The GGAs promote ARF-dependent recruitment of clathrin to the TGN.

The GGAs constitute a family of modular adaptor-related proteins that bind ADP-ribosylation factors (ARFs) and localize to the trans-Golgi network (TGN) via their GAT domains. Here, we show that binding of the GAT domain stabilizes membrane-bound ARF1.GTP due to interference with the action of GTPase-activating proteins. We also show that the hinge and ear domains of the GGAs interact with clathrin in vitro, and that the GGAs promote recruitment of clathrin to liposomes in vitro and to TGN membranes in vivo. These observations suggest that the GGAs could function to link clathrin to membrane-bound ARF.GTP.

ACAPs are arf6 GTPase-activating proteins that function in the cell periphery.

The GTP-binding protein ADP-ribosylation factor 6 (Arf6) regulates endosomal membrane trafficking and the actin cytoskeleton in the cell periphery. GTPase-activating proteins (GAPs) are critical regulators of Arf function, controlling the return of Arf to the inactive GDP-bound state. Here, we report the identification and characterization of two Arf6 GAPs, ACAP1 and ACAP2. Together with two previously described Arf GAPs, ASAP1 and PAP, they can be grouped into a protein family defined by several common structural motifs including coiled coil, pleckstrin homology, Arf GAP, and three complete ankyrin-repeat domains. All contain phosphoinositide-dependent GAP activity. ACAP1 and ACAP2 are widely expressed and occur together in the various cultured cell lines we examined. Similar to ASAP1, ACAP1 and ACAP2 were recruited to and, when overexpressed, inhibited the formation of platelet-derived growth factor (PDGF)-induced dorsal membrane ruffles in NIH 3T3 fibroblasts. However, in contrast with ASAP1, ACAP1 and ACAP2 functioned as Arf6 GAPs. In vitro, ACAP1 and ACAP2 preferred Arf6 as a substrate, rather than Arf1 and Arf5, more so than did ASAP1. In HeLa cells, overexpression of either ACAP blocked the formation of Arf6-dependent protrusions. In addition, ACAP1 and ACAP2 were recruited to peripheral, tubular membranes, where activation of Arf6 occurs to allow membrane recycling back to the plasma membrane. ASAP1 did not inhibit Arf6-dependent protrusions and was not recruited by Arf6 to tubular membranes. The additional effects of ASAP1 on PDGF-induced ruffling in fibroblasts suggest that multiple Arf GAPs function coordinately in the cell periphery.

Phosphoinositide-dependent activation of the ADP-ribosylation factor GTPase-activating protein ASAP1. Evidence for the pleckstrin homology domain functioning as an allosteric site.

The ADP-ribosylation factor (Arf) family of GTP-binding proteins are regulators of membrane traffic and the actin cytoskeleton. Both negative and positive regulators of Arf, the centaurin beta family of Arf GTPase-activating proteins (GAPs) and Arf guanine nucleotide exchange factors, contain pleckstrin homology (PH) domains and are activated by phosphoinositides. To understand how the activities are coordinated, we have examined the role of phosphoinositide binding for Arf GAP function using ASAP1/centaurin beta4 as a model. In contrast to Arf exchange factors, phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P(2)) specifically activated Arf GAP. D3 phosphorylated phosphoinositides were less effective. Activation involved PtdIns-4,5-P(2) binding to the PH domain; however, in contrast to the Arf exchange factors and contrary to predictions based on the current paradigm for PH domains as independently functioning recruitment signals, we found the following: (i) the PH domain was dispensable for targeting to PDGF-induced ruffles; (ii) activation and recruitment could be uncoupled; (iii) the PH domain was necessary for activity even in the absence of phospholipids; and (iv) the Arf GAP domain influenced localization and lipid binding of the PH domain. Furthermore, PtdIns-4,5-P(2) binding to the PH domain caused a conformational change in the Arf GAP domain detected by limited proteolysis. Thus, these data demonstrate that PH domains can function as allosteric sites. In addition, differences from the published properties of the Arf exchange factors suggest a model in which feedforward and feedback loops involving lipid metabolites coordinate GTP binding and hydrolysis by Arf.

