ARF Is Required for Maintenance of Yeast Golgi and Endosome Structure and Function

ADP ribosylation factor (ARF) is thought to play a critical role in recruiting coatomer (COPI) to Golgi membranes to drive transport vesicle budding. Yeast strains harboring mutant COPI proteins exhibit defects in retrograde Golgi to endoplasmic reticulum protein transport and striking cargo-selective defects in anterograde endoplasmic reticulum to Golgi protein transport. To determine whether arf mutants exhibit similar phenotypes, the anterograde transport kinetics of multiple cargo proteins were examined in arf mutant cells, and, surprisingly, both COPI-dependent and COPI-independent cargo proteins exhibited comparable defects. Retrograde dilysine-mediated transport also appeared to be inefficient in the arf mutants, and coatomer mutants with no detectable anterograde transport defect exhibited a synthetic growth defect when combined with arf1Δ, supporting a role for ARF in retrograde transport. Remarkably, we found that early and medial Golgi glycosyltransferases localized to abnormally large ring-shaped structures. The endocytic marker FM4–64 also stained similar, but generally larger ring-shaped structures en route from the plasma membrane to the vacuole in arf mutants. Brefeldin A similarly perturbed endosome morphology and also inhibited transport of FM4–64 from endosomal structures to the vacuole. Electron microscopy of arf mutant cells revealed the presence of what appear to be hollow spheres of interconnected membrane tubules which likely correspond to the fluorescent ring structures. Together, these observations indicate that organelle morphology is significantly more affected than transport in the arf mutants, suggesting a fundamental role for ARF in regulating membrane dynamics. Possible mechanisms for producing this dramatic morphological change in intracellular organelles and its relation to the function of ARF in coat assembly are discussed.     

An eGFP-based genetic screen for defects in light-triggered subcelluar translocation of the Drosophila photoreceptor channel TRPL

Signaling at the plasma membrane is modulated by up- and downregulation of signaling proteins. A prominent example for this type of regulation is the Drosophila TRPL ion channel that changes its spatial distribution within the photoreceptor cell. In dark-raised flies TRPL is localized in the rhabdomeral photoreceptor membrane and it translocates to the cell body upon illumination. It has been shown that TRPL translocation depends on the activation of the phototransduction cascade and requires the presence of functional rhodopsin as well as Ca2+-influx through a second lightactivated ion channel, TRP. However, little is known about the cell biological mechanism underlying TRPL translocation. Here we describe a FRT/FLP screen designed to isolate mutants defective in TRPL internalization based on the localization of eGFP-tagged TRPL in the eyes of living flies. We mutated chromosome arms 2L, 2R and 3R and isolated 12 mutants that failed to internalize TRPL. We found that four mutants did not complement genes known to affect TRPL translocation, which are trp, ninaE and inaD. Two of the isolated mutants represent new alleles of trp and ninaE. The trp allele contains a premature stop codon after amino acid 884, whereas the ninaE allele has a mutation resulting in the substitution P193S. As determined biochemically no TRP or rhodopsin protein, respectively, was expressed in the eyes of these mutants. The absence of TRP or rhodopsin in the isolated mutants readily explains the defect in TRPL internalization and proves the feasibility of our genetic screen.     

An opsin gene expressed in only one photoreceptor cell type of the Drosophila eye

We have isolated an opsin gene from D. melanogaster that is expressed specifically in photoreceptor cell 8 of the Drosophila compound eye. This opsin is 381 amino acid residues long and is 67% homologous to the ninaE opsin, which is expressed in photoreceptor cells 1-6. The gene is divided into four exons; only one of the intron positions is conserved with that of the ninaE gene.     

The small G protein Arl1 directs the trans-Golgi-specific targeting of the Arf1 exchange factors BIG1 and BIG2

The small G protein Arf1 regulates Golgi traffic and is activated by two related types of guanine nucleotide exchange factor (GEF). GBF1 acts at the cis-Golgi, whereas BIG1 and its close paralog BIG2 act at the trans-Golgi. Peripheral membrane proteins such as these GEFs are often recruited to membranes by small G proteins, but the basis for specific recruitment of Arf GEFs, and hence Arfs, to Golgi membranes is not understood. In this paper, we report a liposome-based affinity purification method to identify effectors for small G proteins of the Arf family. We validate this with the Drosophila melanogaster Arf1 orthologue (Arf79F) and the related class II Arf (Arf102F), which showed a similar pattern of effector binding. Applying the method to the Arf-like G protein Arl1, we found that it binds directly to Sec71, the Drosophila ortholog of BIG1 and BIG2, via an N-terminal region. We show that in mammalian cells, Arl1 is necessary for Golgi recruitment of BIG1 and BIG2 but not GBF1. Thus, Arl1 acts to direct a trans-Golgi-specific Arf1 GEF, and hence active Arf1, to the trans side of the Golgi.     

