Regulation of protein transport from the Golgi complex to the endoplasmic reticulum by CDC42 and N-WASP

Regulation of breast cancer cell motility by T-cell lymphoma invasion and metastasis-inducing protein

Introduction

T-cell lymphoma invasion and metastasis-inducing protein (Tiam1) is an Ras-related C3 botulinum toxin substrate (Rac)-specific guanine nucleotide exchange factor that was isolated based on its ability to induce a metastatic phenotype. In polarized migrating keratinocytes, Tiam1 is found at the leading edge, where it cooperates with the protease-activated receptor 1 (Par1) complex to establish front-to-rear polarity. Although a positive correlation has been observed between Tiam1 expression and tumor grade in a variety of human malignancies, including breast, its role in breast cancer cells has not yet been examined.

Methods

Tiam1 expression and Rac activity were examined in a panel of human breast cancer cell lines that exhibit different degrees of cell motility. The contribution of Tiam1 to cell motility was directly examined by using transwell motility and wound-healing assays.

Results

Although we observed a striking, positive correlation between Tiam1 expression and cell motility in the panel of breast cancer cell lines, we did not observe a correlation between Tiam1 expression and overall levels of Rac activity. Consistent with this, small interfering ribonucleic acid (siRNA)-mediated suppression of Tiam1 expression limits the motility of cell lines in which Tiam1 expression is high (MDA-MB-231 and MDA-MB-453) but does not substantially alter the overall levels of activated Rac. Tiam1 overexpression is also not sufficient to increase the motility of more poorly motile cells (T-47D) or to increase Rac activity. Immunofluorescence and cellular fractionations indicate that Tiam1 is found predominantly in the Golgi of breast cancer cells, and in the latter case, Tiam1 was shown to co-fractionate with a limited pool of Rac1. Consistent with this Golgi localization, Tiam1 supports cell motility and Golgi reorientation in response to serum in a wound-healing assay using MDA-MB-231 and MDA-MB-435S cells.

Conclusions

Tiam1 expression correlates with cell motility in human breast cancer cells and is required to support the motile phenotype. Localization of endogenous Tiam1 to the Golgi, and its demonstrated role in Golgi reorientation, suggest that it may support motility through a mechanism that is discrete from its known function in leading-edge dynamics.

Role of EBAG9 protein in coat protein complex I-dependent glycoprotein maturation and secretion processes in tumor cells

Many proteins mature within the secretory pathway by the acquisition of glycans. Failure to maintain the proper distribution of the glycosylation machinery might lead to disease. High expression levels of the ubiquitous Golgi protein estrogen receptor-binding fragment-associated gene 9 (EBAG9) in human tumors correlate with poor clinical prognosis, and EBAG9 overexpression in epithelial cell lines induces truncated glycans, typical of many carcinomas. Here, we addressed the pathogenetic link between EBAG9 expression and the alteration of the cellular glycome. We applied confocal microscopy, live imaging, pulse-chase labeling in conjunction with immunoprecipitation, and enzymatic activity assays in a variety of EBAG9-overexpressing or depleted epithelial tumor cell lines. EBAG9 shuttles between the ER-Golgi intermediate compartment and the cis-Golgi, and we demonstrate association of EBAG9 with coat protein complex I (COPI)-coated transport vesicles. EBAG9 overexpression imposes delay of endoplasmic reticulum-to-Golgi transport and mislocalizes components of the ER quality control and glycosylation machinery. Conversely, EBAG9 down-regulation accelerates glycoprotein transport through the Golgi and enhances mannosidase activity. Thus, EBAG9 acts as a negative regulator of a COPI-dependent ER-to-Golgi transport pathway in epithelial cells and represents a novel pathogenetic principle in which interference with intracellular membrane trafficking results in the emergence of a tumor-associated glycome.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Src-family kinase signaling, actin-mediated membrane trafficking and organellar dynamics in the control of cell fate: lessons to be learned from the adenovirus E4orf4 death factor

Evidence has accumulated that there are different modes of regulated cell death, which share overlapping signaling pathways. Cytoskeletal-dependent inter-organellar communication as a result of protein and lipid trafficking in and out of organelles has emerged as a common, key issue in the regulation of cell death modalities. The movement of proteins and lipids between cell compartments is believed to relay death signals in part through modifications of organelles dynamics. Little is known, however, regarding how trafficking is integrated within stress signaling pathways directing organelle-specific remodeling events. In this review, we discuss emerging evidence supporting a role for regulated changes in actin dynamics and intracellular membrane flow. Based on recent findings using the adenovirus E4orf4 death factor as a probing tool to tackle the mechanistic underpinnings that control alternative modes of cell death, we propose the existence of multifunctional platforms at the endosome-Golgi interface regulated by SFK-signaling. These endosomal platforms could be mobilized during cell activation processes to reorganize cellular membranes and promote inter-organelle signaling.

Phosphatidylinositol- and phosphatidylcholine-transfer activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the endoplasmic reticulum

Vesicles formed by the COPI complex function in retrograde transport from the Golgi to the endoplasmic reticulum (ER). Phosphatidylinositol transfer protein beta (PITPbeta), an essential protein that possesses phosphatidylinositol (PtdIns) and phosphatidylcholine (PtdCho) lipid transfer activity is known to localise to the Golgi and ER but its role in these membrane systems is not clear. To examine the function of PITPbeta at the Golgi-ER interface, RNA interference (RNAi) was used to knockdown PITPbeta protein expression in HeLa cells. Depletion of PITPbeta leads to a decrease in PtdIns(4)P levels, compaction of the Golgi complex and protection from brefeldin-A-mediated dispersal to the ER. Using specific transport assays, we show that anterograde traffic is unaffected but that KDEL-receptor-dependent retrograde traffic is inhibited. This phenotype can be rescued by expression of wild-type PITPbeta but not by mutants defective in docking, PtdIns transfer and PtdCho transfer. These data demonstrate that the PtdIns and PtdCho exchange activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the ER.