The Arf GTPase-activating protein ASAP1 regulates the actin cytoskeleton.

Arf family GTP-binding proteins are best characterized as regulators of membrane traffic, but recent studies indicate an additional role in cytoskeletal organization. An Arf GTPase-activating protein of the centaurin beta family, ASAP1 (also known as centaurin beta4), binds Arf and two other known regulators of the actin cytoskeleton, the tyrosine kinase Src and phosphatidylinositol 4,5-bisphosphate. In this paper, we show that ASAP1 localizes to focal adhesions and cycles with focal adhesion proteins when cells are stimulated to move. Overexpression of ASAP1 altered the morphology of focal adhesions and blocked both cell spreading and formation of dorsal ruffles induced by platelet-derived growth factor (PDGF). On the other hand, ASAP1, with a mutation that disrupted GTPase-activating protein activity, had a reduced effect on cell spreading and increased the number of cells forming dorsal ruffles in response to PDGF. These data support a role for an Arf GTPase-activating protein, ASAP1, as a regulator of cytoskeletal remodeling and raise the possibility that the Arf pathway is a target for PDGF signaling.

Molecular aspects of the cellular activities of ADP-ribosylation factors.

Adenosine diphosphate-ribosylation factor (Arf) proteins are members of the Arf arm of the Ras superfamily of guanosine triphosphate (GTP)-binding proteins. Arfs are named for their activity as cofactors for cholera toxin-catalyzed adenosine diphosphate-ribosylation of the heterotrimeric G protein Gs. Physiologically, Arfs regulate membrane traffic and the actin cytoskeleton. Arfs function both constitutively within the secretory pathway and as targets of signal transduction in the cell periphery. In each case, the controlled binding and hydrolysis of GTP is critical to Arf function. The activities of some guanine nucleotide exchange factors (GEFs) and guanosine triphosphatase (GTPase)-activating proteins (GAPs) are stimulated by phosphoinositides, including phosphatidylinositol 3,4,5-trisphosphate (PIP3) and phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA), likely providing both a means to respond to regulatory signals and a mechanism to coordinate GTP binding and hydrolysis. Arfs affect membrane traffic in part by recruiting coat proteins, including COPI and clathrin adaptor complexes, to membranes. However, Arf function likely involves many additional biochemical activities. Arf activates phospholipase D and phosphatidylinositol 4-phosphate 5-kinase with the consequent production of PA and PIP2, respectively. In addition to mediating Arf's effects on membrane traffic and the actin cytoskeleton, PA and PIP2 are involved in the regulation of Arf. Arf also works with Rho family proteins to affect the actin cytoskeleton. Several Arf-binding proteins suspected to be effectors have been identified in two-hybrid screens. Arf-dependent biochemical activities, actin cytoskeleton changes, and membrane trafficking may be integrally related. Understanding Arf's role in complex cellular functions such as protein secretion or cell movement will involve a description of the temporal and spatial coordination of these multiple Arf-dependent events.

Identification of a new Pyk2 target protein with Arf-GAP activity.

Protein tyrosine kinase Pyk2 is activated by a variety of G-protein-coupled receptors and by extracellular signals that elevate intracellular Ca2+ concentration. We have identified a new Pyk2 binding protein designated Pap. Pap is a multidomain protein composed of an N-terminal alpha-helical region with a coiled-coil motif, followed by a pleckstrin homology domain, an Arf-GAP domain, an ankyrin homology region, a proline-rich region, and a C-terminal SH3 domain. We demonstrate that Pap forms a stable complex with Pyk2 and that activation of Pyk2 leads to tyrosine phosphorylation of Pap in living cells. Immunofluorescence experiments demonstrate that Pap is localized in the Golgi apparatus and at the plasma membrane, where it is colocalized with Pyk2. In addition, in vitro recombinant Pap exhibits strong GTPase-activating protein (GAP) activity towards the small GTPases Arf1 and Arf5 and weak activity towards Arf6. Addition of recombinant Pap protein to Golgi preparations prevented Arf-dependent generation of post-Golgi vesicles in vitro. Moreover, overexpression of Pap in cultured cells reduced the constitutive secretion of a marker protein. We propose that Pap functions as a GAP for Arf and that Pyk2 may be involved in regulation of vesicular transport through its interaction with Pap.

