A regulatory role for ARF6 in receptor-mediated endocytosis

SNAP25, but not syntaxin 1A, recycles via an ARF6-regulated pathway in neuroendocrine cells

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins mediate cellular membrane fusion events and provide a level of specificity to donor-acceptor membrane interactions. However, the trafficking pathways by which individual SNARE proteins are targeted to specific membrane compartments are not well understood. In neuroendocrine cells, synaptosome-associated protein of 25 kDa (SNAP25) is localized to the plasma membrane where it functions in regulated secretory vesicle exocytosis, but it is also found on intracellular membranes. We identified a dynamic recycling pathway for SNAP25 in PC12 cells through which plasma membrane SNAP25 recycles in approximately 3 h. Approximately 20% of the SNAP25 resides in a perinuclear recycling endosome-trans-Golgi network (TGN) compartment from which it recycles back to the plasma membrane. SNAP25 internalization occurs by constitutive, dynamin-independent endocytosis that is distinct from the dynamin-dependent endocytosis that retrieves secretory vesicle constituents after exocytosis. Endocytosis of SNAP25 is regulated by ADP-ribosylation factor (ARF)6 (through phosphatidylinositol bisphosphate synthesis) and is dependent upon F-actin. SNAP25 endosomes, which exclude the plasma membrane SNARE syntaxin 1A, merge with those derived from clathrin-dependent endocytosis containing endosomal syntaxin 13. Our results characterize a robust ARF6-dependent internalization mechanism that maintains an intracellular pool of SNAP25, which is compatible with possible intracellular roles for SNAP25 in neuroendocrine cells.

Phospholipase D2 is required for efficient endocytic recycling of transferrin receptors

RNA interference-mediated depletion of phospholipase D2 (PLD2), but not PLD1, inhibited recycling of transferrin receptors in HeLa cells, whereas the internalization rate was unaffected by depletion of either PLD. Although reduction of both PLD isoforms inhibits PLD activity stimulated by phorbol 12-myristic 13-acetate, only depletion of PLD2 decreased nonstimulated activity. Cells with reduced PLD2 accumulated a greater fraction of transferrin receptors in a perinuclear compartment that was positive for Rab11, a marker of recycling endosomes. EFA6, an exchange factor for Arf6, has been proposed to stimulate the recycling of transferrin receptors. Thus, one consequence of EFA6 overexpression would be a reduction of the internal pool of receptors. We confirmed this observation in control HeLa cells; however, overexpression of EFA6 failed to decrease the internal pool of transferrin receptors that accumulate in cells previously depleted of PLD2. These observations suggest that either PLD2 is required for a constitutive Arf6-mediated recycling pathway or in the absence of PLD2 transferrin receptors accumulate in recycling endosomes that are not responsive to overexpression of EFA6.

Syndecan recycling [corrected] is controlled by syntenin-PIP2 interaction and Arf6

Syndecans are heparan sulfate proteoglycans that modulate the activity of several growth factors and cell adhesion molecules. PDZ domains in the adaptor protein syntenin interact with syndecans and with the phosphoinositide PIP(2), which is involved in the regulation of the actin cytoskeleton and membrane trafficking. Here, we show that the syntenin PDZ domain-PIP(2) interaction controls Arf6-mediated syndecan recycling through endosomal compartments. FGF receptor accompanies syndecan along the syntenin-mediated recycling pathway, in a heparan sulfate- and FGF-dependent manner. Syndecans that cannot recycle via this pathway become trapped intracellularly and inhibit cell spreading. This syntenin-mediated syndecan recycling pathway may regulate the surface availability of a number of cell adhesion and signaling molecules.

An effector domain mutant of Arf6 implicates phospholipase D in endosomal membrane recycling

In this study, we investigated the role of phospholipase D (PLD) in mediating Arf6 function in cells. Expression of Arf6 mutants that are defective in activating PLD, Arf6N48R and Arf6N48I, inhibited membrane recycling to the plasma membrane (PM), resulting in an accumulation of tubular endosomal membranes. Additionally, unlike wild-type Arf6, neither Arf6 mutant could generate protrusions or recruit the Arf6 GTPase activating protein (GAP) ACAP1 onto the endosome in the presence of aluminum fluoride. Remarkably, all of these phenotypes, including accumulated tubular endosomes, blocked recycling, and failure to make protrusions and recruit ACAP effectively, could be recreated in either untransfected cells or cells expressing wild-type Arf6 by treatment with 1-butanol to inhibit the formation of phosphatidic acid (PA), the product of PLD. Moreover, most of the defects present in cells expressing Arf6N48R or N48I could be reversed by treatment with agents expected to elevate PA levels in cells. Together, these observations provide compelling evidence that Arf6 stimulation of PLD is required for endosomal membrane recycling and GAP recruitment.

Actin is required for endocytosis at the apical surface of Madin-Darby canine kidney cells where ARF6 and clathrin regulate the actin cytoskeleton

In epithelial cell lines, apical but not basolateral clathrin-mediated endocytosis has been shown to be affected by actin-disrupting drugs. Using electron and fluorescence microscopy, as well as biochemical assays, we show that the amount of actin dedicated to endocytosis is limiting at the apical surface of epithelia. In part, this contributes to the low basal rate of clathrin-dependent endocytosis observed at this epithelial surface. ARF6 in its GTP-bound state triggers the recruitment of actin from the cell cortex to the clathrin-coated pit to enable dynamin-dependent endocytosis. In addition, we show that perturbation of the apical endocytic system by expression of a clathrin heavy-chain mutant results in the collapse of microvilli. This phenotype was completely reversed by the expression of an ARF6-GTP-locked mutant. These observations indicate that concomitant to actin recruitment, the apical clathrin endocytic system is deeply involved in the morphology of the apical plasma membrane.

Endocytosis resumes during late mitosis and is required for cytokinesis

Recent work has underscored the importance of membrane trafficking events during cytokinesis. For example, targeted membrane secretion occurs at the cleavage furrow in animal cells, and proteins that regulate endocytosis also influence the process of cytokinesis. Nonetheless, the prevailing dogma is that endosomal membrane trafficking ceases during mitosis and resumes after cell division is complete. In this study, we have characterized endocytic membrane trafficking events that occur during mammalian cell cytokinesis. We have found that, although endocytosis ceases during the early stages of mitosis, it resumes during late mitosis in a temporally and spatially regulated pattern as cells progress from anaphase to cytokinesis. Using fixed and live cell imaging, we have found that, during cleavage furrow ingression, vesicles are internalized from the polar region and subsequently trafficked to the midbody area during later stages of cytokinesis. In addition, we have demonstrated that cytokinesis is inhibited when clathrin-mediated endocytosis is blocked using a series of dominant negative mutants. In contrast to previous thought, we conclude that endocytosis resumes during the later stages of mitosis, before cytokinesis is completed. Furthermore, based on our findings, we propose that the proper regulation of endosomal membrane traffic is necessary for the successful completion of cytokinesis.

