Phospholipid- and GTP-dependent activation of cholera toxin and phospholipase D by human ADP-ribosylation factor-like protein 1 (HARL1)

Cellular effects of fluorodeoxyglucose: Global changes in the lipidome and alteration in intracellular transport

2-fluoro-2-deoxy-D-glucose (FDG), labeled with ¹⁸F radioisotope, is the most common imaging agent used for positron emission tomography (PET) in oncology. However, little is known about the cellular effects of FDG. Another glucose analogue, 2-deoxy-D-glucose (2DG), has been shown to affect many cellular functions, including intracellular transport and lipid metabolism, and has been found to improve the efficacy of cancer chemotherapeutic agents in vivo. Thus, in the present study, we have investigated cellular effects of FDG with the focus on changes in cellular lipids and intracellular transport. By quantifying more than 200 lipids from 17 different lipid classes in HEp-2 cells and by analyzing glycosphingolipids from MCF-7, HT-29 and HBMEC cells, we have discovered that FDG treatment inhibits glucosylceramide synthesis and thus reduces cellular levels of glycosphingolipids. In addition, in HEp-2 cells the levels and/or species composition of other lipid classes, namely diacylglycerols, phosphatidic acids and phosphatidylinositols, were found to change upon treatment with FDG. Furthermore, we show here that FDG inhibits retrograde Shiga toxin transport and is much more efficient in protecting cells against the toxin than 2DG. In summary, our data reveal novel effects of FDG on cellular transport and glycosphingolipid metabolism, which suggest a potential clinical application of FDG as an adjuvant for cancer chemotherapy.

The Function of Arf-like Proteins ARL2 and ARL3 in Photoreceptors

Arf-like proteins (ARLs) are ubiquitously expressed small G proteins of the RAS superfamily. In photoreceptors, ARL2 and ARL3 participate in the trafficking of lipidated membrane-associated proteins and colocalize in the inner segment with UNC119A and PDEδ. UNC119A and PDEδ are acyl- and prenyl-binding proteins, respectively, involved in trafficking of acylated (transducin-α subunit, nephrocystin NPHP3) and prenylated proteins (GRK1, PDE6). Germline Arl3 knockout mice do not survive beyond postnatal day 21 and display ciliary defects in multiple organs (kidney, liver and pancreas) as well as retinal degeneration. Conditional knockouts will be necessary to delineate mechanisms of protein transport in retina disease.

Heat-Labile Enterotoxins

Heat-labile enterotoxins (LTs) of Escherichia coli are closely related to cholera toxin (CT), which was originally discovered in 1959 in culture filtrates of the gram-negative bacterium Vibrio cholerae. Several other gram-negative bacteria also produce enterotoxins related to CT and LTs, and together these toxins form the V. cholerae-E. coli family of LTs. Strains of E. coli causing a cholera-like disease were designated enterotoxigenic E. coli (ETEC) strains. The majority of LTI genes (elt) are located on large, self-transmissible or mobilizable plasmids, although there are instances of LTI genes being located on chromosomes or carried by a lysogenic phage. The stoichiometry of A and B subunits in holotoxin requires the production of five B monomers for every A subunit. One proposed mechanism is a more efficient ribosome binding site for the B gene than for the A gene, increasing the rate of initiation of translation of the B gene independently from A gene translation. The three-dimensional crystal structures of representative members of the LT family (CT, LTpI, and LTIIb) have all been determined by X-ray crystallography and found to be highly similar. Site-directed mutagenesis has identified many residues in the CT and LT A subunits, including His44, Val53, Ser63, Val97, Glu110, and Glu112, that are critical for the structures and enzymatic activities of these enterotoxins. For the enzymatically active A1 fragment to reach its substrate, receptor-bound holotoxin must gain access to the cytosol of target cells.

Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery

The Arf small GTP-binding (G) proteins regulate membrane traffic and organelle structure in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. The first function identified for Arf proteins was recruitment of cytosolic coat complexes to membranes to mediate vesicle formation. However, subsequent studies have uncovered additional functions, including roles in plasma membrane signalling pathways, cytoskeleton regulation, lipid droplet function, and non-vesicular lipid transport. In contrast to other families of G proteins, there are only a few Arf proteins in each organism, yet they function specifically at many different cellular locations. Part of this specificity is achieved by formation of complexes with their guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP binding and hydrolysis, respectively. Because these regulators outnumber their Arf substrates by at least 3-to-1, an important aspect of understanding Arf function is elucidating the mechanisms by which a single Arf protein is incorporated into different GEF, GAP, and effector complexes. New insights into these mechanisms have come from recent studies showing GEF–effector interactions, Arf activation cascades, and positive feedback loops. A unifying theme in the function of Arf proteins, carried out in conjunction with their regulators and effectors, is sensing and modulating the properties of the lipids that make up cellular membranes.

Cell density-induced changes in lipid composition and intracellular trafficking

Cell density is one of the extrinsic factors to which cells adapt their physiology when grown in culture. However, little is known about the molecular changes which occur during cell growth and how cellular responses are then modulated. In many cases, inhibitors, drugs or growth factors used for in vitro studies change the rate of cell proliferation, resulting in different cell densities in control and treated samples. Therefore, for a comprehensive data analysis, it is essential to understand the implications of cell density on the molecular level. In this study, we have investigated how lipid composition changes during cell growth, and the consequences it has for transport of Shiga toxin. By quantifying 308 individual lipid species from 17 different lipid classes, we have found that the levels and species distribution of several lipids change during cell growth, with the major changes observed for diacylglycerols, phosphatidic acids, cholesterol esters, and lysophosphatidylethanolamines. In addition, there is a reduced binding and retrograde transport of Shiga toxin in high density cells which lead to reduced intoxication by the toxin. In conclusion, our data provide novel information on how lipid composition changes during cell growth in culture, and how these changes can modulate intracellular trafficking.

Mon2 is a negative regulator of the monomeric G protein, Arl1

Using site-directed mutants of ARL1 predicted to alter nucleotide binding, we examined phenotypes associated with the loss of ARL1 , including effects on membrane traffic and K (+) homeostasis. The GTP-restricted allele, ARL[Q72L] , complemented the membrane traffic phenotype (CPY secretion), but not the K (+) homeostasis phenotypes (sensitivity to hygromycin B, steady-state levels of K (+) , and accumulation of (86) Rb (+) ), while the XTP-restricted mutant, ARL1[D130N] , complemented the ion phenotypes, but not the membrane traffic phenotype. A GDP-restricted allele, ARL1[T32N] , did not effectively complement either phenotype. These results are consistent with a model in which Arl1 has three different conformations in vivo. We also explored the relationship between ARL1 and MON2 using the synthetic lethal phenotype exhibited by these two genes and demonstrated that MON2 is a negative regulator of the GTP-restricted allele of ARL1 , ARL1[Q72L] . Finally, we constructed several new alleles predicted to alter binding of Arl1 to the sole GRIP domain containing protein in yeast, Imh1, and found that ARL1[F52G] and ARL1[Y82G] were unable to complement the loss of ARL1 with respect to either the membrane traffic or K (+) homeostasis phenotypes. Our study expands understanding of the roles of Arl1 in vivo.

ARF family G proteins and their regulators: roles in membrane transport, development and disease

The ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF proteins, ARF-like (ARL) proteins and SAR1, regulates membrane traffic and organelle structure, and each family member is regulated through a cycle of GTP binding and GTP hydrolysis, which activate and inactivate, respectively, the G protein.

Traditionally, ARFs have been characterized for their immediate effects in the recruitment of coat proteins to drive cargo sorting, the recruitment of enzymes that can alter membrane lipid composition and the regulation of cytoskeletal factors. Now, new roles for ARFs have been discovered at the Golgi complex, for example in driving lipid transport. ARL proteins are also being increasingly linked to coordination of trafficking with cytoskeletal processes, for example during ciliogenesis.