Human and Giardia ADP-ribosylation factors (ARFs) complement ARF function in Saccharomyces cerevisiae.

ADP-ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that stimulate the ADP-ribosyltransferase activity of cholera toxin in vitro. ARFs are highly conserved, ubiquitously expressed in eukaryotic cells and appear to be involved in vesicular protein transport. The two yeast ARFs are > 60% identical to mammalian ARFs and are essential for cell viability (Stearns, T., Kahn, R. A., Botstein, D., and Hoyt, M. A. (1990) Mol. Cell. Biol. 10, 6690-6699). Although the two yeast ARF proteins are 96% identical in amino acid sequence, the yeast ARF1 gene is constitutively expressed, whereas the ARF2 gene is repressed by glucose. Human ARF5 and ARF6 and a Giardia ARF differ substantially in size and amino acid identity from other mammalian and eukaryotic ARFs but will, as befits their designation, activate cholera toxin. Expression of human ARF5, ARF6, or Giardia ARF cDNA rescued the lethal yeast ARF double mutant (arf1, arf2). Strains rescued by human ARF5, ARF6, or Giardia ARF grew much more slowly than wild-type yeast or strains rescued with yeast ARF1. We infer from the impaired growth of these rescued strains that the homologous ARFs may have specific targeting information that does not interact effectively or efficiently with the yeast protein membrane trafficking system.     

ADP-Ribosylation Factors Do Not Activate Yeast Phospholipase Ds but Are Required for Sporulation

ADP-ribosylation factor (ARF) proteins in Saccharomyces cerevisiae are encoded by two genes, ARF1 and ARF2. The addition of the c-myc epitope at the C terminus of Arf1 resulted in a mutant (arf1-myc arf2) that supported vegetative growth and rescued cells from supersensitivity to fluoride, but homozygous diploids failed to sporulate. arf1-myc arf2 mutants completed both meiotic divisions but were unable to form spores. The SPO14 gene encodes a phospholipase D (PLD), whose activity is essential for mediating the formation of the prospore membrane, a prerequisite event for spore formation. Spo14 localized normally to the developing prospore membrane in arf1-myc arf2 mutants; however, the synthesis of the membrane was attenuated. This was not a consequence of reduced PLD catalytic activity, because the enzymatic activity of Spo14 was unaffected in meiotic arf1-myc arf2 mutants. Although potent activators of mammalian PLD1, Arf1 proteins did not influence the catalytic activities of either Spo14 or ScPld2, a second yeast PLD. These results demonstrate that ARF1 is required for sporulation, and the mitotic and meiotic functions of Arf proteins are not mediated by the activation of any known yeast PLD activities. The implications of these results are discussed with respect to current models of Arf signaling.     

Rhodopsin activation causes retinal degeneration in Drosophila rdgC mutant

Drosophila rdgC (retinal degeneration-C) mutants show normal retinal morphology and photoreceptor physiology at young ages. Dark-reared rdgC flies retain this wild-type phenotype, but light-reared mutants undergo retinal degeneration. rdgC photoreceptors with low levels of rhodopsin as a result of vitamin A deprivation or a mutant rhodopsin (ninaE) gene fail to show rdgC-induced degeneration even after prolonged light treatment, demonstrating that degeneration occurs as a result of light stimulation of rhodopsin. Analysis of norpA; rdgC flies shows that the norpA-encoded phospholipase C, the target enzyme of the G protein activated by rhodopsin, is not required for rdgC-induced degeneration. Thus the rdgC+ gene product is required to prevent retinal degeneration that results from a previously unrecognized consequence of rhodopsin stimulation.     

Eyes closed, a Drosophila p47 homolog, is essential for photoreceptor morphogenesis.

Starting with a mutation impacting photoreceptor morphogenesis, we identify here a Drosophila gene, eyes closed (eyc), as a fly homolog of p47, a protein co-factor of the p97 ATPase implicated in membrane fusion. Temporal misexpression of Eyc during rhabdomere extension early in pupal life results in inappropriate retention of normally transient adhesions between developing rhabdomeres. Later Eyc misexpression results in endoplasmic reticulum proliferation and inhibits rhodopsin transport to the developing photosensitive membrane. Loss of Eyc function results in a lethal failure of nuclear envelope assembly in early zygotic divisions. Phenotypes resulting from eyc mutations provide the first in vivo evidence for a role for p47 in membrane biogenesis.     

