Evidence for a symmetrical requirement for Rab5-GTP in in vitro endosome-endosome fusion

Pathophysiological Potentials of NRF3-Regulated Transcriptional Axes in Protein and Lipid Homeostasis

NRF3 (NFE2L3) belongs to the CNC-basic leucine zipper transcription factor family. An NRF3 homolog, NRF1 (NFE2L1), induces the expression of proteasome-related genes in response to proteasome inhibition. Another homolog, NRF2 (NFE2L2), induces the expression of genes related to antioxidant responses and encodes metabolic enzymes in response to oxidative stress. Dysfunction of each homolog causes several diseases, such as neurodegenerative diseases and cancer development. However, NRF3 target genes and their biological roles remain unknown. This review summarizes our recent reports that showed NRF3-regulated transcriptional axes for protein and lipid homeostasis. NRF3 induces the gene expression of POMP for 20S proteasome assembly and CPEB3 for NRF1 translational repression, inhibiting tumor suppression responses, including cell-cycle arrest and apoptosis, with resistance to a proteasome inhibitor anticancer agent bortezomib. NRF3 also promotes mevalonate biosynthesis by inducing SREBP2 and HMGCR gene expression, and reduces the intracellular levels of neural fatty acids by inducing GGPS1 gene expression. In parallel, NRF3 induces macropinocytosis for cholesterol uptake by inducing RAB5 gene expression. Finally, this review mentions not only the pathophysiological aspects of these NRF3-regulated axes for cancer cell growth and anti-obesity potential but also their possible role in obesity-induced cancer development.

NRF3 upregulates gene expression in SREBP2-dependent mevalonate pathway with cholesterol uptake and lipogenesis inhibition

Lipids, such as cholesterol and fatty acids, influence cell signaling, energy storage, and membrane formation. Cholesterol is biosynthesized through the mevalonate pathway, and aberrant metabolism causes metabolic diseases. The genetic association of a transcription factor NRF3 with obesity has been suggested, although the molecular mechanisms remain unknown. Here, we show that NRF3 upregulates gene expression in SREBP2-dependent mevalonate pathway. We further reveal that NRF3 overexpression not only reduces lanosterol, a cholesterol precursor, but also induces the expression of the GGPS1 gene encoding an enzyme in the production of GGPP from farnesyl pyrophosphate (FPP), a lanosterol precursor. NRF3 overexpression also enhances cholesterol uptake through RAB5-mediated macropinocytosis process, a bulk and fluid-phase endocytosis pathway. Moreover, we find that GGPP treatment abolishes NRF3 knockdown-mediated increase of neutral lipids. These results reveal the potential roles of NRF3 in the SREBP2-dependent mevalonate pathway for cholesterol uptake through macropinocytosis induction and for lipogenesis inhibition through GGPP production.

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NRF3 upregulates gene expression of enzymes in the mevalonate pathway

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NRF3 induces SREBP2 gene expression and interacts with active SREBP2 proteins

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NRF3 reduces neutral lipid levels through GGPS1-mediated GGPP production

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NRF3 enhances cholesterol uptake through RAB5-mediated macropinocytosis

Distribution and Co-localization of endosome markers in cells

Clathrin mediated endocytosis is one pathway for internalization of extracellular nano materials into cells [1, 2]. In this pathway, proteins attached to receptors and the internalized materials are encapsulated in clathrin coated membrane vesicles that subsequently fuse with or transform into intracellular compartments (early and late endosomes) as their contents are being directed to the lysosomes for degradation. The following proteins are commonly used to mark the pathway at various stages: Rab5 (early endosome), Rab7 (late endosome), and LAMP-1 (lysosome). In this work, we studied the distribution and co-localization of these marker proteins in two cell lines (C2C12 and A549) to determine whether these markers are unique for specific endosome types or whether they can co-exist with other markers. We estimate the densities and sizes of the endosomes containing the three markers, as well as the number of marker antibodies attached to each endosome. We determine that the markers are not unique to one endosome type but that the extent of co-localization is different for the two cell types. In fact, we find endosomes that contain all three markers simultaneously. Our results suggest that the use of these proteins as specific markers for specific endosome types should be reevaluated. This was the first successful use of triple image cross correlation spectroscopy to qualitatively and quantitatively study the extent of interaction among three different species in cells and also the first experimental study of three-way interactions of clathrin mediated endocytic markers.

RLIP76 regulates Arf6-dependent cell spreading and migration by linking ARNO with activated R-Ras at recycling endosomes

R-Ras small GTPase enhances cell spreading and motility via RalBP1/RLIP76, an R-Ras effector that links GTP-R-Ras to activation of Arf6 and Rac1 GTPases. Here, we report that RLIP76 performs these functions by binding cytohesin-2/ARNO, an Arf GTPase guanine exchange factor, and connecting it to R-Ras at recycling endosomes. RLIP76 formed a complex with R-Ras and ARNO by binding ARNO via its N-terminus (residues 1-180) and R-Ras via residues 180-192. This complex was present in Rab11-positive recycling endosomes and the presence of ARNO in recycling endosomes required RLIP76, and was not supported by RLIP76(Δ1-180) or RLIP76(Δ180-192). Spreading and migration required RLIP76(1-180), and RLIP76(Δ1-180) blocked ARNO recruitment to recycling endosomes, and spreading. Arf6 activation with an ArfGAP inhibitor overcame the spreading defects in RLIP76-depleted cells or cells expressing RLIP76(Δ1-180). Similarly, RLIP76(Δ1-180) or RLIP76(Δ180-192) suppressed Arf6 activation. Together these results demonstrate that RLIP76 acts as a scaffold at recycling endosomes by binding activated R-Ras, recruiting ARNO to activate Arf6, thereby contributing to cell spreading and migration.