Digital holographic microscopy real-time monitoring of cytoarchitectural alterations during simulated microgravity

Previous investigations on mammalian cells have shown that microgravity, either that experienced in space, or simulated on earth, causes severe cellular modifications that compromise tissue determination and function. The aim of this study is to investigate, in real time, the morphological changes undergone by cells experiencing simulated microgravity by using digital holographic microscopy (DHM). DHM analysis of living mouse myoblasts (C2C12) is undertaken under simulated microgravity with a random positioning machine. The DHM analysis reveals cytoskeletal alterations similar to those previously reported with conventional methods, and in agreement with conventional brightfield fluorescence microscopy a posteriori investigation. Indeed, DHM is shown to be able to noninvasively and quantitatively detect changes in actin reticular formation, as well as actin distribution, in living unstained samples. Such results were previously only obtainable with the use of labeled probes in conjunction with conventional fluorescence microscopy, with all the classically described limitations in terms of bias, bleaching, and temporal resolution.

In vivo roles for Arp2/3 in cortical actin organization during C. elegans gastrulation

The Arp2/3 complex is important for morphogenesis in various developmental systems, but specific in vivo roles for this complex in cells that move during morphogenesis are not well understood. We have examined cellular roles for Arp2/3 in the Caenorhabditis elegans embryo. In C. elegans, the first morphogenetic movement, gastrulation, is initiated by the internalization of two endodermal precursor cells. These cells undergo a myosin-dependent apical constriction, pulling a ring of six neighboring cells into a gap left behind on the ventral surface of the embryo. In agreement with a previous report, we found that in Arp2/3-depleted C. elegans embryos, membrane blebs form and the endodermal precursor cells fail to fully internalize. We show that these cells are normal with respect to several key requirements for gastrulation: cell cycle timing, cell fate, apicobasal cell polarity and apical accumulation and activation of myosin-II. To further understand the function of Arp2/3 in gastrulation, we examined F-actin dynamics in wild-type embryos. We found that three of the six neighboring cells extend short, dynamic F-actin-rich processes at their apical borders with the internalizing cells. These processes failed to form in embryos that were depleted of Arp2/3 or the apical protein PAR-3. Our results identify an in vivo role for Arp2/3 in the formation of subcellular structures during morphogenesis. The results also suggest a new layer to the model of C. elegans gastrulation: in addition to apical constriction, internalization of the endoderm might involve dynamic Arp2/3-dependent F-actin-rich extensions on one side of a ring of cells.

Alterations in molecular chaperones and eIF2alpha during lung endothelial cell apoptosis

We have previously demonstrated that inhibition of CAAX carboxyl methylation with AGGC caused redistribution and condensation of the ER molecular chaperones, glucose-regulated protein (GRP)-94 and calnexin; an effect that was attenuated by overexpression of dominant active RhoA. We have also shown that AGGC decreased GRP94 protein level; an effect that was dependent on caspase activity. In the present study, we tested the effects of inhibition of posttranslational processing of CAAX proteins on localization and protein levels of molecular chaperones and phosphorylation and protein level of eIF2alpha. We found that both AGGC, which inhibits CAAX carboxyl methylation, and simvastatin, which inhibits CAAX geranylgeranylation, caused relocalization of GRP94, calnexin, and calreticulin, effects that were not seen during endothelial apoptosis induced by TNF-alpha or ultraviolet (UV) irradiation. These results suggest that posttranslational processing of CAAX proteins is important in maintaining localization of molecular chaperones normally found in the ER. We also noted that AGGC, but not simvastatin, TNF-alpha, or UV irradiation, decreased protein levels of most molecular chaperones. Increased eIF2alpha phosphorylation was observed in the early stages of apoptosis, which was independent of the cause of apoptosis. These results suggest that eIF2alpha phosphorylation is a common early response to apoptosis-inducing stimuli. Interestingly, eIF2alpha protein level was decreased in the late stages of apoptosis induced by AGGC, TNF-alpha, and UV irradiation: an effect that was prevented by caspase inhibition. Thus we speculate that caspase(s)-dependent proteolysis of molecular chaperones and eIF2alpha may be novel signaling pathways of apoptosis. We also speculate that increased eIF2alpha phosphorylation is a defensive response against endothelial cell apoptosis.

Retrograde Shiga toxin trafficking is regulated by ARHGAP21 and Cdc42

Shiga-toxin-producing Escherichia coli remain a food-borne health threat. Shiga toxin is endocytosed by intestinal epithelial cells and transported retrogradely through the secretory pathway. It is ultimately translocated to the cytosol where it inhibits protein translation. We found that Shiga toxin transport through the secretory pathway was dependent on the cytoskeleton. Recent studies reveal that Shiga toxin activates signaling pathways that affect microtubule reassembly and dynein-dependent motility. We propose that Shiga toxin alters cytoskeletal dynamics in a way that facilitates its transport through the secretory pathway. We have now found that Rho GTPases regulate the endocytosis and retrograde motility of Shiga toxin. The expression of RhoA mutants inhibited endocytosis of Shiga toxin. Constitutively active Cdc42 or knockdown of the Cdc42-specific GAP, ARHGAP21, inhibited the transport of Shiga toxin to the juxtanuclear Golgi apparatus. The ability of Shiga toxin to stimulate microtubule-based transferrin transport also required Cdc42 and ARHGAP21 function. Shiga toxin addition greatly decreases the levels of active Cdc42-GTP in an ARHGAP21-dependent manner. We conclude that ARHGAP21 and Cdc42-based signaling regulates the dynein-dependent retrograde transport of Shiga toxin to the Golgi apparatus.

Direct interaction between the COG complex and the SM protein, Sly1, is required for Golgi SNARE pairing

The crucial roles of Sec1/Munc18 (SM)-like proteins in membrane fusion have been evidenced in genetic and biochemical studies. SM proteins interact directly with SNAREs and contribute to SNARE pairing by a yet unclear mechanism. Here, we show that the SM protein, Sly1, interacts directly with the conserved oligomeric Golgi (COG) tethering complex. The Sly1-COG interaction is mediated by the Cog4 subunit, which also interacts with Syntaxin 5 through a different binding site. We provide evidence that disruption of Cog4-Sly1 interaction impairs pairing of SNAREs involved in intra-Golgi transport thereby markedly attenuating Golgi-to-ER retrograde transport. These results highlight the mechanism by which SM proteins link tethering to SNAREpin assembly.

Faciogenital dysplasia protein (FGD1) regulates export of cargo proteins from the golgi complex via Cdc42 activation

Mutations in the FGD1 gene are responsible for the X-linked disorder known as faciogenital dysplasia (FGDY). FGD1 encodes a guanine nucleotide exchange factor that specifically activates the GTPase Cdc42. In turn, Cdc42 is an important regulator of membrane trafficking, although little is known about FGD1 involvement in this process. During development, FGD1 is highly expressed during bone growth and mineralization, and therefore a lack of the functional protein leads to a severe phenotype. Whether the secretion of proteins, which is a process essential for bone formation, is altered by mutations in FGD1 is of great interest. We initially show here that FGD1 is preferentially associated with the trans-Golgi network (TGN), suggesting its involvement in export of proteins from the Golgi. Indeed, expression of a dominant-negative FGD1 mutant and RNA interference of FGD1 both resulted in a reduction in post-Golgi transport of various cargoes (including bone-specific proteins in osteoblasts). Live-cell imaging reveals that formation of post-Golgi transport intermediates directed to the cell surface is inhibited in FGD1-deficient cells, apparently due to an impairment of TGN membrane extension along microtubules. These effects depend on FGD1 regulation of Cdc42 activation and its association with the Golgi membranes, and they may contribute to FGDY pathogenesis.

Bicaudal-C associates with a Trailer Hitch/Me31B complex and is required for efficient Gurken secretion

Bicaudal-C (Bic-C) is a multiple KH-domain RNA-binding protein required for Drosophila oogenesis and, maternally, for embryonic patterning. In early oogenesis, Bic-C negatively regulates target mRNAs, including Bic-C, by recruiting the CCR4 deadenylase through a direct association with its NOT3 subunit. Here, we identify a novel function for Bic-C in secretion of the TGF-alpha homolog Gurken (Grk). In Bic-C mutant egg chambers, Grk is sequestered within actin-coated structures during mid-oogenesis. As a consequence, Egfr signalling is not efficiently activated in the dorsal-anterior follicle cells. This phenotype is strikingly similar to that of trailer hitch (tral) mutants. Consistent with the idea that Bic-C and Tral act together in Grk secretion, Bic-C co-localizes with Tral within cytoplasmic granules, and can be co-purified with multiple protein components of a Tral mRNP complex. Taken together, our results implicate translational regulation by Bic-C and Tral in the secretory pathway.

GM130-dependent control of Cdc42 activity at the Golgi regulates centrosome organization

The physical proximity of the Golgi apparatus and the centrosome is a unique feature of mammalian cells whose functional significance is only poorly understood. Here, we demonstrate that the previously described regulation of centrosome organization and function by the Golgi protein, GM130, involves a Golgi-associated complex consisting of GM130, the Rho GTPase, Cdc42, and its guanine nucleotide exchange factor, Tuba. We identified Tuba as a novel GM130-interacting protein and showed that this association controls Tuba-mediated activation of Cdc42 at the Golgi apparatus. Blocking either Tuba or Cdc42 activity reproduced the GM130 depletion phenotype of aberrant, nonfunctional centrosomes. Expression of constitutively active Cdc42 bypassed the requirement for GM130 in centrosome regulation, indicating that Cdc42 functions downstream of GM130. Our studies demonstrate that Cdc42 has a novel role in controlling centrosome organization in unstimulated cells in addition to its known function as a regulator of centrosome reorientation in stimulated cells. This first description of a regulatory pathway between the Golgi apparatus and the interphase centrosome that complements the known role of Golgi proteins in controlling spindle formation during mitosis and may provide an explanation for the pericentriolar position of the mammalian Golgi apparatus during interphase.

LIM kinase 1 and cofilin regulate actin filament population required for dynamin-dependent apical carrier fission from the trans-Golgi network

The functions of the actin cytoskeleton in post-Golgi trafficking are still poorly understood. Here, we report the role of LIM Kinase 1 (LIMK1) and its substrate cofilin in the trafficking of apical and basolateral proteins in Madin-Darby canine kidney cells. Our data indicate that LIMK1 and cofilin organize a specialized population of actin filaments at the Golgi complex that is selectively required for the emergence of an apical cargo route to the plasma membrane (PM). Quantitative pulse-chase live imaging experiments showed that overexpression of kinase-dead LIMK1 (LIMK1-KD), or of LIMK1 small interfering RNA, or of an activated cofilin mutant (cofilin S3A), selectively slowed down the exit from the trans-Golgi network (TGN) of the apical PM marker p75-green fluorescent protein (GFP) but did not interfere with the apical PM marker glycosyl phosphatidylinositol-YFP or the basolateral PM marker neural cell adhesion molecule-GFP. High-resolution live imaging experiments of carrier formation and release by the TGN and analysis of peri-Golgi actin dynamics using photoactivatable GFP suggest a scenario in which TGN-localized LIMK1-cofilin regulate a population of actin filaments required for dynamin-syndapin-cortactin-dependent generation and/or fission of precursors to p75 transporters.

DOCK10-mediated Cdc42 activation is necessary for amoeboid invasion of melanoma cells

BACKGROUND

Tumor cells can move in a three-dimensional (3D) environment in either mesenchymal-type or amoeboid modes. In mesenchymal-type movement, cells have an elongated morphology with Rac-induced protrusions at the leading edge. Amoeboid cells have high levels of actomyosin contractility, and movement is associated with deformation of the cell body through the matrix without proteolysis. Because signaling pathways that control the activation of GTPases for amoeboid movement are poorly understood, we sought to identify regulators of amoeboid movement by screening an siRNA library targeting guanine nucleotide exchange factors (GEFs) for Rho-family GTPases.

RESULTS

We identified DOCK10, a Cdc42 GEF, as a key player in amoeboid migration; accordingly, we find that expression of activated Cdc42 induces a mesenchymal-amoeboid transition and increases cell invasion. Silencing DOCK10 expression promotes conversion to mesenchymal migration and is associated with decreased MLC2 phosphorylation and increased Rac1 activation. Consequently, abrogating DOCK10 and Rac1 expression suppresses both amoeboid and mesenchymal migration and results in decreased invasion. We show that the Cdc42 effectors N-WASP and Pak2 are required for the maintenance of the rounded-amoeboid phenotype. Blocking Cdc42 results in loss of mesenchymal morphology, arguing that Cdc42 is also involved in mesenchymal morphology through different activation and effector pathways.

CONCLUSIONS

Previous work has identified roles of Rho and Rac signaling in tumor cell movement, and we now elucidate novel roles of Cdc42 signaling in amoeboid and mesenchymal movement and tumor cell invasion.

PX-RICS mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex

Cadherins mediate Ca2+-dependent cell-cell adhesion. Efficient export of cadherins from the endoplasmic reticulum (ER) is known to require complex formation with beta-catenin. However, the molecular mechanisms underlying this requirement remain elusive. Here we show that PX-RICS, a beta-catenin-interacting GTPase-activating protein (GAP) for Cdc42, mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex. Knockdown of PX-RICS expression induced the accumulation of the N-cadherin/beta-catenin complex in the ER and ER exit site, resulting in a decrease in cell-cell adhesion. PX-RICS was also required for ER-to-Golgi transport of the fibroblast growth factor-receptor 4 (FGFR4) associated with N-cadherin. PX-RICS-mediated ER-to-Golgi transport was dependent on its interaction with beta-catenin, phosphatidylinositol-4-phosphate (PI4P), Cdc42, and its novel binding partner gamma-aminobutyric acid type A receptor-associated protein (GABARAP). These results suggest that PX-RICS ensures the efficient entry of the N-cadherin/beta-catenin complex into the secretory pathway, and thereby regulates the amount of N-cadherin available for cell adhesion and FGFR4-mediated signaling.

GTPase-mediated regulation of the unfolded protein response in Caenorhabditis elegans is dependent on the AAA+ ATPase CDC-48

When endoplasmic reticulum (ER) homeostasis is perturbed, an adaptive mechanism is triggered and named the unfolded protein response (UPR). Thus far, three known UPR signaling branches (IRE-1, PERK, and ATF-6) mediate the reestablishment of ER functions but can also lead to apoptosis if ER stress is not alleviated. However, the understanding of the molecular mechanisms integrating the UPR to other ER functions, such as membrane traffic or endomembrane signaling, remains incomplete. We consequently sought to identify new regulators of UPR-dependent transcriptional mechanisms and focused on a family of proteins known to mediate, among other, ER-related functions: the small GTP-binding proteins of the RAS superfamily. To this end, we used transgenic UPR reporter Caenorhabditis elegans strains as a model to specifically silence small-GTPase expression. We show that the Rho subfamily member CRP-1 is an essential component of UPR-induced transcriptional events through its physical and genetic interactions with the AAA+ ATPase CDC-48. In addition, we describe a novel signaling module involving CRP-1 and CDC-48 which may directly link the UPR to DNA remodeling and transcription control.

Regulation of actin cytoskeleton dynamics by Arf-family GTPases

Small GTPases of the Arf family are best known for their role in vesicular transport, wherein they nucleate the assembly of coat proteins at sites of carrier vesicle formation. However, accumulating evidence indicates that the Arfs are also important regulators of actin cytoskeleton dynamics and are involved in a variety of actin-based processes, including cell adhesion, migration and neurite outgrowth. The mechanisms of this regulation are remarkably diverse, ranging from the integration of vesicular transport with cytoskeleton assembly to the direct regulation of Rho-family GTPase function. Here, we review recent progress in our understanding of how Arfs and their interacting proteins function to integrate membrane and cytoskeletal dynamics.

Cdc42-interacting protein-4 functionally links actin and microtubule networks at the cytolytic NK cell immunological synapse

An essential function of the immunological synapse (IS) is directed secretion. NK cells are especially adept at this activity, as they direct lytic granules to the synapse for secretion, which enables cytotoxicity and facilitates host defense. This initially requires rearrangement of the actin cytoskeleton and, subsequently, microtubule-dependent trafficking of the lytic granules. As these two steps are sequential, specific linkages between them are likely to serve as critical regulators of cytotoxicity. We studied Cdc42-interacting protein-4 (CIP4), which constitutively interacts with tubulin and microtubules but focuses to the microtubule organizing center (MTOC) after NK cell activation, when it is able to associate with Wiskott-Aldrich syndrome protein (WASp) and the actin filament-rich IS. WASp deficiency, overexpression of CIP4, or parts of CIP4 interfere with this union and block normal CIP4 localization, MTOC polarization to the IS, and cytotoxicity. Reduction of endogenous CIP4 expression using small interfering RNA similarly inhibits MTOC polarization and cytotoxic activity but does not impair actin filament accumulation at the IS, or Cdc42 activation. Thus, CIP4 is an important cytoskeletal adaptor that functions after filamentous actin accumulation and Cdc42 activation to enable MTOC polarization and NK cell cytotoxicity.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Actin filaments are involved in the maintenance of Golgi cisternae morphology and intra-Golgi pH

Here we examine the contribution of actin dynamics to the architecture and pH of the Golgi complex. To this end, we have used toxins that depolymerize (cytochalasin D, latrunculin B, mycalolide B, and Clostridium botulinum C2 toxin) or stabilize (jasplakinolide) filamentous actin. When various clonal cell lines were examined by epifluorescence microscopy, all of these actin toxins induced compaction of the Golgi complex. However, ultrastructural analysis by transmission electron microscopy and electron tomography/three-dimensional modelling of the Golgi complex showed that F-actin depolymerization first induces perforation/fragmentation and severe swelling of Golgi cisternae, which leads to a completely disorganized structure. In contrast, F-actin stabilization results only in cisternae perforation/fragmentation. Concomitantly to actin depolymerization-induced cisternae swelling and disorganization, the intra-Golgi pH significantly increased. Similar ultrastructural and Golgi pH alkalinization were observed in cells treated with the vacuolar H+ -ATPases inhibitors bafilomycin A1 and concanamycin A. Overall, these results suggest that actin filaments are implicated in the preservation of the flattened shape of Golgi cisternae. This maintenance seems to be mediated by the regulation of the state of F-actin assembly on the Golgi pH homeostasis.

Requirement for Akt-mediated survival in cell transformation by the dbl oncogene

The dbl oncogene product is the founding member of a large family of oncogenic proteins that function by activating the small GTP-binding proteins Cdc42, Rac and Rho. Through its substrate GTPases, Dbl transduces proliferative signals from cell-surface receptors to diverse cellular effectors and signaling pathways. The mechanisms by which these multiple signals are integrated, as well as their relative contribution to Dbl-induced cell transformation, are presently poorly understood. We investigated the role of the survival regulators PI3-kinase and Akt in Dbl-induced cell transformation. We found that Dbl induced the phosphorylation of Akt on threonine 308, through the GTPases Rac and Cdc42 and in a PI3-kinase dependent manner. Pharmacological or biochemical interference with this pathway lead to a marked, dose-dependent inhibition of the focus formation activity exhibited by Dbl-expressing cells. Dbl expression stimulated the phosphorylation of the anti-apoptotic Akt substrate Bad, and caused a marked decrease in basal levels of apoptosis. Finally, we found that activated Cdc42 existed in cells in complex with phosphoionositide-dependent kinase-1 (PDK1), the downstream mediator of PI3-kinase action. The data indicate that Dbl signaling stimulate the formation of a novel survival complex, through which anti-apoptotic signals are generated and propagated.

The ARP2/3 complex: an actin nucleator comes of age

The cellular functions of the actin cytoskeleton require precise regulation of both the initiation of actin polymerization and the organization of the resulting filaments. The actin-related protein-2/3 (ARP2/3) complex is a central player in this regulation. A decade of study has begun to shed light on the molecular mechanisms by which this powerful machine controls the polymerization, organization and recycling of actin-filament networks, both in vitro and in the living cell.

Rho GTPases and actin dynamics in membrane protrusions and vesicle trafficking

Rho GTPases are well known to regulate actin dynamics. They activate two types of actin nucleators, WASP/WAVE proteins and Diaphanous-related formins (DRFs), which induce different types of actin organization. Their ability to interact with membranes allows them to target actin polymerization to discrete sites on the plasma membrane and to intracellular membrane compartments and thereby induce membrane protrusions or regulate vesicle movement. Most studies have concentrated on just three of the 22 mammalian Rho proteins, RhoA, Rac1 and Cdc42. However, recent research indicates that several other members of the Rho family, including Rif, RhoD, TC10 and Wrch1, and also related Rho-of-plants proteins (ROPs) in plants, stimulate actin polymerization and affect plasma membrane protrusion and/or vesicular traffic.

Adenovirus E4orf4 hijacks rho GTPase-dependent actin dynamics to kill cells: a role for endosome-associated actin assembly

The adenovirus early region 4 ORF4 protein (E4orf4) triggers a novel death program that bypasses classical apoptotic pathways in human cancer cells. Deregulation of the cell cytoskeleton is a hallmark of E4orf4 killing that relies on Src family kinases and E4orf4 phosphorylation. However, the cytoskeletal targets of E4orf4 and their role in the death process are unknown. Here, we show that E4orf4 translocates to cytoplasmic sites and triggers the assembly of a peculiar juxtanuclear actin-myosin network that drives polarized blebbing and nuclear shrinkage. We found that E4orf4 activates the myosin II motor and triggers de novo actin polymerization in the perinuclear region, promoting endosomes recruitment to the sites of actin assembly. E4orf4-induced actin dynamics requires interaction with Src family kinases and involves a spatial regulation of the Rho GTPases pathways Cdc42/N-Wasp, RhoA/Rho kinase, and Rac1, which make distinct contributions. Remarkably, activation of the Rho GTPases is required for induction of apoptotic-like cell death. Furthermore, inhibition of actin dynamics per se dramatically impairs E4orf4 killing. This work provides strong support for a causal role for endosome-associated actin dynamics in E4orf4 killing and in the regulation of cancer cell fate.

Dictyostelium RacH Regulates Endocytic Vesicular Trafficking and is Required for Localization of Vacuolin

Dictyostelium RacH localizes predominantly to membranes of the nuclear envelope, endoplasmic reticulum and Golgi apparatus. To investigate the role of this protein, we generated knockout and overexpressor strains. RacH-deficient cells displayed 50% reduced fluid-phase uptake and a moderate exocytosis defect, but phagocytosis was unaffected. Detailed examination of the endocytic pathway revealed defective acidification of early endosomes and reduced secretion of acid phosphatase in the presence of sucrose. The distribution of the post-lysosomal marker vacuolin was altered, with a high proportion of cells showing a diffuse vesicular pattern in contrast to the wild-type strain, where few intensely stained vacuoles predominate. Cytokinesis, cell motility, chemotaxis and development appeared largely unaffected. In a cell-free system, RacH stimulates actin polymerization, suggesting that this protein is involved in actin-based trafficking of vesicular compartments. We also investigated the determinants of subcellular localization of RacH by expression of green-fluorescent-protein-tagged chimeras in which the C-terminus of RacH and the plasma-membrane-targeted RacG were exchanged, the insert region was deleted or the net positive charge of the hypervariable region was increased. We show that several regions of the molecule, not only the hypervariable region, determine targeting of RacH. Overexpression of mistargeted RacH mutants did not recapitulate the phenotypes of a strain overexpressing nonmutated RacH, indicating that the function of this protein is in great part related to its subcellular localization.

The physiology of membrane transport and endomembrane-based signalling

Some of the important open questions concerning the physiology of the secretory pathway relate to its homeostasis. Secretion involves a number of separate compartments for which their transport activities should be precisely cross-coordinated to avoid gross imbalances in the trafficking system. Moreover, the membrane fluxes across these compartments should be able to adapt to environmental 'requests' and to respond to extracellular signals. How is this regulation effected? Here, we consider evidence that endomembrane-based signalling cascades that are similar in organization to those used at the plasma membrane coordinate membrane traffic. If this is the case, this would also represent a model for a more general inter-organelle signalling network for functionally interconnecting different intracellular activities, a necessity for the maintenance of cellular homeostasis and to express harmonic global cellular responses.

