTP[S] to activate Na(+) influx was prevented by including GTP in the pipette solution, indicating the involvement of small G-proteins. Brefeldin A and Arf-1-(2-17), inhibitors of Arf-1 (ADP-ribosylation factor-1) proteins, suppressed the activation of Na(+) entry by GTP[S]. However, non-active succinylated Arf-1-(2-17) or an N-terminal myristoylated peptide directed towards Arf-5 were ineffective. Arf proteins modulate the cytoskeleton, and disruption of the cytoskeleton with cytochalasin D or its stabilization with phalloidin impaired the development of the Na(+) current. Disaggregation of microtubules was without effect. Dialysis with cAMP or inhibition of cAMP phosphodiesterase with caffeine both decreased the extent of Na(+) entry, and this was not prevented by pre-treatment with broad-spectrum protein kinase inhibitors. Collectively, our results suggest that the mechanism of activation of a mammalian non-voltage-activated Na(+)-selective current requires an Arf small G-protein, most probably Arf-1.

In vitro transport on cis and trans sides of the Golgi involves two distinct types of coatomer and ADP-ribosylation factor-independent transport intermediates

The cisternal maturation model proposes that secretory proteins transit the Golgi in cisternae that mature by the continuous retrograde transport of Golgi enzymes in vesicles. We have tested the hypothesis that de novo generation of transport intermediates containing medial, trans, and trans Golgi network (TGN) enzymes is reconstituted in vitro. Our analysis shows that the majority of transport is mediated by a steady state of transport intermediate production and consumption by Golgi cisternae, with only a minor contribution of pre-existing transport intermediates. Transport in the medial and trans regions of the stack involved intermediates containing Golgi enzymes, apparently moving in a retrograde direction. In contrast, transport between the trans Golgi and TGN was exclusively mediated by intermediates containing secretory protein, as expected for anterograde transport. These intermediates may be physiologically relevant, because only these two specific types of intermediates can be detected in cell homogenates. By analogy to the coatomer (COPI)-independent transport of Golgi enzymes to the endoplasmic reticulum, the steady-state production of intra-Golgi transport intermediates was not impaired by inhibition of COPI vesicle formation. These data suggest a model for COPI-independent intra-Golgi transport by cisternal maturation with a shift in mechanism to anterograde transport at the trans Golgi and TGN boundary.

Regulation and recruitment of phosphatidylinositol 4-kinase on immature secretory granules is independent of ADP-ribosylation factor 1

Heterotrimeric G-proteins, as well as small GTPases of the Rho and ADP-ribosylation factor (ARF) family, are implicated in the regulation of lipid kinases, including PtdIns 4-kinases and PtdIns(4)P 5-kinases. Here, we describe a PtdIns 4-kinase activity on immature secretory granules (ISGs), regulated secretory organelles formed from the trans-Golgi network (TGN), and investigate the regulation of PtdIns4P levels on these membranes. Over 50% of the PtdIns 4-kinase activity on ISGs is inhibited by both a low concentration of adenosine and the monoclonal antibody 4C5G, a specific inhibitor of the type II PtdIns 4-kinase. Treatment of ISGs with mastoparan 7 (M7) stimulates the type II PtdIns 4-kinase via pertussis-toxin-sensitive G(i)/G(0) proteins, which, in contrast with previous results obtained with chromaffin granules [Gasman, Chasserot-Golaz, Hubert, Aunis and Bader (1998) J. Biol. Chem. 273, 16913-16920], does not require Rho A, B or C. M7 treatment also leads to an inhibition in the recruitment of ARF to ISG membranes: this inhibition is not dependent on G(i)/G(0) activation, and is not linked to the stimulation of PtdIns 4-kinase observed with M7. PtdIns 4-kinase activity on ISGs is not regulated by myristoylated ARF1-GTP, in contrast with results obtained with Golgi membranes [Godi, Pertile, Meyers, Marra, Di Tullio, Iurisci, Luini, Corda and De Matteis (1999) Nat. Cell Biol. 1, 280-287; Jones, Morris, Morgan, Kondo, Irvine and Cockcroft (2000) J. Biol. Chem. 275, 13962-13170], whereas ARF1-GTP does regulate the production of PtdIns(4,5)P(2). Our results suggest that the regulation of PtdIns 4-kinase on the ISGs differs in comparison with that on the TGN, and might be related to a specific requirement of ISG maturation.

