Rab 7: an important regulator of late endocytic membrane traffic

Expression of rab11a N124I in gastric parietal cells inhibits stimulatory recruitment of the H+-K+-ATPase

Stimulation of the gastric parietal cell results in a massive redistribution of H+-K+-ATPase from cytoplasmic tubulovesicles to the apical plasma membrane. Previous studies have implicated the small GTPase rab11 in this process. Using matrix-assisted laser desorption mass spectrometry, we confirmed that rab11 is associated with H+-K+-ATPase-enriched gastric microsomes. A stoichiometry of one rab11 per six copies of H+-K+-ATPase was estimated. Furthermore, rab11 exists in at least three forms on rabbit gastric microsomes: the two most prominent resemble rab11a, whereas the third resembles rab11b. Using an adenoviral expression system, we expressed the dominant negative mutant rab11a N124I in primary cultures of rabbit parietal cells under the control of the tetracycline transactivator protein (tTA). The mutant was well expressed with a distribution similar to that of the H+-K+-ATPase. Stimulation of these cultures with histamine and IBMX was assessed by measuring the aminopyrine (AP) uptake relative to resting cells (AP index). In experiments on six culture preparations, stimulated uninfected cells gave an AP index of 10.0 +/- 2.9, whereas parallel cultures expressing rab11a N124I were poorly responsive to stimulation, with a mean AP index of 3.2 +/- 0. 9. Control cultures expressing tTA alone or tTA plus actin responded equally well to stimulation, giving AP index values of 9.0 +/- 3.1 and 9.6 +/- 0.9, respectively. Thus inhibition by rab11a N124I is not simply due to adenoviral infection. The AP uptake data were confirmed by immunocytochemistry. In uninfected cells, H+-K+-ATPase demonstrated a broad cytoplasmic distribution, but it was cleared from the cytoplasm and associated with apically derived membranes on stimulation. In cells expressing rab11a N124I, H+-K+-ATPase maintained its resting localization on stimulation. Furthermore, this effect could be alleviated by culturing infected cells in the presence of tetracycline, which prevents expression of the mutant rab11. We therefore conclude that rab11a is the prominent GTPase associated with gastric microsomes and that it plays a role in parietal cell activation.

Rab15 mediates an early endocytic event in Chinese hamster ovary cells

Rab GTPases comprise a large family of monomeric proteins that regulate a diverse number of membrane trafficking events, including endocytosis. In this paper, we examine the subcellular distribution and function of the GTPase Rab15. Our biochemical and confocal immunofluorescence studies demonstrate that Rab15 associates with the transferrin receptor, a marker for the early endocytic pathway, but not with Rab7 or the cation-independent mannose 6-phosphate receptor, markers for late endosomal membranes. Furthermore, Rab15 colocalizes with Rab4 and -5 on early/sorting endosomes, as well as Rab11 on pericentriolar recycling endosomes. Consistent with its localization to early endosomal membranes, overexpression of the constitutively active mutant HArab15Q67L reduces receptor-mediated and fluid phase endocytosis. Therefore, our functional studies suggest that Rab15 may function as an inhibitory GTPase in early endocytic trafficking.

Impaired recruitment of the small GTPase rab7 correlates with the inhibition of phagosome maturation by Leishmania donovani promastigotes

We have shown recently that one of the survival strategies used by Leishmania donovani promastigotes during the establishment of infection in macrophages consists in inhibiting phagosome-endosome fusion. This inhibition requires the expression of lipophosphoglycan (LPG), the predominant surface glycoconjugate of promastigotes, as parasites expressing truncated forms of LPG reside in phagosomes that fuse extensively with endocytic organelles. In the present study, we developed a single-organelle fluorescence analysis approach to study and analyse the intracellular trafficking of 'fusogenic' and 'low-fusogenic' phagosomes induced by an LPG repeating unit-defective mutant (Ipg2 KO) or by wild-type L. donovani promastigotes respectively. The results obtained indicate that phagosomes containing mutant parasites fuse extensively with endocytic organelles and transform into phagolysosomes by losing the early endosome markers EEA1 and transferrin receptor, and acquiring the late endocytic and lysosomal markers rab7 and LAMP1. In contrast, a majority of 'low-fusogenic' phagosomes containing wild-type L. donovani promastigotes do not acquire rab7, wheres they acquire LAMP1 with slower kinetics. These results suggest that L. donovani parasites use LPG to restrict phagosome-endosome fusion at the onset of infection in order to prevent phagosome maturation. This is likely to permit the transformation of hydrolase-sensitive promastigotes into hydrolase-resistant amastigotes within a hospitable vacuole not displaying the harsh environment of phagolysosomes.