There is particular interest in the mechanisms that control recruitment of the ARF guanine nucleotide exchange factors (GEFs) that mediate GTP binding to ARFs and, in the case of the cytohesin (also known as ARNO) GEF, membrane recruitment is coupled to relief of autoinhibition. GEFs such as cytohesin may also participate in a cascade of activation between particular pairs of ARFs.

Traditionally, G protein signalling has been viewed as a linear pathway, with the GDP-bound form of an ARF protein being inactive; however, more recent studies have highlighted novel roles for these GDP-bound forms and have also shown that GEFs and GTPase-activating proteins (GAPs) themselves can engage in distinct signalling responses through scaffolding functions.

The ADP-ribosylation factor (ARF) and ARF-like (ARL) family of G proteins, which are known to regulate membrane traffic and organelle structure, are emerging as regulators of diverse processes, including lipid and cytoskeletal transport. Although traditionally viewed as part of a linear signalling pathway, ARFs and their regulators must now be considered to exist within functional networks, in which both the 'inactive' ARF and the regulators themselves can mediate distinct effects.

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF-like (ARL) proteins and SAR1, regulate membrane traffic and organelle structure by recruiting cargo-sorting coat proteins, modulating membrane lipid composition, and interacting with regulators of other G proteins. New roles of ARF and ARL proteins are emerging, including novel functions at the Golgi complex and in cilia formation. Their function is under tight spatial control, which is mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that catalyse GTP exchange and hydrolysis, respectively. Important advances are being gained in our understanding of the functional networks that are formed not only by the GEFs and GAPs themselves but also by the inactive forms of the ARF proteins.

Crystal structure of the ARL2-GTP-BART complex reveals a novel recognition and binding mode of small GTPase with effector

ARL2 is a member of the ADP-ribosylation factor family but has unique biochemical features. BART is an effector of ARL2 that is essential for nuclear retention of STAT3 and may also be involved in mitochondria transport and apoptosis. Here we report the crystal structure and biochemical characterization of human ARL2-GTP-BART complex. ARL2-GTP assumes a typical small GTPase fold with a unique N-terminal alpha helix conformation. BART consists of a six alpha helix bundle. The interactions between ARL2 and BART involve two interfaces: a conserved N-terminal LLXIL motif of ARL2 is embedded in a hydrophobic cleft of BART and the switch regions of ARL2 interact with helix alpha3 of BART. Both interfaces are essential for the binding as verified by mutagenesis study. This novel recognition and binding mode is different from that of other small GTPase-effector interactions and provides molecular basis for the high specificity of ARL2 for BART.

The leishmania ARL-1 and Golgi traffic

We present here the characterisation of the Leishmania small G protein ADP-Ribosylation Factor-Like protein 1 (ARL-1). The ARL-1 gene is present in one copy per haploid genome and conserved among trypanosomatids. It encodes a protein of 20 kDa, which is equally expressed in the insect promastigote and mammalian amastigote forms of the parasite. ARL-1 localises to the Trans-Golgi Network (TGN); N-terminal myristoylation is essential for TGN localisation. In vivo expression of the LdARL-1/Q74L and LdARL-1/T51N mutants (GTP- and GDP-bound blocked forms respectively) shows that GDP/GTP cycling occurs entirely within the TGN. This is contrary to previous reports in yeast and mammals, where the mutant empty form devoid of nucleotide has been considered as the GDP-blocked form. The dominant-negative empty form mutant LdARL-1/T34N inhibits endocytosis and intracellular trafficking from the TGN to the Lysosome/Multivesicular Tubule and to the acidocalcisomes; these defects are probably related to a mislocalisation of the GRIP domain-containing vesicle tethering factors which cannot be recruited to the TGN by the cytoplasmic LdARL-1/T34N. Thus, besides the functional characterization of a new mutant and a better understanding of ARL-1 GDP/GTP cycling, this work shows that Leishmania ARL-1 is a key component of an essential pathway worth future study.