Phospholipase D-dependent and -independent mechanisms are involved in milk protein secretion in rabbit mammary epithelial cells

Phospholipase D has been implicated in membrane traffic in the secretory pathway of yeast and of some mammalian cell lines. Here we investigated the involvement of phospholipase D in protein transport at various steps of the secretory pathway of mammary epithelial cells. Treatment of rabbit mammary explants with butanol, which blocks the formation of phosphatidic acid, decreased the secretion of caseins and to a lesser extent that of whey acidic protein. Butanol interfered with both the endoplasmic reticulum to Golgi complex transport of the caseins and secretory vesicle formation from the trans-Golgi network. In contrast, the transport of whey acidic protein to the Golgi was less affected. Activation of protein kinase C enhanced the overall secretion of both markers and interestingly, this stimulation of secretion was maintained for whey acidic protein in the presence of butanol. Transphosphatidylation assays demonstrated the existence of a constitutive phospholipase D activity which was stimulated by the activation of protein kinase C. We conclude that phospholipase D plays a role in casein transport from the endoplasmic reticulum to the Golgi and in the secretory vesicle formation from the trans-Golgi network. Moreover, our results suggest a differential requirement for phospholipase D in the secretion of caseins and that of whey acidic protein.     

Arf1 GTPase plays roles in the protein traffic between the endoplasmic reticulum and the Golgi apparatus in tobacco and Arabidopsis cultured cells

Arf GTPases are known to be key regulators of vesicle budding in various steps of membrane traffic in yeast and animal cells. We cloned the Arabidopsis Arf1 homologue, AtArf1, and examined its function. AtArf1 complements yeast arf1 arf2 mutants and its GFP-fusion is localized to the Golgi apparatus in plant cells like its animal counterpart. The expression of dominant negative mutants of AtArf1 in tobacco and Arabidopsis cultured cells affected the localization of co-expressed GFP-tagged proteins in a variety of ways. AtArf1 Q71L and AtArf1 T31N, GTP- and GDP-fixed mutants, respectively, changed the localization of a cis-Golgi marker, AtErd2-GFP, from the Golgi apparatus to the endoplasmic reticulum but not that of GFP-AtRer1B or GFP-AtSed5. GFP-AtRer1B and GFP-AtSed5 were accumulated in aberrant structures of the Golgi by AtArf1 Q71L. A soluble vacuolar protein, sporamin-GFP, was also located to the ER by AtArf1 Q71L. These results indicate that AtArf1 play roles in the vesicular transport between the ER and the Golgi and in the maintenance of the normal Golgi organization in plant cells.     

Retrograde transport from the yeast Golgi is mediated by two ARF GAP proteins with overlapping function.

ARF proteins, which mediate vesicular transport, have little or no intrinsic GTPase activity. They rely on the actions of GTPase-activating proteins (GAPs) for their function. The in vitro GTPase activity of the Saccharomyces cerevisiae ARF proteins Arf1 and Arf2 is stimulated by the yeast Gcs1 protein, and in vivo genetic interactions between arf and gcs1 mutations implicate Gcs1 in vesicular transport. However, the Gcs1 protein is dispensable, indicating that additional ARF GAP proteins exist. We show that the structurally related protein Glo3, which is also dispensable, also exhibits ARF GAP activity. Genetic and in vitro approaches reveal that Glo3 and Gcs1 have an overlapping essential function at the endoplasmic reticulum (ER)-Golgi stage of vesicular transport. Mutant cells deficient for both ARF GAPs cannot proliferate, undergo a dramatic accumulation of ER and are defective for protein transport between ER and Golgi. The glo3Delta and gcs1Delta single mutations each interact with a sec21 mutation that affects a component of COPI, which mediates vesicular transport within the ER-Golgi shuttle, while increased dosage of the BET1, BOS1 and SEC22 genes encoding members of a v-SNARE family that functions within the ER-Golgi alleviates the effects of a glo3Delta mutation. An in vitro assay indicates that efficient retrieval from the Golgi to the ER requires these two proteins. These findings suggest that Glo3 and Gcs1 ARF GAPs mediate retrograde vesicular transport from the Golgi to the ER.     