Mechanisms of phagocytosis in macrophages

Phagocytosis of pathogens by macrophages initiates the innate immune response, which in turn orchestrates the adaptive response. In order to discriminate between infectious agents and self, macrophages have evolved a restricted number of phagocytic receptors, like the mannose receptor, that recognize conserved motifs on pathogens. Pathogens are also phagocytosed by complement receptors after relatively nonspecific opsonization with complement and by Fc receptors after specific opsonization with antibodies. All these receptors induce rearrangements in the actin cytoskeleton that lead to the internalization of the particle. However, important differences in the molecular mechanisms underlying phagocytosis by different receptors are now being appreciated. These include differences in the cytoskeletal elements that mediate ingestion, differences in vacuole maturation, and differences in inflammatory responses. Infectious agents, such as M. tuberculosis, Legionella pneumophila, and Salmonella typhimurium, enter macrophages via heterogeneous pathways and modify vacuolar maturation in a manner that favors their survival. Macrophages also play an important role in the recognition and clearance of apoptotic cells; a notable feature of this process is the absence of an inflammatory response.

Biogenesis of transport intermediates in the endocytic pathway

Evidence is accumulating that membrane traffic between organelles can be achieved by different types of intermediates. Small (< 100 nm) and short-lived vesicles mediate transport from the plasma membrane or the trans-Golgi network to endosomes, and formation of these vesicles depends on specific adapter complexes. In contrast, transport from early to late endosomes is achieved by relatively large (approximately 0.5 microm), long-lived and multivesicular intermediates, and their biogenesis depends on endosomal COP-I proteins. Here, we review recent work on the formation of these different transport intermediates, and we discuss, in particular, coat proteins, sorting signals contained in cargo molecules and the emerging role of lipid in vesicle biogenesis.

An arrested late endosome-lysosome intermediate aggregate observed in a Chinese hamster ovary cell mutant isolated by novel three-step screening

Chinese hamster ovary cell mutants defective in the post-uptake degradation of low-density lipoprotein (LDL) in lysosomes were selected from mutagenized cells by novel three-step screening. First, in the presence of LDL, clones sensitive to an inhibitor of the rate-limiting enzyme of the cholesterol biosynthetic pathway, 3-hydroxy-3-methylglutaryl-CoA reductase, were isolated. Second, from the selected clones, those lacking in the degradation of a constituent of a fluorescent LDL were qualitatively screened by microscopy. Third, the clones were further screened by previously established quantitative analytical flow cytometry that detects the early-phase disintegration of LDL by lysosomal acid hydrolases. One of the isolated mutant clones, LEX1 (Lysosome-Endosome X 1), was a recessive mutant, and exhibited a specific disorder in the late endocytic pathway. LEX1 cells showed an unusual perinuclear aggregate of vesicles, heterogeneously positive for lysosomal glycoprotein-B/cathepsin D and rab7, yet negative for the cation-independent mannose 6-phosphate receptor. The aggregate was formed around the microtubule organizing center, and was disrupted by nocodazole treatment. Internalized octadecyl rhodamine B-labeled LDL (R18-LDL) was accumulated in the perinuclear rab7-positive vesicles. In a Percoll density gradient, neither internalized R18-LDL nor internalized horseradish peroxidase was efficiently chased into heavy lysosomal fractions positive for beta-hexosaminidase. LEX1 cells showed differences in the activity and subcellular distribution of lysosomal enzymes. These characteristics of LEX1 cells are consistent with the ideas that the perinuclear vesicle aggregate is an arrested intermediate of direct fusion or divergence between lysosomes and rab7-positive, cation-independent mannose 6-phosphate receptor-negative late endosomes, and that equilibrium between the lysosomes and the late endosomes is shifted towards the late endosomes in LEX1 cells. Such fusion or divergence between the late endosomes and the lysosomes would determine an appropriate equilibrium between them, and might thereby play an important role for proper lysosomal digestive functions. LEX1 mutant cells would be helpful for the dissection of the as yet unrevealed details of the late endocytic membrane dynamics and for the identification of factors involved in the process arrested by the mutation.