ASAP1, a phospholipid-dependent arf GTPase-activating protein that associates with and is phosphorylated by Src.

Membrane trafficking is regulated in part by small GTP-binding proteins of the ADP-ribosylation factor (Arf) family. Arf function depends on the controlled exchange and hydrolysis of GTP. We have purified and cloned two variants of a 130-kDa phosphatidylinositol 4, 5-biphosphate (PIP2)-dependent Arf1 GTPase-activating protein (GAP), which we call ASAP1a and ASAP1b. Both contain a pleckstrin homology (PH) domain, a zinc finger similar to that found in another Arf GAP, three ankyrin (ANK) repeats, a proline-rich region with alternative splicing and SH3 binding motifs, eight repeats of the sequence E/DLPPKP, and an SH3 domain. Together, the PH, zinc finger, and ANK repeat regions possess PIP2-dependent GAP activity on Arf1 and Arf5, less activity on Arf6, and no detectable activity on Arl2 in vitro. The cDNA for ASAP1 was independently identified in a screen for proteins that interact with the SH3 domain of the tyrosine kinase Src. ASAP1 associates in vitro with the SH3 domains of Src family members and with the Crk adapter protein. ASAP1 coprecipitates with Src from cell lysates and is phosphorylated on tyrosine residues in cells expressing activated Src. Both coimmunoprecipitation and tyrosine phosphorylation depend on the same proline-rich class II Src SH3 binding site required for in vitro association. By directly interacting with both Arfs and tyrosine kinases involved in regulating cell growth and cytoskeletal organization, ASAP1 could coordinate membrane remodeling events with these processes.

Resolution of two ADP-ribosylation factor 1 GTPase-activating proteins from rat liver.

ADP-ribosylation factor 1 (ARF1) is a 21 kDa GTP-binding protein that regulates multiple steps in membrane traffic. Here, two ARF1 GTPase-activating proteins (GAPs) from rat liver were resolved. The GAPs were antigenically distinct. One reacted with a polyclonal antibody raised against the GAP catalytic peptide previously purified by Makler et al. [Makler, Cukierman, Rotman, Admon and Cassel (1995) J. Biol. Chem. 270, 5232-5237], and here is referred to as GAP1. The other GAP (GAP2) did not react with the antibody. These GAPs differed in phospholipid dependencies. GAP1 was activated 3-7-fold by the acid phospholipids phosphatidylinositol 4, 5-bisphosphate (PIP2), phosphatidic acid (PA) and phosphatidylserine (PS). In contrast, GAP2 was stimulated 20-40-fold by PIP2. PA and PS had no effect by themselves but PA increased GAP2 activity in the presence of PIP2. The GAPs were otherwise similar in activity. In the presence of phosphoinositides, the Km of GAP1 for ARF1-GTP was estimated to be 8.1+/-1.6 microM and the dissociation constant for ARF1-guanosine 5',3-O-(thio)triphosphate (GTP[S]) was 7.4+/-2.2 microM. GAP2 was similar with a Km for ARF1-GTP of 5.4+/-1.2 microM and a dissociation constant for ARF1-GTP[S] of 4.8+/-0.3 microM. Similarly, no differences were found in substrate preferences. Both GAP1 and GAP2 used ARF1 and ARF5 as substrates but not ARF6 or ARF-like protein-2. The potential role of multiple ARF GAPs in the independent regulation of ARF at specific steps in membrane traffic is discussed.