Involvement of ADP-ribosylation factor 1 in cholera toxin-induced morphological changes of Chinese hamster ovary cells

ADP-ribosylation factor 1 (ARF1) was originally found as a cofactor in CT-catalyzed ADP-ribosylation of Galpha(s) but is now known to participate in vesicle trafficking. We asked whether ARF1 function in vesicular trafficking is necessary for CT-induced morphological changes in Chinese hamster ovary (CHO) cells, which result from increased intracellular cAMP. Brefeldin A treatment of cells suppressed CT action, confirming a requirement for Golgi integrity. Overexpression of a GFP-ARF1 fusion protein did not affect the morphological changes induced by CT, but changes were reduced in cells overexpressing guanine nucleotide exchange-defective ARF1(T31N) or GTP hydrolysis-deficient ARF1(Q71L) mutants. In cells expressing these mutants, 8-bromo-cAMP induced changes similar to those seen in cells transfected with ARF1 or vector. Inhibition of CT action was specific for mutants of ARF1 and not reproduced by analogous mutants of ARF5 or ARF6. ARF1(Q71L) was mostly colocalized with betaCOP, but ARF5(Q71L) less so. ARF6(Q67L) did not colocalize with betaCOP and was partially associated with the plasma membrane. These data are consistent with the conclusion that ARF1 influenced CT action in cells by its specific function in the vesicular transport pathway used by CT to travel from plasma membrane to Golgi to ER.

Mastoparan transiently permeabilizes Swiss 3T3 cells and induces c-fos proto-oncogene expression. Role of calcium and G protein activation

Mastoparan, a widely used tetradecapeptide activator of Gi/Go G proteins, has been reported to be a potent co-mitogen for Swiss 3T3 fibroblasts. However, we have previously shown that the peptide promotes the release of lactate dehydrogenase from Swiss 3T3 cells and evokes only a modest and delayed increase in DNA. We suggested that the ability of the peptide to permeabilise these cells may account for its mitogenic action. Here we show that mastoparan caused a rapid release of fluorescein from cells which had been pre-incubated with fluorescein diacetate, indicating that the peptide increases membrane permeability to small molecules. Furthermore, the release of lactate dehydrogenase evoked by mastoparan was lost after prolonged (24 h) incubation of cells with the peptide. Together, these data indicate that mastoparan-induced cell permeabilisation is both rapid and transient. We have also shown that mastoparan increased c-fos mRNA accumulation and that this response was not influenced by pertussis toxin or indomethacin. Although mastoparan increased the intracellular calcium concentration, the removal of extracellular calcium had no effect on mastoparan stimulated c-fos mRNA accumulation. These data show that mastoparan-induced c-fos mRNA accumulation is not mediated by activation of a G protein and subsequent activation of phospholipase D nor by a non-selective increase in calcium influx. The data have significance for the interpretation of studies in which mastoparan is, or has been, used as an activator of Gi/Go.

Coatomer vesicles are not required for inhibition of Golgi transport by G-protein activators

The G-protein activators guanosine 5'-O-(3-thiodiphosphate) (GTP gamma S) and aluminum fluoride (AIF) are thought to inhibit transport between Golgi cisternae by causing the accumulation of nonfunctional coatomer-coated transport vesicles on the Golgi. Although GTP gamma S and AIF inhibit transport in cell-free intra-Golgi transport systems, blocking coatomer vesicle formation does not. We therefore determined whether inhibition of in vitro Golgi transport by these agents requires coatomer vesicle formation. Depletion of coatomer was found to completely block coated vesicle formation on Golgi cisternae without affecting inhibition of in vitro transport by either GTP gamma S or AIF. Depletion of ADP-ribosylation factor (ARF) prevented inhibition of transport by GTP gamma S, but not by AIF, suggesting that the AIF-sensitive component in transport may not be a GTP-binding protein. Surprisingly, depletion of cytosolic ARF did not prevent the GTP gamma S-induced formation of Golgi-coated vesicles, whereas ARF was required for AIF-induced vesicle formation. Although ARF or coatomer depletion caused an increase in the fenestration of cisternae, no other ultrastructural changes were observed that might explain the inhibition of transport by GTP gamma S or AIF. These findings suggest that ARF-GTP gamma S and AIF act by distinct and coatomer-independent mechanisms to inhibit membrane fusion in cell-free intra-Golgi transport.