Oncogenic Ras triggers cell suicide through the activation of a caspase-independent cell death program in human cancer cells

To prevent neoplasia, cells of multicellular organisms activate cellular disposal programs such as apoptosis in response to deregulated oncogene expression, making the suppression of such programs an essential step for potentially neoplastic cells to become established as clinically relevant tumors. Since the mutation of ras proto-oncogenes, the most frequently mutated proto-oncogenes in human tumors, is very rare in some tumor types such as glioblastomas and gastric cancers, we hypothesized that mutated ras genes might activate a cell death program that cannot be overcome by these tumor types. Here we show that the expression of oncogenically mutated ras gene induces cellular degeneration accompanied by cytoplasmic vacuoles in human glioma and gastric cancer cell lines. Cells dying as a result of oncogenic Ras expression had relatively well-preserved nuclei that were negative for TUNEL staining. An immunocytochemical analysis demonstrated that the cytoplasmic vacuoles are derived mainly from lysosomes. This oncogenic Ras-induced cell death occurred in the absence of caspase activation, and was not inhibited by the overexpression of anti-apoptotic Bcl-2 protein. These observations suggested that oncogenic Ras-induced cell death is most consistent with a type of programmed cell death designated 'type 2 physiological cell death' or 'autophagic degeneration', and that this cell death is regulated by a molecular mechanism distinct from that of apoptosis. Our findings suggest a possible role for this non-apoptotic cell death in the prevention of neoplasia, and the activation of the non-apoptotic cell death program may become a potential cancer therapy complementing apoptosis-based therapies. In addition, the approach used in this study may be a valuable way to find genetically-regulated cell suicide programs that cannot be overcome by particular tumor types.

Specific changes in beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation

Agonist-activated beta2-adrenoceptors rapidly internalize and then recycle to the cell surface, however chronic agonist eventually causes receptor downregulation. To characterize beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation, human embryonic kidney cells expressing epitope-tagged receptors were examined by radioligand binding with (+/-)-[3H]4-(3-tertiarybutylamino-2-hydroxypropoxy)-benzimidazole- 2-on hydrochloride ([3H]CGP12177) and immunofluorescence microscopy. The first-order receptor recycling rate constant declined after 18 h of agonist compared with 15 min (0.05 min(-1) vs. 0.12 min(-1)), thus increasing the intracellular transit time (20.0 min vs. 8.3 min). There was also a reduction in the rate of receptor endocytosis and a decline in the total number of receptors. Although the intracellular receptor fraction did not increase between 15 min and 18 h of agonist, some receptors moved irreversibly into a protease-containing compartment while retaining radioligand binding activity. Our results indicate that beta2-adrenoceptor downregulation is associated principally with an increased intracellular transit time during recycling. This could promote the diversion of receptors into protease-containing compartments, where there is an irreversible commitment to downregulation prior to loss of radioligand binding activity.

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.

Neisseria gonorrhoeae porin modulates phagosome maturation

The porin (PorB) of Neisseria gonorrhoeae has been implicated in the pathogenesis of this species. Porin is believed to translocate from the bacterial outer membrane into target cell membranes affecting various cell functions. Here we investigated the effect of porin on phagosome maturation. Phagocytosis of latex beads by human macrophages was allowed in the presence or absence of purified porin. Isolation of latex bead-containing phagosomes and subsequent two-dimensional gel electrophoresis revealed substantial differences in the phagosomal protein composition. Immunoblotting detected higher amounts of annexin II and the early endocytic markers Rab5 and transferrin receptor and decreased levels of the late endocytic markers Rab7 and cathepsin D in phagosomes obtained in the presence of porin compared with those obtained in its absence. Furthermore, association of Rab4 with the latex bead-containing phagosomes was revealed by flow cytometry. The amount of this small GTPase was markedly higher in the phagosomes isolated in the presence of porin. The data thus indicate that neisserial porin is itself able to arrest phagosome maturation within macrophages.

Interferon-gamma selectively induces Rab5a synthesis and processing in mononuclear cells

Macrophage activation by interferon (IFN)-gamma is characterized by enhanced phagocytosis and killing of internalized pathogens. We studied the effects of IFN-gamma on Rab5a, a GTPase involved in both endocytosis and phagocytosis. IFN-gamma induced the synthesis of Rab5a in mononuclear cells as detected by immunoprecipitation and by Western blotting. Rab5a messenger RNA levels were also increased. Elevated protein expression was detected as early as 6 h following IFN-gamma and was maximal at 24 h. Following IFN-gamma, membrane association of Rab5a:GTP was substantially increased. Rab5b and Rab5c, as well as Rab7 and Rab11, Rab GTPases localized in the endosomal-lysosomal pathway, were unaffected by IFN-gamma. Moreover, Rab5a expression in non-macrophages was unaltered by IFN-gamma. Rab5a is a prenylated protein, and newly synthesized Rab5a was rapidly processed following IFN-gamma. However, elevated geranylgeranylation was not Rab5a-specific since all the Rab5 isoforms were more rapidly prenylated in vitro using cytosol from IFN-gamma-treated cells. Last, guanine nucleotide exchange on Rab5a was elevated about 3-fold in the presence of cytosol from IFN-gamma-treated cells. The selective effect of IFN-gamma on Rab5a, synthesis, processing, and nucleotide exchange suggests that Rab isoforms have closely associated but not identical functions and that selective enhancement of membrane trafficking may play a key role in intracellular killing.