Functional interaction of ADP-ribosylation factor 1 with phosphatidylinositol 4,5-bisphosphate.

The relationship between ADP-ribosylation factor (Arf) 1 and phosphoinositides, which have been independently implicated as regulators of membrane traffic, was examined. Because both Arf-dependent phospholipase D and Arf1 GTPase-activating protein (GAP) require phosphatidylinositol 4,5-bisphosphate (PIP2), Arf1 complexed with PIP2 has been proposed to interact with target proteins. This hypothesis was tested using Arf1 GAP as a model system. Arf1 was shown to bind to PIP2 in Triton X-100 micelles with a Kd of 45 +/- 13 microM. Arf1 also bound phosphatidic acid but with 10-fold lower affinity. PIP2 binding was specifically disrupted by mutating lysines 15, 16, and 181 and arginine 178 to leucines. Decreased PIP2 binding resulted in an increased EC50 of PIP2 for activation of Arf GAP. None of the mutations that decreased PIP2 binding affected binding to or activation of GAP by phosphatidic acid, which acts at a functionally distinct site. These data support the hypothesis that PIP2 binding to Arf1 promotes interaction with Arf GAP. The implications of lipid-directed protein-protein interactions for membrane traffic are discussed.

Effects of acid phospholipids on ARF activities: potential roles in membrane traffic.

ADP-ribosylation factors are a family of approximately 21 kDa GTP binding proteins which have been implicated as ubiquitous regulators of multiple steps in both exocytic and endocytic membrane traffic in mammals and yeast. Reversible membrane associations are thought to be an essential component in the physiological actions of ARF and are regulated by GTP binding. ARFs are unique among the superfamily of GTP binding proteins in having a strict dependence on phospholipids for nucleotide exchange. In addition, ARF proteins were found to bind phospatidylinositol 4,5-bisphosphate (PIP2) specifically. PIP2 was found to increase the rate of GDP dissociation and stabilize the nucleotide-free form of the protein. The previously described requirements for PIP2 in the ARF stimulated phospholipase D (PLD) activity and ARF GTPase activating protein (ARF GAP) assays provide the basis for a model in which PIP2 acts as a cofactor in one or more ARF pathways. There are potentially two distinct phospholipid binding sites each of which are coupled to the nucleotide binding site of ARFs.

Antiproliferative effect in vitro and antitumor activity in vivo of brefeldin A.

PURPOSE: An empiric in vitro screen of human tumor cell lines found brefeldin A inhibited the growth of immortalized human cell lines, with particular sensitivity to brefeldin in a series of immortalized melanoma cell lines and nonimmortalized prostate carcinoma explants. Brefeldin A alters the morphology and function of the Golgi apparatus, endosomal, and trans-Golgi compartments in different cell types. The studies presented here sought to obtain evidence of in vivo antitumor activity by brefeldin A. METHODS: Antiproliferative activity was studied in prostate carcinoma cells in vitro using cell counts, protein, and viable stains. Activity was also studied in vivo against subcutaneous and subrenal capsule melanoma models. RESULTS: Protracted exposures in vitro (between 24 and 72 hours) are necessary to cause persistent growth inhibition of immortalized PC3 prostate carcinoma cells. In human melanoma athymic mouse xenografts, brefeldin A showed antitumor activity in vivo when given 16 to 64 mg/kg/injection intraperitoneally q 7 h x 2, daily for 5 days. Activity was also observed in the intraperitoneal LOX IMVI (65%-100% increase in life span, with 17%-50% day 60 survivors); early-stage subcutaneous LOX IMVI and SK-MEL-5 (86%-100% growth inhibition), and subrenal capsule SK-MEL-5 and M19-MEL models. CONCLUSIONS: Brefeldin A possesses noteworthy antitumor activity in vivo and antiproliferative effects in vitro in certain cell types. Strategies to allow protracted exposure of tumor cells to brefeldin A while preserving a therapeutic index are needed to assess the clinical potential of brefeldin A.