Golgi targeting of ARF-like GTPase Arl3p requires its Nalpha-acetylation and the integral membrane protein Sys1p

Myristoylation of ARF family GTPases is required for their association with Golgi and endosomal membranes, where they regulate protein sorting and the lipid composition of these organelles. The Golgi-localized ARF-like GTPase Arl3p/ARP lacks a myristoylation signal, indicating that its targeting mechanism is distinct from myristoylated ARFs. We demonstrate that acetylation of the N-terminal methionine of Arl3p requires the NatC N(alpha)-acetyltransferase and that this modification is required for its Golgi localization. Chemical crosslinking and fluorescence microscopy experiments demonstrate that localization of Arl3p also requires Sys1p, a Golgi-localized integral membrane protein, which may serve as a receptor for acetylated Arl3p.     

PDMP blocks the BFA-induced ADP-ribosylation of BARS-50 in isolated Golgi membranes.

We reported that an inhibitor of sphingolipid biosynthesis, D, L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), blocks brefeldin A (BFA)-induced retrograde membrane transport from the Golgi complex to the endoplasmic reticulum (ER) (Kok et al., 1998, J. Cell Biol. 142, 25-38). We now show that PDMP partially blocks the BFA-induced ADP-ribosylation of the cytosolic protein BARS-50. Moreover, PDMP does not interfere with the BFA-induced inhibition of the binding of ADP-ribosylation factor (ARF) and the coatomer component beta-coat protein to Golgi membranes. These results are consistent with a role of ADP-ribosylation in the action of BFA and with the involvement of BARS-50 in the regulation of membrane trafficking.     

Actin microfilaments facilitate the retrograde transport from the Golgi complex to the endoplasmic reticulum in mammalian cells

The morphology and subcellular positioning of the Golgi complex depend on both microtubule and actin cytoskeletons. In contrast to microtubules, the role of actin cytoskeleton in the secretory pathway in mammalian cells has not been clearly established. Using cytochalasin D, we have previously shown that microfilaments are not involved in the endoplasmic reticulum–Golgi membrane dynamics. However, it has been reported that, unlike botulinum C2 toxin and latrunculins, cytochalasin D does not produce net depolymerization of actin filaments. Therefore, we have reassessed the functional role of actin microfilaments in the early steps of the biosynthetic pathway using C2 toxin and latrunculin B. The anterograde endoplasmic reticulum-to-Golgi transport monitored with the vesicular stomatitis virus-G protein remained unaltered in cells treated with cytochalasin D, latrunculin B or C2 toxin. Conversely, the brefeldin A-induced Golgi membrane fusion into the endoplasmic reticulum, the Golgi-to-endoplasmic reticulum transport of a Shiga toxin mutant form, and the subcellular distribution of the KDEL receptor were all impaired when actin microfilaments were depolymerized by latrunculin B or C2 toxin. These findings, together with the fact that COPI-coated and uncoated vesicles contain β/γ-actin isoforms, indicate that actin microfilaments are involved in the endoplasmic reticulum/Golgi interface, facilitating the retrograde Golgi-to-endoplasmic reticulum membrane transport, which could be mediated by the orchestrated movement of transport intermediates along microtubule and microfilament tracks.     

The GTPase ARF1p controls the sequence-specific vacuolar sorting route to the lytic vacuole

We have studied the transport of soluble cargo molecules by inhibiting specific transport steps to and from the Golgi apparatus. Inhibition of export from the Golgi via coexpression of a dominant-negative GTP-restricted ARF1 mutant (Q71L) inhibits the secretion of alpha-amylase and simultaneously induces the secretion of the vacuolar protein phytepsin to the culture medium. By contrast, specific inhibition of endoplasmic reticulum export via overexpression of Sec12p or coexpression of a GTP-restricted form of Sar1p inhibits the anterograde transport of either cargo molecule in a similar manner. Increased secretion of the vacuolar protein was not observed after incubation with the drug brefeldin A or after coexpression of the GDP-restricted mutant of ARF1 (T31N). Therefore, the differential effect of inducing the secretion of one cargo molecule while inhibiting the secretion of another is dependent on the GTP hydrolysis by ARF1p and is not caused by a general inhibition of Golgi-derived COPI vesicle traffic. Moreover, we demonstrate that GTP-restricted ARF1-stimulated secretion is observed only for cargo molecules that are expected to be sorted in a BP80-dependent manner, exhibiting sequence-specific, context-independent, vacuolar sorting signals. Induced secretion of proteins carrying C-terminal vacuolar sorting signals was not observed. This finding suggests that ARF1p influences the BP80-mediated transport route to the vacuole in addition to transport steps of the default secretory pathway to the cell surface.     