In vitro fusion of phagosomes with different endocytic organelles from J774 macrophages

We describe novel biochemical and electron microscopy assays to investigate in vitro fusion of latex bead phagosomes with three different endocytic organelle fractions from J774 macrophages. After formation, early phagosomes fuse avidly with early and late endosomes and for a longer period of time with lysosomes, but they subsequently become fusion-incompetent. The fusion of early, but not late, phagosomes with all three endocytic fractions could be significantly stimulated by Rab5. In contrast to other cell types investigated, this Rab is uniquely enriched on both early and late endosomes in J774 macrophages. Moreover, exogenous Rab5 stimulates homotypic fusion between both sets of organelles. This was shown by a quantitative electron microscopy fusion assay that can directly assay fusion between any combination of morphologically defined organelles. By the same approach, we discovered an unexpected Rab5-stimulatable fusion between early and late endosomes in J774, but not in BHK cells. Thus, in J774 cells both Rab5 and the endocytic pathway seem to have evolved additional functions not yet seen in nonphagocytic cells.

Rab11 is required for trans-golgi network-to-plasma membrane transport and a preferential target for GDP dissociation inhibitor

The rab11 GTPase has been localized to both the Golgi and recycling endosomes; however, its Golgi-associated function has remained obscure. In this study, rab11 function in exocytic transport was analyzed by using two independent means to perturb its activity. First, expression of the dominant interfering rab11S25N mutant protein led to a significant inhibition of the cell surface transport of vesicular stomatitis virus (VSV) G protein and caused VSV G protein to accumulate in the Golgi. On the other hand, the expression of wild-type rab11 or the activating rab11Q70L mutant had no adverse effect on VSV G transport. Next, the membrane association of rab11, which is crucial for its function, was perturbed by modest increases in GDP dissociation inhibitor (GDI) levels. This led to selective inhibition of the trans-Golgi network to cell surface delivery, whereas endoplasmic reticulum-to-Golgi and intra-Golgi transport were largely unaffected. The transport inhibition was reversed specifically by coexpression of wild-type rab11 with GDI. Under the same conditions two other exocytic rab proteins, rab2 and rab8, remained membrane bound, and the transport steps regulated by these rab proteins were unaffected. Neither mutant rab11S25N nor GDI overexpression had any impact on the cell surface delivery of influenza hemagglutinin. These data show that functional rab11 is critical for the export of a basolateral marker but not an apical marker from the trans-Golgi network and pinpoint rab11 as a sensitive target for inhibition by excess GDI.

Post-Golgi trafficking of rhodopsin in retinal photoreceptors

Rod outer segment renewal in retinal rod photoreceptors is mediated by polarised sorting of rhodopsin, and its associated proteins and lipids, on post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialised domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. The intracellular sorting machinery that regulates budding, targeting and fusion of rhodopsin carrier vesicles has been studied using multiple means including a newly developed cell-free assay that reconstitutes vesicle budding. These studies have revealed an essential role for small GTP-binding protein rab6, as well as the carboxyl-terminal domain of rhodopsin, in the formation of post-Golgi vesicles. In this report their role in post-Golgi trafficking of rhodopsin and the maintenance of photoreceptor cell polarity and health is discussed.

Maturation of phagosomes is accompanied by specific patterns of small GTPases

In this study we purified phagosomes of the ciliated protozoan Tetrahymena thermophila to analyze aspects of the maturation pathway of phagocytotic vesicles. Phagosomes were labeled with magnetic microparticles and then purified in high amounts with the help of a permanent magnet. By combining a pulse-chase labeling protocol with the magnetic separation procedure we were able to isolate phagosomes of defined ages, which represent distinct stages of their maturation pathway. GTP-overlay assays showed that a set of small GTPases of the ras superfamily is associated with these phagosomes. Phagosomes isolated at different stages of maturation revealed a change in the pattern of the small GTPases. Some small GTPases identified by the GTPase overlay assays could be aligned to India ink stained protein spots in two-dimensional gels of isolated phagosomes. The results presented here are a first step to identify the members of small GTPases associated with phagosomes during their maturation pathway. Microsequencing of pooled polypeptides by mass-spectrometric techniques will identify the primary structure of these small GTPases.