The myristoylated amino terminus of ADP-ribosylation factor 1 is a phospholipid- and GTP-sensitive switch.

ADP-ribosylation factor 1 (Arf1) is an essential N-myristoylated 21-kDa GTP-binding protein with activities that include the regulation of membrane traffic and phospholipase D activity. Both the N terminus of the protein and the N-myristate bound to glycine 2 have previously been shown to be essential to the function of Arf in cells. We show that the bound nucleotide affects the conformation of either the N terminus or residues of Arf1 that are in direct contact with the N terminus. This was demonstrated by examining the effects of mutations in this N-terminal domain on guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) and GDP binding and dissociation kinetics. Arf1 mutants, lacking 13 or 17 residues from the N terminus or mutated at residues 3-7, had a greater affinity for GTP gamma S and a lower affinity for GDP than did the wild-type protein. As the N terminus is required for interactions with target proteins, we conclude that the N terminus of Arf1 is a GTP-sensitive effector domain. When Arf1 was acylated, the GTP-dependent conformational changes were codependent on added phospholipids. In the absence of phospholipids, myristoylated Arf1 has a lower affinity for GTP gamma S than for GDP, and in the presence of phospholipids, the myristoylated protein has a greater affinity for GTP gamma S than for GDP. Thus, N-myristoylation is a critical component in the construction of this phospholipid- and GTP-dependent switch.

Mutational analysis of Saccharomyces cerevisiae ARF1.

Wild type and eight point mutants of Saccharomyces cerevisiae ARF1 were expressed in yeast and bacteria to determine the roles of specific residues in in vivo and in vitro activities. Mutations at either Gly2 or Asp26 resulted in recessive loss of function. It was concluded that N-myristoylation is required for Arf action in cells but not for either nucleotide exchange or cofactor activities in vitro. Asp26 (homologous to Gly12 of p21ras) was essential for the binding of the activating nucleotide, guanosine 5'-3-O-(thio)triphosphate. This is in marked contrast to results obtained after mutagenesis of the homologous residue in p21ras or Gs alpha, and suggests a fundamental difference in the guanine nucleotide binding site of Arf with respect to these other GTP-binding proteins. Two dominant alleles were also identified, one activating dominant ([Q71L]Arf1) and the other ([N126I]) a negative dominant. A conditional allele, [W66R]Arf1, was characterized and shown to have approximately 300-fold lower specific activity in an in vitro Arf assay. Two high-copy suppressors of this conditional phenotype were cloned and sequenced. One of these suppressors, SFS4, was found to be identical to PBS2/HOG4, recently shown to encode a microtubule-associated protein kinase kinase in yeast.

Effects of acid phospholipids on nucleotide exchange properties of ADP-ribosylation factor 1. Evidence for specific interaction with phosphatidylinositol 4,5-bisphosphate.

ADP-ribosylation factors (ARFs) have been implicated as ubiquitous regulators of multiple steps in both exocytic and endocytic membrane traffic in yeast and mammalian cells. More specific interactions have also been described for ARF proteins with an ARF-specific GTP-ase-activating protein and as activators of phospholipase D activity. These protein interactions have defined requirements for phosphatidylinositol 4,5-bisphosphate (PIP2). Direct interactions between ARF1 and PIP2 or other phospholipids were tested by examining effects on guanine nucleotide binding kinetics. PIP2 (400 microM) increased the rate of GDP dissociation > 100-fold. Several other acid phospholipids had more modest effects (4-7-fold) on GDP dissociation rates, while other phospholipids had no effect. PIP2 also had the greatest effect on the rate of binding of guanosine 5'-(gamma-thio)triphosphate (GTP gamma S), increasing it almost 100-fold at early time points. However, at later times (> 5 min), PIP2 caused a paradoxical loss of nucleotide binding to ARF1. PIP2 was found to stabilize the nucleotide-free form of ARF1 as subsequent dilution of PIP2 allowed ARF1 to bind GTP gamma S to high stoichiometry. The demonstration of direct interaction between ARF1 and PIP2 provides the basis for a model in which PIP2 acts as a cofactor in some of the interactions between ARF1 and other proteins.