The nutrient stress-induced small GTPase Rab5 contributes to the activation of vesicle trafficking and vacuolar activity

Rab family small GTPases regulate membrane trafficking by spatiotemporal recruitment of various effectors. However, it remains largely unclear how the expression and functions of Rab proteins are regulated in response to extracellular or intracellular stimuli. Here we show that Ypt53, one isoform of Rab5 in Saccharomyces cerevisiae, is up-regulated significantly under nutrient stress. Under non-stress conditions, Vps21, a constitutively expressed Rab5 isoform, is crucial to Golgi-vacuole trafficking and to vacuolar hydrolase activity. However, when cells are exposed to nutrient stress for an extended period of time, the up-regulated Ypt53 and the constitutive Vps21 function redundantly to maintain these activities, which, in turn, prevent the accumulation of reactive oxygen species and maintain mitochondrial respiration. Together, our results clarify the relative roles of these constitutive and nutrient stress-inducible Rab5 proteins that ensure adaptable vesicle trafficking and vacuolar hydrolase activity, thereby allowing cells to adapt to environmental changes.

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.

Intrinsic tethering activity of endosomal Rab proteins

Rab small G-proteins control membrane trafficking events required for a multitude of processes including secretion, lipid metabolism, antigen presentation, and growth factor signaling. Rabs recruit effectors that mediate diverse functions including vesicle tethering and fusion. However, many mechanistic questions about Rab-regulated vesicle tethering are unresolved. Using chemically defined reaction systems we discovered that Vps21, a Saccharomyces cerevisiae ortholog of mammalian endosomal Rab5, functions in trans with itself and with at least two other endosomal Rabs to directly mediate GTP-dependent tethering. Vps21-mediated tethering was stringently and reversibly regulated by an upstream activator, Vps9, and an inhibitor, Gyp1, which were sufficient to drive dynamic cycles of tethering and de-tethering. These experiments reveal an unexpected mode of tethering by endocytic Rabs. In our working model, the intrinsic tethering capacity Vps21 operates in concert with conventional effectors and SNAREs to drive efficient docking and fusion.

SNARE function is not involved in early endosome docking

Docking and fusion of transport vesicles constitute elementary steps in intracellular membrane traffic. While docking is thought to be initiated by Rab-effector complexes, fusion is mediated by SNARE (N-ethylmaleimide-sensitive factor [NSF] attachment receptor) proteins. However, it has been recently debated whether SNAREs also play a role in the establishment or maintenance of a stably docked state. To address this question, we have investigated the SNARE dependence of docking and fusion of early endosomes, one of the central sorting compartments in the endocytic pathway. A new, fluorescence-based in vitro assay was developed, which allowed us to investigate fusion and docking in parallel. Similar to homotypic fusion, docking of early endosomes is dependent on the presence of ATP and requires physiological temperatures. Unlike fusion, docking is insensitive to the perturbation of SNARE function by means of soluble SNARE motifs, SNARE-specific F(ab) fragments, or by a block of NSF activity. In contrast, as expected, docking is strongly reduced by interfering with the synthesis of phosphatidyl inositol (PI)-3 phosphate, with the function of Rab-GTPases, as well as with early endosomal autoantigen 1 (EEA1), an essential tethering factor. We conclude that docking of early endosomes is independent of SNARE function.

Identification of rab20 as a potential regulator of connexin 43 trafficking

Connexin oligomerization and trafficking are regulated processes. To identify proteins that control connexin 43 (Cx43), a screen was designed using HeLa cells expressing a Cx43 construct with di-lysine endoplasmic reticulum (ER)-retention/retrieval motif, Cx43-HKKSL. At moderate levels of expression, Cx43-HKKSL is retained in the ER as monomers; however, Cx43-HKKSL stably overexpressed by HeLa cells localizes to the perinuclear region and oligomerizes. HeLa/Cx43-HKKSL overexpressors were transiently transfected with pooled clones from a human kidney cDNA library and used immunofluorescence microscopy to identify cDNAs that enabled overexpressed Cx43-HKKSL to convert from a perinuclear to ER localization pattern. Using this approach, a small molecular weight GTPase, rab20, was identified as a candidate protein with the ability to regulate Cx43 trafficking. Enhanced green fluorescent protein (EGFP)-tagged rab20 showed a predominantly perinuclear and ER localization pattern and caused wild-type Cx43 to be retained inside the cell. By contrast, mutant EGFP-rab20T19N, which lacks the ability to bind GTP, had no effect on Cx43. These results suggest Cx43 is transported through an intracellular compartment regulated by rab20 along the secretory pathway.

Regulation of early endosomal entry by the Drosophila tumor suppressors Rabenosyn and Vps45

The small GTPase Rab5 has emerged as an important regulator of animal development, and it is essential for endocytic trafficking. However, the mechanisms that link Rab5 activation to cargo entry into early endosomes remain unclear. We show here that Drosophila Rabenosyn (Rbsn) is a Rab5 effector that bridges an interaction between Rab5 and the Sec1/Munc18-family protein Vps45, and we further identify the syntaxin Avalanche (Avl) as a target for Vps45 activity. Rbsn and Vps45, like Avl and Rab5, are specifically localized to early endosomes and are required for endocytosis. Ultrastructural analysis of rbsn, Vps45, avl, and Rab5 null mutant cells, which show identical defects, demonstrates that all four proteins are required for vesicle fusion to form early endosomes. These defects lead to loss of epithelial polarity in mutant tissues, which overproliferate to form neoplastic tumors. This work represents the first characterization of a Rab5 effector as a tumor suppressor, and it provides in vivo evidence for a Rbsn-Vps45 complex on early endosomes that links Rab5 to the SNARE fusion machinery.