Size of IgG-opsonized particles determines macrophage response during internalization

It is generally assumed that particles > 1 micron elicit a phagocytic response. To determine whether this is the case, we examined the uptake and transport of IgG-opsonized polystyrene beads of defined size, ranging from 0.2 to 3 microns, by mouse bone marrow-derived macrophages. The kinetics of opsonized bead internalization were comparable for each of the different beads examined. We used rhodamine phalloidin to examine particle-induced assembly of F-actin phagocytic cups by fluorescence microscopy. Phagocytic cup formation was size dependent in a nonlinear fashion. Less than 30% of 0.2- to 0.75-micron particles and greater than 80% of 2- and 3-micron particles were associated with F-actin. Cells treated with 0.25 micron cytochalasin D showed decreased phagocytic cup formation and a linear decrease in bead uptake as a function of particle surface area. In contrast, potassium depletion, which preferentially inhibits clathrin-mediated endocytosis, was more effective at inhibiting the uptake of smaller beads. Thus, with increasing particle size, IgG-opsonized particle uptake became less clathrin dependent and more actin dependent. The kinetics of ligand delivery to lysosomes was measured using an immunoprecipitation assay based on the intermixing of internalized anti-dinitrophenol (DNP) IgG with DNP-derivitized beta-glucuronidase (DNP-beta-glu) incorporated into lysosomes. Soluble mannosylated anti-DNP IgG was delivered to lysosomes after an 8-min lag period. The kinetics of anti-DNP IgG-opsonized beads showed a size-dependent response, where beads sized 0.2, 0.5, and 0.75 micron showed a lag period prior to delivery to lysosomes. In contrast, beads 1.0 micron or larger showed no lag in delivery to lysosomes. Since beads that had no lag in delivery to lysosomes also showed high levels of phagocytic cup formation, this suggests that phagocytic cups may be important in the rapid delivery of internalized particles to lysosomes.

Hydrolysis of GTP on rab11 is required for the direct delivery of transferrin from the pericentriolar recycling compartment to the cell surface but not from sorting endosomes

Rab11 is a small GTP-binding protein that in cultured mammalian cells has been shown to be concentrated in the pericentriolar endosomal recycling compartment and to play a key role in passage of the recycling transferrin receptor through that compartment [Ullrich, O., Reinsch, S., Urbé, S., Zerial, M. & Parton, R. G. (1996) J. Cell Biol. 135, 913-924]. To obtain insights into the site(s) of action of rab11 within the recycling pathway, we have now compared the effects on recycling at 37 degreesC of overexpression of wild-type rab11 and various mutant forms of this protein in cells that had been loaded with transferrin at either 37 degreesC or 16 degreesC. We show that incubation at 16 degreesC blocks passage of endocytosed transferrin into the recycling compartment and that, whereas the rab11 dominant negative mutant form (S25N) inhibits transferrin recycling after interiorization at either temperature, the wild-type rab11 and constitutively active mutant (Q70L) have no inhibitory effect on the recycling of molecules that were interiorized at 16 degreesC. This differential inhibitory effect shows that two distinct pathways for recycling are followed by the bulk of the transferrin molecules interiorized at the two different temperatures. The incapacity of the constitutively active form of rab11 (Q70L) to inhibit recycling of molecules interiorized at 16 degreesC is consistent with their recycling taking place directly from sorting endosomes, in a process that does not require hydrolysis of GTP on rab11. The fact that the dominant negative (S25N) form of rab11 inhibits recycling of molecules interiorized at both temperatures indicates that activation of rab11 by GTP is required for exit of transferrin from sorting endosomes, regardless of whether this exit is toward the recycling compartment or directly to the plasma membrane.