Sec7p directs the transitions required for yeast Golgi biogenesis

Endoplasmic reticulum (ER)-to-Golgi traffic in yeast proceeds by the maturation of membrane compartments from post-ER vesicles to intermediate small vesicle tubular clusters (VTCs) to Golgi nodular membrane networks (Morin-Ganet et al., Traffic 2000; 1: 56-68). The balance between ER and Golgi compartments is maintained by COPII- and COPI-mediated anterograde and retrograde traffic, which are dependent on Sec7p and ARF function. The sec7-4 temperature-sensitive allele is a mutation in the highly conserved Sec7 domain (Sec7d) found in all ARF-guanine nucleotide exchange factor proteins. Post-ER trafficking is rapidly inactivated in sec7-4 mutant yeast at the restrictive temperature. This conditional defect prevented the normal production of VTCs and instead generated Golgi-like tubes emanating from the ER exit sites. These tubes progressively developed into stacked cisternae defining the landmark sec7 mutant phenotype. Consistent with the in vivo results, a Sec7d peptide inhibited ER-to-Golgi transport and displaced Sec7p from its membrane anchor in vitro. The similarities in the consequences of inactivating Sec7p or ARFs in vivo was revealed by genetic disruption of yeast ARFs or by addition of brefeldin A (BFA) to whole cells. These treatments, as in sec7-4 yeast, affected the morphology of membrane compartments in the ER-Golgi transition. Further evidence for Sec7p involvement in the transition for Golgi biogenesis was revealed by in vitro binding between distinct domains of Sec7p with ARFs, COPI and COPII coat proteins. These results suggest that Sec7p coordinates membrane transitions in Golgi biogenesis by directing and scaffolding the binding and disassembly of coat protein complexes to membranes, both at the VTC transition from ER exit sites to form Golgi elements and for later events in Golgi maturation.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

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.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.

Brefeldin A inhibits cell-free, de novo synthesis of poliovirus

Brefeldin A (BFA), an inhibitor of intracellular vesicle-dependent secretory transport, is a potent inhibitor of poliovirus RNA replication in infected cells. We have determined that the unknown mechanism of BFA inhibition of replication is reproduced in the cell-free poliovirus translation, replication, and encapsidation system. Furthermore, we provide evidence suggesting that the cellular mechanism targeted by BFA, the GTP-dependent synthesis of secretory transport vesicles, may be involved in viral RNA replication in the system via a soluble cellular GTP-binding and -hydrolyzing activity. This activity is related to the ARF (ADP-ribosylation factor) family of GTP-binding proteins. ARFs are required for the formation of several classes of secretory vesicles, and some family members are indirectly inactivated by BFA. Peptides that function as competitive inhibitors of ARF activity in cell-free transport systems also inhibit poliovirus RNA replication, and this inhibitory effect can be countered by the addition of exogenous ARF. We suggest that BFA inhibition of replication is diagnostic of a requirement for ARF activity in the cell-free system.

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.

Signalling molecules and the regulation of intracellular transport

A variety of signalling molecules has been implicated over the past 8 years in the regulation of intracellular transport pathways. Those molecules include heterotrimeric GTP binding proteins, members of the protein kinase C family, and members of the Rho subfamily of small GTPases. Until recently, no common theme among the three classes of regulators was apparent. The finding that all three can influence the activity of phospholipase D (PLD), and the fact that members of the Arf subfamily of GTPases (with established roles in intracellular transport) are potent activators of PLD suggests the hypothesis that PLD is a focal point for integration of cellular responses to hormone signalling and for membrane homeostasis. Work during the past 2 years is beginning to uncover some transport pathways where PLD involvement is inferred. It is proposed that, if signalling is required to monitor and adjust transport rates to and from the various membrane organelles, the most economical way to achieve this would be to regulate recycling and allow the concentration of cargo receptors to determine forward transport.