The amino terminus of ADP-ribosylation factor (ARF) 1 is essential for interaction with Gs and ARF GTPase-activating protein.

The role of the amino terminus in the actions of ADP-ribosylation factor 1 (ARF1) was examined by comparing wild type ARF1, a 13-residue NH2-terminal deletion mutant ([delta 13]ARF1), and a 17-residue NH2-terminal deletion mutant ([delta 17]ARF1). The amino-terminal 13 residues of ARF1 are required for cofactor activity in the ADP-ribosylation by cholera toxin when Gs is the substrate. This is in marked contrast to the finding that cofactor activity is the same for wild type and [delta 13]ARF1 when agmatine is substrate (Hong, J.-X., Haun, R. S., Tsai, S.-C., Moss, J., and Vaughan, M. (1994) J. Biol. Chem. 269, 9743-9745). These data support the conclusion that ARF1 interacts with both cholera toxin and Gs and that the amino terminus of ARF1 is required specifically for binding Gs. Surprisingly, this result also clearly revealed that the two principal assays for ARF activity, cofactor activity for cholera toxin using either Gs or agmatine as substrates, used for over 10 years in different laboratories, can yield quite different results. While both NH2-terminal deletion mutants failed to support the ADP-ribosylation of Gs by cholera toxin, [delta 13]ARF1, but not [delta 17]ARF1, inhibited the activity of the wild type protein. The GTPase activity of [delta 13]ARF1 was activated to a small extent by ARF GTPase-activating protein (GAP), whereas that of [delta 17]ARF1 was unaffected. We conclude that residues 14-17 are involved in the interaction of ARF with both cholera toxin and ARF GAP. The co-purifying nucleotides, nucleotide exchange kinetics, and dependence of exchange on phospholipids for the mutant proteins were all different from the wild type ARF1 proteins. The importance of monitoring the nucleotide binding to ARF proteins under the conditions used in the ARF assay and expressing ARF activities as specific activities, normalized to GTP binding sites, particularly when comparisons between different proteins or preparations are made, is discussed.

ADP-ribosylation factor (ARF)-like 3, a new member of the ARF family of GTP-binding proteins cloned from human and rat tissues.

The human and rat homologues of a new member of the ADP-ribosylation factor (ARF) family of 21-kDa GTP-binding proteins, termed Arl3, were identified as an expressed sequence tag (human) and as a product of polymerase chain reaction amplification using degenerate probes derived from conserved sequences in members of the ARF family (rat). Alignments of the full-length open reading frames of the human and rat homologues revealed the encoded proteins to be over 97% identical to each other and 43% identical to human ARF1. Northern blots of mRNA from seven human tissues and four rat tissues revealed the presence of a ubiquitous band of about 1 kilobase in length that hybridized with the corresponding Arl3 probes. A number of human tumor cell lines expressed Arl3, as determined by immunoblotting with an Arl-specific antibody, raised against a peptide derived from the human Arl3 sequence. The level of Arl3 expressed in these cell lines was on the order of 0.01% of total cell protein. Purified recombinant human Arl3 was shown to bind guanine nucleotides but lacks ARF activity and intrinsic or ARF GTPase-activating protein-stimulated GTPase activity. In contrast to ARF proteins, the Arl3 protein has reduced dependence on phospholipids and magnesium for guanine nucleotide exchange. Thus, Arl3 is a ubiquitously expressed GTP-binding protein in the ARF family with distinctive biochemical properties consistent with its having unique, but unknown, role(s) in cell physiology.