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

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

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

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

Isolation of a new member of the ADP-ribosylation like factor gene family, ARL8, from a cartilage cDNA library

ADP-ribosylation factors (ARFs) and ARF-like proteins (ARLs) are part of the ARF family within the RAS superfamily of regulatory GTPases. Guanine nucleotide binding proteins or GTPases are involved in a diverse spectrum of cellular activities, including regulating cell growth and signal transduction, organization of the cytoskeleton and regulating membrane trafficking along the exocytic and endocytic pathways. ARL proteins share 40-60% sequence identity with the ARF proteins, but ARLs can be distinguished from ARFs based on expression patterns and biological functions. We have identified a new ARL, ARL8, from a fetal cartilage cDNA library. ARL8 contains six exons and five introns, and encodes a 179 amino acid protein that shares homology to the other ARL proteins, especially ARL5. It also shows significant homology with orthologous proteins found in Mus musculus and Drosophila melanogaster. The expression pattern of the mouse ortholog revealed differential tissue expression and an alternate transcript was seen in brain that was age-dependent. ARL8 is an additional member of a family of closely related proteins that are conserved both within the family and across species.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

Photoreceptor synaptic protein HRG4 (UNC119) interacts with ARL2 via a putative conserved domain

Human retinal gene 4 (HRG4) (UNC119) is a photoreceptor synaptic protein of unknown function, shown when mutated to cause retinal degeneration in a patient and in a confirmatory transgenic model. ADP-ribosylation factor-like protein 2 (ARL2) was identified as an interactor of HRG4 by the yeast two-hybrid strategy. The presence of ARL2 in the retina and co-localization with HRG4 was confirmed by Western blot and double immunofluorescence analysis, respectively. The interaction of ARL2 with HRG4 was further confirmed by co-immunoprecipitation and direct binding analysis. Phosphodiesterase delta (PDEdelta) is an ARL2-binding protein homologous to HRG4. Amino acid residues of PDEdelta involved in binding ARL2 and forming a hydrophobic pocket were shown to be highly conserved in HRG4, suggesting similarity in binding mechanism and function.

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

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

Arf, Arl, Arp and Sar proteins: a family of GTP-binding proteins with a structural device for 'front-back' communication

Arf proteins are important regulators of cellular traffic and the founding members of an expanding family of homologous proteins and genomic sequences. They depart from other small GTP-binding proteins by a unique structural device, which we call the 'interswitch toggle', that implements front-back communication from the N-terminus to the nucleotide binding site. Here we define the sequence and structural determinants that propagate information across the protein and identify them in all of the Arf family proteins other than Arl6 and Arl4/Arl7. The positions of these determinants lead us to propose that Arf family members with the interswitch toggle device are activated by a bipartite mechanism acting on opposite sides of the protein. The presence of this communication device might provide a more useful basis for unifying Arf homologs as a family than do the cellular functions of these proteins, which are mostly unrelated. We review available genomic sequences and functional data from this perspective, and identify a novel subfamily that we call Arl8.

The complex of Arl2-GTP and PDE delta: from structure to function

Arf-like (Arl) proteins are close relatives of the Arf regulators of vesicular transport, but their function is unknown. Here, we present the crystal structure of full-length Arl2-GTP in complex with its effector PDE delta solved in two crystal forms (Protein Data Bank codes 1KSG, 1KSH and 1KSJ). Arl2 shows a dramatic conformational change from the GDP-bound form, which suggests that it is reversibly membrane associated. PDE delta is structurally closely related to RhoGDI and contains a deep empty hydrophobic pocket. Further experiments show that H-Ras, Rheb, Rho6 and G alpha(i1) interact with PDE delta and that, at least for H-Ras, the intact C-terminus is required. We suggest PDE delta to be a specific soluble transport factor for certain prenylated proteins and Arl2-GTP a regulator of PDE delta-mediated transport.