Cell-free transport to distinct Golgi cisternae is compartment specific and ARF independent

The small GTPase ADP-ribosylation factor (ARF) is absolutely required for coatomer vesicle formation on Golgi membranes but not for anterograde transport to the medial-Golgi in a mammalian in vitro transport system. This might indicate that the in vivo mechanism of intra-Golgi transport is not faithfully reproduced in vitro, or that intra-Golgi transport occurs by a nonvesicular mechanism. As one approach to distinguishing between these possibilities, we have characterized two additional cell-free systems that reconstitute transport to the trans-Golgi (trans assay) and trans-Golgi network (TGN assay). Like in vitro transport to the medial-Golgi (medial assay), transport to the trans-Golgi and TGN requires cytosol, ATP, and N-ethylmaleimide-sensitive fusion protein (NSF). However, each assay has its own distinct characteristics of transport. The kinetics of transport to late compartments are slower, and less cytosol is needed for guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS) to inhibit transport, suggesting that each assay reconstitutes a distinct transport event. Depletion of ARF from cytosol abolishes vesicle formation and inhibition by GTPgammaS, but transport in all assays is otherwise unaffected. Purified recombinant myristoylated ARF1 restores inhibition by GTPgammaS, indicating that the GTP-sensitive component in all assays is ARF. We also show that asymmetry in donor and acceptor membrane properties in the medial assay is a unique feature of this assay that is unrelated to the production of vesicles. These findings demonstrate that characteristics specific to transport between different Golgi compartments are reconstituted in the cell-free system and that vesicle formation is not required for in vitro transport at any level of the stack.

Cell penetration by transportan

Transportan is a 27 amino acid-long peptide containing 12 functional amino acids from the amino terminus of the neuropeptide galanin and mastoparan in the carboxyl terminus, connected via a lysine. Transportan is a cell-penetrating peptide as judged by indirect immunofluorescence using N epsilon13-biotinyl-transportan. The internalization of biotinyl-transportan is energy independent and takes place efficiently at 37 degrees, 4 degrees, and 0 degrees C. Cellular uptake of transportan is probably not mediated by endocytosis, since it cannot be blocked by treating the cells with phenylarsine oxide or hyperosmolar sucrose solution and is nonsaturable. The kinetics of internalization was studied with the aid of the 125I-labeled peptide. At 37 degrees C, the maximal intracellular concentration is reached in about 20 min. The internalized transportan is protected from trypsin. The cell-penetrating ability of transportan is not restricted by cell type, but seems to be a general feature of this peptide. In Bowes' melanoma cells, transportan first localizes in the outer membrane and cytoplasmatic membrane structures. This is followed by a redistribution into the nuclear membrane and uptake into the nuclei where transportan concentrates in distinct substructures, probably the nucleoli.

Different domains of mammalian ADP-ribosylation factor 1 mediate interaction with selected target proteins

Mammalian ADP-ribosylation factor 1 (mARF1) is a small GTP-binding protein that is activated by a Golgi guanine nucleotide exchange factor. Once bound to the Golgi membranes in the GTP form, mARF1 initiates the recruitment of the adaptor protein 1 (AP-1) complex and coatomer (COPI) onto these membranes and activates phospholipase D1 (PLD1). To map the domains of mARF1 that are important for these activities, we constructed chimeras between mARF1 and Saccharomyces cerevisiae ARF2, which functions poorly in all of these processes except COPI recruitment. The carboxyl half of mARF1 (amino acids 95-181) was essential for activation by the Golgi guanine nucleotide exchange factor, whereas a separate domain (residues 35-94) was required to effectively activate PLD1 and to promote efficient AP-1 recruitment. Since residues 35-94 of mARF1 are critical for optimal activity in both PLD1 activation and AP-1 recruitment, we hypothesize that this region of ARF contains residues that interact with effector molecules.

Evidence that a low-molecular-mass GTP-binding protein is required for store-activated Ca2+ inflow in hepatocytes