Four distinct pathways of hemoglobin uptake in the malaria parasite Plasmodium falciparum

During the bloodstage of malaria infection, the parasite internalizes and degrades massive amounts of hemoglobin from the host red blood cell. Using serial thin-section electron microscopy and three-dimensional reconstruction, we demonstrate four independent, but partially overlapping, hemoglobin-uptake processes distinguishable temporally, morphologically, and pharmacologically. Early ring-stage parasites undergo a profound morphological transformation in which they fold, like a cup, onto themselves and in so doing take a large first gulp of host cell cytoplasm. This event, which we term the "Big Gulp," appears to be independent of actin polymerization and marks the first step in biogenesis of the parasite's lysosomal compartment-the food vacuole. A second, previously identified uptake process, uses the cytostome, a well characterized and morphologically distinct structure at the surface of the parasite. This process is more akin to classical endocytosis, giving rise to small (<0.004 fl) vesicles that are marked by the early endosomal regulatory protein Rab5a. A third process, also arising from cytostomes, creates long thin tubes previously termed cytostomal tubes in an actin-dependent manner. The fourth pathway, which we term phagotrophy, is similar to the Big Gulp in that it more closely resembles phagocytosis, except that phagotrophy does not require actin polymerization. Each of these four processes has aspects that are unique to Plasmodium, thus opening avenues to antimalarial therapy.

Tyrphostin A8 stimulates a novel trafficking pathway of apically endocytosed transferrin through Rab11-enriched compartments in Caco-2 cells

The potential application of transferrin receptors as delivery vehicles for transport of macromolecular drugs across intestinal epithelial cells is limited by several factors, including the low level of transferrin receptor-mediated transcytosis, particularly in the apical-to-basolateral direction. The GTPase inhibitor, AG10 (tyrphostin A8), has been shown previously to increase the apical-to-basolateral transcytosis of transferrin in Caco-2 cells. However, the mechanism of the increased transcytosis has not been established. In this report, the effect of AG10 on the trafficking of endocytosed transferrin among different endosomal compartments as well as the involvement of Rab11 in the intracellular trafficking of transferrin was investigated. Confocal microscopy studies showed a high level of colocalization of FITC-transferrin with Rab5 and Rab11 in Caco-2 cells pulsed at 16 degrees C and 37 degrees C, which indicated the presence of apically endocytosed FITC-transferrin in early endosomes and apical recycling endosomes at 16 degrees C and 37 degrees C, respectively. The effect of AG10 on the accumulation of transferrin within different endosomal compartment was studied, and an increase in the transcytosis and recycling of internalized (125)I-labeled transferrin, as well as a decrease in cell-associated (125)I-labeled transferrin, was observed in AG10-treated Caco-2 cells pulsed at 37 degrees C for 30 min and chased for 30 min. Moreover, confocal microscopy showed that FITC-transferrin exhibited an increased level of colocalization with Rab11, but not with Rab5, in the presence of AG10. These results suggest an effect of AG10 on the later steps of transferrin receptor trafficking, which are involved in subsequent recycling, and possibly transcytosis, of endocytosed transferrin in Caco-2 cells.

The Rab5 Activator ALS2/alsin Acts as a Novel Rac1 Effector through Rac1-activated Endocytosis

Mutations in the ALS2 gene cause a number of recessive motor neuron diseases, indicating that the ALS2 protein (ALS2/alsin) is vital for motor neurons. ALS2 acts as a guanine nucleotide exchange factor (GEF) for Rab5 (Rab5GEF) and is involved in endosome dynamics. However, the spatiotemporal regulation of the ALS2-mediated Rab5 activation is unclear. Here we identified an upstream activator for ALS2 and showed a functional significance of the ALS2 activation in endosome dynamics. ALS2 preferentially interacts with activated Rac1. In the cells activated Rac1 recruits cytoplasmic ALS2 to membrane ruffles and subsequently to nascent macropinosomes via Rac1-activated macropinocytosis. At later endocytic stages macropinosomal ALS2 augments fusion of the ALS2-localized macropinosomes with the transferrin-positive endosomes, depending on the ALS2-associated Rab5GEF activity. These results indicate that Rac1 promotes the ALS2 membranous localization, thereby rendering ALS2 active via Rac1-activated endocytosis. Thus, ALS2 is a novel Rac1 effector and is involved in Rac1-activated macropinocytosis. All together, loss of ALS2 may perturb macropinocytosis and/or the following membrane trafficking, which gives rise to neuronal dysfunction in the ALS2-linked motor neuron diseases.

Rabs and their effectors: achieving specificity in membrane traffic

Rab proteins constitute the largest branch of the Ras GTPase superfamily. Rabs use the guanine nucleotide-dependent switch mechanism common to the superfamily to regulate each of the four major steps in membrane traffic: vesicle budding, vesicle delivery, vesicle tethering, and fusion of the vesicle membrane with that of the target compartment. These different tasks are carried out by a diverse collection of effector molecules that bind to specific Rabs in their GTP-bound state. Recent advances have not only greatly extended the number of known Rab effectors, but have also begun to define the mechanisms underlying their distinct functions. By binding to the guanine nucleotide exchange proteins that activate the Rabs certain effectors act to establish positive feedback loops that help to define and maintain tightly localized domains of activated Rab proteins, which then serve to recruit other effector molecules. Additionally, Rab cascades and Rab conversions appear to confer directionality to membrane traffic and couple each stage of traffic with the next along the pathway.

Varp is a Rab21 guanine nucleotide exchange factor and regulates endosome dynamics

The small GTPases Rab5 and Rab21 are closely related, and play essential roles in endocytic trafficking. Rab5 is regulated by VPS9-domain-containing guanine nucleotide exchange factors. Here, we describe a new VPS9-domain protein with ankyrin repeats, the VPS9-ankyrin-repeat protein (Varp). Varp interacts preferentially with GDP-bound Rab21 and has a much stronger guanine nucleotide exchange activity towards Rab21 than Rab5. Furthermore, RNAi-mediated depletion of endogenous Varp significantly disrupts the activity of Rab21 in HeLa cells. Ectopically expressed Varp mainly localizes to early endosomes and causes enlargement of early endosomes and giant late endosomes. Both the VPS9 domain and ankyrin-repeats are required for the endosomal localization and the activity of Varp in vivo. These results suggest that Varp is a potential Rab21 guanine nucleotide exchange factor and might regulate endosome dynamics in vivo.

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.