Legionella pneumophila DotA protein is required for early phagosome trafficking decisions that occur within minutes of bacterial uptake

Numerous intracellular bacterial pathogens modulate the nature of the membrane-bound compartment in which they reside, although little is known about the molecular basis for this control. Legionella pneumophila is a bacterial pathogen able to grow within human alveolar macrophages and residing in a phagosome that does not fuse with lysosomes. This study demonstrates that the dotA product is required to regulate trafficking of the L. pneumophila phagosome. Phagosomes containing L. pneumophila dotA+ bacteria exhibited differential trafficking profiles when compared with isogenic dotA mutants. Phagosomes containing dotA mutants showed rapid accumulation of the lysosomal glycoprotein LAMP-1 as early as 5 min after uptake, whereas the majority of wild-type L. pneumophila phagosomes did not acquire LAMP-1. The association of LAMP-1 with phagosomes containing dotA mutant bacteria was concomitant with the appearance of the small GTP-binding protein Rab7 on the vacuolar membrane. These data demonstrate that phagosomes containing replication-competent L. pneumophila evade early endocytic fusion events. In contrast, the kinetics of LAMP-1 and Rab7 association indicate that the dotA mutants are routed along a well-characterized endocytic pathway leading to fusion with lysosomes. Genetic studies show that L. pneumophila requires DotA expression before macrophage uptake in order to establish an intracellular site for replication. However, the bacteria do not appear to require continuous expression of the DotA protein to maintain a replicative phagosome. These data indicate that DotA is one factor that plays a fundamental role in regulating initial phagosome trafficking decisions either upon or immediately after macrophage uptake.

Localization of atypical protein kinase C isoforms into lysosome-targeted endosomes through interaction with p62

An increasing number of independent studies indicate that the atypical protein kinase C (PKC) isoforms (aPKCs) are critically involved in the control of cell proliferation and survival. The aPKCs are targets of important lipid mediators such as ceramide and the products of the PI 3-kinase. In addition, the aPKCs have been shown to interact with Ras and with two novel proteins, LIP (lambda-interacting protein; a selective activator of lambda/iotaPKC) and the product of par-4 (a gene induced during apoptosis), which is an inhibitor of both lambda/iotaPKC and zetaPKC. LIP and Par-4 interact with the zinc finger domain of the aPKCs where the lipid mediators have been shown to bind. Here we report the identification of p62, a previously described phosphotyrosine-independent p56(lck) SH2-interacting protein, as a molecule that interacts potently with the V1 domain of lambda/iotaPKC and, albeit with lower affinity, with zetaPKC. We also show in this study that ectopically expressed p62 colocalizes perfectly with both lambda/iotaPKC and zetaPKC. Interestingly, the endogenous p62, like the ectopically expressed protein, displays a punctate vesicular pattern and clearly colocalizes with endogenous lambda/iotaPKC and endogenous zetaPKC. P62 colocalizes with Rab7 and partially with lamp-1 and limp-II as well as with the epidermal growth factor (EGF) receptor in activated cells, but not with Rab5 or the transferrin receptor. Of functional relevance, expression of dominant negative lambda/iotaPKC, but not of the wild-type enzyme, severely impairs the endocytic membrane transport of the EGF receptor with no effect on the transferrin receptor. These findings strongly suggest that the aPKCs are anchored by p62 in the lysosome-targeted endosomal compartment, which seems critical for the control of the growth factor receptor trafficking. This is particularly relevant in light of the role played by the aPKCs in mitogenic cell signaling events.

Mutant Rab7 causes the accumulation of cathepsin D and cation-independent mannose 6-phosphate receptor in an early endocytic compartment

Stable BHK cell lines inducibly expressing wild-type or dominant negative mutant forms of the rab7 GTPase were isolated and used to analyze the role of a rab7-regulated pathway in lysosome biogenesis. Expression of mutant rab7N125I protein induced a dramatic redistribution of cation-independent mannose 6-phosphate receptor (CI-MPR) from its normal perinuclear localization to large peripheral endosomes. Under these circumstances approximately 50% of the total receptor and several lysosomal hydrolases cofractionated with light membranes containing early endosome and Golgi markers. Late endosomes and lysosomes were contained exclusively in well-separated, denser gradient fractions. Newly synthesized CI-MPR and cathepsin D were shown to traverse through an early endocytic compartment, and functional rab7 was crucial for delivery to later compartments. This observation was evidenced by the fact that 2 h after synthesis, both markers were more prevalent in fractions containing light membranes. In addition, both were sensitive to HRP-DAB- mediated cross-linking of early endosomal proteins, and the late endosomal processing of cathepsin D was impaired. Using similar criteria, the lysosomal membrane glycoprotein 120 was not found accumulated in an early endocytic compartment. The data are indicative of a post-Golgi divergence in the routes followed by different lysosome-directed molecules.