Functional overlap between retinitis pigmentosa 2 protein and the tubulin-specific chaperone cofactor C

Mutations in the X-linked retinitis pigmentosa 2 gene cause progressive degeneration of photoreceptor cells. The retinitis pigmentosa 2 protein (RP2) is similar in sequence to the tubulin-specific chaperone cofactor C. Together with cofactors D and E, cofactor C stimulates the GTPase activity of native tubulin, a reaction regulated by ADP-ribosylation factor-like 2 protein. Here we show that in the presence of cofactor D, RP2 protein also stimulates the GTPase activity of tubulin. We find that this function is abolished by mutation in an arginine residue that is conserved in both cofactor C and RP2. Notably, mutations that alter this arginine codon cause familial retinitis pigmentosa. Our data imply that this residue acts as an "arginine finger" to trigger the tubulin GTPase activity and suggest that loss of this function in RP2 contributes to retinal degeneration. We also show that in Saccharomyces cerevisiae, both cofactor C and RP2 partially complement the microtubule phenotype resulting from deletion of the cofactor C homolog, demonstrating their functional overlap in vivo. Finally, we find that RP2 interacts with GTP-bound ADP ribosylation factor-like 3 protein, providing a link between RP2 and several retinal-specific proteins, mutations in which also cause retinitis pigmentosa.

Identification and characterization of nine novel human small GTPases showing variable expressions in liver cancer tissues

Digestion and detoxification are the two most important functions of the liver, and liver cells always keep a high metabolism level and active vesicular traffic. The malfunction of the vesicular traffic system might be a cause of the abnormal biological behavior of cancerous liver cells. The Ras superfamily is known to regulate various steps of vesicular traffic in eukaryotic cells. It would be significant to determine the change of vesicular transport molecules such as the members of Ras superfamily in carcinogenesis of liver cells. In the present study, we have cloned nine novel genes encoding human small GTPases: RAB1B, RAB4B, RAB10, RAB22A, RAB24, RAB25 ARL5, SARA1, and SARA2, among which the former six belong to the RAB family and the latter three belong to the ARF/SAR1 family. The identification of these new genes has greatly enlarged the pool of the Ras superfamily. It is interesting to find that they are upregulated in most of the 11 hepatocellular carcinoma and 1 cholangiohepatoma cases. Furthermore, the expression in 16 normal human adult tissues, the chromosome loci, and the gene structures of the nine genes are also described. The above findings could be valuable for understanding the vesicular transport system and elucidating the molecular basis of liver cancer carcinogenesis.

ADP-ribosylation factors (ARFs) and ARF-like 1 (ARL1) have both specific and shared effectors: characterizing ARL1-binding proteins

Despite the 40-60% identity between ADP-ribosylation factors (ARFs) and ARF-like (ARL) proteins, distinct functional roles have been inferred from findings that ARLs lack the biochemical or genetic activities characteristic of ARFs. The potential for functional overlap between ARFs and ARLs was examined by comparing effects of expression on intact cells and the ability to bind effectors. Expression of [Q71L]ARL1 in mammalian cells led to altered Golgi structure similar to, but less dramatic than, that reported previously for [Q71L]ARF1. Two previously identified partners of ARFs, MKLP1 and Arfaptin2/POR1, also bind ARL1 but not ARL2 or ARL3. Two-hybrid screens of human cDNA libraries with dominant active mutants of human ARL1, ARL2, and ARL3 identified eight different but overlapping sets of binding partners. Specific interactions between ARL1 and two binding proteins, SCOCO and Golgin-245, are defined and characterized in more detail. Like ARFs and ARL1, the binding of SCOCO to Golgi membranes is rapidly reversed by brefeldin A, suggesting the presence of a brefeldin A-sensitive ARL1 exchange factor. These data reveal a complex network of interactions between GTPases in the ARF family and their effectors and reveal a potential for cross-talk not demonstrated previously.