The roles of a monomeric GTP-binding regulatory protein in the activation of store-activated plasma membrane Ca2+ channels and in the release of Ca2+ from the smooth endoplasmic reticulum (SER) in rat liver parenchymal cells were investigated with the use of freshly isolated rat hepatocytes and rat liver microsomes. A low concentration (approx. 130 microM intracellular) of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) activated Ca2+ inflow in intact hepatocytes in the absence of an agonist, whereas a high concentration (approx. 530 microM intracellular) of GTP-S- or guanosine 5'-[betagamma-imido]triphosphate (p[NH]ppG) inhibited the Ca2+ inflow induced by inhibitors of the activity of the endoplasmic-reticulum Ca2+-ATPase (SERCA) and by vasopressin. GTP (530 microM) prevented the inhibition of Ca2+ inflow by GTP-S- and p[NH]ppG. Brefeldin A and the peptide human Arf-1-(2-17), which inhibit many functions of ADP ribosylation factor (Arf) proteins, inhibited the Ca2+ inflow induced by SERCA inhibitors and vasopressin, and altered the profile of Ca2+ release from the SER. These effects were observed at concentrations of Brefeldin A and Arf-1-(2-17) comparable with those that inhibit the functions of Arf proteins in other systems. Succinylated Arf-1-(2-17) had a negligible effect on Ca2+ inflow. GTP[S] and Arf-1-(2-17) completely inhibited the synergistic action of GTP and Ins(1,4,5)P3 in releasing 45Ca2+ from rat liver microsomes loaded with 45Ca2+. AlF4(-) (under conditions expected to activate trimeric G-proteins) and succinylated Arf-1-(2-17) had no effect on GTP/Ins(1,4,5))3-induced 45Ca2+ release, and a mastoparan analogue caused partial inhibition. Arf-1-(2-17) did not inhibit 45Ca2+ release induced by either thapsigargin or ionomycin. It is concluded that a low-molecular-mass G-protein, most probably a member of the Arf protein family, is required for store-activated Ca2+ inflow in rat hepatocytes. The idea that the role of this G-protein is to maintain a region of the SER in the correct intracellular location is discussed briefly.

Trimeric G proteins control exocytosis in chromaffin cells. Go regulates the peripheral actin network and catecholamine secretion by a mechanism involving the small GTP-binding protein Rho

Besides having a role in signal transduction, heterotrimeric G proteins may be involved in membrane trafficking events. In chromaffin cells, Go is associated with secretory organelles and its activation by mastoparan inhibits the ATP-dependent priming of exocytosis. The effectors by which Go controls exocytosis are currently unknown. The subplasmalemmal actin network is one candidate, since it modulates secretion by controlling the movement of secretory granules to the plasma membrane. In streptolysin-O-permeabilized chromaffin cells, activation of exocytosis produces disassembly of cortical actin filaments. Mastoparan blocks the calcium-evoked disruption of cortical actin, and this effect is specifically inhibited by antibodies against Galphao and by a synthetic peptide corresponding to the COOH-terminal domain of Galphao. Disruption of actin filaments with cytochalasin E and Clostridium perfringens iota toxin partially reverses the mastoparan-induced inhibition of secretion. Furthermore, the effects of mastoparan on cortical actin and exocytosis are greatly reduced in cells treated with Clostridium botulinum C3 exoenzyme, which specifically inactivates the small G protein Rho. We propose that the control exerted by the granule-associated Go on exocytosis may be related to effects on the cortical actin network through a sequence of events which eventually involves the participation of Rho.

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.

Rab2 is essential for the maturation of pre-Golgi intermediates

The small GTPase Rab2 is a resident of pre-Golgi intermediates and required for protein transport from the endoplasmic reticulum (ER) to the Golgi complex (Tisdale, E. J., Bourne, J. R., Khosravi-Far, R. , Der, C. J., and Balch, W. E. (1992) J. Cell Biol. 119, 749-761). The Rab2 protein, like all small GTPases, contains conserved GTP-binding domains as well as hypervariable carboxyl-terminal and amino-terminal domains. While the role of the carboxyl terminus in specific membrane localization is well recognized, the potential role of the variable NH2 terminus remains to be clarified. To determine whether the NH2 terminus of Rab2 was required for its activity in vivo, a trans dominant mutant of Rab2 that inhibits ER to Golgi transport was progressively truncated and analyzed for its effect on vesicular stomatitis virus glycoprotein transport in a vaccinia-based transient expression system. Deletion of the first 14 amino-terminal residues resulted in the loss of the inhibitory properties of the mutant without affecting its post-translational processing or membrane association. To assess the potential role of the NH2 terminus in Rab2 function, a peptide corresponding to the first 13 amino acids following the initiator methionine was introduced into an in vitro assay that efficiently reconstitutes transport of vesicular stomatitis virus glycoprotein from the ER to the Golgi stack. This peptide was a potent inhibitor of transport. Biochemical and morphological studies revealed that the peptide strongly interfered with assembly of pre-Golgi intermediates which mediate segregation of anterograde and retrograde transported proteins en route to the Golgi. The combined results suggest that the NH2 terminus of Rab2 is required for its function and for direct interaction with components of the transport machinery involved in the maturation of pre-Golgi intermediates.