Signaling through the B cell antigen receptor regulates discrete steps in the antigen processing pathway

Antigen processing in B cells is initiated by antigen binding to the surface B cell antigen receptor (BCR). The BCR is a signaling receptor which also functions to endocytose bound antigen for subsequent intracellular processing and presentation with class II molecules. Previously, using subcellular fractionation, we showed that although the surface BCR constitutively traffics from the cell surface to the class II peptide-loading compartment (IIPLC), cross-linking the BCR regulates trafficking, resulting in a more rapid movement of the BCR to the IIPLC (Song et al., 1995, J. Immunol. 155, 4255). The rate of degradation of both the BCR and the bound antigen was also accelerated following BCR cross-linking. Here we provide evidence that the effect of cross-linking the BCR on antigen processing is in part dependent on signal cascades initiated by the BCR. We show that the protein kinase inhibitors Genistein and Chelerythrine, which block BCR signaling, reduce BCR-enhanced antigen processing in a dose-dependent manner. The kinase inhibitors have a small effect on the rate of internalization of the BCR and antigen following BCR cross-linking and significantly decrease the accelerated trafficking to the IIPLC. The increased rate of degradation of the BCR and antigen induced by BCR cross-linking is also decreased by the kinase inhibitors. BCR signaling does not appear to have a global effect on intracellular membrane trafficking as cross-linking the BCR did not alter the rate of trafficking of newly synthesized class II molecules to the IIPLC. Thus, the signaling function of the BCR appears to play a significant role in regulating discrete steps in the intracellular antigen processing pathway.

Fusion of lysosomes with late endosomes produces a hybrid organelle of intermediate density and is NSF dependent

Using a cell-free content mixing assay containing rat liver endosomes and lysosomes in the presence of pig brain cytosol, we demonstrated that after incubation at 37 degrees C, late endosome-lysosome hybrid organelles were formed, which could be isolated by density gradient centrifugation. ImmunoEM showed that the hybrids contained both an endocytosed marker and a lysosomal enzyme. Formation of the hybrid organelles appeared not to require vesicular transport between late endosomes and lysosomes but occurred as a result of direct fusion. Hybrid organelles with similar properties were isolated directly from rat liver homogenates and thus were not an artifact of cell-free incubations. Direct fusion between late endosomes and lysosomes was an N-ethylmaleimide-sensitive factor-dependent event and was inhibited by GDP-dissociation inhibitor, indicating a requirement for a rab protein. We suggest that in cells, delivery of endocytosed ligands to an organelle where proteolytic digestion occurs is mediated by direct fusion of late endosomes with lysosomes. The consequences of this fusion to the maintenance and function of lysosomes are discussed.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.

NotI linking/jumping clones of human chromosome 3: mapping of the TFRC, RAB7 and HAUSP genes to regions rearranged in leukemia and deleted in solid tumors

By applying the 'recognition mask' strategy to 300 mammalian sequences containing NotI sites we demonstrated that 5' ends of genes are highly enriched in NotI sites. A NotI linking clone NL2-252 (D3S1678) containing transferrin receptor (TFRC) gene was used as an initial point for chromosomal jumping. One of the jumping clones, J21-045 traverses 210 kbp and links NL2-252 to NL26 (D3S1632), a NotI linking clone containing highly polymorphic sequences. The TFRC gene was mapped to 3q29, close to the telomeric marker D3S2344, by linkage analysis, a panel of hybrid cell lines, GeneBridge 4 panel and FISH. Clone NLM-007 (D3S4302) was found to contain ras-homologous gene RAB7. By FISH and a panel of hybrid cell lines this gene was mapped to 3q21. This region is of particular interest due to frequent rearrangements in different types of leukemia. Clone L2-081 (D3S4283) containing new member of ubiquitin-specific proteases (HAUSP gene) was localized in 3p21 inspiring further investigation of involvement of this gene in development of lung and renal carcinomas.

Role of Rab GTPases in membrane traffic

Small GTPases of the Rab subfamily have been known to be key regulators of intracellular membrane traffic since the late 1980s. Today this protein group amounts to more than 40 members in mammalian cells which localize to distinct membrane compartments and exert functions in different trafficking steps on the biosynthetic and endocytic pathways. Recent studies indicate that cycles of GTP binding and hydrolysis by the Rab proteins are linked to the recruitment of specific effector molecules on cellular membranes, which in turn impact on membrane docking/fusion processes. Different Rabs may, nevertheless, have slightly different principles of action. Studies performed in yeast suggest that connections between the Rabs and the SNARE machinery play a central role in membrane docking/fusion. Further elucidation of this linkage is required in order to fully understand the functional mechanisms of Rab GTPases in membrane traffic.