ApRab11, a cnidarian homologue of the recycling regulatory protein Rab11, is involved in the establishment and maintenance of the Aiptasia-Symbiodinium endosymbiosis

Endosymbiotic association of the Symbiodinium dinoflagellates (zooxanthellae) with their cnidarian host cells involves an alteration in the development of the alga-enclosing phagosomes. To uncover its molecular basis, we previously investigated and established that the intracellular persistence of the zooxanthella-containing phagosomes involves specific alga-mediated interference with the expression of ApRab5 and ApRab7, two key endocytic regulatory Rab proteins, which results in the selective retention of the former on and exclusion of the later from the organelles. Here we examined the role of ApRab11, a cnidarian homologue of the key endocytic recycling regulator, Rab11, in the Aiptasia-Symbiodinium endosymbiosis. ApRab11 protein shared 88% overall sequence identity with human Rab11A and contained all Rab-specific signature motifs. Co-localization and mutagenesis studies showed that EGFP-tagged ApRab11 was predominantly associated with recycling endosomes and functioned in the recycling of internalized transferrin. In phagocytosis of latex beads, ApRab11 was quickly recruited to and later gradually removed from the developing phagosomes. Significantly, although ApRab11 immunoreactivity was rapidly detected on the phagosomes containing either newly internalized, heat-killed zooxanthellae, or resident zooxanthellae briefly treated with the photosynthesis inhibitor DCMU, it was rarely observed in the majority of phagosomes containing either newly internalized live, or healthy resident, zooxanthellae. It was concluded that through active exclusion of ApRab11 from the phagosomes in which they reside, zooxanthellae interfere with the normal recycling process required for efficient phagosome maturation, and thereby, secure their intracellular persistence, and consequently their endosymbiotic relationship with their cnidarian hosts.

Inhibition of Rab5a exchange activity is a key step for Listeria monocytogenes survival

Listeria monocytogenes (LM) modifies the phagocytic compartment by targeting Rab5a function through an unknown mechanism. Inhibition of Rab5a exchange by LM can be considered the main virulence mechanism as it favours viability of the parasite within the phagosome as well as the exclusion of putative listericidal lysosomal proteases such as cathepsin-D. The significance of this survival mechanism is evidenced by the overexpression of Rab5a mutants in CHO cells that promoted GDP exchange on Rab5a and eliminated pathogenic LM. The following mutants showed listericidal effects: Rab5a:Q79L, a constitutively active mutant with accelerated GDP exchange and Rab5a GEF, Vps9, which overactivates the endogenous protein. Clearance of LM from these phagosomes was controlled by the hydrolytic action of cathepsin-D as suggested by the lysosomal protease inhibitor chloroquine, or the cathepsin-D inhibitor, pepstatin A, which caused a reversion of listericidal activity. Moreover, the effects of LM on Rab5a phagocytic function mimics those reported for the GDP locked dominant negative Rab5a mutant, S34N. Transfection of these mutants into CHO cells increased pathogen survival as they showed higher numbers of viable bacteria, complete inhibition of GDP exchange on Rab5a and impairment of the listericidal action probably exerted by cathepsin-D. We cotransfected functional Rab5a GEF into this dominant negative mutant and restored normal LM intraphagosomal viability, Rab5a exchange and listericidal action of cathepsin-D.

Cross-species characterization of the ALS2 gene and analysis of its pattern of expression in development and adulthood

Mutations in the ALS2 gene, which encodes alsin, cause autosomal recessive juvenile-onset amyotrophic lateral sclerosis (ALS2) and related conditions. Using both a novel monoclonal antibody and LacZ knock-in mice, we demonstrate that alsin is widely expressed in neurons of the CNS, including the cortex, brain stem and motor neurons of the spinal cord. Interestingly, the highest levels of alsin are found in the molecular layer of the cerebellum, a brain region not previously implicated in ALS2. During development, alsin is expressed by day E9.5, but CNS expression does not become predominant until early postnatal life. At the subcellular level, alsin is tightly associated with endosomal membranes and is likely to be part of a large protein complex that may include the actin cytoskeleton. ALS2 is present in primates, rodents, fish and flies, but not in the nematode worm or yeast, and is more highly conserved than expected among mammals. Additionally, the product of a second, widely expressed gene, ALS2 C-terminal like (ALS2CL), may subserve or modulate some of the functions of alsin as an activator of Rab and Rho GTPases.

Distribution of Rab GTPases in mouse kidney and comparison with vacuolar H+-ATPase

BACKGROUND

Vacuolar H+-ATPases (V-ATPases) are essential for renal bicarbonate transport in both the proximal and distal nephron. Regulation of proton transport occurs, in part, by vesicle-mediated traffic of V-ATPases between intracellular vacuoles and the plasma membrane. Although the proteins involved in regulated V-ATPase traffic are largely unknown, Rab GTPases have a central role in the traffic and recycling of other membrane proteins.

METHODS

To identify candidate Rab GTPases potentially involved in V-ATPase traffic, immunocytochemical and subcellular fractionation studies were used to evaluate the distribution to sites of abundant V-ATPase of 5 Rab GTPases expressed in kidney, Rab5a, Rab11, Rab13, Rab18, and Rab20.

RESULTS

The immunocytochemical distribution of Rab5a and Rab13 and the subcellular distribution of Rab18 were not compatible with a role in V-ATPase traffic. In contrast, Rab11 colocalized with V-ATPase in apical regions of proximal tubule, and Rab20 colocalized with the enzyme in intercalated cells. Rab11 and Rab20 were enriched in membrane fractions that were also enriched in V-ATPase B2 and B1 subunit isoforms, respectively.

CONCLUSIONS

The immunohistochemical data in combination with the membrane fractionation studies are consistent with a potential role for Rab11 and Rab20 in regulating V-ATPase traffic in specific segments of the nephron.