Potential roles of heterotrimeric G proteins of the endomembrane system

Heterotrimeric G proteins are recognized as versatile switches linking cell surface receptors to cellular effectors. Beside their location at the plasma membrane G proteins are found on intracellular membranes. Studies with modulators of G protein activity suggest that G proteins associated with organelle membranes are involved in various steps of secretion and vesicular function. In contrast to hormonal responses involving G proteins little is currently known about possible receptors or activators and effectors interacting with intracellular G proteins. This short review focuses on recent developments elucidating the role of organelle-associated G proteins.

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.

ADP-ribosylation factor-1 stimulates formation of nascent secretory vesicles from the trans-Golgi network of endocrine cells

ADP-ribosylation factor (ARF) is a small GTP-binding protein that has been implicated in intracellular vesicular transport. ARF regulates the budding of vesicles that mediate endoplasmic reticulum to Golgi and intra-Golgi transport. It also plays an important role in maintaining the function and morphology of the Golgi apparatus. Using a permeabilized cell system derived from GH3 cells, we provide evidence that ARF-1 regulates the formation of nascent secretory vesicles from the trans-Golgi network. Both myristoylated and non-myristoylated forms of recombinant human ARF-1 enhanced secretory vesicle budding about 2-fold. A mutant lacking the first 17 N-terminal residues, as well as one that preferentially binds GDP (T31N) did not stimulate vesicle formation. In contrast, a mutant defective in GTP hydrolysis (Q71L) promoted vesicle budding. Strikingly, a peptide corresponding to the N terminus of human ARF-1 (amino acids 2-17) also stimulated vesicle budding from the trans-Golgi network, in marked contrast to its inhibitory effect on vesicular transport from the endoplasmic reticulum to Golgi. These data demonstrate that in endocrine cells, ARF-1 and in particular its N terminus play an essential role in the formation of secretory vesicles.

Intra-Golgi transport inhibition by megalomicin

Megalomicin (MGM) is a macrolide antibiotic which has been demonstrated previously to cause an anomalous glycosylation of viral proteins. Here we show that MGM produces profound alterations on Golgi morphology and function. The addition of MGM at 50 microM for 1 h caused a dilation of the Golgi detected by immunofluorescence staining for medial- and trans-Golgi markers. The effect of MGM was clearly more intense on the trans-side of the Golgi, as evidenced in electron microscope preparations. The effect on Golgi morphology was reversible and correlated with an impairment of glycoprotein processing in the trans-Golgi. Thus, although the vesicular stomatitis virus G protein was processed in the presence of MGM to an endoglycosidase H-resistant form, it was poorly sialylated. The sialylation of cellular proteins was also inhibited, resulting in cells with low level of sialylation on the cell surface. However MGM did not inhibit the activities of the galactosyl- or sialyltransferase as measured in vitro. MGM inhibited cis- to medial-, and more strongly, medial- to trans-Golgi transport of vesicular stomatitis virus G protein in an in vitro system, suggesting that the impairment in glycoprotein maturation observed in vivo is the result of intra-Golgi transport inhibition.

Role of heterotrimeric GTP binding proteins in vesicular protein transport: indications for both classical and alternative G protein cycles

Heterotrimeric G proteins are involved in hormonal signal transduction across the plasma membrane. Recent evidence suggests that they have a role in vesicular protein transport as well. Biochemical probes that interfere with the classical G protein cycle have been applied to the field of intracellular membrane transport to study their mechanism of action. Evidence has been obtained that intracellular G proteins act both through classical and alternative G protein cycles.

The role of coat proteins in the biosynthesis of secretory proteins

The biosynthesis of secretory proteins requires vesicle-mediated transport between the organelles of the secretory pathway. Biochemical and genetic analysis of the secretory pathway has identified two non-clathrin coats--COPI and COPII--that drive the formation of vesicles that mediate transport between the endoplasmic reticulum and the Golgi apparatus, and through the compartments of the Golgi. Recently, a molecular description of the subunits of these coats and the development of biochemical reagents to study their function has yielded new information on how these proteins share the task of organizing vesicle traffic early in the secretory pathway.

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.