Uncoupling of brefeldin a-mediated coatomer protein complex-I dissociation from Golgi redistribution

The Golgi complex functions in transport of molecules from the endoplasmic reticulum (ER) to the plasma membrane and other distal organelles as well as in retrograde transport to the ER. The fungal metabolite brefeldin A (BFA) promotes dissociation of ADP-ribosylation-factor-1 (ARF1) and the coatomer protein complex-I (COP-I) from Golgi membranes, followed by Golgi tubulation and fusion with the ER. Here we demonstrate that the cationic ionophore monensin inhibited the BFA-mediated Golgi redistribution to the ER without interfering with ARF1 and COP-I dissociation. Preservation of a perinuclear Golgi despite COP-I and ARF1 dissociation enables addressing the involvement of these proteins in anterograde ER to Golgi transport. The thermo-reversible folding mutant of vesicular stomatitis virus G protein (VSVGtsO45) was retained in the ER in the presence of both monensin and BFA, thus supporting ARF1/COP-I participation in ER-exit processes. Live-cell imaging revealed that BFA-induced Golgi tubulation persisted longer in the presence of monensin, suggesting that monensin inhibits tubule fusion with the ER. Moreover, monensin also augmented Golgi-derived tubules that contained the ER-Golgi-intermediate compartment marker, p58, in the absence of BFA, signifying the generality of this effect. Taken together, we propose that monensin inhibits membrane fusion processes in the presence or absence of BFA.     

dGRASP-mediated noncanonical integrin secretion is required for Drosophila epithelial remodeling

Integral plasma membrane proteins are typically transported in the secretory pathway from the endoplasmic reticulum and the Golgi complex. Here we show that at specific stages of Drosophila development corresponding to morphological changes in epithelia, apposed basolateral membranes separate slightly, allowing new plasma membrane contacts with basal extracellular matrix. At these sites, newly synthesized integrin alpha subunits are deposited via a mechanism that appears to bypass the Golgi. We show that the Drosophila Golgi resident protein dGRASP localizes to these membrane domains and that, in the absence of dGRASP, the integrin subunit is retained intracellularly in both follicular and wing epithelia that are found disrupted. We propose that this dGRASP-mediated noncanonical secretion route allows for developmental regulation of integrin function upon epithelial remodeling. We speculate that this mechanism might be used during development as a means of targeting a specific subset of transmembrane proteins to the plasma membrane.     

Structural basis for recruitment of GRIP domain golgin-245 by small GTPase Arl1

Recruitment of the GRIP domain golgins to the trans-Golgi network is mediated by Arl1, a member of the ARF/Arl small GTPase family, through interaction between their GRIP domains and Arl1-GTP. The crystal structure of Arl1-GTP in complex with the GRIP domain of golgin-245 shows that Arl1-GTP interacts with the GRIP domain predominantly in a hydrophobic manner, with the switch II region conferring the main recognition surface. The involvement of the switch and interswitch regions in the interaction between Arl1-GTP and GRIP accounts for the specificity of GRIP domain for Arl1-GTP. Mutations that abolished the Arl1-mediated Golgi localization of GRIP domain golgins have been mapped on the interface between Arl1-GTP and GRIP. Notably, the GRIP domain forms a homodimer in which each subunit interacts separately with one Arl1-GTP. Mutations disrupting the GRIP domain dimerization also abrogated its Golgi targeting, suggesting that the dimeric form of GRIP domain is a functional unit.     

Protein kinase CK2 is required for Wntless internalization and Wnt secretion

Wnt proteins are lipid modified signaling molecules that have essential functions in development and adult tissue homeostasis. Secretion of Wnt is mediated by the transmembrane protein Wntless, which binds Wnt and transports it from the endoplasmic reticulum to the cell surface for release. To maintain efficient Wnt secretion, Wntless is recycled back to the Golgi and the endoplasmic reticulum through endocytosis and retromer dependent endosome to Golgi transport. We have previously identified protein kinase CK2 (CK2) in a genome-wide screen for regulators of Wnt signaling in Caenorhabditis elegans. Here, we show that CK2 function is required in Wnt producing cells for Wnt secretion. This function is evolutionarily conserved, as inhibition of CK2 activity interferes with Wnt5a secretion from mammalian cells. Mechanistically, we show that inhibition of CK2 function results in enhanced plasma membrane localization of Wls in C. elegans and mammalian cells, consistent with the notion that CK2 is involved in the regulation of Wls internalization.