Overexpression of Rab22a hampers the transport between endosomes and the Golgi apparatus

The transport and sorting of soluble and membrane-associated macromolecules arriving at endosomal compartments require a complex set of Rab proteins. Rab22a has been localized to the endocytic compartment; however, very little is known about the function of Rab22a and inconsistent results have been reported in studies performed in different cell lines. To characterize the function of Rab22a in endocytic transport, the wild-type protein (Rab22a WT), a hydrolysis-deficient mutant (Rab22a Q64L), and a mutant with reduced affinity for GTP (Rab22a S19N) were expressed in CHO cells. None of the three Rab22a constructs affected the transport of rhodamine-dextran to lysosomes, the digestion of internalized proteins, or the lysosomal localization of cathepsin D. In contrast with the mild effect of Rab22a on the endosome-lysosome route, cells expressing Rab22a WT and Rab22a Q64L presented a strong delay in the retrograde transport of cholera toxin from endosomes to the Golgi apparatus. Moreover, these cells accumulated the cation independent mannose 6-phosphate receptor in endosomes. These observations indicate that Rab22a can affect the trafficking from endosomes to the Golgi apparatus probably by promoting fusion among endosomes and impairing the proper segregation of membrane domains required for targeting to the trans-Golgi network (TGN).

Local control of AMPA receptor trafficking at the postsynaptic terminal by a small GTPase of the Rab family

The delivery of neurotransmitter receptors into the synaptic membrane is essential for synaptic function and plasticity. However, the molecular mechanisms of these specialized trafficking events and their integration with the intracellular membrane transport machinery are virtually unknown. Here, we have investigated the role of the Rab family of membrane sorting proteins in the late stages of receptor trafficking into the postsynaptic membrane. We have identified Rab8, a vesicular transport protein associated with trans-Golgi network membranes, as a critical component of the cellular machinery that delivers AMPA-type glutamatergic receptors (AMPARs) into synapses. Using electron microscopic techniques, we have found that Rab8 is localized in close proximity to the synaptic membrane, including the postsynaptic density. Electrophysiological studies indicated that Rab8 is necessary for the synaptic delivery of AMPARs during plasticity (long-term potentiation) and during constitutive receptor cycling. In addition, Rab8 is required for AMPAR delivery into the spine surface, but not for receptor transport from the dendritic shaft into the spine compartment or for delivery into the dendritic surface. Therefore, Rab8 specifically drives the local delivery of AMPARs into synapses. These results demonstrate a new role for the cellular secretory machinery in the control of synaptic function and plasticity directly at the postsynaptic membrane.

Role of Rab5 in EGF receptor-mediated signal transduction

Activated epidermal growth factor receptor (EGFR) recruits intracellular proteins that mediate receptor trafficking and signaling. Rab5 and Rin1, a multifunctional protein with a Rab5 guanine nucleotide exchange factor domain, have been shown to regulate EGFR endocytosis (Barbieri et al., 2000; Tall et al., 2001). In this study, we demonstrate that overexpression of both dominant negative Rab5 (Rab5:S34N) and full-length Rin1 selectively block EGF activation of the Raf-Erk1/2 kinase pathway and EGF-stimulated incorporation of [3H]thymidine into DNA without affecting the activity of JN and p38 kinase pathways. Expression of Rab5:S34N and Rin1 also block EGF induction of cyclin D1 transcription. In contrast, expression of Rin1:delta, a natural splice variant of Rin1 lacking 47 amino acids in the Vps9p domain or Rab5, increase both activation of Raf-Erk1/2- and cyclin D1 transcription in response to EGF. These results indicate that Rab5 and the Raf/Erk signal transduction pathway play essential and selective roles in EGF-induced cell proliferation, and highlight a new function for Rab5 in EGF signaling.

Xanthine nucleotide-specific G-protein alpha-subunits: a novel approach for the analysis of G-protein-mediated signal transduction

Pro- and eukaryotic cells express multiple GTP-binding proteins that play crucial roles in signal transduction. GTP-binding proteins possess a highly conserved NKX D motif critically involved in guanine binding. In order to selectively activate a defined GTP-binding protein, base-specificity can be switched from guanine to xanthine by mutating the conserved aspartate into asparagine (D/N-mutation). This approach was very successful at elucidating the function of structurally diverse GTP-binding proteins in complex systems. However, attempts to generate functional xanthine nucleotide-specific alpha-subunits of heterotrimeric GTP-binding proteins (G-proteins) met more difficulties. Recent studies have shown that a sufficiently high GDP-affinity is critical for functional expression of xanthine nucleotide-selective G-protein mutants. Moreover, xanthosine 5'-[gamma-thio]triphosphate and xanthosine 5'-[gamma, beta-imido]triphosphate are not functionally equivalent activators of D/N-G-protein mutants. We are now in the position to exploit xanthine nucleotide-specific G-proteins to dissect signaling pathways activated by a given G-protein in complex systems.

Phosphatidylinositol 3-phosphate is found in microdomains of early endosomes

Phosphatidylinositol 3-phosphate [PI(3)P] is a phosphatidylinositol 3-kinase product whose localisation is restricted to the limiting membranes of early endosomes and to the internal vesicles of multivesicular bodies. In this study the intracellular distribution of PI(3)P was compared with those of another phosphoinositide and a number of endosomal proteins. Using a 2xFYVE probe specific for PI(3)P we found that PI(3)P is present in microdomains within the endosome membrane, whereas a phosphoinositide required for clathrin-mediated endocytosis, PI(4,5)P2, was only detected at the plasma membrane. The small GTPase Rab5 as well as the PI(3)P-binding proteins EEA1, SARA and CISK were found to be abundant within PI(3)P-containing endosomal microdomains. In contrast, another PI(3)P-binding protein, Hrs, was found concentrated in clathrin-coated endosomal microdomains with low levels of PI(3)P. While PI(3)P-containing microdomains could be readily distinguished on enlarged endosomes in cells transfected with a constitutively active Rab5 mutant, such domains could also be detected in endosomes of non-transfected cells. We conclude that the membranes of early endosomes consist of microdomains in which PI(3)P and specific proteins are concentrated. These microdomains may be necessary for the assembly of distinct multimolecular complexes that specify organelle identity, membrane trafficking and receptor signalling.