Flow cytometric sorting and biochemical characterization of the late endosomal rab7-containing compartment

Rab7 is a small molecular weight GTPase that is known to be associated with late endocytic compartments. Studies in which wild-type or mutant forms of this protein have been overexpressed in mammalian cells have indicated that rab7 plays a role in controlling membrane transport between late endocytic compartments. However, both the precise site(s) of action and localization of rab7 remain unclear. In the present study, we have used density-gradient centrifugation in combination with a new epitope-specific flow cytometric sorting method to isolate rab7-containing vesicles from baby hamster kidney (BHK) cells. Electron-micrographs of sorted elements showed a homogeneous population of vesicles that resembles late endosomes. The polypeptide composition of rab7-containing vesicles was then analyzed by two-dimensional (2-D) gel electrophoresis. Rab7-containing vesicles were enriched in the cation-independent mannose 6-phosphate receptor and especially in the precursor forms of cathepsin D. Taken together, these results show that the rab7-containing vesicles are a component of the endocytic pathway that connects late endosomes and lysosomes and in which precursor forms of lysosomal hydrolases, segregated from their receptor, might be included.

Mycobacterial phagosome maturation, rab proteins, and intracellular trafficking

One of the most prominent features of pathogenic mycobacteria, which include the potent human pathogens Mycobacterium tuberculosis and Mycobacterium leprae and their opportunistic relatives Mycobacterium avium and Mycobacterium marinum, is their ability to survive and multiply in phagosomes of mononuclear phagocytic cells. The phagocytosed mycobacteria reside in a vacuolar compartment which is exempted from maturation into the phagolysosome. Recently, the arrest of the maturation of phagosomes containing M. tuberculosis complex organisms (Mycobacterium bovis BCG) has been linked to the accumulation on the phagosomal membrane of the small GTP binding protein rab5, specific for the control of fusion within the early endosomal compartment. Furthermore, M. bovis BCG phagosome is devoid of rab7, a rab protein associated with the late endosome. The selective accumulation of rab5 and exclusion of rab7 defines the check point that has been compromised in mycobacterial phagosome maturation. Here we summarize these observations and relates them to other phenomena in the area of membrane and protein trafficking with the emphasis on phagosomes containing intracellular pathogens.

Homotypic lysosome fusion in macrophages: analysis using an in vitro assay

Lysosomes are dynamic structures capable of fusing with endosomes as well as other lysosomes. We examined the biochemical requirements for homotypic lysosome fusion in vitro using lysosomes obtained from rabbit alveolar macrophages or the cultured macrophage-like cell line, J774E. The in vitro assay measures the formation of a biotinylated HRP-avidin conjugate, in which biotinylated HRP and avidin were accumulated in lysosomes by receptor-mediated endocytosis. We determined that lysosome fusion in vitro was time- and temperature-dependent and required ATP and an N-ethylmaleimide (NEM)-sensitive factor from cytosol. The NEM-sensitive factor was NSF as purified recombinant NSF could completely replace cytosol in the fusion assay whereas a dominant-negative mutant NSF inhibited fusion. Fusion in vitro was extensive; up to 30% of purified macrophage lysosomes were capable of self-fusion. Addition of GTPgammas to the in vitro assay inhibited fusion in a concentration-dependent manner. Purified GDP-dissociation inhibitor inhibited homotypic lysosome fusion suggesting the involvement of rabs. Fusion was also inhibited by the heterotrimeric G protein activator mastoparan, but not by its inactive analogue Mas-17. Pertussis toxin, a Galphai activator, inhibited in vitro lysosome fusion whereas cholera toxin, a Galphas activator did not inhibit the fusion reaction. Addition of agents that either promoted or disrupted microtubule function had little effect on either the extent or rate of lysosome fusion. The high value of homotypic fusion was supported by in vivo experiments examining lysosome fusion in heterokaryons formed between cells containing fluorescently labeled lysosomes. In both macrophages and J774E cells, almost complete mixing of the lysosome labels was observed within 1-3 h of UV sendai-mediated cell fusion. These studies provide a model system for identifying the components required for lysosome fusion.

Rab17 and rab18, small GTPases with specificity for polarized epithelial cells: genetic mapping in the mouse

The Rab subfamily of small GTPases plays an important role in the regulation of membrane traffic in eukaryotic cells. While most Rab proteins are equally expressed in polarized and nonpolarized cells, Rab17 and Rab18 show epithelial cell specificity. