Rab5, an early acting endosomal GTPase, supports in vitro endosome fusion without GTP hydrolysis

IqgC is a potent regulator of macropinocytosis in the presence of NF1 and its loading to macropinosomes is dependent on RasG

RasG is a major regulator of macropinocytosis in Dictyostelium discoideum. Its activity is under the control of an IQGAP-related protein, IqgC, which acts as a RasG-specific GAP (GTPase activating protein). IqgC colocalizes with the active Ras at the macropinosome membrane during its formation and for some time after the cup closure. However, the loss of IqgC induces only a minor enhancement of fluid uptake in axenic cells that already lack another RasGAP, NF1. Here, we show that IqgC plays an important role in the regulation of macropinocytosis in the presence of NF1 by restricting the size of macropinosomes. We further provide evidence that interaction with RasG is indispensable for the recruitment of IqgC to forming macropinocytic cups. We also demonstrate that IqgC interacts with another small GTPase from the Ras superfamily, Rab5A, but is not a GAP for Rab5A. Since mammalian Rab5 plays a key role in early endosome maturation, we hypothesized that IqgC could be involved in macropinosome maturation via its interaction with Rab5A. Although an excessive amount of Rab5A reduces the RasGAP activity of IqgC in vitro and correlates with IqgC dissociation from endosomes in vivo, the physiological significance of the Rab5A-IqgC interaction remains elusive.

Assembly and disassembly dynamics of nonmuscle myosin II control endosomal fission

Endocytic vesicular trafficking requires merging of two lipid bilayers, but how the two lipid bilayers can come close together during fusion and fission in endocytic trafficking is not well explored. Here, we establish that knocking down nonmuscle myosin IIs (NM IIs) by small interfering RNA (siRNA) or inhibition of their activities by (-) blebbistatin causes the formation of a ring-like assembly of early endosomes (raEE). Inhibition of NM II assembly by an inhibitor of regulatory light-chain (RLC) kinase results in the formation of raEE, whereas inhibition of NM II disassembly by inhibitors of heavy chain kinases, protein kinase C (PKC) and casein kinase 2 (CK2), causes the dispersion of early endosomes. The raEEs retain EEA1, Rab7, and LAMP2 markers. Overexpression of an assembly incompetent form, RLC-AA, and disassembly incompetent form, NMHCIIB-S6A or NMHCIIA-1916A, induces such defects, respectively. Altogether, these data support that NM II assembly and disassembly dynamics participate in endocytic trafficking by regulating fission to maintain the size of early endosomes.

Testis developmental related gene 1 regulates the chemosensitivity of seminoma TCam-2 cells to cisplatin via autophagy

We previously identified testis developmental related gene 1 (TDRG1), a gene implicated in proliferation of TCam-2 seminoma cells. Recent evidence has revealed that autophagy influences the chemosensitivity of cancer cells to chemotherapy. However, whether TDRG1 protein regulates autophagy in seminoma cells and influences their sensitivity to cis-dichlorodiammine platinum (CDDP) remains unknown. In this study, we used TCam-2 cells and male athymic BALB/c nude mice with xenografts of TCam-2 cells to investigate autophagy, cell viability, apoptosis and the p110β/Rab5/Vps34 (PI3-kinase Class III) pathway under the conditions of TDRG1 overexpression or knockdown and with or without CDDP treatment. We found that TDRG1 upregulation promoted autophagy in both TCam-2 cells and seminoma xenografts via p110β/Rab5/Vps34 activation. Inhibition of autophagy reduced cell viability and promoted apoptosis during CDDP treatment of TCam-2 cells. Similarly, TDRG1 knockdown inhibited autophagy, reduced cell viability and promoted apoptosis during CDDP treatment of TCam-2 cells. TDRG1 knockdown inhibited tumour growth and promoted apoptosis in TCam-2 cell xenografts, whereas TDRG1 overexpression had the opposite effect. According to these results, we propose that high expression of TDRG1 promotes autophagy through the p110β/Rab5/Vps34 pathway in TCam-2 cells. TDRG1 overexpression promotes autophagy and leads to CDDP resistance, whereas TDRG1 knockdown inhibits autophagy and promotes chemosensitivity to CDDP both in vivo and in vitro. This study has uncovered a novel role of TDRG1 in reducing chemoresistance during CDDP treatment and provides potential therapeutic strategies for the treatment of human seminoma.

Dissecting the Cell Entry Pathway of Baculovirus by Single-Particle Tracking and Quantitative Electron Microscopic Analysis

The budded virus of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infects insect cells through mainly clathrin-mediated endocytosis. However, the cell entry pathway of AcMNPV remains unclear. In this study, by using population-based analysis of single-virus tracking and electron microscopy, we investigated the internalization, fusion behavior, and endocytic trafficking of AcMNPV. AcMNPV internalization into host insect cells was facilitated by actin polymerization and dynamin. After incorporation into early endosomes, the AcMNPV envelope fused with the membranes of early endosome, allowing for nucleocapsid release into the cytoplasm. Microtubules were implicated in the bidirectional and long-range transport of virus-containing endosomes. In addition, microtubule depolymerization reduced the motility of virus-bearing early endosomes, impairing the progression of infection beyond enlarged early endosomes. These findings demonstrated that AcMNPV internalization was facilitated by actin polymerization in a dynamin-dependent manner, and nucleocapsid release occurred in early endosomes in a microtubule-dependent manner. This study provides mechanistic and kinetic insights into AcMNPV infection and enhance our understanding of the infection pathway of baculoviruses.IMPORTANCE Baculoviruses are used widely as environmentally benign pesticides, protein expression systems, and potential mammalian gene delivery vectors. Despite the significant application value, little is known about the cell entry and endocytic trafficking pathways of baculoviruses. In this study, we demonstrated that the alphabaculovirus AcMNPV exhibited actin- and microtubule-dependent transport for nucleocapsid release predominantly from within early endosomes. In contrast to AcMNPV transduction in mammalian cells, its infection in host insect cells is facilitated by actin polymerization for internalization and microtubules for endocytic trafficking within early endosomes, implying that AcMNPV exhibits cell type specificity in the requirement of the cytoskeleton network. In addition, experimental depolymerization of microtubules impaired the progression of infection beyond enlarged early endosomes. This is the first study that dissects the cell entry pathway of baculoviruses in host cells at the single-particle level, which advances our understanding of the early steps of baculovirus entry.

Comprehending a Killer: The Akt/mTOR Signaling Pathways Are Temporally High-Jacked by the Highly Pathogenic 1918 Influenza Virus

Previous transcriptomic analyses suggested that the 1918 influenza A virus (IAV1918), one of the most devastating pandemic viruses of the 20th century, induces a dysfunctional cytokine storm and affects other innate immune response patterns. Because all viruses are obligate parasites that require host cells for replication, we globally assessed how IAV1918 induces host protein dysregulation. We performed quantitative mass spectrometry of IAV1918-infected cells to measure host protein dysregulation. Selected proteins were validated by immunoblotting and phosphorylation levels of members of the PI3K/AKT/mTOR pathway were assessed. Compared to mock-infected controls, >170 proteins in the IAV1918-infected cells were dysregulated. Proteins mapped to amino sugar metabolism, purine metabolism, steroid biosynthesis, transmembrane receptors, phosphatases and transcription regulation. Immunoblotting demonstrated that IAV1918 induced a slight up-regulation of the lamin B receptor whereas all other tested virus strains induced a significant down-regulation. IAV1918 also strongly induced Rab5b expression whereas all other tested viruses induced minor up-regulation or down-regulation. IAV1918 showed early reduced phosphorylation of PI3K/AKT/mTOR pathway members and was especially sensitive to rapamycin. These results suggest the 1918 strain requires mTORC1 activity in early replication events, and may explain the unique pathogenicity of this virus.

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Proteomic analyses of influenza 1918 virus-infected cells identified >170 dysregulated host proteins.

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Dysregulated proteins mapped to numerous important cellular pathways.

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1918 virus infection showed prominent early reduced phosphorylation of PI3K/Akt/mTOR.

The 1918 influenza pandemic was one of the most devastating infectious disease events of the 20th century, resulting in 20–100 million deaths. Gene-based assays showed severe dysregulation of the host's cytokine responses, but little was known about global protein responses to virus infection. This work identifies unique and temporal alterations in phosphorylation of the PI3K/AKT/mTOR signaling pathway, which is important in determining cell death. This work paves the way for further research on how this pathway influences host mechanisms responsible for aiding virus replication and in determining levels and severity of influenza virus-induced patho

Dysregulation of Rab5-mediated endocytic pathways in Alzheimer's disease

Increasing evidence has pointed to that dysregulation of the endo-lysosomal system is an early cellular phenotype of pathogenesis for Alzheimer's disease (AD). Rab5, a small GTPase, plays a critical role in mediating these processes. Abnormal overactivation of Rab5 has been observed in post-mortem brain samples of Alzheimer's patients as well as brain samples of mouse models of AD. Recent genome-wide association studies of AD have identified RIN3 (Ras and Rab interactor 3) as a novel risk factor for the disease. RIN3 that functions as a guanine nucleotide exchange factor for Rab5 may serve as an important activator for Rab5 in AD pathogenesis. In this review, we present recent research highlights on the possible roles of dysregulation of Rab5-mediated endocytic pathways in contributing to early pathogenesis of AD.

Distinct biochemical and functional properties of two Rab5 homologs from the rice blast fungus Magnaporthe oryzae

Rab5 is a key regulator of early endocytosis by promoting early endosomal fusion and motility. In this study, we have unexpectedly found distinct properties of the two Rab5 homologs (MoRab5A and MoRab5B) from Magnaporthe oryzae, a pathogenic fungus in plants whose infection causes rice blast disease. Like mammalian Rab5, MoRab5A and MoRab5B can bind to several Rab5 effectors in a GTP-dependent manner, including EEA1, Rabenosyn-5, and Rabaptin-5. However, MoRab5A shows distinct binding characteristics in the sense that both the wild-type and the GTP hydrolysis-defective constitutively active mutant bind the effectors equally well in GST pull-down assays, suggesting that MoRab5A is defective in GTP hydrolysis and mostly in the GTP-bound conformation in the cell. Indeed, GTP hydrolysis assays indicate that MoRab5A GTPase activity is dramatically lower than MoRab5B and human Rab5 and is insensitive to RabGAP5 stimulation. We have further identified a Pro residue in the switch I region largely responsible for the distinct MoRab5A properties by characterization of MoRab5A and MoRab5B chimeras and mutagenesis. The differences between MoRab5A and MoRab5B extend to their functions in the cell. Although they both target to early endosomes, only MoRab5B closely resembles human Rab5 in promoting early endosome fusion and stimulating fluid phase endocytosis. In contrast, MoRab5A correlates with another related early endosomal Rab, Rab22, in terms of the presence of the switch I Pro residue and the blocked GTPase activity. Our data thus identify MoRab5B as the Rab5 ortholog and suggest that MoRab5A specializes to perform a non-redundant function in endosomal sorting.

Rab5 isoforms orchestrate a "division of labor" in the endocytic network; Rab5C modulates Rac-mediated cell motility

Rab5, the prototypical Rab GTPase and master regulator of the endocytic pathway, is encoded as three differentially expressed isoforms, Rab5A, Rab5B and Rab5C. Here, we examined the differential effects of Rab5 isoform silencing on cell motility and report that Rab5C, but neither Rab5A nor Rab5B, is selectively associated with the growth factor-activation of Rac1 and with enhanced cell motility. Initial observations revealed that silencing of Rab5C expression, but neither Rab5A nor Rab5C, led to spindle-shaped cells that displayed reduced formation of membrane ruffles. When subjected to a scratch wound assay, cells depleted of Rab5C, but not Rab5A or Rab5B, demonstrated reduced cell migration. U937 cells depleted of Rab5C also displayed reduced cell motility in a Transwell plate migration assay. To examine activation of Rac, HeLa cells stably expressing GFP-Rac1 were independently depleted of Rab5A, Rab5B or Rab5C and seeded onto coverslips imprinted with a crossbow pattern. 3-D GFP-Rac1 images of micro-patterned cells show that GFP-Rac1 was less localized to the cell periphery in the absence of Rab5C. To confirm the connection between Rab5C and Rac activation, HeLa cells depleted of Rab5 isoforms were starved and then stimulated with EGF. Rac1 pull-down assays revealed that EGF-stimulated Rac1 activity was significantly suppressed in Rab5C-suppressed cells. To determine whether events upstream of Rac activation were affected by Rab5C, we observed that EGF-stimulated Akt phosphorylation was suppressed in cells depleted of Rab5C. Finally, since spatio-temporal assembly/disassembly of adhesion complexes are essential components of cell migration, we examined the effect of Rab5 isoform depletion on the formation of focal adhesion complexes. Rab5C-depleted HeLa cells have significantly fewer focal adhesion foci, in accordance with the lack of persistent lamellipodial protrusions and reduced directional migration. We conclude that Rab5 isoforms selectively oversee the multiple signaling and trafficking events associated with the endocytic network.

Wide-area scanner for high-speed atomic force microscopy

High-speed atomic force microscopy (HS-AFM) has recently been established. The dynamic processes and structural dynamics of protein molecules in action have been successfully visualized using HS-AFM. However, its maximum scan ranges in the X- and Y-directions have been limited to ~1 μm and ~4 μm, respectively, making it infeasible to observe the dynamics of much larger samples, including live cells. Here, we develop a wide-area scanner with a maximum XY scan range of ~46 × 46 μm(2) by magnifying the displacements of stack piezoelectric actuators using a leverage mechanism. Mechanical vibrations produced by fast displacement of the X-scanner are suppressed by a combination of feed-forward inverse compensation and the use of triangular scan signals with rounded vertices. As a result, the scan speed in the X-direction reaches 6.3 mm/s even for a scan size as large as ~40 μm. The nonlinearity of the X- and Y-piezoelectric actuators' displacements that arises from their hysteresis is eliminated by polynomial-approximation-based open-loop control. The interference between the X- and Y-scanners is also eliminated by the same technique. The usefulness of this wide-area scanner is demonstrated by video imaging of dynamic processes in live bacterial and eukaryotic cells.

Class IA PI3K p110β subunit promotes autophagy through Rab5 small GTPase in response to growth factor limitation

Autophagy is an evolutionarily conserved membrane trafficking process. Induction of autophagy in response to nutrient limitation or cellular stress occurs by similar mechanisms in organisms from yeast to mammals. Unlike yeast, metazoan cells rely more on growth factor signaling for a wide variety of cellular activities including nutrient uptake. How growth factor availability regulates autophagy is poorly understood. Here we show that, upon growth factor limitation, the p110β catalytic subunit of the class IA phosphoinositide 3-kinases (PI3Ks) dissociates from growth factor receptor complexes and increases its interaction with the small GTPase Rab5. This p110β-Rab5 association maintains Rab5 in its guanosine triphosphate (GTP)-bound state and enhances the Rab5-Vps34 interaction that promotes autophagy. p110β mutants that fail to interact with Rab5 are defective in autophagy promotion. Hence, in mammalian cells, p110β acts as a molecular sensor for growth factor availability and induces autophagy by activating a Rab5-mediated signaling cascade.

Gαs promotes EEA1 endosome maturation and shuts down proliferative signaling through interaction with GIV (Girdin)

EEA1 endosomes are believed to function mainly in down-regulating EGFR signaling, and APPL endosomes are regarded as signaling endosomes. Evidence is given that EGF-induced, proliferative signaling occurs from EEA1 endosomes and is regulated by interaction between the signal-transducing protein GIV and the trimeric G protein Gαs.

The organization of the endocytic system into biochemically distinct subcompartments allows for spatial and temporal control of the strength and duration of signaling. Recent work has established that Akt cell survival signaling via the epidermal growth factor receptor (EGFR) occurs from APPL early endosomes that mature into early EEA1 endosomes. Less is known about receptor signaling from EEA1 endosomes. We show here that EGF-induced, proliferative signaling occurs from EEA1 endosomes and is regulated by the heterotrimeric G protein Gαs through interaction with the signal transducing protein GIV (also known as Girdin). When Gαs or GIV is depleted, activated EGFR and its adaptors accumulate in EEA1 endosomes, and EGFR signaling is prolonged, EGFR down-regulation is delayed, and cell proliferation is greatly enhanced. Our findings define EEA1 endosomes as major sites for proliferative signaling and establish that Gαs and GIV regulate EEA1 but not APPL endosome maturation and determine the duration and strength of proliferative signaling from this compartment.

Effect of EGF-receptor tyrosine kinase inhibitor on Rab5 function during endocytosis

Tyrosine autophosphorylation within the cytoplasmic tail of EGF-receptor is a key event, which in turn recruits several factors including Shc, Grb2 and Rin1 that are essential activities for receptor-mediated endocytosis and signaling. In this study, we demonstrated that treatment with AG1478, an EGF-receptor kinase inhibitor, blocked the formation of Rab5-positive endosomes as well as the activation of Rab5 upon addition of EGF. We also found that EGF-receptor catalytically inactive mutant failed to activate Rab5 upon EGF stimulation. Additionally, endosomal co-localization of Rab5 and EGF-receptor was inhibited by AG1478. Interestingly, AG1478 inhibitor did not block the formation of enlarged Rab5-positive endosomes in cells expressing Rab5 GTP hydrolysis defective mutant (Rab5:Q79L). AG1478 inhibitor also blocked the in vitro endosome fusion in a concentration-dependent manner, and more importantly, Rab5:Q79L mutant rescued it. Furthermore, addition of Rin1, a Rab5 guanine nucleotide exchange factor, partially restored endosome fusion in the presence of AG1478 inhibitor. Consistent with these observations, we also observed that Rin1 was unable to localize to membranes upon EGF-stimulation in the presence of AG1478 inhibitor. These results constitute first evidence that the enzymatic activity of a tyrosine kinase receptor is required endosome fusion via the activation of Rab5.

The ubiquitin ligase RPM-1 and the p38 MAPK PMK-3 regulate AMPA receptor trafficking

Ubiquitination occurs at synapses, yet its role remains unclear. Previous studies demonstrated that the RPM-1 ubiquitin ligase organizes presynaptic boutons at neuromuscular junctions in C. elegans motorneurons. Here we find that RPM-1 has a novel postsynaptic role in interneurons, where it regulates the trafficking of the AMPA-type glutamate receptor GLR-1 from synapses into endosomes. Mutations in rpm-1 cause the aberrant accumulation of GLR-1 in neurites. Moreover, rpm-1 mutations enhance the endosomal accumulation of GLR-1 observed in mutants for lin-10, a Mint2 ortholog that promotes GLR-1 recycling from Syntaxin-13 containing endosomes. As in motorneurons, RPM-1 negatively regulates the pmk-3/p38 MAPK pathway in interneurons by repressing the protein levels of the MAPKKK DLK-1. This regulation of PMK-3 signaling is critical for RPM-1 function with respect to GLR-1 trafficking, as pmk-3 mutations suppress both lin-10 and rpm-1 mutations. Positive or negative changes in endocytosis mimic the effects of rpm-1 or pmk-3 mutations, respectively, on GLR-1 trafficking. Specifically, RAB-5(GDP), an inactive mutant of RAB-5 that reduces endocytosis, mimics the effect of pmk-3 mutations when introduced into wild-type animals, and occludes the effect of pmk-3 mutations when introduced into pmk-3 mutants. By contrast, RAB-5(GTP), which increases endocytosis, suppresses the effect of pmk-3 mutations, mimics the effect of rpm-1 mutations, and occludes the effect of rpm-1 mutations. Our findings indicate a novel specialized role for RPM-1 and PMK-3/p38 MAPK in regulating the endosomal trafficking of AMPARs at central synapses.

Membrane-permeant Rab3A triggers acrosomal exocytosis in living human sperm

The acrosome reaction is a regulated Ca2+-dependent secretion event required for sperm-egg interaction. Previous studies indicate that the process requires Rab3-dependent tethering of membranes, SNARE complex assembly, and Ca2+-mediated activation of synaptotagmin. Sperm are transcriptionally and translationally inactive; hence, most studies of the exocytosis mechanism are limited to membrane-permeant reagents. The effect of proteins involved in exocytosis has been assessed only in permeabilized cells. Polyarginine peptides are a powerful tool for delivering macromolecules to cells. Most reports indicate that membrane translocation of arginine-containing proteins requires endocytosis; therefore, this strategy might not be useful in sperm. However, our results indicate that GST and Rab3A, when fused with an arginine-rich peptide, were able to translocate into sperm. Moreover, membrane-permeant Rab3A initiated exocytosis when prenylated and activated with GTP. We show here that a key event after the cytoplasmic Ca2+ increase caused by progesterone is the activation of Rab3A. When active Rab3A is introduced into sperm, Ca2+ in the extracellular medium and in the cytoplasm is dispensable. However, a Ca2+ efflux from inside the acrosome is still required to achieve exocytosis. In conclusion, arginine-containing proteins can penetrate the sperm plasma membrane and thus are valuable tools to study sperm physiology in intact cells.

Roles of RabGEF1/Rabex-5 domains in regulating Fc epsilon RI surface expression and Fc epsilon RI-dependent responses in mast cells

RabGEF1/Rabex-5, a guanine nucleotide exchange factor (GEF) for the endocytic pathway regulator, Rab5, contains a Vps9 domain, an A20-like zinc finger (ZnF) domain, and a coiled coil domain. To investigate the importance of these domains in regulating receptor internalization and cell activation, we lentivirally delivered RabGEF1 mutants into RabGEF1-deficient (-/-) mast cells and examined Fc epsilon RI-dependent responses. Wild-type RabGEF1 expression corrected phenotypic abnormalities in -/- mast cells, including decreased basal Fc epsilon RI expression, slowed Fc epsilon RI internalization, elevated IgE + Ag-induced degranulation and IL-6 production, and the decreased ability of -/- cytosol to support endosome fusion. We showed that RabGEF1's ZnF domain has ubiquitin ligase activity. Moreover, the coiled coil domain of RabGEF1 is required for Rabaptin-5 binding and for maintaining basal levels of Rabaptin-5 and surface Fc epsilon RI. However, mutants lacking either of these domains normalized phenotypic abnormalities in IgE + antigen-activated -/- mast cells. By contrast, correction of these -/- phenotypes required a functional Vps9 domain. Thus, Fc epsilon RI-mediated mast cell functional activation is dependent on RabGEF1's GEF activity.

Hepatitis C virus entry requires a critical postinternalization step and delivery to early endosomes via clathrin-coated vesicles

Hepatitis C virus (HCV) is a major human pathogen associated with life-threatening liver disease. Entry into hepatocytes requires CD81 and a putative second receptor. In this study, we elucidated the postreceptor attachment stages of HCV entry using HCV pseudoparticles (HCVpp) as a model system. By means of dominant-negative mutants and short interfering RNAs of various cellular proteins, we showed that HCVpp enter via clathrin-coated vesicles and require delivery to early but not to late endosomes. However, the kinetics of HCV envelope glycoprotein-mediated fusion are delayed compared to those of other viruses that enter in early endosomes. Entry of HCVpp can be efficiently blocked by bafilomycin A1, which neutralizes the pH in early endosomes and impairs progression of endocytosis beyond this stage. However, low-pH exposure of bafilomycin A1-treated target cells does not induce entry of HCVpp at the plasma membrane or in the early stages of endocytosis. These observations indicate that, subsequent to internalization, HCVpp entry necessitates additional, low-pH-dependent interactions, modifications, or trafficking, and that these events are irreversibly disrupted by bafilomycin A1 treatment.

Endocytosis-dominated membrane area decrease requires Rab5 protein in rat melanotrophs

Eukaryotic cells internalize extracellular macromolecules by endocytosis and it was shown that Rab5 protein is required for this process. While it is clear that endocytosis consists of vesicle fission from the plasma membrane, the role of Rab5 protein in the plasma membrane surface area changes is still unclear. Here we studied whether Rab5 is required for membrane surface area changes in rat melanotrophs-cells deriving from the pituitary pars intermedia. The presence of this protein in melanotrophs was probed by immunocytochemistry and its putative role in membrane area dynamics was monitored electrophysiologically with membrane capacitance measurements as this parameter directly reflects changes in membrane surface area. We found that Rab5 protein exists in melanotrophs. At [Ca(2+)](i) < 3 microM, endocytosis-dominated membrane capacitance decrease was found to be blocked by microinjection of specific Rab5 antibody. At high [Ca(2+)](i), Rab5 antibody did not affect the steady-state increase in membrane capacitance, while it elevated the rate of membrane capacitance increase, which is consistent with an inhibition of endocytosis.

IL4/PGE2 induction of an enlarged early endosomal compartment in mouse macrophages is Rab5-dependent

The endosomal compartment and the plasma membrane form a complex partnership that controls signal transduction and trafficking of different molecules. The specificity and functionality of the early endocytic pathway are regulated by a growing number of Rab GTPases, particularly Rab5. In this study, we demonstrate that IL4 (a Th-2 cytokine) and prostaglandin E2 (PGE2) synergistically induce Rab5 and several Rab effector proteins, including Rin1 and EEA1, and promote the formation of an enlarged early endocytic (EEE) compartment. Endosome enlargement is linked to a substantial induction of the mannose receptor (MR), a well-characterized macrophage endocytic receptor. Both MR levels and MR-mediated endocytosis are enhanced approximately 7-fold. Fluid-phase endocytosis is also elevated in treated cells. Light microscopy and fractionation studies reveal that MR colocalizes predominantly with Rab5a and partially with Rab11, an endosomal recycling pathway marker. Using retroviral expression of Rab5a:S34N, a dominant negative mutant, and siRNA Rab5a silencing, we demonstrate that Rab5a is essential for the large endosome phenotype and for localization of MR in these structures. We speculate that the EEE is maintained by activated Rab5, and that the EEE phenotype is part of some macrophage developmental program such as cell fusion, a characteristic of IL4-stimulated cells.

Als2-deficient mice exhibit disturbances in endosome trafficking associated with motor behavioral abnormalities

ALS2 is an autosomal recessive form of spastic paraparesis (motor neuron disease) with juvenile onset and slow progression caused by loss of function of alsin, an activator of Rac1 and Rab5 small GTPases. To establish an animal model of ALS2 and derive insights into the pathogenesis of this illness, we have generated alsin-null mice. Cytosol from brains of Als2(-/-) mice shows marked diminution of Rab5-dependent endosome fusion activity. Furthermore, primary neurons from Als2(-/-) mice show a disturbance in endosomal transport of insulin-like growth factor 1 (IGF1) and BDNF receptors, whereas neuronal viability and endocytosis of transferrin and dextran seem unaltered. There is a significant decrease in the size of cortical motor neurons, and Als2(-/-) mice are mildly hypoactive. Altered trophic receptor trafficking in neurons of Als2(-/-) mice may underlie the histopathological and behavioral changes observed and the pathogenesis of ALS2.

Role of Rab5 in insulin receptor-mediated endocytosis and signaling

Activated insulin receptors recruit various intracellular proteins leading to signal generation and endocytic trafficking. Although activated receptors are rapidly internalized into the endocytic compartment and subsequently degraded in lysosomes, the linkage between insulin receptor signaling and endocytosis is not well understood. This study utilizes both overexpression and depletion of Rab5 proteins to show that they play a critical role in both insulin-stimulated fluid phase and receptor-mediated endocytosis. Specifically, Rab5:WT and Rab5:Q79L (a GTP-hydrolysis defective mutant) enhance both types of endocytosis in response to insulin, while Rab5:S34N (a GTP-binding defective mutant) has the opposite effect. Morphological analysis indicates that both Rab5 and insulin receptor are found on early endosomes, but not at the plasma membrane. In addition, expression of Rab5:WT and Rab5:Q79L enhance both Erk1/2 and Akt activation without affecting JN- and p38-kinase activities, while the expression of Rab5:S34N blocks both Erk1/2 and Akt activation. Consistent with these observations, DNA synthesis is also altered by the expression of Rab5:S34N. Taken together, these results demonstrate that Rab5 is required for insulin receptor membrane trafficking and signaling.

Rab15 effector protein: a novel protein for receptor recycling from the endocytic recycling compartment

Sorting endosomes and the endocytic recycling compartment are critical intracellular stores for the rapid recycling of internalized membrane receptors to the cell surface in multiple cell types. However, the molecular mechanisms distinguishing fast receptor recycling from sorting endosomes and slow receptor recycling from the endocytic recycling compartment remain poorly understood. We previously reported that Rab15 differentially regulates transferrin receptor trafficking through sorting endosomes and the endocytic recycling compartment, suggesting a role for distinct Rab15-effector interactions at these endocytic compartments. In this study, we identified the novel protein Rab15 effector protein (REP15) as a binding partner for Rab15-GTP. REP15 is compartment specific, colocalizing with Rab15 and Rab11 on the endocytic recycling compartment but not with Rab15, Rab4, or early endosome antigen 1 on sorting endosomes. REP15 interacts directly with Rab15-GTP but not with Rab5 or Rab11. Consistent with its localization, REP15 overexpression and small interfering RNA-mediated depletion inhibited transferrin receptor recycling from the endocytic recycling compartment, without affecting receptor entry into or recycling from sorting endosomes. Our data identify REP15 as a compartment-specific protein for receptor recycling from the endocytic recycling compartment, highlighting that the rapid and slow modes of transferrin receptor recycling are mechanistically distinct pathways.

Molecular cloning of Rab5 (ApRab5) in Aiptasia pulchella and its retention in phagosomes harboring live zooxanthellae

The intracellular association of symbiotic dinoflagellates (zooxanthellae) with marine cnidarians is the very foundation of the highly productive and diversified coral reef ecosystems. To reveal its underlying molecular mechanisms, we previously cloned ApRab7, a Rab7 homologue of the sea anemone Aiptasia pulchella, and demonstrated its selective exclusion from phagosomes containing live zooxanthellae, but not from those containing either dead or photosynthesis-impaired algae. In this study, Rab5 was characterized, due to its key role in endocytosis and phagocytosis acting upstream of Rab7. The Aiptasia Rab5 homologue (ApRab5) is 79.5% identical to human Rab5C and contains all Rab-specific signature motifs. Subcellular fractionation study showed that ApRab5 is mainly cytosolic. EGFP reporter and phagocytosis studies indicated that membrane-associated ApRab5 is present in early endocytic and phagocytic compartments, and is able to promote their fusion. Significantly, immunofluorescence study showed that the majority of phagosomes containing either resident or newly internalized live zooxanthellae were labeled with ApRab5, while those containing either heat-killed or photosynthesis-impaired algae were mostly negative for ApRab5 staining whereas the opposite was observed for ApRab7. We propose that active phagosomal retention of ApRab5 is part of the mechanisms employed by live zooxanthellae to: (1) persist inside their host cells and (2) exclude ApRab7 from their phagosomes, thereby, establishing and/or maintaining an endosymbiotic relationship with their cnidarian hosts.

Expression of dominant negative rab5 in HeLa cells regulates endocytic trafficking distal from the plasma membrane

Ligand-mediated endocytosis is an important regulatory mechanism of epidermal growth factor (EGF) receptor (EGFR) signal transduction. Coordinated EGFR internalization and degradation function to regulate the spatial and temporal components of EGFR-effector interactions. In an effort to better understand the molecular mechanisms that control these events, we examined the role of rab5 in the endocytic trafficking of the EGFR. Rab5 is a 25-kDa guanine nucleotide binding protein that has previously been shown to be involved in the early stages of endocytic trafficking. Using adenovirally expressed dominant negative and constitutively active rab5 [rab5(S34N) and rab5(Q79L)] in cells with endogenous EGFRs, we have found that the guanine nucleotide binding state of rab5 has no bearing on the rate of EGFR endocytosis. However, expression of dominant negative rab5 affects downstream endocytic trafficking by slowing the ligand-induced disappearance of total cellular EGFR. Using confocal microscopy to examine EGF/EGFR and rab5 localization indicates that the activity of rab5 governs whether internalized EGF/EGFR and rab5 co-localize. Transferrin, which internalizes via a constitutively internalized cell surface receptor, co-sediments with rab5(WT), but not rab5(S34N) on sucrose gradients. Taken together, these data are consistent with rab5 functioning to regulate intracellular endocytic trafficking distal from the plasma membrane.

Dynein-mediated vesicle transport controls intracellular Salmonella replication

Salmonella typhimurium survives and replicates intracellular in a membrane-bound compartment, the Salmonella-containing vacuole (SCV). In HeLa cells, the SCV matures through interactions with the endocytic pathway, but Salmonella avoids fusion with mature lysosomes. The exact mechanism of the inhibition of phagolysosomal fusion is not understood. Rab GTPases control several proteins involved in membrane fusion and vesicular transport. The small GTPase Rab7 regulates the transport of and fusion between late endosomes and lysosomes and associates with the SCV. We show that the Rab7 GTPase cycle is not affected on the SCV. We then manipulated a pathway downstream of the small GTPase Rab7 in HeLa cells infected with Salmonella. Expression of the Rab7 effector RILP induces recruitment of the dynein/dynactin motor complex to the SCV. Subsequently, SCV fuse with lysosomes. As a result, the intracellular replication of Salmonella is inhibited. Activation of dynein-mediated vesicle transport can thus control intracellular survival of Salmonella.

Membrane trafficking along the phagocytic pathway

Phagosome maturation involves extensive remodelling of the phagosomal membrane as a result of intracellular transport events. Newly formed phagosomes exchange membrane-associated and soluble proteins with early endosomes by fusion. Budding of vesicles from the phagosome and fusion with Golgi-derived vesicles may also contribute to the remodelling of the phagosomal compartment. As a consequence of changes in membrane composition, phagosomes acquire the ability to fuse with late endocytic compartments. In vitro reconstitution and other studies suggest that the trafficking events underlying phagosome maturation require several GTP-binding proteins, including Rab5 and Galphas', NSF-SNAP-SNARE complexes and coatomers.

Intracellular delivery of drugs to macrophages

Toxic side effects which often complicate successful therapy in a number of diseases possibly arise due to the fact that at therapeutically effective concentrations the non-target cells in the body are also exposed to the cytotoxic effects of the drugs. Minimization of such adverse reactions might be feasible through drug delivery modalities that would reduce the uptake of the drugs by non-target cells and selectively deliver the drug only to the target cells (and/or intracellular sites) at relatively low extracellular concentrations. The current generic approach to site-specific drug delivery consists of attaching the therapeutic agent to a carrier recognized only by the cells where the pharmacological action is desired. Two types of recognition elements on the surface of target cells are being exploited for this purpose, viz., (i) antigens capable of generating specific, non-cross reactive antibodies, and (ii) receptors on the cell surface capable of efficient transport of the ligands. In general, incomplete specificity for the target cells and poor internalization of antibody-drug conjugates still limit the usefulness of antibodies for site-specific drug delivery applications necessitating exploration of alternatives. The alternate possibility is to exploit the exquisite cell type specificity and high efficiency of endocytosis of macromolecules mediated by specific receptors present on the surface of target cells for delivering drugs. A large number of infectious, metabolic, and neoplastic diseases are associated with macrophages leading to morbidities and mortalities to millions of people worldwide, thus an appropriate design of a drug delivery system to macrophages will be of tremendous help.

The SRC homology 2 domain of Rin1 mediates its binding to the epidermal growth factor receptor and regulates receptor endocytosis

Activated epidermal growth factor receptors (EGFRs) recruit intracellular proteins that mediate receptor signaling and endocytic trafficking. Rin1, a multifunctional protein, has been shown to regulate EGFR internalization (1). Here we show that EGF stimulation induces a specific, rapid, and transient membrane recruitment of Rin1 and that recruitment is dependent on the Src homology 2 (SH2) domain of Rin1. Immunoprecipitation of EGFR is accompanied by co-immunoprecipitation of Rin1 in a time- and ligand-dependent manner. Association of Rin1 and specifically the SH2 domain of Rin1 with the EGFR was dependent on tyrosine phosphorylation of the intracellular domain of the EGFR. The recruitment of Rin1, observed by light microscopy, indicated that although initially cytosolic, Rin1 was recruited to both plasma membrane and endosomes following EGF addition. Moreover, the expression of the SH2 domain of Rin1 substantially impaired the internalization of EGF without affecting internalization of transferrin. Finally, we found that Rin1 co-immunoprecipitated with a number of tyrosine kinase receptors but not with cargo endocytic receptors. These results indicate that Rin1 provides a link via its SH2 domain between activated tyrosine kinase receptors and the endocytic pathway through the recruitment and activation of Rab5a.

Mycobacterium's arrest of phagosome maturation in macrophages requires Rab5 activity and accessibility to iron

Many mycobacteria are intramacrophage pathogens that reside within nonacidified phagosomes that fuse with early endosomes but do not mature to phagolysosomes. The mechanism by which mycobacteria block this maturation process remains elusive. To gain insight into whether fusion with early endosomes is required for mycobacteria-mediated inhibition of phagosome maturation, we investigated how perturbing the GTPase cycles of Rab5 and Rab7, GTPases that regulate early and late endosome fusion, respectively, would affect phagosome maturation. Retroviral transduction of the constitutively activated forms of both GTPases into primary murine macrophages had no effect on Mycobacterium avium retention in an early endosomal compartment. Interestingly, expression of dominant negative Rab5, Rab5(S34N), but not dominant negative Rab7, resulted in a significant increase in colocalization of M. avium with markers of late endosomes/lysosomes and increased mycobacterial killing. This colocalization was specific to mycobacteria since Rab5(S34N) expressing cells showed diminished trafficking of endocytic tracers to lysosomes. We further demonstrated that maturation of M. avium phagosomes was halted in Rab5(S34N) expressing macrophages supplemented with exogenous iron. These findings suggest that fusion with early endosomes is required for mycobacterial retention in early phagosomal compartments and that an inadequate supply of iron is one factor in mycobacteria's inability to prevent the normal maturation process in Rab5(S34N)-expressing macrophages.

High resolution crystal structures of human Rab5a and five mutants with substitutions in the catalytically important phosphate-binding loop

GTPase domain crystal structures of Rab5a wild type and five variants with mutations in the phosphate-binding loop are reported here at resolutions up to 1.5 A. Of particular interest, the A30P mutant was crystallized in complexes with GDP, GDP+AlF(3), and authentic GTP, respectively. The other variant crystals were obtained in complexes with a non-hydrolyzable GTP analog, GppNHp. All structures were solved in the same crystal form, providing an unusual opportunity to compare structures of small GTPases with different catalytic rates. The A30P mutant exhibits dramatically reduced GTPase activity and forms a GTP-bound complex stable enough for crystallographic analysis. Importantly, the A30P structure with bound GDP plus AlF(3) has been solved in the absence of a GTPase-activating protein, and it may resemble that of a transition state intermediate. Conformational changes are observed between the GTP-bound form and the transition state intermediate, mainly in the switch II region containing the catalytic Gln(79) residue and independent of A30P mutation-induced local alterations in the P-loop. The structures suggest an important catalytic role for a P-loop backbone amide group, which is eliminated in the A30P mutant, and support the notion that the transition state of GTPase-mediated GTP hydrolysis is of considerable dissociative character.

Dynamics of endosomal sorting

The endocytic pathway receives cargo from the cell surface via endocytosis, biosynthetic cargo from the late Golgi complex, and various molecules from the cytoplasm via autophagy. This review focuses on the dynamics of the endocytic pathway in relationship to these processes and covers new information about the sorting events and molecular complexes involved. The following areas are discussed: dynamics at the plasma membrane, sorting within early endosomes and recycling to the cell surface, the role of the cytoskeleton, transport to late endosomes and sorting into multivesicular bodies, anterograde and retrograde Golgi transport, as well as the autophagic pathway.

Mammalian suppressor of Sec4 modulates the inhibitory effect of Rab15 during early endocytosis

Rab15 is a novel endocytic Rab that counters the stimulatory effect of Rab5-GTP on early endocytic trafficking. Rab15 may interfere with Rab5 function directly by sequestering Rab5 effectors or indirectly through novel sets of effector interactions. To distinguish between these possibilities, we examined the effector binding properties of Rab15. Rab15 does not interact directly with the Rab5 effectors rabex-5 and rabaptin-5 in a yeast two-hybrid binding assay. Rather mammalian suppressor of Sec4 (Mss4) was identified as a binding partner for Rab15. Mss4 preferentially binds GDP-bound (T22N) and nucleotide-free (N121I) Rab15, consistent with the proposed role of Mss4 as a chaperone that stabilizes target Rabs in their nucleotide-free form. Mutational analysis of Rab15 indicates that lysine at position 48 (K48Q) is important for the binding of Rab15-GDP to Mss4. Moreover, the mutation K48Q counters the inhibitory phenotype of wild type Rab15 on receptor-mediated endocytosis in HeLa cells and homotypic endosome fusion in vitro without altering the relative amount of cell surface-associated transferrin receptor. Together, these data indicate a novel role for Mss4 as an effector for Rab15 in early endocytic trafficking.

The cytoplasmic tail of invariant chain regulates endosome fusion and morphology

The major histocompatibility complex class II associated invariant chain (Ii) has been shown to inhibit endocytic transport and to increase the size of endosomes. We have recently found that this property has a significant impact on antigen processing and presentation. Here, we show in a cell-free endosome fusion assay that expression of Ii can increase fusion after phosphatidylinositol 3-kinase activity is blocked by wortmannin. In live cells wortmannin was also not able to block formation of the Ii-induced enlarged endosomes. The effects of Ii on endosomal transport and morphology depend on elements within the cytoplasmic tail. Data from mutagenesis analysis and nuclear magnetic resonance-based structure calculations of the Ii cytoplasmic tail demonstrate that free negative charges that are not involved in internal salt bridges are essential for both interactions between the tails and for the formation of enlarged endosomes. This correlation indicates that it is interactions between the Ii cytoplasmic tails that are involved in endosome fusion. The combined data from live cells, cell-free assays, and molecular dynamic simulations suggest that Ii molecules on different vesicles can promote endosome docking and fusion and thereby control endosomal traffic of membrane proteins and endosomal content.

Rab5a GTPase regulates fusion between pathogen-containing phagosomes and cytoplasmic organelles in human neutrophils

Biogenesis of phagolysosomes proceeds through a sequential series of interactions with endocytic organelles, a process known to be regulated by Rab and SNARE proteins. The molecular mechanisms underlying phagosome maturation in neutrophils are, however, not clearly understood. We investigated fusion between phagosomes containing the intracellular pathogen Mycobacterium tuberculosis versus the extracellular pathogen Staphylococcus aureus (designated MCP for mycobacteria-containing phagosome and SCP for S. aureus-containing phagosome) and cytoplasmic compartments in human neutrophils. Western blot analysis of phagosomes isolated after internalisation revealed that lactoferrin (a constituent of secondary granules) and LAMP-1 were incorporated into both SCP and MCP, whereas hck(marker of azurophil granules) interacted solely with SCP. The subcellular distribution of the proteins Rab5a and syntaxin-4 suggested a role in docking of granules and/or endosomes to the target membrane in the neutrophil. We observed that during phagocytosis, Rab5a in GTP-bound form interacted with syntaxin-4 on the membrane of MCP and were retained for up to 90 minutes,whereas the complex was recruited to the SCP within 5 minutes but was selectively depleted from these vacuoles after 30 minutes of phagocytosis. Downregulation of Rab5a by antisense oligonucleotides efficiently reduced the synthesis of Rab5a, the binding of syntaxin-4 to MCP and SCP and the capacity for fusion exhibited by the pathogen-containing phagosomes, but it had no effect on bacteria internalisation. These data indicate that the difference in granule fusion is correlated with a difference in the association of Rab5a and syntaxin-4 with the phagosomes. Intracellular pathogen-containing phagosomes retain Rab5a and syntaxin-4, whereas extracellular pathogen-containing phagosomes bind briefly to this complex. These results also identified Rab5a as a key regulator of phagolysosome maturation in human neutrophils.

Sequential roles for phosphatidylinositol 3-phosphate and Rab5 in tethering and fusion of early endosomes via their interaction with EEA1

Early endosome antigen 1 (EEA1) is a 170-kDa polypeptide required for endosome fusion in mammalian cells. The COOH terminus of EEA1 contains a FYVE domain that interacts specifically with phosphatidylinositol 3-phosphate (PtdIns-3-P) and a Rab5 GTPase binding region adjacent to the FYVE domain. The dual interaction of EEA1 with both PtdIns-3-P and Rab5 has been hypothesized to provide the specificity required to target EEA1 to early endosomes. To test this hypothesis, we generated truncated (amino acids 1277--1411) and full-length EEA1 constructs containing point mutations in the COOH terminus that impair Rab5 but not PtdIns-3-P binding. These constructs localized to endosomes in intact cells as efficiently as their wild-type counterparts. Furthermore, overexpression of the truncated constructs, both wild-type and mutated, impaired the function of endogenous EEA1 resulting in the accumulation of small, untethered endosomes. These results suggest that association with Rab5 is not necessary for the initial binding and tethering functions of EEA1. A role for Rab5 binding was revealed, however, upon comparison of endosomes in cells expressing full-length wild-type or mutated EEA1. The mutant full-length EEA1 caused the accumulation of endosome clusters and suppressed the enlargement of endosomes caused by a persistently active form of Rab5 (Rab5Q79L). In contrast, expression of wild-type EEA1 with Rab5Q79L enhanced this enlargement. Thus, endosome tethering depends on the interaction of EEA1 with PtdIns-3-P, and its interaction with Rab5 appears to regulate subsequent fusion.

Lysosome proteins are redistributed during expression of a GTP-hydrolysis-defective rab5a

The functioning of the endocytic pathway is influenced by a distinct set of rab GTPases, including rab5a, which regulates homotypic fusion of early endosomes. Expression of a dominant active, GTPase-defective rab5a accelerates endosome fusion, causing the formation of a greatly enlarged endocytic compartment. Here we present evidence that rab5a also regulates trafficking between endosomes and lysosomes and may play a role in lysosome biogenesis. The GTPase defective rab5aQ79L mutant was inducibly expressed as an EGFP fusion in HEK293 cells, and the distribution of lysosome proteins and endocytic markers then assessed by deconvolution fluorescence microscopy. During expression of EGFP-rab5aQ79L, the lysosome proteins LAMP-1, LAMP-2 and cathepsin D were found in dilated EGFP-rab5aQ79L-positive vesicles, which also rapidly labeled with transferrin Texas Red. Exogenous tracers that normally traffic to lysosomes after prolonged chase (dextran Texas Red and DiI-LDL) also accumulated in these vesicles. Dextran Texas Red preloaded into lysosomes localized with subsequently expressed EGFP-rab5a Q79L, suggesting the existence of lysosome to endosome traffic. Cells expressing EGFP-rab5a wt or the dominant negative EGFP-rab5aS34N did not exhibit these abnormalities. Despite the dramatic alterations in lysosome protein distribution caused by expression of EGFP-rab5a Q79L, there was little change in the endocytosis or recycling of a cell-surface receptor (β2-adrenergic receptor). However, there was a deficiency of dense β-hexosaminidase-containing lysosomes in cells expressing EGFP-rab5aQ79L, as assessed by Percoll gradient fractionation. These results suggest that expression of a GTPase-defective rab5a affects lysosome biogenesis by alteration of traffic between lysosomes and endosomes.