Mycobacterium tuberculosis phagosome maturation arrest: mycobacterial phosphatidylinositol analog phosphatidylinositol mannoside stimulates early endosomal fusion

Mycobacterium tuberculosis is a facultative intracellular pathogen that parasitizes macrophages by modulating properties of the Mycobacterium-containing phagosome. Mycobacterial phagosomes do not fuse with late endosomal/lysosomal organelles but retain access to early endosomal contents by an unknown mechanism. We have previously reported that mycobacterial phosphatidylinositol analog lipoarabinomannan (LAM) blocks a trans-Golgi network-to-phagosome phosphatidylinositol 3-kinase-dependent pathway. In this work, we extend our investigations of the effects of mycobacterial phosphoinositides on host membrane trafficking. We present data demonstrating that phosphatidylinositol mannoside (PIM) specifically stimulated homotypic fusion of early endosomes in an ATP-, cytosol-, and N-ethylmaleimide sensitive factor-dependent manner. The fusion showed absolute requirement for small Rab GTPases, and the stimulatory effect of PIM increased upon partial depletion of membrane Rabs with RabGDI. We found that stimulation of early endosomal fusion by PIM was higher when phosphatidylinositol 3-kinase was inhibited by wortmannin. PIM also stimulated in vitro fusion between model phagosomes and early endosomes. Finally, PIM displayed in vivo effects in macrophages by increasing accumulation of plasma membrane-endosomal syntaxin 4 and transferrin receptor on PIM-coated latex bead phagosomes. In addition, inhibition of phagosomal acidification was detected with PIM-coated beads. The effects of PIM, along with the previously reported action of LAM, suggest that M. tuberculosis has evolved a two-prong strategy to modify its intracellular niche: its products block acquisition of late endosomal/lysosomal constituents, while facilitating fusion with early endosomal compartments.

RIN3: a novel Rab5 GEF interacting with amphiphysin II involved in the early endocytic pathway

The small GTPase Rab5, which cycles between active (GTP-bound) and inactive (GDP-bound) states, plays essential roles in membrane budding and trafficking in the early endocytic pathway. However, the molecular mechanisms underlying the Rab5-regulated processes are not fully understood other than the targeting event to early endosomes. Here, we report a novel Rab5-binding protein, RIN3, that contains many functional domains shared with other RIN members and additional Pro-rich domains. RIN3 displays the same biochemical properties as RIN2, the stimulator and stabilizer of GTP-Rab5. In addition, RIN3 exhibits its unique intracellular localization. RIN3 expressed in HeLa cells localized to cytoplasmic vesicles and the RIN3-positive vesicles contained Rab5 but not the early endosomal marker EEA1. Transferrin appeared to be transported partly through the RIN3-positive vesicles to early endosomes. RIN3 was also capable of interacting via its Pro-rich domain with amphiphysin II, which contains SH3 domain and participates in receptor-mediated endocytosis. Interestingly, cytoplasmic amphiphysin II was translocated into the RIN3- and Rab5-positive vesicles when co-expressed with RIN3. These results indicate that RIN3 biochemically characterized as the stimulator and stabilizer for GTP-Rab5 plays an important role in the transport pathway from plasma membrane to early endosomes.

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.

Platelet-derived growth factor-BB-mediated glycosaminoglycan synthesis is transduced through Akt

Previously we have demonstrated that the phosphoinositide 3-kinase (PI-3K) signal-transduction pathway mediates platelet-derived growth factor (PDGF)-BB-induced glycosaminoglycan (GAG) synthesis in fetal lung fibroblasts. In the present study we further investigated the signal-transduction pathway(s) that results in PDGF-BB-induced GAG synthesis. Over-expression of a soluble PDGF beta-receptor as well as a mutated form of the beta-receptor, unable to bind PI-3K, diminished GAG synthesis in fetal lung fibroblasts subsequent to PDGF-BB stimulation. The PI-3K inhibitor wortmannin blocked PDGF-BB-induced Akt activity as well as significantly diminishing PDGF-BB-mediated GAG synthesis. Expression of dominant-negative PI-3K also abrogated Akt activity and GAG synthesis. Furthermore, expression of dominant-negative Akt abrogated endogenous Akt activity, Rab3D phosphorylation and GAG synthesis, whereas expression of constitutively activated Akt stimulated Rab3D phosphorylation and GAG synthesis in the absence of PDGF-BB. Over-expression of wild-type PTEN (phosphatase and tensin homologue deleted in chromosome 10) inhibited Akt activity and concomitantly attenuated GAG synthesis in fibroblasts stimulated with PDGF-BB. These data suggest that Akt is an integral protein involved in PDGF-BB-mediated GAG regulation in fetal lung fibroblasts.

Kinetic timing: a novel mechanism that improves the accuracy of GTPase timers in endosome fusion and other biological processes

The GTPase superfamily contains a large number of proteins that function as molecular switches by binding and hydrolyzing GTP molecules. They are localized at various intracellular organelles and control diverse cellular processes. For many GTPases, the lifetime of the activated, GTP-bound state is believed to serve as a timer in determining the activation time of a biological event such as membrane fusion and signal transduction. However, such a timer is intrinsically stochastic due to thermal noise at the level of single GTPase molecules. Here, we describe a mathematical model that shows how a directional GTPase cycle, in a nonequilibrium steady-state driven by GTP hydrolysis, can significantly reduce the variance in the lifetime of an activated GTPase molecule and thereby increase the accuracy and efficiency of the timer. This mechanism, termed kinetic timing, articulates a clear function for the energy consumption in GTPase-controlled biological processes. It provides a rationale for why biological timers utilize a GTP hydrolysis cycle rather than a simple GTP binding-dissociation equilibrium, and why the GTP-bound state is a better timer than the GDP-bound state. It also explains the necessity for the existence of multiple GTP-bound intermediates identified by fluorescence spectroscopy and nuclear magnetic resonance studies.