2.0 A crystal structure of human ARL5-GDP3'P, a novel member of the small GTP-binding proteins

ARL5 is a member of ARLs, which is widespread in high eukaryotes and homologous between species. But no structure or biological function of this member is reported. We expressed, purified, and resolved the structure of human ARL5 with bound GDP3'P at 2.0 A resolution. A comparison with the known structures of ARFs shows that besides the typical features of ARFs, human ARL5 has specific features of its own. Bacterially expressed human ARL5 contains bound GDP3'P which is seldom seen in other structures. The hydrophobic tail of the introduced detergent Triton X-305 binds at the possible myristoylation site of Gly2, simulating the myristoylated state of N-terminal amphipathic helix in vivo. The structural features of the nucleotide binding motifs and the switch regions prove that ARL5 will undergo the typical GDP/GTP structural cycle as other members of ARLs, which is the basis of their biological functions.

Isoform-selective effects of the depletion of ADP-ribosylation factors 1-5 on membrane traffic

The ADP-ribosylation factors (Arfs) are six proteins within the larger Arf family and Ras superfamily that regulate membrane traffic. Arfs all share numerous biochemical activities and have very similar specific activities. The use of dominant mutants and brefeldin A has been important to the discovery of the cellular functions of Arfs but lack specificity between Arf isoforms. We developed small interference RNA constructs capable of specific depletion of each of the cytoplasmic human Arfs to examine the specificity of Arfs in live cells. No single Arf was required for any step of membrane traffic examined in HeLa cells. However, every combination of the double knockdowns of Arf1, Arf3, Arf4, and Arf5 yielded a distinct pattern of defects in secretory and endocytic traffic, demonstrating clear specificity for Arfs at multiple steps. These results suggest that the cooperation of two Arfs at the same site may be a general feature of Arf signaling and provide candidates at several cellular locations that when paired with data on the localization of the many different Arf guanine nucleotide exchange factors, Arf GTPase activating proteins, and effectors will aid in the description of the mechanisms of specificity in this highly conserved and primordial family of regulatory GTPases.

Arf6: a new player in FcγRIIIA lymphocyte-mediated cytotoxicity

The activation of phosphoinositide metabolism represents a critical step in the signaling pathways leading to the activation of cytolytic machinery, but its regulation is partially understood. We report here that the stimulation of the low-affinity receptor for immunoglobulin G (IgG) (FcγRIIIA, CD16) on primary human natural killer (NK) cells induces a phosphatidylinositol 3-kinase (PI3K)–dependent activation of the small G protein Arf6. We first demonstrate a functional role for Arf6-dependent signals in the activation of the antibody-dependent cellular cytotoxicity (ADCC) attributable to the control of secretion of lytic granule content. We also show that Arf6 couples CD16 to the lipid-modifying enzymes phosphatidylinositol4phosphate 5-kinase type I alpha (PI5KIα) and phospholipase D (PLD) that are involved in the control of granule secretion; Arf6, but not Rho family small G proteins RhoA and Rac1, is required for receptor-induced PI5KIα membrane targeting as well as for PI5KIα and PLD activation. Our findings suggest that Arf6 plays a crucial role in the generation of a phosphatidylinositol4,5-bisphosphate (PIP2) plasma membrane pool required for cytolytic granule-mediated target cell killing.

Interaction of the Salmonella-containing vacuole with the endocytic recycling system

Upon entry of the pathogen Salmonella enterica serovar Typhimurium into host cells, the majority of bacteria reside in a membrane-bound compartment called the Salmonella-containing vacuole (SCV). Previous studies have established that the SCV transiently interacts with early endosomes but only acquires a subset of late endosomal/lysosomal proteins. However, the complete set of interactions between the SCV and the endocytic machinery has yet to be characterized. In this study, we have shown that four characterized regulators of endocytic recycling were present on the SCV after invasion. Interaction kinetics were different for each of the regulators; ARF6 and Rab4 associated immediately, but their presence was diminished 60 min post-infection, whereas syntaxin13 and Rab11 association peaked at 60 min. Using a dominant negative approach, we determined that Rab11 regulates the recycling of CD44 from the vacuole but had no effect on major histocompatibility complex (MHC) class I recycling. In contrast, syntaxin13 regulated the recycling of MHC class I but not of CD44. We also determined that maturation of the SCV, measured by the acquisition of lysosomal associated membrane protein-1, slowed when recycling was impaired. These findings suggest that protein movement through the endocytic recycling system is regulated through at least two concurrent pathways and that efficient interaction with these pathways is necessary for maturation of the Salmonella-containing vacuole. We also demonstrate the utility of using Salmonella invasion as a model of endosomal recycling events.

Cellular and subcellular localization of EFA6C, a third member of the EFA6 family, in adult mouse Purkinje cells

EFA6C is a third member of the EFA6 family of guanine nucleotide exchange factors (GEFs) for ADP-ribosylation factor 6 (ARF6). In this study, we first demonstrated that EFA6C indeed activated ARF6 more selectively than ARF1 by ARF pull-down assay. In situ hybridization histochemistry revealed that EFA6C mRNA was expressed predominantly in mature Purkinje cells and the epithelial cells of the choroid plexus in contrast to the ubiquitous expression of ARF6 mRNA throughout the brain. EFA6C mRNA was already detectable in the Purkinje cells at embryonic day 13, increased progressively during post-natal development and peaked during post-natal second week. In Purkinje cells, the immunoreactivity for EFA6C was localized particularly in the post-synaptic density as well as the plasma membranes of the cell somata, dendritic shafts and spines, while the immunoreactivity in their axon terminals in the deep cerebellar nuclei was very faint. These findings suggest that EFA6C may be involved in the regulation of the membrane dynamics of the somatodendritic compartments of Purkinje cells through the activation of ARF6.

ARF6 GTPase controls bacterial invasion by actin remodelling

The obligate intracellular bacterium Chlamydia penetrates the host epithelial cell by inducing cytoskeleton and membrane rearrangements reminiscent of phagocytosis. Here we report that Chlamydia induces a sharp and transient activation of the endogenous small GTP-binding protein ARF6, which is required for efficient uptake. We also show that a downstream effector of ARF6, phosphatidylinositol 4-phosphate 5-kinase and its product, phosphatidylinositol 4,5-bisphosphate were instrumental for bacterial entry. By contrast, ARF6 activation of phospholipase D was not required for Chlamydia uptake. ARF6 activation was necessary for extensive actin reorganization at the invasion sites. Remarkably, these signalling players gathered with F-actin in a highly organized three-dimensional concentric calyx-like protrusion around invasive bacteria. These results indicate that ARF6, which controls membrane delivery during phagocytosis of red blood cells in macrophages, has a different role in the entry of this small bacterium, controlling cytoskeletal reorganization.

Functional genomic analysis of the ADP-ribosylation factor family of GTPases: phylogeny among diverse eukaryotes and function in C. elegans

ADP-ribosylation factor (Arf) and Arf-like (Arl) proteins are a family of highly conserved 21 kDa GTPases that emerged early in the evolution of eukaryotes. These proteins serve regulatory roles in vesicular traffic, lipid metabolism, microtubule dynamics, development, and likely other cellular processes. We found evidence for the presence of 6 Arf family members in the protist Giardia lamblia and 22 members in mammals. A phylogenetic analysis was performed to delineate the evolutionary relationships among Arf family members and to attempt to organize them by both their evolutionary origins and functions in cells and/or organisms. The approximately 100 protein sequences analyzed from animals, fungi, plants, and protists clustered into 11 groups, including Arfs, nine Arls, and Sar proteins. To begin functional analyses of the family in a metazoan model organism, we examined roles for all three C. elegans Arfs (Arf-1, Arf-3, and Arf-6) and three Arls (Arl-1, Arl-2, and Arl-3) by use of RNA-mediated interference (RNAi). Injection of double-stranded RNA (dsRNA) encoding Arf-1 or Arf-3 into N2 hermaphrodites produced embryonic lethality in their offspring and, later, sterility in the injected animals themselves. Injection of Arl-2 dsRNA resulted in a disorganized germline and sterility in early offspring, with later offspring exhibiting an early embryonic arrest. Thus, of the six Arf family members examined in C. elegans, at least three are required for embryogenesis. These data represent the first analysis of the role(s) of multiple members of this family in the development of a multicellular organism.

ARF6-dependent interaction of the TWIK1 K+ channel with EFA6, a GDP/GTP exchange factor for ARF6

TWIK1 belongs to a family of K(+) channels involved in neuronal excitability and cell volume regulation. Its tissue distribution suggests a role in epithelial potassium transport. Here we show that TWIK1 is expressed in a subapical compartment in renal proximal tubules and in polarized MDCK cells. In nonpolarized cells, this compartment corresponds to pericentriolar recycling endosomes. We identified EFA6, an exchange factor for the small G protein ADP-ribosylation factor 6 (ARF6), as a protein binding to TWIK1. EFA6 interacts with TWIK1 only when it is bound to ARF6. Because ARF6 modulates endocytosis at the apical surface of epithelial cells, the ARF6/EFA6/TWIK1 association is probably important for channel internalization and recycling.

Thyrotropin receptor trafficking relies on the hScrib-betaPIX-GIT1-ARF6 pathway

G protein-coupled receptors are regulated by ligand stimulation, endocytosis, degradation of recycling to the cell surface. Little information is available on the molecular mechanisms underlying G protein-coupled receptors recycling. We have investigated recycling of the G protein-coupled thyroid stimulating hormone receptor (TSHR) and found that it relies on hScrib, a membrane-associated PDZ protein. hScrib directly binds to TSHR, inhibits basal receptor endocytosis and promotes recycling, and thus TSHR signalling, at the cell membrane. We previously demonstrated that hScrib is associated with a betaPIX-GIT1 complex comprised of a guanine nucleotide exchange factor and a GTPase-activating protein for ADP ribosylation factors that is involved in vesicle trafficking. We used dominant-negative constructs and small interfering RNA to show that TSHR recycling is regulated by the interaction between hScrib and betaPIX, and by the activity of GIT1. In addition, ARF6, a major target for GIT1, is activated during TSH stimulation of HEK293 and FRTL-5 thyroid cells, and plays a key role in TSHR recycling. Thus, we have uncovered an hScrib-betaPIX-GIT1-ARF6 pathway devoted to TSHR trafficking and function.

Human T lymphotropic virus type 1 accessory protein p12I modulates calcium-mediated cellular gene expression and enhances p300 expression in T lymphocytes

Human T-lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T cell leukemia/lymphoma (ATLL), an aggressive CD4+ T lymphocyte malignancy. Activation of T lymphocytes is required for effective retroviral integration into the host cell genome and subsequent viral replication, but the molecular mechanisms involved in HTLV-1-mediated T cell activation remain unclear. HTLV-1 encodes various accessory proteins such as p12I, which has been demonstrated to be critical for HTLV-1 infectivity in vivo in rabbits and in vitro in quiescent primary human T lymphocytes. This hydrophobic protein localizes in the endoplasmic reticulum, increases intracellular calcium, and activates nuclear factor of activated T cell-mediated transcription. To further elucidate the role of p12I in regulation of cellular gene expression, we performed gene array analysis on stable p12I-expressing Jurkat T cells, using Affymetrix U133A arrays. Our data indicate that p12I altered the expression of genes associated with a network of interrelated pathways including T cell signaling, cell proliferation, and apoptosis. Expression of several calcium-regulated genes was found to be altered by p12I, consistent with known properties of the viral protein. Gene array findings were confirmed by semiquantitative RT-PCR in Jurkat T cells and primary CD4+ T lymphocytes. Furthermore, dose-dependent expression of p12I in Jurkat T cells resulted in significant increases in p300 and p300-dependent transcription. This is the first report of a viral protein influencing the transcription of p300, a rate-limiting coadapter critical in HTLV-1-mediated T cell activation. Collectively, our data strongly indicate that HTLV-1 p12I modulates cellular gene expression patterns to hasten the activation of T lymphocytes and thereby promote efficient viral infection.

The small G-protein Arf6GTP recruits the AP-2 adaptor complex to membranes

The small GTP-binding protein ADP-ribosylation factor 6 (Arf6) is involved in plasma membrane/endosomes trafficking. However, precisely how the activation of Arf6 regulates vesicular transport is still unclear. Here, we show that, in vitro, recombinant Arf6GTP recruits purified clathrin-adaptor complex AP-2 (but not AP-1) onto phospholipid liposomes in the absence of phosphoinositides. We also show that phosphoinositides and Arf6 tightly cooperate to translocate AP-2 to the membrane. In vivo, Arf6GTP (but not Arf6GDP) was found associated to AP-2. The expression of the GTP-locked mutant of Arf6 leads to the plasma membrane redistribution of AP-2 in Arf6GTP-enriched areas. Finally, we demonstrated that the expression of the GTP-locked mutant of Arf6 inhibits transferrin receptor internalization without affecting its recycling. Altogether, our results demonstrated that Arf6GTP interacts specifically with AP-2 and promotes its membrane recruitment. These findings strongly suggest that Arf6 plays a major role in clathrin-mediated endocytosis by directly controlling the assembly of the AP-2/clathrin coat.

The nature of the phagosomal membrane: endoplasmic reticulum versus plasmalemma

For decades, the vacuole that surrounds particles engulfed by phagocytosis was believed to originate from the plasma membrane. Conversion of the nascent phagosome into a microbicidal organelle was thought to result from the subsequent, orderly fusion of early endosomes, late endosomes, and ultimately, lysosomes with the original plasma membrane-derived vacuole. This conventional model has been challenged, if not superseded, by a revolutionary model that regards phagosome formation as resulting from the particle sliding into the endoplasmic reticulum via an opening at the base of the phagocytic cup. The merits and implications of these two hypotheses are summarized here and analyzed in light of recent results.

A novel GTPase-activating protein for ARF6 directly interacts with clathrin and regulates clathrin-dependent endocytosis

ADP-ribosylation factor 6 (Arf6) is a small-GTPase that regulates the membrane trafficking between the plasma membrane and endosome. It is also involved in the reorganization of the actin cytoskeleton. GTPase-activating protein (GAP) is a critical regulator of Arf function as it inactivates Arf. Here, we identified a novel species of GAP denoted as SMAP1 that preferentially acts on Arf6. Although overexpression of SMAP1 did not alter the subcellular distribution of the actin cytoskeleton, it did block the endocytosis of transferrin receptors. Knock down of endogenous SMAP1 also abolished transferrin internalization, which confirms that SMAP1 is needed for this endocytic process. SMAP1 overexpression had no effect on clathrin-independent endocytosis, however. Intriguingly, SMAP1 binds directly to the clathrin heavy chain via its clathrin-box and mutation studies revealed that its GAP domain and clathrin-box both contribute to the role SMAP1 plays in clathrin-dependent endocytosis. These observations suggest that SMAP1 may be an Arf6GAP that specifically regulates one of the multiple functions of Arf6, namely, clathrin-dependent endocytosis, and that it does so by binding directly to clathrin.

The GDP-bound form of Arf6 is located at the plasma membrane

The function of Arf6 has been investigated largely by using the T27N and the Q67L mutants, which are thought to be blocked in GDP- and GTP-bound states, respectively. However, these mutants have been poorly characterized biochemically. Here, we found that Arf6(T27N) is not an appropriate marker of the inactive GDP-bound form because it has a high tendency to lose its nucleotide in vitro and to denature. As a consequence, most of the protein is aggregated in vivo and localizes to detergent-insoluble structures. However, a small proportion of Arf6(T27N) is able to form a stable complex with its exchange factor EFA6 at the plasma membrane, accounting for its dominant-negative phenotype. To define the cellular localization of Arf6-GDP, we designed a new mutant, Arf6(T44N). In vitro, this mutant has a 30-fold decreased affinity for GTP. In vivo, it is mostly GDP bound and, in contrast to the wild type, does not switch to the active conformation when expressed with EFA6. This GDP-locked mutant is found at the plasma membrane, where it localizes with EFA6 and Ezrin in actin- and phosphatidylinositol (4,5)-bisphosphate-enriched domains. From these results, we conclude that the Arf6 GDP-GTP cycle takes place at the plasma membrane.

ACAP1 promotes endocytic recycling by recognizing recycling sorting signals

Cargo sorting that promotes the transport of cargo proteins from a membrane compartment has been predicted to be unlikely in the endocytic recycling pathways. We now show that ACAP1 binds specifically and directly to recycling cargo proteins. Reducing this interaction for TfR inhibits its recycling. Moreover, ACAP1 binds to two distinct phenylalanine-based sequences in the cytoplasmic domain of TfR that function as recycling sorting signals to promote its transport from the recycling endosome. Taken together, these findings indicate that ACAP1 promotes cargo sorting by recognizing recycling sorting signals.

The TBC (Tre-2/Bub2/Cdc16) domain protein TRE17 regulates plasma membrane-endosomal trafficking through activation of Arf6

TBC (Tre-2/Bub2/Cdc16) domains are predicted to encode GTPase-activating proteins (GAPs) for Rab family G proteins. While approximately 50 TBC proteins are predicted to exist in humans, little is known about their substrate specificity. Here we show that TRE17 (also called Tre-2 and USP6), a founding member of the TBC family, targets the Arf family GTPase Arf6, which regulates plasma membrane-endosome trafficking. Surprisingly, TRE17 does not function as a GAP for Arf6 but rather promotes its activation in vivo. TRE17 associates directly with Arf6 in its GDP- but not GTP-bound state. Mapping experiments pinpoint the site of interaction to the TBC domain of TRE17. Forced expression of TRE17 promotes the localization of Arf6 to the plasma membrane, leading to Arf6 activation, presumably due to facilitated access to membrane-associated guanine nucleotide exchange factors (GEFs). Furthermore, TRE17 cooperates with Arf6 GEFs to induce GTP loading of Arf6 in vivo. Finally, short interfering RNA-mediated loss of TRE17 leads to attenuated Arf6 activation. These studies identify TRE17 as a novel regulator of the Arf6-regulated plasma membrane recycling system and reveal an unexpected function for TBC domains.

G protein-coupled receptor endocytosis in ADP-ribosylation factor 6-depleted cells

The internalization of G protein-coupled receptors is regulated by several important proteins that act in concert to finely control this complex cellular process. Here, we have applied the RNA interference approach to demonstrate that ADP-ribosylation factor 6 (ARF6) is essential for the endocytosis of a broad variety of receptors. Reduction of endogenous expression of ARF6 in HEK 293 cells resulted in a correlated inhibition of the beta(2) -adrenergic receptor internalization previously characterized as being sequestered via the clathrin-coated vesicle pathway. Furthermore, other receptors internalizing via this endocytic route, namely the angiotensin type 1 receptor and the vasopressin type 2 receptor, were also impaired in their ability to be sequestered when levels of endogenous ARF6 in cells were reduced. Interestingly, endocytosis of the endothelin type B receptor, characterized as being internalized via the caveolae pathway, was also markedly inhibited in ARF6-depleted cells. In contrast, internalization of the vasoactive intestinal peptide receptor was unaffected by reduced levels of ARF6. Finally, internalization of the acetylcholine-muscarinic type 2 receptor via the non-clathrin-coated vesicle pathway was also inhibited in ARF6-depleted cells. Taken together, our results demonstrate that ARF6 proteins play an essential role in the internalization process of most G protein-coupled receptors regardless of the endocytic route being utilized. However, this phenomenon is not general. In some cases, another ARF isoform or other proteins may be essential to regulate the endocytic process.

Functional differentiation of endosomes in Arabidopsis cells

Endocytosis plays an important role in plant physiology, but how endocytic organelles are organized remains unknown. We present the evidence that endosomes are functionally differentiated in Arabidopsis cells. Two types of Rab5-related GTPases are localized on distinct population of endosomes in a partially overlapping manner. Ara7 and Rha1 are on an early type of endosomes with AtVamp727, where recycling of plasma membrane proteins occurs. In contrast, the plant-unique Rab5, Ara6, resides on distinct endosomes with the prevacuolar SNAREs. Partially overlapping localization of Ara6 and Ara7/Rha1 with reciprocal gradients suggests maturation of endosomes from one to the other.

Regulation of G protein-coupled receptor endocytosis by ARF6 GTP-binding proteins

The function of G protein-coupled receptors is regulated by a broad variety of membrane-bound and intracellular proteins. These act in concert to activate signaling pathways that will lead to the desensitization of activated receptors and, for most receptor types, their trafficking to intracellular compartments. This review focuses mainly on the endocytic pathways used by a G protein-coupled receptor and on the proteins that play an essential role in the regulation of the internalization process, most specifically the ADP-ribosylation factors. This family of proteins has been shown to be important for vesicle trafficking between different cellular membranes. The latest findings regarding the molecular mechanisms that regulate internalization of an agonist-stimulated receptor are presented here. Finally, a perspective on how ARF6 proteins might regulate the internalization process is also proposed.Key words: G protein-coupled receptors, endocytosis, ADP-ribosylation factor.

High glucose-induced membrane translocation of PKC betaI is associated with Arf6 in glomerular mesangial cells

Protein kinase C (PKC)-induced changes in glomerular mesangial cell (MC) phenotypic behavior has been implicated in diabetes. The activity of diacylglycerol-sensitive PKC isoforms in MCs is altered by ambient changes in glucose, but the regulation of PKC activity and subsequent intracellular signaling events are not yet clearly defined. Small GTP-binding proteins of the ADP-ribosylation factor (Arfs) family, may regulate protein kinase membrane recruitment and hence its activity in signaling events of non-polarized cells. Members of the ARF family may coordinate membrane dynamics and other cellular functions through their interaction with PKC. We studied the activation of Arf, PKC betaI and phospholipase D (PLD) in MCs cultured under normal or high glucose conditions. MCs cultured in high glucose medium exhibited predominantly cytosolic localization of PKC betaI, Arf3 and Arf6. However, phorbol ester (PMA) stimulation of cells cultured in high glucose significantly enhanced membrane association of PKC betaI and Arf6, but not Arf3. Using [3H]choline chloride to prelabel MCs and measuring [3H]choline-containing metabolite release as PLD activity, PMA stimulated a significant increase of PLD activity under high glucose condition. Our data suggest that Arf6 plays a specific role in activation of PKC betaI and PLD under high glucose condition, and may be a significant intracellular event in the change of the mesangial cell phenotype associated with diabetic nephropathy.

Phosphatidylinositol (4,5) bisphosphate regulates HIV-1 Gag targeting to the plasma membrane

A critical early event in the HIV type 1 (HIV-1) particle assembly pathway is the targeting of the Gag protein to the site of virus assembly. In many cell types, assembly takes place predominantly at the plasma membrane. Cellular factors that regulate Gag targeting remain undefined. The phosphoinositide phosphatidylinositol (4,5) bisphosphate [PI(4,5)P2] controls the plasma membrane localization of a number of cellular proteins. To explore the possibility that this lipid may be involved in Gag targeting and virus particle production, we overexpressed phosphoinositide 5-phosphatase IV, an enzyme that depletes cellular PI(4,5)P2, or overexpressed a constitutively active form of Arf6 (Arf6/Q67L), which induces the formation of PI(4,5)P2-enriched endosomal structures. Both approaches severely reduced virus production. Upon 5-phosphatase IV overexpression, Gag was no longer localized on the plasma membrane but instead was retargeted to late endosomes. Strikingly, in cells expressing Arf6/Q67L, Gag was redirected to the PI(4,5)P2-enriched vesicles and HIV-1 virions budded into these vesicles. These results demonstrate that PI(4,5)P2 plays a key role in Gag targeting to the plasma membrane and thus serves as a cellular determinant of HIV-1 particle production.

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.

A novel mode of action of an ArfGAP, AMAP2/PAG3/Papa lpha, in Arf6 function

Previously we reported that AMAP2/PAG3/Papalpha/KIAA0400, a GTPase-activating protein (GAP), acts to antagonize Arf6 function when overexpressed, whereas it was shown to exhibit efficient GAP activities for other Arf isoforms in vitro. Here, we found that AMAP2, through its ArfGAP domain, binds to GTP-Arf6 but not to GDP-Arf6 or other Arfs irrespective of nucleotide status. The majority of AMAP2 was localized to intracellular tubulovesicular structures and redistributed to Arf6-enriched membrane areas upon Arf6 activation. In HeLa cells, Arf6 has been shown to be involved in the clathrin-independent endocytosis of Tac, but not the clathrin-dependent endocytosis of transferrin. We found that Arf6 silencing inhibited the internalization of Tac, but not transferrin, in HeLa cells. Internalization of Tac, but not transferrin, was also significantly inhibited by AMAP2 silencing and overexpression. AMAP2 was moreover found to bind to amphiphysin IIm, a component of the endocytic machinery, via its proline-rich domain. We propose that AMAP2 has dual mechanisms for its function; it exhibits efficient catalytic GAP activity for the class I and II Arfs and yet is involved in the cellular function of the class III Arf without immediate GAP activity. These dual mechanisms of AMAP2 may be important for the cellular function of GTP-Arf6.

Annexin 2 is a phosphatidylinositol (4,5)-bisphosphate binding protein recruited to actin assembly sites at cellular membranes

Annexin 2 is a Ca(2+)-regulated membrane protein and an F-actin-binding protein enriched at actin assembly sites both, on the plasma membrane and on endosomal vesicles. Here, we identify annexin 2 as a phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P(2))-interacting protein, thereby explaining this specific membrane association. Using the pleckstrin-homology (PH) domain of phospholipase Cdelta1 fused to yellow fluorescent protein as a marker for PtdIns(4,5)P(2), we show that annexin 2 and its ligand p11 (S100A10) are targeted to sites of PtdIns(4,5)P(2) enrichment where F-actin accumulates. At the plasma membrane, adhesion of pedestal-forming enteropathogenic Escherichia coli induces a recruitment of 1-phosphatidylinositol-4-phosphate 5-kinase (PtdIns4P 5-kinase) and an enrichment of PtdIns(4,5)P(2) and annexin 2-p11 at sites of bacterial adhesion. Induction of PtdIns(4,5)P(2)-enriched ruffles and PtdIns(4,5)P(2)-positive, actin-coated vacuoles by Arf6-mediated activation of PtdIns4P 5-kinase also leads to a concomitant accumulation of the annexin 2-p11 complex and the PH domain. Binding studies with immobilized phosphoinositides and phosphoinositide-containing liposomes reveal that the purified annexin 2-p11 complex directly and specifically binds to PtdIns(4,5)P(2) with an affinity comparable to that of the PH domain of phospholipase Cdelta1. Experiments using individual subunits identify annexin 2 as the PtdIns(4,5)P(2)-binding entity. Thus, the direct interaction of annexin 2 with PtdIns(4,5)P(2) is a means of specifically recruiting the annexin 2-p11 complex to sites of membrane-associated actin assembly.

The cell fate determinant numb interacts with EHD/Rme-1 family proteins and has a role in endocytic recycling

The adaptor protein Numb is necessary for the cell fate specification of progenitor cells in the Drosophila nervous system. Numb is evolutionarily conserved and previous studies have provided evidence for a similar functional role during mammalian development. The Numb protein has multiple protein-protein interaction regions including a phosphotyrosine binding (PTB) domain and a carboxy-terminal domain that contains conserved interaction motifs including an EH (Eps15 Homology) domain binding motif and alpha-adaptin binding site. In this study we identify the EHD/Rme-1/Pincher family of endocytic proteins as Numb interacting partners in mammals and Drosophila. The EHD/Rme-1 proteins function in recycling of plasma membrane receptors internalized by both clathrin-mediated endocytosis and a clathrin-independent pathway regulated by ADP ribosylation factor 6 (Arf6). Here we report that Numb colocalizes with endogenous EHD4/Pincher and Arf6 and that Arf6 mutants alter Numb subcellular localization. In addition, we present evidence that Numb has a novel function in endosomal recycling and intracellular trafficking of receptors.

Requirement for Arf6 in breast cancer invasive activities

In most human breast cancer cell lines, there is a direct correlation between their in vivo invasive phenotypes and in vitro invasion activities. Here, we found that ADP-ribosylation factor 6 (Arf6) is localized at the invadopodia of the cultured breast cancer cells MDA-MB-231, and its suppression by a small-interfering RNA duplex effectively blocks the invasive activities of the cells, such as invadopodia formation, localized matrix degradation and Matrigel transmigration but not the cell-adhesion activity. We also found that the GTP hydrolysis-defective mutant Arf6(Q67L) and the GTP-binding defective mutant Arf6(T27N) both blocked these invasive activities but not cell adhesion, suggesting the necessity of continued activation and cycling of the Arf6 GTPase cycle in invasion. Among the different human breast cancer cell lines that we examined, cell lines with high invasive activities expressed higher amounts of Arf6 protein than those in weakly invasive and noninvasive cell lines, although no notable correlation was found between Arf6 mRNA expression levels and invasive activities. Moreover, Matrigel-transmigration activity of all of these invasive cells was blocked effectively by an Arf6 small-interfering RNA duplex. Hence, Arf6 appears to be an integral component of breast cancer invasive activities, and we propose that Arf6 and the intracellular machinery regulating Arf6 during invasion should be considered as therapeutic targets for the prevention of breast cancer invasion.

Arf GAPs: multifunctional proteins that regulate membrane traffic and actin remodelling

The ADP-ribosylation factor (Arf) Arf GTPase-activating proteins (GAPs) are a family of proteins that induce hydrolysis of GTP bound to Arf. A conserved domain containing a zinc finger motif mediates catalysis. The substrate, Arf.GTP, affects membrane trafficking and actin remodelling. Consistent with activity as an Arf regulator, the Arf GAPs affect both of these pathways. However, the Arf GAPs are likely to have Arf-independent activities that contribute to their cellular functions. Structures of the Arf GAPs are diverse containing catalytic, protein-protein interaction and lipid interaction domains in addition to the Arf GAP domain. Some Arf GAPs have been identified and characterized on the basis of activities other than Arf GAP. Here, we describe the Arf GAP family, enzymology of some members of the Arf GAP family and known functions of the proteins. The results discussed illustrate roles for both Arf-dependent and -independent activities in the regulation of cellular architecture.

Differential expression and targeting of endogenous Arf1 and Arf6 small GTPases in kidney epithelial cells in situ

ADP-ribosylation factors (Arfs) are small GTPases that regulate vesicular trafficking in exo- and endocytotic pathways. As a first step in understanding the role of Arfs in renal physiology, immunocytochemistry and Western blotting were performed to characterize the expression and targeting of Arf1 and Arf6 in epithelial cells in situ. Arf1 and Arf6 were associated with apical membranes and subapical vesicles in proximal tubules, where they colocalized with megalin. Arf1 was also apically expressed in the distal tubule, connecting segment, and collecting duct (CD). Arf1 was abundant in intercalated cells (IC) and colocalized with V-ATPase in A-IC (apical) and B-IC (apical and/or basolateral). In contrast, Arf6 was associated exclusively with basolateral membranes and vesicles in the CD. In the medulla, basolateral Arf6 was detectable mainly in A-IC. Expression in principal cells became weaker throughout the outer medulla, and Arf6 was not detectable in principal cells in the inner medulla. In some kidney epithelial cells Arf1 but not Arf6 was also targeted to a perinuclear patch, where it colocalized with TGN38, a marker of the trans-Golgi network. Quantitative Western blotting showed that expression of endogenous Arf1 was 26–180 times higher than Arf6. These data indicate that Arf GTPases are expressed and targeted in a cell- and membrane-specific pattern in kidney epithelial cells in situ. The results provide a framework on which to base and interpret future studies on the role of Arf GTPases in the multitude of cellular trafficking events that occur in renal tubular epithelial cells.

Somatodendritic localization of the mRNA for EFA6A, a guanine nucleotide exchange protein for ARF6, in rat hippocampus and its involvement in dendritic formation

EFA6A is a guanine nucleotide exchange protein (GEP) that can specifically activate ADP-ribosylation factor 6 (ARF6) in vitro. A recent study has demonstrated that ARF6 is involved in the dendritic formation of developing hippocampal neurons [Hernandez-Deviez et al. (2002) Nature Neurosci., 5, 623-624]. This study examined a potential role for EFA6A in hippocampal development in Wistar rats. Our results provided definitive evidence for somatodendritic localization of EFA6A mRNA in both cultured and in vivo hippocampal neurons by nonradioactive in situ hybridization. During postnatal development, EFA6A mRNA was dramatically increased and its dendritic localization was most evident between P7 and P14. In contrast, ARF6 mRNA was confined to the neuronal layers of the hippocampus throughout development. In addition, the overexpression of a GEP-defective mutant of EFA6A enhanced the dendritic formation of the primary hippocampal neurons. The present findings suggest that EFA6A is intimately involved in the regulation of the dendritic development of hippocampal neurons.

ARF6 controls post-endocytic recycling through its downstream exocyst complex effector

The small guanosine triphosphate (GTP)–binding protein ADP-ribosylation factor (ARF) 6 regulates membrane recycling to regions of plasma membrane remodeling via the endocytic pathway. Here, we show that GTP–bound ARF6 interacts with Sec10, a subunit of the exocyst complex involved in docking of vesicles with the plasma membrane. We found that Sec10 localization in the perinuclear region is not restricted to the trans-Golgi network, but extends to recycling endosomes. In addition, we report that depletion of Sec5 exocyst subunit or dominant inhibition of Sec10 affects the function and the morphology of the recycling pathway. Sec10 is found to redistribute to ruffling areas of the plasma membrane in cells expressing GTP-ARF6, whereas dominant inhibition of Sec10 interferes with ARF6-induced cell spreading. Our paper suggests that ARF6 specifies delivery and insertion of recycling membranes to regions of dynamic reorganization of the plasma membrane through interaction with the vesicle-tethering exocyst complex.

Role of ARF4L in recycling between endosomes and the plasma membrane

The human ADP-ribosylation factor-like protein, ARF4L is a member of the ARF family, which are small GTP-binding proteins that play significant roles in vesicle transport and protein secretion. However, little is known about the physiological roles of ARF4L. In this study, to understand the biological functions of ARF4L, we carried out immunocytochemical analysis of ARF4L molecules with mutations in the functional domains. ARF4L was shown to be distributed to the plasma membrane following binding to GTP (Q80L), and into endosomes following binding to GDP (T35N). Moreover, the inactive-form of ARF4L (T35N) causes localization of transferrin receptors to the endosomal compartment, while the active form (Q80L) causes transport to the plasma membrane. These findings indicate that ARF4L drive the transport of cargo protein and subsequent fusion of recycling vesicles with the plasma membrane for maintenance of the cell surface.

Phylogenetic analysis of Sec7-domain-containing Arf nucleotide exchangers

The eukaryotic family of ADP-ribosylation factor (Arf) GTPases plays a key role in the regulation of protein trafficking, and guanine-nucleotide exchange is crucial for Arf function. Exchange is stimulated by members of another family of proteins characterized by a 200-amino acid Sec7 domain, which alone is sufficient to catalyze exchange on Arf. Here, we analyzed the phylogeny of Sec7-domain-containing proteins in seven model organisms, representing fungi, plants, and animals. The phylogenetic tree has seven main groups, of which two include members from all seven model systems. Three groups are specific for animals, whereas two are specific for fungi. Based on this grouping, we propose a phylogenetically consistent set of names for members of the Sec7-domain family. Each group, except for one, contains proteins with known Arf exchange activity, implying that all members of this family have this activity. Contrary to the current convention, the sensitivity of Arf exchange activity to the inhibitor brefeldin A probably cannot be predicted by group membership. Multiple alignment reveals group-specific domains outside the Sec7 domain and a set of highly conserved amino acids within it. Determination of the importance of these conserved elements in Arf exchange activity and other cellular functions is now possible.

Actin microfilaments et al.--the many components, effectors and regulators of epithelial cell endocytosis

The aim of this review is to introduce the advances made over the past several years regarding the participation of actin and actin-associated proteins in clathrin-mediated endocytosis in simple cell models, and then to consider the evidence for the involvement of these effectors in apical clathrin-mediated endocytosis in epithelial cells. Basic mechanisms of clathrin-mediated endocytosis are initially addressed, followed by a detailed description of the actin cytoskeleton: its organization, function and, most importantly, the essential role played by proteins and signaling pathways responsible for the regulation of actin filament dynamics. Our focus then shifts to the GTPase, dynamin and its pivotal role as a bridge between various components of the clathrin endocytic machinery and the actin cytoskeleton. Mechanisms and effectors of dynamin-dependent endocytosis are then described, with a particular emphasis on novel proteins, which link dynamin to actin filaments. We consider additional effectors proposed to interact with actin to facilitate clathrin-mediated endocytosis in a dynamin-independent manner. The multiple roles which actin filaments are thought to play in endocytosis are addressed followed by a more detailed characterization of actin filament participation specifically in apical endocytosis. We conclude by discussing how these concepts may be integrated to improve drug internalization at the apical plasma membrane of epithelial cells.

HIV Nef-mediated major histocompatibility complex class I down-modulation is independent of Arf6 activity

HIV Nef has a number of important biological effects, including the down-modulation of several immunological important molecules (CD4, major histocompatibility complex [MHC] class I). Down-modulation of CD4 seems to be via clathrin-dependent endocytosis, whereas down-modulation of MHC class I remains unexplained. Several mutant proteins, including mutations in the small GTPase Arf6, have been used to probe membrane traffic pathways. One such mutant has recently been used to propose that Nef acts through Arf6 to activate the endocytosis of MHC class I. Here, we show that MHC class I down-modulation is unaffected by other Arf6 mutants that provide more specific perturbations in the GDP-GTP cycling of Arf6. Inhibition of phosphatidylinositol-3-phosphate kinase, an upstream activator of Arf6, also had no effect on the internalization step, but its activity is required to direct MHC class I to the trans-Golgi network. We conclude that the apparent Arf6 dependency of Nef-mediated MHC class I down-modulation is due to nonspecific perturbations in membrane traffic.

Selective interaction of ARF1 with the carboxy-terminal tail domain of the 5-HT2A receptor

The 5-hydroxytryptamine 2A receptor (5-HT2AR) is a member of the class I family of rhodopsin-related G protein-coupled receptors. The receptor is known to activate phospholipase C via the heterotrimeric G proteins Gq/11, but we showed previously that it can also signal through the phospholipase D (PLD) pathway in an ADP-ribosylation factor (ARF)-dependent manner that seems to be independent of Gq/11 (Mitchell et al., 1998). Both coimmunoprecipitation experiments and the effects of negative mutant ARF constructs on 5-HT2AR-induced PLD activation here suggested that ARF1 may play a greater role than ARF6 in the function of this receptor. Furthermore, we demonstrated using glutathione S-transferase (GST)-fusion proteins of receptor domains that ARF1 and ARF6 bind to the third intracellular loop (i3) and the carboxy terminal tail (ct) of the 5-HT2AR. The association of ARF1 with the ct domain of the receptor was stronger than its interaction with i3, or the interactions of ARF6 with either construct. Experiments using ARF mutants that are deficient in GTP loading, and the in vitro addition of GTPgammaS suggested that GTP loading enhances ARF1 binding to the receptor. The N376PxxY motif in the transmembrane 7 domain of the receptor (rather than a N376DPxxY mutant form) was shown to be essential for ARF-dependent PLD signaling and ARF1 coimmunoprecipitation. In GST-fusion proteins of the 5-HT2AR ct domain, mutation of Asn376 to Asp also markedly reduced ARF1-HA binding, although additional motifs in the Asn376-Asn384 sequence and to a lesser extent elsewhere, seem also to contribute to the interaction.

A Role for GEA1 and GEA2 in the Organization of the Actin Cytoskeleton in Saccharomyces cerevisiae

Profilin is an actin monomer-binding protein implicated in the polymerization of actin filaments. In the budding yeast Saccharomyces cerevisiae, the pfy1-111 rho2Δ double mutant has severe growth and actin cytoskeletal defects. The GEA1 and GEA2 genes, which code for paralog guanosine exchange factors for Arf proteins, were identified as multicopy suppressors of the mutant phenotype. These two genes restored the polarized distribution of actin cortical patches and produced visible actin cables in both the pfy1-111 rho2Δ and pfy1Δ cells. Thus, overexpression of GEA1 or GEA2 bypassed the requirement for profilin in actin cable formation. In addition, gea1 gea2 double mutants showed defects in budding and in actin cytoskeleton organization, while overexpression of GEA1 or GEA2 led to the formation of supernumerary actin cable-like structures in a Bni1p/Bnr1p-dependent manner. The ADP-ribosylation factor Arf3p may be a target of Gea1p/Gea2p, since overexpression of ARF3 partially suppressed the profilin-deficient phenotype and a deletion of ARF3 exacerbated the phenotype of a pfy1-111 mutant. Gea1p, Gea2p, Arf1p, and Arf2p but not Arf3p are known to function in vesicular transport between the endoplasmic reticulum and the Golgi. In this work, we demonstrate a role for Gea1p, Gea2p, and Arf3p in the organization of the actin cytoskeleton.

The intracellular trafficking of opioid receptors directed by carboxyl tail and a di-leucine motif in Neuro2A cells

The mu- and delta-opioid receptors (MOR and DOR) differ significantly in their intracellular trafficking. MORs recycle back to the cell surface upon agonist treatment, whereas most internalized DORs are targeted to lysosomes for degradation. By exchanging the carboxyl tail domains of MOR and DOR and expressing the receptor chimeras in mouse neuroblastoma Neuro2A cells, it could be demonstrated that the carboxyl tail domain is not the sole determinant in directing the intracellular trafficking in these Neuro2A cells. Deletion of the dileucine motif (Leu245-Leu246) within the third intracellular loop of DOR or the mutation of Leu245 to Met slowed the lysosomal targeting of these delta-opioid receptors. Meanwhile the mutation of Met264 to Leu increased the rate of agonist-induced receptor internalization and the lysosomal targeting of the wild type and the delta-opioid receptor carboxyl tail chimera of the mu-opioid receptor. These studies suggest interplay between a di-leucine motif and the carboxyl tail in the lysosomal targeting of the receptor.