ADP-ribosylation of Rab5 by ExoS of Pseudomonas aeruginosa affects endocytosis

Pseudomonas aeruginosa exoenzyme S (ExoS) is an ADP-ribosyltransferase that modifies low-molecular-weight GTPases. Here we studied the effect of Rab5 ADP-ribosylation by ExoS on its cellular function, i.e., regulation of early endocytic events. Coculture of CHO cells with P. aeruginosa induced a marked decrease in horseradish peroxidase (HRP) uptake compared to noninfected cells, while coculture with a P. aeruginosa mutant strain that fails to produce ExoS did not lead to any change in HRP uptake. Microinjection of recombinant ExoS into Xenopus oocytes induced strong inhibition of basal HRP uptake by oocytes. Moreover, coinjection of recombinant ExoS with Rab5 abolished the typical stimulation of HRP uptake obtained after GTPase microinjection. Cytosols prepared from injected oocytes were used in an endosome-endosome fusion assay. Cytosol from ExoS-microinjected oocytes was ineffective in promoting endosome-endosome fusion. However, in these conditions, the addition of Rab5 to the assay led to fusion recovery. Finally, we found that the interaction of Rab5 with EEA1 was markedly diminished after Rab5 ADP-ribosylation by ExoS.

Rab7 regulates phagosome maturation in Dictyostelium

A Dictyostelium Rab7 homolog has been demonstrated to regulate fluid-phase influx, efflux, retention of lysosomal hydrolases and phagocytosis. Since Rab7 function appeared to be required for efficient phagocytosis, we sought to further characterize the role of Rab7 in phagosomal maturation. Expression of GFP-Rab7 resulted in labeling of both early and late phagosomes containing yeast, but not forming phagocytic cups. In order to determine if Rab7 played a role in regulating membrane traffic between the endo/lysosomal system and maturing phagosomes, latex bead containing (LBC) phagosomes were purified from wild-type cells at various times after internalization. Glycosidases, cysteine proteinases, Rab7 and lysosomally associated membrane proteins were delivered rapidly to nascent phagosomes in control cells. LBC phagosomes isolated from cells overexpressing dominant negative (DN) Rab7 contained very low levels of LmpA (lysosomal integral membrane protein) and α-mannosidase was not detectable. Interestingly, cysteine proteinases were delivered to phagosomes as apparent pro-forms in cells overexpressing DN Rab7. Despite these defects, phagosomes in cells overexpressing DN Rab7 matured to form multi-particle spacious phagosomes, except that these phagosomes remained significantly more acidic than control phagosomes. These results suggested that Rab7 regulates both an early and late steps of phagosomal maturation, similar to its role in the endo/lysosomal system.

Membrane targeting of a Rab GTPase that fails to associate with Rab escort protein (REP) or guanine nucleotide dissociation inhibitor (GDI)

The targeting of various Rab proteins to different subcellular compartments appears to be determined by variable amino acid sequences located upstream from geranylgeranylated cysteine residues in the C-terminal tail. All nascent Rab proteins are prenylated by geranylgeranyltransferase II, which recognizes the Rab substrate only when it is bound to Rab escort protein (REP). After prenylation, REP remains associated with the modified Rab until it is delivered to the appropriate subcellular membrane. It remains unclear whether docking of the Rab with the correct membrane is solely a function of features contained within the prenylated Rab itself (with REP serving as a "passive" carrier) or whether REP actively participates in the targeting process. To address this issue, we took advantage of a mutation in the alpha2 helix of Rab1B (i.e. Y78D) that abolishes REP and GDI interaction without disrupting nucleotide binding or hydrolysis. These studies demonstrate that replacing the C-terminal GGCC residues of Rab1B(Y78D) with a CLLL motif permits this protein to be prenylated by geranylgeranyltransferase I but not II both in cell-free enzyme assays and in transfected cells. Subcellular fractionation and immunofluorescence studies reveal that the prenylated Rab1B(Y78D)CLLL, which remains deficient in REP and GDI association is, nonetheless, delivered to the Golgi and endoplasmic reticulum (ER) membranes. When the dominant-negative S22N mutation was inserted into Rab1B-CLLL, the resulting monoprenylated construct suppressed ER --> Golgi protein transport. However, when the Y78D mutation was added to the latter construct, its inhibitory effect on protein trafficking was lost despite the fact that it was localized to the ER/Golgi membrane. Therefore, protein interactions mediated by the alpha2 helical domain of Rab1B(S22N) appear to be essential for its functional interaction with components of the ER --> Golgi transport machinery.

Receptor and membrane recycling can occur with unaltered efficiency despite dramatic Rab5(q79l)-induced changes in endosome geometry

Current models for sorting in the endosomal compartment suggest that endosomal geometry plays a significant role as membrane-bound proteins accumulate in tubular regions for recycling, and lumenal markers accumulate in large vacuolar portions for delivery to lysosomes. Rab5, a small molecular weight GTPase, functions in the formation and maintenance of the early/sorting endosome. Overexpression of the constitutively active form, Rab5(Q79L), leads to enhanced endosome fusion resulting in the enlargement of early endosomes. Using an adenoviral expression system to regulate the time and level of Rab5(Q79L) overexpression in HeLa cells, we find that although endosomes are dramatically enlarged, the rates of transferrin receptor-mediated endocytosis and recycling are unaffected. Moreover, despite the enlarged endosome phenotype, neither the rate of internalization of a fluid phase marker nor the rate of recycling of a bulk lipid marker were affected. These results suggest that GTP hydrolysis by Rab5 is rate-limiting for endosome fusion but not for endocytic trafficking and that early endosome geometry may be a less critical determinant of sorting efficiencies than previously thought.

Mycobacterium tuberculosis and Legionella pneumophila phagosomes exhibit arrested maturation despite acquisition of Rab7

Rab7 is a small GTPase that regulates vesicular traffic from early to late endosomal stages of the endocytic pathway. Phagosomes containing inert particles have also been shown to transiently acquire Rab7 as they mature. Disruption in the pathway prior to the acquisition of Rab7 has been suggested as playing a role in the altered maturation of Mycobacterium bovis BCG phagosomes. As a first step to determine whether disruption in the delivery or function of Rab7 could play a role in the altered maturation of Legionella pneumophila and M. tuberculosis phagosomes, we have examined the distribution of wild-type Rab7 and the GTPase-deficient, constitutively active mutant form of Rab7 in HeLa cells infected with L. pneumophila or M. tuberculosis. We have found that the majority of L. pneumophila and M. tuberculosis phagosomes acquire relatively abundant staining for Rab7 and for the constitutively active mutant Rab7 in HeLa cells that overexpress these proteins. Nevertheless, despite acquisition of wild-type or constitutively active Rab7, both the L. pneumophila and the M. tuberculosis phagosomes continue to exhibit altered maturation as manifested by a failure to acquire lysosome-associated membrane glycoprotein 1. These results demonstrate that L. pneumophila and M. tuberculosis phagosomes have receptors for Rab7 and that the altered maturation of these phagosomes is not due to a failure to acquire Rab7.

A novel membrane-anchored Rab5 interacting protein required for homotypic endosome fusion

The ras-related GTPase rab5 is rate-limiting for homotypic early endosome fusion. We used a yeast two-hybrid screen to identify a rab5 interacting protein, rab5ip. The cDNA sequence encodes a ubiquitous 75-kDa protein with an N-terminal transmembrane domain (TM), a central coiled-coil structure, and a C-terminal region homologous to several centrosome-associated proteins. rab5ip lacking the transmembrane domain (rab5ipTM(-)) had a greater affinity in vitro for rab5-guanosine 5'-O-2-(thio)diphosphate than for rab5-guanosine 5'-3-O-(thio)triphosphate. In transfected HeLa cells, rab5ipTM(-) was partly cytosolic and localized (by immunofluorescence) with a rab5 mutant believed to be in a GDP conformation (GFP-rab5(G78A)) but not with GFP-rab5(Q79L), a GTPase-deficient mutant. rab5ip with the transmembrane domain (rab5ipTM(+)) was completely associated with the particulate fraction and localized extensively with GFP-rab5(wt) in punctate endosome-like structures. Overexpression of rab5ipTM(+) using Sindbis virus stimulated the accumulation of fluid-phase horseradish peroxidase by BHK-21 cells, and homotypic endosome fusion in vitro was inhibited by antibody against rab5ip. rab5ipTM(-) inhibited rab5(wt)-stimulated endosome fusion but did not inhibit fusion stimulated by rab5(Q79L). rab5ip represents a novel rab5 interacting protein that may function on endocytic vesicles as a receptor for rab5-GDP and participate in the activation of rab5.