Regulation of Trypanosoma cruzi invasion of nonphagocytic cells by the endocytically active GTPases dynamin, Rab5, and Rab7

During invasion of nonphagocytic cells by Trypanosoma cruzi (T. cruzi), host cell lysosomes are recruited to the plasma membrane attachment site followed by lysosomal enzyme secretion. The membrane trafficking events involved in invasion have not been delineated. We demonstrate here that T. cruzi invasion of nonphagocytic cells was completely abolished by overexpression of a dominant negative mutant of dynamin. Likewise, overexpression of a dominant negative mutant of Rab5, the rate-limiting GTPase for endocytosis, resulted in reduced infection rates compared with cells expressing Rab5 wild-type. Moreover, cells expressing the activated mutant of Rab5 experienced higher infection rates. A similar pattern was also observed when Rab7-transfected cells were examined. Confocal microscopy experiments showed that parasites colocalized with green fluorescent protein-Rab5-positive early endosomes after 5 min of invasion. These data clearly indicate that newly forming T. cruzi phagosomes first interact with an early endosomal compartment and subsequently with other late component markers before lysosomal interaction occurs.

Identification of rabaptin-5, rabex-5, and GM130 as putative effectors of rab33b, a regulator of retrograde traffic between the Golgi apparatus and ER

The role of rab33b, a Golgi-specific rab protein, was investigated. Microinjection of rab33b mutants stabilised in the GTP-specific state resulted in a marked inhibition of anterograde transport within the Golgi and in the recycling of glycosyltransferases from the Golgi to the ER, respectively. A GST-rab33b fusion protein stabilised in its GTP form was found to interact by Western blotting or mass spectroscopy with Golgi protein GM130 and rabaptin-5 and rabex-5, two rab effector molecules thought to function exclusively in the endocytic pathway. A similar binding was seen to rab1 but not to rab6, both Golgi rabs. In contrast, rab5 was as expected, shown to bind rabaptin-5 and rabex-5 as well as the endosomal effector protein EEA1 but not GM130. No binding of EEA1 was seen to any of the Golgi rabs.

Evolution of the Rab family of small GTP-binding proteins

Rab proteins are small GTP-binding proteins that form the largest family within the Ras superfamily. Rab proteins regulate vesicular trafficking pathways, behaving as membrane-associated molecular switches. Here, we have identified the complete Rab families in the Caenorhabditis elegans (29 members), Drosophila melanogaster (29), Homo sapiens (60) and Arabidopsis thaliana (57), and we defined criteria for annotation of this protein family in each organism. We studied sequence conservation patterns and observed that the RabF motifs and the RabSF regions previously described in mammalian Rabs are conserved across species. This is consistent with conserved recognition mechanisms by general regulators and specific effectors. We used phylogenetic analysis and other approaches to reconstruct the multiplication of the Rab family and observed that this family shows a strict phylogeny of function as opposed to a phylogeny of species. Furthermore, we observed that Rabs co-segregating in phylogenetic trees show a pattern of similar cellular localisation and/or function. Therefore, animal and fungi Rab proteins can be grouped in "Rab functional groups" according to their segregating patterns in phylogenetic trees. These functional groups reflect similarity of sequence, localisation and/or function, and may also represent shared ancestry. Rab functional groups can help the understanding of the functional evolution of the Rab family in particular and vesicular transport in general, and may be used to predict general functions for novel Rab sequences.

Ypt/rab gtpases: regulators of protein trafficking

Ypt/Rab guanosine triphosphatases (GTPases) have emerged in the last decade as key regulators of protein transport in all eukaryotic cells. They seem to be involved in all aspects of vesicle trafficking: vesicle formation, motility, and docking, and membrane remodeling and fusion. The functions of Ypt/Rabs are themselves controlled by upstream regulators that stimulate both their nucleotide cycling and their cycling between membranes. Ypt/Rabs transmit signals to downstream effectors in a guanosine triphosphate (GTP)-dependent manner. The identity of upstream regulators and downstream effectors is known for a number of Ypt/Rabs, and models for their mechanisms of action are emerging. In at least two cases, Ypt/Rab upstream regulators and downstream effectors are found together in a single complex. In agreement with the idea that Ypt/Rabs function in all aspects of vesicular transport, their diverse effectors have recently been shown to function in all identified aspects of vesicle transport. Activators and effectors for individual Ypt/Rabs share no similarity, but are conserved between yeast and mammalian cells. Finally, cross talk demonstrated among the various Ypt/Rabs, and between Ypt/Rabs and other signaling factors, suggests possible coordination among secretory steps, as well as between protein transport and other cellular processes.

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.

Ras-activated endocytosis is mediated by the Rab5 guanine nucleotide exchange activity of RIN1

RIN1 was originally identified by its ability to inhibit activated Ras and likely participates in multiple signaling pathways because it binds c-ABL and 14-3-3 proteins, in addition to Ras. RIN1 also contains a region homologous to the catalytic domain of Vps9p-like Rab guanine nucleotide exchange factors (GEFs). Here, we show that this region is necessary and sufficient for RIN1 interaction with the GDP-bound Rabs, Vps21p, and Rab5A. RIN1 is also shown to stimulate Rab5 guanine nucleotide exchange, Rab5A-dependent endosome fusion, and EGF receptor-mediated endocytosis. The stimulatory effect of RIN1 on all three of these processes is potentiated by activated Ras. We conclude that Ras-activated endocytosis is facilitated, in part, by the ability of Ras to directly regulate the Rab5 nucleotide exchange activity of RIN1.

Cellular functions of phosphatidylinositol 3-phosphate and FYVE domain proteins

PtdIns3P is a phosphoinositide 3-kinase product that has been strongly implicated in regulating membrane trafficking in both mammalian and yeast cells. PtdIns3P has been shown to be specifically located on membranes associated with the endocytic pathway. Proteins that contain FYVE zinc-finger domains are recruited to PtdIns3P-containing membranes. Structural information is now available concerning the interaction between FYVE domains and PtdIns3P. A number of proteins have been identified which contain a FYVE domain, and in this review we discuss the functions of PtdIns3P and its FYVE-domain-containing effector proteins in membrane trafficking, cytoskeletal regulation and receptor signalling.