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

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

The ARF-like 2 (ARL2)-binding protein, BART. Purification, cloning, and initial characterization

ARF-like proteins (ARLs) comprise a functionally distinct group of incompletely characterized members in the ARF family of RAS-related GTPases. We took advantage of the GTP binding characteristics of human ARL2 to develop a specific, high affinity binding assay that allowed the purification of a novel ARL2-binding protein. A 19-kDa protein (BART, Binder of Arl Two) was identified and purified from bovine brain homogenate. BART binding is specific to ARL2.GTP with high affinity but does not interact with ARL2.GDP or activated ARF or RHO proteins. Based on peptide sequences of purified bovine BART, the human cDNA sequence was determined. The 489-base pair BART open reading frame encodes a novel 163-amino acid protein with a predicted molecular mass of 18,822 Da. Recombinant BART was found to bind ARL2.GTP in a manner indistinguishable from native BART. Northern and Western analyses indicated BART is expressed in all tissues sampled. The lack of detectable membrane association of ARL2 or BART upon activation of ARL2 is suggestive of actions quite distinct from those of the ARFs. The lack of ARL2 GTPase-activating protein activity in BART led us to conclude that the specific interaction with ARL2.GTP is most consistent with BART being the first identified ARL2-specific effector.

ADP-ribosylation factor (ARF)-like 4, 6, and 7 represent a subgroup of the ARF family characterization by rapid nucleotide exchange and a nuclear localization signal

The novel ARF-like GTPase ARL7 is a close relative of ARL4 and ARL6 (71% and 59%) identical amino acids). A striking characteristic of these GTPases is their basic C-terminus which, when fused to the C-terminus of green fluorescent protein (GFP), targets the constructs to the nucleus of transfected COS-7 cells. Full length ARL4 was detected in both nuclear and extranuclear compartments, whereas a construct of ARL4 lacking its C-terminus was excluded from the nucleus. Nucleotide exchange rates of recombinant ARL4, ARL6 and ARL7 were similar and appeared considerably higher than those of other members of the ARF family (ARF1, ARP). It is concluded that ARL4, ARL6 and ARL7 form a subgroup within the ARF family with similar, possibly nuclear, function.

Structural elements of ADP-ribosylation factor 1 required for functional interaction with cytohesin-1

ADP-ribosylation factor 1 (ARF1) is a 20-kDa guanine nucleotide-binding protein involved in vesicular trafficking. Conversion of inactive ARF-GDP to active ARF-GTP is catalyzed by guanine nucleotide exchange proteins such as cytohesin-1. Cytohesin-1 and its Sec7 domain (C-1Sec7) exhibit guanine nucleotide exchange protein activity with ARF1 but not ARF-like protein 1 (ARL1), which is 57% identical in amino acid sequence. With chimeric proteins composed of ARF1 (F) and ARL1 (L) sequences we identified three structural elements responsible for this specificity. Cytohesin-1 increased [35S]guanosine 5'-(gamma-thio)triphosphate binding to L28/F (first 28 residues of L, remainder F) and to a much lesser extent F139/L, and mut13F139/L (F139/L with random sequence in the first 13 positions) but not Delta13ARF1 that lacks the first 13 amino acids; therefore, a nonspecific ARF N terminus was required for cytohesin-1 action. The N terminus was not, however, required for that of C-1Sec7. Both C-1Sec7 and cytohesin-1 effectively released guanosine 5'-(gamma-thio)triphosphate from ARF1, but only C-1Sec7 displaced the nonhydrolyzable GTP analog bound to mut13F139/L, again indicating that structure in addition to the Sec7 domain is involved in cytohesin-1 interaction. Some element(s) of the C-terminal region is also involved, because replacement of the last 42 amino acids with ARL sequence in F139L decreased markedly the interaction with cytohesin-1. Participation of both termini is consistent with the crystallographic structure of ARF in which the two terminal alpha-helices are in close proximity. ARF1 residues 28-50 are also important in the interaction with cytohesin-1; replacement of Lys-38 with Gln, the corresponding residue in ARL1, abolished the ability to serve as substrate for cytohesin-1 or C-1Sec7. These studies have defined multiple structural elements in ARF1, including switch 1 and the N and C termini, that participate in functional interactions with cytohesin-1 (or its catalytic domain C-1Sec7), which were not apparent from crystallographic analysis.