ARF proteins: roles in membrane traffic and beyond

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

ARP2 and ARP3 are localized to sites of actin filament nucleation in tobacco BY-2 cells

Complete depolymerization of actin filaments (AFs) at low temperature (0 degrees C) is followed by the formation of transient actin structures at 25 degrees C in tobacco BY-2 cells (Nicotiana tabacum L.). Using antibodies against fission yeast actin-related proteins (ARP2 and ARP3), we show here that transient actin structures (dots, dotted filaments, rods) colocalize with epitopes stained by these antibodies and thus are likely to represent sites of actin filament nucleation (SANs). In contrast to the cold-induced disassembly of AFs, no transient actin structures were detectable during recovery of AFs from latrunculin B-induced depolymerization. However, the staining pattern obtained with ARP antibodies in latrunculin B-treated cells was similar to that in controls and cold-treated cells. This suggests that, in addition to the complete depolymerization of AFs, disruption of other cellular structures is needed for the formation of transient actin structures during the early phase of recovery from cold treatment.

The Actin-depolymerizing Factor Homology and Charged/Helical Domains of Drebrin and mAbp1 Direct Membrane Binding and Localization via Distinct Interactions with Actin

Cytoskeletal dynamics are important for efficient function of the secretory pathway. ADP-ribosylation factor, ARF1, triggers vesicle coat assembly and, in concert with Cdc42, regulates actin polymerization and molecular motor-based motility. Drebrin and mammalian Abp1 (mAbp1) are actin-binding proteins found previously to bind to Golgi membranes in an ARF1-dependent manner in vitro. Despite sharing homology through two shared actin binding domains, drebrin and mAbp1 have different subcellular localization and bind to distinct actin structures on the Golgi apparatus. We find that the actin-depolymerizing factor homology (ADFH) and charged/helical actin binding domains of drebrin and mAbp1 are sufficient for regulated binding to Golgi membranes and subcellular localization. We have used mutant proteins and chimeras between mAbp1 and drebrin to identify motifs that direct targeting. We find that a linker region between the ADFH and charged/helical domains confers Golgi binding properties to mAbp1. mAbp1 binds to a specific actin pool through its ADFH/linker domain that is not bound by drebrin. Drebrin localization to the cell surface was found to involve motifs within the charged/helical domain. Our results indicate that targeting of these proteins is directed through multiple distinct interactions with the actin cytoskeleton. The mechanisms for selective recruitment of mAbp1 and drebrin to Golgi membranes indicate how actin-based structures are able to select specific actin-binding proteins and, thus, carry out multiple different functions within cells.

In vitro reconstitution of ARF-regulated cytoskeletal dynamics on Golgi membranes

Function of the secretory pathway is intimately connected to the cytoskeleton. Cytoskeletal dynamics and molecular motors are involved in organelle morphology and positioning, as well as the formation and translocation of trafficking intermediates such as vesicles. At least three classes of small GTPases, the ADP-ribosylation factor (ARF), Rho, and Rab families, have been implicated in regulating cytoskeletal dynamics and molecular motor function within the secretory pathway. We have used the reconstitution of transport vesicle formation on isolated Golgi membranes to characterize mechanisms of ARF1 regulated actin polymerization. ARF1 affects cytoskeletal function in part by recruiting a complex between the vesicle-coat protein, coatomer, and the Rho-related GTPase, Cdc42, to the Golgi apparatus. Cdc42 can activate actin polymerization on Golgi membranes through an Arp2/3-dependent mechanism. Coatomer-bound Cdc42 plays a further role in regulating vesicle motility via the motor protein, dynein. Future studies elucidating the molecular mechanisms connecting vesicular transport with actin dynamics will provide important clues to the overall contribution of the cytoskeleton and molecular motors to protein transport. This article describes methods and reagents for characterizing cytoskeletal regulation at the Golgi apparatus through the cell-free reconstitution of vesicle formation.

Actin dynamics at the Golgi complex in mammalian cells

Secretion and endocytosis are highly dynamic processes that are sensitive to external stimuli. Thus, in multicellular organisms, different cell types utilize specialised pathways of intracellular membrane traffic to facilitate specific physiological functions. In addition to the complex internal molecular factors that govern sorting functions and fission or fusion of transport carriers, the actin cytoskeleton plays an important role in both the endocytic and secretory pathways. The interaction between the actin cytoskeleton and membrane trafficking is not restricted to transport processes: it also appears to be directly involved in the biogenesis of Golgi-derived transport carriers (budding and fission processes) and in the maintenance of the unique flat shape of Golgi cisternae.

Protein complexes regulating Arp2/3-mediated actin assembly

Key steps in regulating actin dynamics are the de novo nucleation and elongation of actin filaments, which can be catalysed by a limited number of proteins and protein complexes. Among these, Arp2/3 complex and formins are the best studied. Arp2/3-complex activity is controlled through signalling-dependent association with nucleation-promoting factors, such as the WASP/WAVE family proteins. A common theme for these molecules, which is well established for WAVEs but is only just beginning to emerge for WASPs, is that they act as coincident detectors of a variety of signalling pathways through the formation of large multi-molecular complexes.

Cdc42-dependent localization of polarisome component Spa2 to the incipient bud site is independent of the GDP/GTP exchange factor Cdc24

Cdc42, a member of the Rho subfamily of small GTPases, is highly conserved in both sequence and function across eukaryotic species. In budding yeast, Cdc42 triggers polarized growth necessary for bud emergence via rearrangement of the actin cytoskeleton. It has been shown that the role of Cdc42 in bud emergence requires both Cdc28-Cln (G1) kinase and the passage through START. In this report, we show that Cdc42 also serves an essential function in the establishment of bud site prior to START by catalyzing the translocation of bud-site components such as Spa2 to the cell cortex. Our analysis of various conditional alleles of CDC42 suggests that these two functions (bud site establishment and bud emergence) are genetically separable. Surprisingly, the role of Cdc42 in the cortical localization of Spa2 appears to be independent of its well known GTP/GDP exchange factor Cdc24. We also provide evidence that this role of Cdc42 requires the function of the COPI coatomer complex.

Secramine inhibits Cdc42-dependent functions in cells and Cdc42 activation in vitro

Inspired by the usefulness of small molecules to study membrane traffic, we used high-throughput synthesis and phenotypic screening to discover secramine, a molecule that inhibits membrane traffic out of the Golgi apparatus by an unknown mechanism. We report here that secramine inhibits activation of the Rho GTPase Cdc42, a protein involved in membrane traffic, by a mechanism dependent upon the guanine dissociation inhibitor RhoGDI. RhoGDI binds Cdc42 and antagonizes its membrane association, nucleotide exchange and effector binding. In vitro, secramine inhibits Cdc42 binding to membranes, GTP and effectors in a RhoGDI-dependent manner. In cells, secramine mimics the effects of dominant-negative Cdc42 expression on protein export from the Golgi and on Golgi polarization in migrating cells. RhoGDI-dependent Cdc42 inhibition by secramine illustrates a new way to inhibit Rho GTPases with small molecules and provides a new means to study Cdc42, RhoGDI and the cellular processes they mediate.

The leukocyte podosome

Myeloid leukocytes are the first line of host defence. When they sense perturbations in tissue homeostasis such as infection, inflammation and ischemia, they respond by trafficking. Whilst neutrophils and macrophages migrate to sites of infection, dendritic cells (DC) migrate from tissue-resident sites back into lymph nodes where they activate T and B lymphocytes. The directed migration of these leukocytes through peripheral tissues is thus crucial for their function. This article considers recent advances in our understanding of the adhesive and motile behaviour of macrophages and DC, with particular emphasis on the podosomes that appear to be required for normal migration through extracellular matrices.

Membrane trafficking at the ER/Golgi interface: Functional implications of RhoA and Rac1

N-WASP and Arp2/3, the components of the actin nucleation/polymerization signaling pathway governed by Cdc42, are located in Golgi membranes and regulate ER/Golgi interface protein transport. In the present study, we examined whether RhoA and Rac1, like Cdc42, are also involved in this early secretory pathway. Unlike Cdc42, RhoA and Rac1 were not observed in the Golgi complex of different clonal cell lines nor were they present in isolated Golgi membranes. Expression of constitutively active or inactive mutants of RhoA or Rac1 proteins in HeLa cells did not alter either the disassembly or the assembly of the Golgi complex following the addition or withdrawal of BFA, respectively, the ER-to-Golgi VSV-G transport or the Sar1(dn)-induced ER accumulation of Golgi proteins. Moreover, unlike Cdc42-expressing cells, the 15 degrees C-induced subcellular redistribution of the KDEL receptor remained unaltered. Only cells that constitutively express the activated Cdc42 mutant (Cdc42Q61L), or that were microinjected with activated Cdc42Q61L protein, exhibited a significant change in Golgi complex morphology. Collectively, our results demonstrate that RhoA and Rac1 are not located in the Golgi complex, nor do they directly or indirectly regulate membrane trafficking at the ER/Golgi interface. This finding, in turn, confirms that Cdc42 is the only Rho GTPase to have a specific function on the Golgi complex.

Coatomer-bound Cdc42 regulates dynein recruitment to COPI vesicles

Cytoskeletal dynamics at the Golgi apparatus are regulated in part through a binding interaction between the Golgi-vesicle coat protein, coatomer, and the regulatory GTP-binding protein Cdc42 (Wu, W.J., J.W. Erickson, R. Lin, and R.A. Cerione. 2000. Nature. 405:800–804; Fucini, R.V., J.L. Chen, C. Sharma, M.M. Kessels, and M. Stamnes. 2002. Mol. Biol. Cell. 13:621–631). The precise role of this complex has not been determined. We have analyzed the protein composition of Golgi-derived coat protomer I (COPI)–coated vesicles after activating or inhibiting signaling through coatomer-bound Cdc42. We show that Cdc42 has profound effects on the recruitment of dynein to COPI vesicles. Cdc42, when bound to coatomer, inhibits dynein binding to COPI vesicles whereas preventing the coatomer–Cdc42 interaction stimulates dynein binding. Dynein recruitment was found to involve actin dynamics and dynactin. Reclustering of nocodazole-dispersed Golgi stacks and microtubule/dynein-dependent ER-to-Golgi transport are both sensitive to disrupting Cdc42 mediated signaling. By contrast, dynein-independent transport to the Golgi complex is insensitive to mutant Cdc42. We propose a model for how proper temporal regulation of motor-based vesicle translocation could be coupled to the completion of vesicle formation.

Involvement of the Wiskott-Aldrich syndrome protein and other actin regulatory adaptors in T cell activation

The actin cytoskeleton is a dynamic structure recognized for many years as integral to the coupling of external stimuli to cell activation and ensuing changes in morphology and movement. It is only recently, however, that a molecular understanding of actin involvement in these activities has emerged coincident with the identification of cytosolic signaling effectors that couple extracellular stimuli to induction of actin nucleation. Notable among these actin regulatory effectors are members of the Wiskott-Aldrich syndrome protein (WASp) family, a group of cytoskeletal adaptors imbued with the capacity to connect various signal transduction pathways to the Arp 2/3 complex and Arp 2/3-mediated actin polymerization. In T cells, the functional characterization of WASp and other actin-modulatory adaptors has proved instrumental in delineating the molecular interactions evoking actin cytoskeletal reorganization downstream of antigen receptor engagement and in clarifying the influence of actin-based processes on T cell activation. In this review, the structural and functional properties of the major actin regulatory cytoskeletal adaptors in T cells are described with an emphasis on the roles of these proteins in fostering the TCR actin cytoskeletal interplay required for induction of T cell activation and expression of dynamic effector responses.

Actin and Arf1-dependent recruitment of a cortactin-dynamin complex to the Golgi regulates post-Golgi transport

Cortactin is an actin-binding protein that has recently been implicated in endocytosis. It binds directly to dynamin-2 (Dyn2), a large GTPase that mediates the formation of vesicles from the plasma membrane and the Golgi. Here we show that cortactin associates with the Golgi to regulate the actin- and Dyn2-dependent transport of cargo. Cortactin antibodies stain the Golgi apparatus, labelling peripheral buds and vesicles that are associated with the cisternae. Notably, in vitro or intact-cell experiments show that activation of Arf1 mediates the recruitment of actin, cortactin and Dyn2 to Golgi membranes. Furthermore, selective disruption of the cortactin-Dyn2 interaction significantly reduces the levels of Dyn2 at the Golgi and blocks the transit of nascent proteins from the trans-Golgi network, resulting in swollen and distended cisternae. These findings support the idea of an Arf1-activated recruitment of an actin, cortactin and Dyn2 complex that is essential for Golgi function.

Spatially distinct binding of Cdc42 to PAK1 and N-WASP in breast carcinoma cells

While a significant amount is known about the biochemical signaling pathways of the Rho family GTPase Cdc42, a better understanding of how these signaling networks are coordinated in cells is required. In particular, the predominant subcellular sites where GTP-bound Cdc42 binds to its effectors, such as p21-activated kinase 1 (PAK1) and N-WASP, a homolog of the Wiskott-Aldritch syndrome protein, are still undetermined. Recent fluorescence resonance energy transfer (FRET) imaging experiments using activity biosensors show inconsistencies between the site of local activity of PAK1 or N-WASP and the formation of specific membrane protrusion structures in the cell periphery. The data presented here demonstrate the localization of interactions by using multiphoton time-domain fluorescence lifetime imaging microscopy (FLIM). Our data here establish that activated Cdc42 interacts with PAK1 in a nucleotide-dependent manner in the cell periphery, leading to Thr-423 phosphorylation of PAK1, particularly along the lengths of cell protrusion structures. In contrast, the majority of GFP-N-WASP undergoing FRET with Cy3-Cdc42 is localized within a transferrin receptor- and Rab11-positive endosomal compartment in breast carcinoma cells. These data reveal for the first time distinct spatial association patterns between Cdc42 and its key effector proteins controlling cytoskeletal remodeling.

Golgi-localized GAP for Cdc42 functions downstream of ARF1 to control Arp2/3 complex and F-actin dynamics

The small GTP-binding ADP-ribosylation factor 1 (ARF1) acts as a master regulator of Golgi structure and function through the recruitment and activation of various downstream effectors. It has been proposed that members of the Rho family of small GTPases also control Golgi function in coordination with ARF1, possibly through the regulation of Arp2/3 complex and actin polymerization on Golgi membranes. Here, we identify ARHGAP10--a novel Rho GTPase-activating protein (Rho-GAP) that is recruited to Golgi membranes through binding to GTP-ARF1. We show that ARHGAP10 functions preferentially as a GAP for Cdc42 and regulates the Arp2/3 complex and F-actin dynamics at the Golgi through the control of Cdc42 activity. Our results establish a role for ARHGAP10 in Golgi structure and function at the crossroads between ARF1 and Cdc42 signalling pathways.

Bi-directional protein transport between the ER and Golgi

The endoplasmic reticulum (ER) and the Golgi comprise the first two steps in protein secretion. Vesicular carriers mediate a continuous flux of proteins and lipids between these compartments, reflecting the transport of newly synthesized proteins out of the ER and the retrieval of escaped ER residents and vesicle machinery. Anterograde and retrograde transport is mediated by distinct sets of cytosolic coat proteins, the COPII and COPI coats, respectively, which act on the membrane to capture cargo proteins into nascent vesicles. We review the mechanisms that govern coat recruitment to the membrane, cargo capture into a transport vesicle, and accurate delivery to the target organelle.

Requirement for the Rac GTPase in Chlamydia trachomatis invasion of non-phagocytic cells

Chlamydiae are gram-negative obligate intracellular pathogens to which access to an intracellular environment is paramount to their survival and replication. To this end, chlamydiae have evolved extremely efficient means of invading nonphagocytic cells. To elucidate the host cell machinery utilized by Chlamydia trachomatis in invasion, we examined the roles of the Rho GTPase family members in the internalization of chlamydial elementary bodies. Upon binding of elementary bodies on the cell surface, actin is rapidly recruited to the sites of internalization. Members of the Rho GTPase family are frequently involved in localized recruitment of actin. Clostridial Toxin B, which is a known enzymatic inhibitor of Rac, Cdc42 and Rho GTPases, significantly reduced chlamydial invasion of HeLa cells. Expression of dominant negative constructs in HeLa cells revealed that chlamydial uptake was dependent on Rac, but not on Cdc42 or RhoA. Rac but not Cdc42 was found to be activated by chlamydial attachment. The effect of dominant negative Rac expression on chlamydial uptake is manifested through the inhibition of actin recruitment to the sites of chlamydial entry. Studies utilizing Green Fluorescent Protein fusion constructs of Rac, Cdc42 and RhoA, showed Rac to be the sole member of the Rho GTPase family recruited to the site of chlamydial entry.

Association of Cdc42/N-WASP/Arp2/3 Signaling Pathway with Golgi Membranes

Recent findings indicate that Cdc42 regulates Golgi-to-ER (endoplasmic reticulum) protein transport through N-WASP and Arp2/3 (Luna et al. 2002, Mol. Biol. Cell, 13:866-879). To analyse the components of the Cdc42-governed signaling pathway in the secretory pathway, we localized Cdc42, N-WASP and Arp2/3 in the Golgi complex by cryoimmunoelectron microscopy. Cdc42 is found throughout the Golgi stack, particularly in cis/middle cisternae, whereas N-WASP and Arp3 (a component of the Arp2/3 complex) are restricted to cis cisternae. Arp3 also colocalized in peri-Golgi tubulovesicular structures with either KDEL receptor or GM130. Even though Arp3 is not found in TGN46-positive cisternal elements, a small fraction of Arp3-labeled tubulo-vesicular elements showed TGN46 labeling. Active Cdc42 (GTP-bound form) induced relocation of N-WASP and Arp3 to the lateral rims of Golgi cisternae. These results show that the actin nucleation and polymerization signaling pathway governed by Cdc42/N-WASP/Arp operates in the Golgi complex of mammalian cells, further implicating actin dynamics in Golgi-associated membrane trafficking.