Deviant expression of Rab5 on phagosomes containing the intracellular pathogens Mycobacterium tuberculosis and Legionella pneumophila is associated with altered phagosomal fate

The intracellular human pathogens Legionella pneumophila and Mycobacterium tuberculosis reside in altered phagosomes that do not fuse with lysosomes and are only mildly acidified. The L. pneumophila phagosome exists completely outside the endolysosomal pathway, and the M. tuberculosis phagosome displays a maturational arrest at an early endosomal stage along this pathway. Rab5 plays a critical role in regulating membrane trafficking involving endosomes and phagosomes. To determine whether an alteration in the function or delivery of Rab5 could play a role in the aberrant development of L. pneumophila and M. tuberculosis phagosomes, we have examined the distribution of the small GTPase, Rab5c, in infected HeLa cells overexpressing Rab5c. Both pathogens formed phagosomes in HeLa cells with molecular characteristics similar to their phagosomes in human macrophages and multiplied in these host cells. Phagosomes containing virulent wild-type L. pneumophila never acquired immunogold staining for Rab5c, whereas phagosomes containing an avirulent mutant L. pneumophila (which ultimately fused with lysosomes) transiently acquired staining for Rab5c after phagocytosis. In contrast, M. tuberculosis phagosomes exhibited abundant staining for Rab5c throughout its life cycle. To verify that the overexpressed, recombinant Rab5c observed on the bacterial phagosomes was biologically active, we examined the phagosomes in HeLa cells expressing Rab5c Q79L, a fusion-promoting mutant. Such HeLa cells formed giant vacuoles, and after incubation with various particles, the giant vacuoles acquired large numbers of latex beads, M. tuberculosis, and avirulent L. pneumophila but not wild-type L. pneumophila, which consistently remained in tight phagosomes that did not fuse with the giant vacuoles. These results indicate that whereas Rab5 is absent from wild-type L. pneumophila phagosomes, functional Rab5 persists on M. tuberculosis phagosomes. The absence of Rab5 on the L. pneumophila phagosome may underlie its lack of interaction with endocytic compartments. The persistence of functional Rab5 on the M. tuberculosis phagosomes may enable the phagosome to retard its own maturation at an early endosomal stage.

Rab3A triggers the acrosome reaction in permeabilized human spermatozoa

The acrosome reaction is a regulated exocytotic process leading to a massive fusion between the outer acrosomal membrane and the cell membrane. In spite of the great amount of information available related to the acrosome reaction in several species, there is a remarkable paucity about the role of monomeric guanosine triphosphatases (GTPases) of the Rab family-well-established participants in exocytosis in other cell types-in the acrosome reaction. Western blot and immunofluorescence analysis indicate that Rab3A is present in human spermatozoa and localizes to the acrosomal region in the sperm head. One difficulty in studying the role of proteins in intact cells is the fact that they are unable to cross the cell membrane. Therefore, we established a working model of streptolysin O-permeabilized human spermatozoa. Permeabilized spermatozoa were able to respond in a regulated way to different stimuli, such as G protein activators and calcium. An acrosomal reaction was also triggered by a Rab3A peptide corresponding to the effector region. More important, recombinant Rab3A protein in the GTP-bound form caused acrosome exocytosis. The same protein loaded with GDP or Rab11 in the GTP-bound form was inactive. Also, recombinant GDI (GDP dissociation inhibitor)-a protein that releases Rab proteins from membrane-inhibited a GTPgammaS-stimulated acrosome reaction. Our results indicate that 1) permeabilized spermatozoa can be used to study the role of macromolecules in the acrosome reaction, 2) Rab3A is present in human spermatozoa, and 3) Rab3A or another Rab3 isoform is involved in the exocytosis of the acrosomal granule in human spermatozoa.

Live Salmonella recruits N-ethylmaleimide-sensitive fusion protein on phagosomal membrane and promotes fusion with early endosome

To understand intracellular trafficking modulations by live Salmonella, we investigated the characteristics of in vitro fusion between endosomes and phagosomes containing live (LSP) or dead Salmonella (DSP). We observed that fusion of both DSP and LSP were time, temperature and cytosol dependent. GTPgammaS and treatment of the phagosomes with Rab-GDI inhibited fusion, indicating involvement of Rab-GTPases. LSP were rich in rab5, alpha-SNAP, and NSF, while DSP mainly contained rab7. Fusion of endosomes with DSP was inhibited by ATP depletion, N-ethylmaleimide (NEM) treatment, and in NEM-sensitive factor (NSF)-depleted cytosol. In contrast, fusion of endosomes with LSP was not inhibited by ATP depletion or NEM treatment, and occurred in NSF-depleted cytosol. However, ATPgammaS inhibited both fusion events. Fusion of NEM-treated LSP with endosomes was abrogated in NSF- depleted cytosol and was restored by adding purified NSF, whereas no fusion occurred with NEM-treated DSP, indicating that NSF recruitment is dependent on continuous signals from live Salmonella. Binding of NSF with LSP required prior presence of rab5 on the phagosome. We have also shown that rab5 specifically binds with Sop E, a protein from Salmonella. Our results indicate that live Salmonella help binding of rab5 on the phagosomes, possibly activate the SNARE which leads to further recruitment of alpha-SNAP for subsequent binding with NSF to promote fusion of the LSP with early endosomes and inhibition of their transport to lysosomes.

rab5 GTPase regulates adenovirus endocytosis

Adenovirus interaction with alphav integrins is important for virus entry. We have examined the effects of adenovirus attachment on intracellular signaling in HeLa cells, with an emphasis on pathways known to be activated following integrin interaction with other ligands. We found no evidence for [Ca(2+)](c)-mediated signaling or for tyrosine phosphorylation of pp125(FAK), p130(CAS), and paxillin. However, adenovirus attachment is known to activate phosphatidylinositol-3 kinase, which in turn may regulate endocytosis via rab5 GTPase. We found that adenovirus uptake was increased by overexpression of wild-type rab5 and decreased by dominant-negative rab5. These results indicate a role for rab5 in adenovirus entry.

Increased expression of Rab5a correlates directly with accelerated maturation of Listeria monocytogenes phagosomes

Previous studies have shown that Listeria monocytogenes (LM) modulates phagocytic membrane traffic. Here we explore whether Rab5a, a GTPase associated with phagosome-endosome fusion, is related to phagosome maturation and to the intracellular survival of LM. Stable transfection of Rab5a cDNA into macrophages accelerates intracellular degradation of LM. Morphological studies confirmed that phagosome maturation and phagosome-lysosome fusion is enhanced by overexpression of Rab5a. Down-regulation experiments using antisense oligonucleotides targeted to the Rab5a mRNA efficiently reduced Rab5a synthesis, reduced phagosome-endosome traffic, blocked phagosome-lysosome fusion, and extended intraphagosomal survival of LM. Down-regulation of Rab5a had no effect on LM internalization. Down-regulation of Rab5c had no effect on phagosome maturation and phagosome-lysosome fusion. The results indicate that Rab5a controls early phagosome-endosome interactions and governs the maturation of the early phagosome leading to phagosome-lysosome fusion.

Regulation of membrane transport through the endocytic pathway by rabGTPases

Small GTP binding proteins of the rab family are associated with the cytoplasmic surface of compartments of the central vacuolar system. Several of them, including rab5, rab4 and rab11, are localized to early endocytic organelles where they regulate distinct events in the transferrin receptor pathway. Whereas rab5 is controlling transport to early endosomes, rab4 and rab11 are involved in the regulation of recycling back to the plasma membrane. How GTP-hydrolysis of rab bound GTP is related to the role of these proteins in endocytosis is not yet known, but quick progress is being made towards this goal through the identification of proteins regulating the activity of these rab proteins.