Role for Arf3p in development of polarity, but not endocytosis, in Saccharomyces cerevisiae

ADP-ribosylation factors (ARFs) are ubiquitous regulators of virtually every step of vesicular membrane traffic. Yeast Arf3p, which is most similar to mammalian ARF6, is not essential for cell viability and not required for endoplasmic reticulum-to-Golgi protein transport. Although mammalian ARF6 has been implicated in the regulation of early endocytic transport, we found that Arf3p was not required for fluid-phase, membrane internalization, or mating-type receptor-mediated endocytosis. Arf3p was partially localized to the cell periphery, but was not detected on endocytic structures. The nucleotide-binding, N-terminal region, and N-terminal myristate of Arf3p are important for its proper localization. C-Terminally green fluorescent protein-tagged Arf3, expressed from the endogenous promoter, exhibited a polarized localization to the cell periphery and buds, in a cell cycle-dependent manner. Arf3-GFP achieved its proper localization during polarity growth through an actin-independent pathway. Both haploid and homologous diploid arf3 mutants exhibit a random budding defect, and the overexpression of the GTP-bound form Arf3p(Q71L) or GDP-binding defective Arf3p(T31N) mutant interfered with budding-site selection. We conclude that the GTPase cycle of Arf3p is likely to be important for the function of Arf3p in polarizing growth of the emerging bud and/or an unidentified vesicular trafficking pathway.

ARF6 regulates a plasma membrane pool of phosphatidylinositol(4,5)bisphosphate required for regulated exocytosis

ADP-ribosylation factor (ARF) 6 regulates endosomal plasma membrane trafficking in many cell types, but is also suggested to play a role in Ca2+-dependent dense-core vesicle (DCV) exocytosis in neuroendocrine cells. In the present work, expression of the constitutively active GTPase-defective ARF6Q67L mutant in PC12 cells was found to inhibit Ca2+-dependent DCV exocytosis. The inhibition of exocytosis was accompanied by accumulation of ARFQ67L, phosphatidylinositol 4,5-bisphosphate (PIP2), and the phosphatidylinositol 4-phosphate 5-kinase type I (PIP5KI) on endosomal membranes with their corresponding depletion from the plasma membrane. That the depletion of PIP2 and PIP5K from the plasma membrane caused the inhibition of DCV exocytosis was demonstrated directly in permeable cell reconstitution studies in which overexpression or addition of PIP5KIgamma restored Ca2+-dependent exocytosis. The restoration of exocytosis in ARF6Q67L-expressing permeable cells unexpectedly exhibited a Ca2+ dependence, which was attributed to the dephosphorylation and activation of PIP5K. Increased Ca2+ and dephosphorylation stimulated the association of PIP5KIgamma with ARF6. The results reveal a mechanism by which Ca2+ influx promotes increased ARF6-dependent synthesis of PIP2. We conclude that ARF6 plays a role in Ca2+-dependent DCV exocytosis by regulating the activity of PIP5K for the synthesis of an essential plasma membrane pool of PIP2.

Differential localization and function of ADP-ribosylation factor-6 in anergic human T cells: a potential marker for their identification

Anergy is a state of immunologic tolerance in which T cells are viable but incapable of responding to antigenic stimulation. Recent data indicate that anergic cells have a distinct gene expression program that determines their unique function. In this study we show that anergic human T cells selectively express the small GTPase ADP-ribosylation factor-6 (ARF6), which is involved in membrane traffic and regulation of the cortical actin cytoskeleton. ARF6 was expressed in the GTP-bound form that localizes at the plasma membrane, resulting in a distinct morphologic appearance of anergic cells. Forced expression of ARF6-GTP in Jurkat T cells prevented TCR-mediated reorganization of cortical actin, extracellular signal-regulated kinase1/2 activation, and IL-2 transcription. Forced expression of ARF6-GTP in primary human T cells inhibited extracellular signal-regulated kinase1/2 activation and proliferative responses. Importantly, T cells with the distribution pattern of ARF6-GTP were detected in peripheral blood, suggesting that anergic T cells may constitutively exist in vivo.

Regulation of phospholipase D1 subcellular cycling through coordination of multiple membrane association motifs

The signaling enzyme phospholipase D1 (PLD1) facilitates membrane vesicle trafficking. Here, we explore how PLD1 subcellular localization is regulated via Phox homology (PX) and pleckstrin homology (PH) domains and a PI4,5P2-binding site critical for its activation. PLD1 localized to perinuclear endosomes and Golgi in COS-7 cells, but on cellular stimulation, translocated to the plasma membrane in an activity-facilitated manner and then returned to the endosomes. The PI4,5P2-interacting site sufficed to mediate outward translocation and association with the plasma membrane. However, in the absence of PX and PH domains, PLD1 was unable to return efficiently to the endosomes. The PX and PH domains appear to facilitate internalization at different steps. The PH domain drives PLD1 entry into lipid rafts, which we show to be a step critical for internalization. In contrast, the PX domain appears to mediate binding to PI5P, a lipid newly recognized to accumulate in endocytosing vesicles. Finally, we show that the PH domain-dependent translocation step, but not the PX domain, is required for PLD1 to function in regulated exocytosis in PC12 cells. We propose that PLD1 localization and function involves regulated and continual cycling through a succession of subcellular sites, mediated by successive combinations of membrane association interactions.

Arfophilins are dual Arf/Rab 11 binding proteins that regulate recycling endosome distribution and are related to Drosophila nuclear fallout

Arfophilin is an ADP ribosylation factor (Arf) binding protein of unknown function. It is identical to the Rab11 binding protein eferin/Rab11-FIP3, and we show it binds both Arf5 and Rab11. We describe a related protein, arfophilin-2, that interacts with Arf5 in a nucleotide-dependent manner, but not Arf1, 4, or 6 and also binds Rab11. Arfophilin-2 localized to a perinuclear compartment, the centrosomal area, and focal adhesions. The localization of arfophilin-2 to the perinuclear compartment was selectively blocked by overexpression of Arf5-T31N. In contrast, a green fluorescent protein-arfophilin-2 chimera or arfophilin-2 deletions were localized around the centrosome in a region that was also enriched for transferrin receptors and Rab11 but not early endosome markers, suggesting that the distribution of the endosomal recycling compartment was altered. The arfophilins belong to a conserved family that includes Drosophila melanogaster nuclear fallout, a centrosomal protein required for cellularization. Expression of green fluorescent protein-nuclear fallout in HeLa cells resulted in a similar phenotype, indicative of functional homology and thus implicating the arfophilins in mitosis/cytokinesis. We suggest that the novel dual GTPase-binding capacity of the arfophilins could serve as an interface of signals from Rab and Arf GTPases to regulate membrane traffic and integrate distinct signals in the late endosomal recycling compartment.

ADP ribosylation factor 6 is activated and controls membrane delivery during phagocytosis in macrophages

Engulfment of particles by phagocytes is induced by their interaction with specific receptors on the cell surface, which leads to actin polymerization and the extension of membrane protrusions to form a closed phagosome. Membrane delivery from internal pools is considered to play an important role in pseudopod extension during phagocytosis. Here, we report that endogenous ADP ribosylation factor 6 (ARF6), a small GTP-binding protein, undergoes a sharp and transient activation in macrophages when phagocytosis was initiated via receptors for the Fc portion of immunoglobulins (FcRs). A dominant-negative mutant of ARF6 (T27N mutation) dramatically affected FcR-mediated phagocytosis. Expression of ARF6-T27N lead to a reduction in the focal delivery of vesicle-associated membrane protein 3+ endosomal recycling membranes at phagocytosis sites, whereas actin polymerization was unimpaired. This resulted in an early blockade in pseudopod extension and accumulation of intracellular vesicles, as observed by electron microscopy. We conclude that ARF6 is a major regulator of membrane recycling during phagocytosis.