Rab1 interaction with a GM130 effector complex regulates COPII vesicle cis--Golgi tethering

Members of the Rab family of small molecular weight GTPases regulate the fusion of transport intermediates to target membranes along the biosynthetic and endocytic pathways. We recently demonstrated that Rab1 recruitment of the tethering factor p115 into a cis-SNARE complex programs coat protein II vesicles budding from the endoplasmic reticulum (donor compartment) for fusion with the Golgi apparatus (acceptor compartment) (Allan BB, Moyer BD, Balch WE. Science 2000; 289: 444-448). However, the molecular mechanism(s) of Rab regulation of Golgi acceptor compartment function in endoplasmic reticulum to Golgi transport are unknown. Here, we demonstrate that the cis-Golgi tethering protein GM130, complexed with GRASP65 and other proteins, forms a novel Rab1 effector complex that interacts with activated Rab1-GTP in a p115-independent manner and is required for coat protein II vesicle targeting/fusion with the cis-Golgi. We propose a 'homing hypothesis' in which the same Rab interacts with distinct tethering factors at donor and acceptor membranes to program heterotypic membrane fusion events between transport intermediates and their target compartments.

Traffic control: Rab GTPases and the regulation of interorganellar transport

Membrane proteins, membrane lipids, and luminal contents are exchanged among the intracellular compartments of eukaryotic cells by vesicular transport. This process must be highly ordered to maintain cellular architecture in the face of rapid membrane turnover. The Ras-related Rab GTPases play multiple roles in regulating this traffic.

Yeast homotypic vacuole fusion: a window on organelle trafficking mechanisms

Homotypic (self) fusion of yeast vacuoles, which is essential for the low copy number of this organelle, uses catalytic elements similar to those used in heterotypic vesicular trafficking reactions between different organelles throughout nature. The study of vacuole inheritance has benefited from the ease of vacuole isolation, the availability of the yeast genome sequence and numerous mutants, and from a rapid, quantitative in vitro assay of fusion. The soluble proteins and small molecules that support fusion are being defined, conserved membrane proteins that catalyze the reaction have been identified, and the vacuole membrane has been solubilized and reconstituted into fusion-competent proteoliposomes, allowing the eventual purification of all needed factors. Studies of homotypic vacuole fusion have suggested a modified paradigm of membrane fusion in which integral membrane proteins termed "SNAREs" can form stable complexes in cis (when on the same membrane) as well as in trans (when anchored to opposing membranes). Chaperones (NSF/Sec18p, LMA1, and -SNAP/Sec17p) disassemble cis-SNARE complexes to prepare for the docking of organelles rather than to drive fusion. The specificity of organelle docking resides in a cascade of trans-interactions (involving Rab-like GTPases), "tethering factors," and trans-SNARE pairing. Fusion itself, the mixing of the membrane bilayers and the organelle contents, is triggered by calcium signaling.

Unnatural ligands for engineered proteins: new tools for chemical genetics

Small molecules that modulate the activity of biological signaling molecules can be powerful probes of signal transduction pathways. Highly specific molecules with high affinity are difficult to identify because of the conserved nature of many protein active sites. A newly developed approach to discovery of such small molecules that relies on protein engineering and chemical synthesis has yielded powerful tools for the study of a wide variety of proteins involved in signal transduction (G-proteins, protein kinases, 7-transmembrane receptors, nuclear hormone receptors, and others). Such chemical genetic tools combine the advantages of traditional genetics and the unparalleled temporal control over protein function afforded by small molecule inhibitors/activators that act at diffusion controlled rates with targets.

Rab proteins as membrane organizers

Cellular organelles in the exocytic and endocytic pathways have a distinctive spatial distribution and communicate through an elaborate system of vesiculo-tubular transport. Rab proteins and their effectors coordinate consecutive stages of transport, such as vesicle formation, vesicle and organelle motility, and tethering of vesicles to their target compartment. These molecules are highly compartmentalized in organelle membranes, making them excellent candidates for determining transport specificity and organelle identity.

The stress-induced MAP kinase p38 regulates endocytic trafficking via the GDI:Rab5 complex

Early endocytic membrane traffic is regulated by the small GTPase Rab5, which cycles between GTP- and GDP-bound states as well as between membrane and cytosol. The latter cycle depends on GDI, which functions as a Rab vehicle in the aqueous environment of the cytosol. Here, we report that formation of the GDI:Rab5 complex is stimulated by a cytosolic factor that we purified and then identified as p38 MAPK. We find that p38 regulates GDI in the cytosolic cycle of Rab5 and modulates endocytosis in vivo. Our observations reveal the existence of a cross-talk between endocytosis and the p38-dependent stress response, thus providing molecular evidence that endocytosis can be regulated by the environment.

Epidermal growth factor and membrane trafficking. EGF receptor activation of endocytosis requires Rab5a

Activated epidermal growth factor 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 signaling and endocytosis is not well understood. Here we show that EGF stimulation of NR6 cells induces a specific, rapid and transient activation of Rab5a. EGF also enhanced translocation of the Rab5 effector, early endosomal autoantigen 1 (EEA1), from cytosol to membrane. The activation of endocytosis, fluid phase and receptor mediated, by EGF was enhanced by Rab5a expression, but not by Rab5b, Rab5c, or Rab5a truncated at the NH(2) and/or COOH terminus. Dominant negative Rab5a (Rab5:N34) blocked EGF-stimulated receptor-mediated and fluid-phase endocytosis. EGF activation of Rab5a function was dependent on tyrosine residues in the COOH-terminal domain of the EGF receptor (EGFR). Removal of the entire COOH terminus by truncation (c'973 and c'991) abrogated ligand-induced Rab5a activation of endocytosis. A "kinase-dead" EGFR failed to stimulate Rab5a function. However, another EGF receptor mutant (c'1000), with the kinase domain intact and a single autophosphorylation site effectively signaled Rab5 activation. These results indicate that EGFR and Rab5a are linked via a cascade that results in the activation of Rab5a and that appears essential for internalization. The results point to an interdependent relationship between receptor activation, signal generation and endocytosis.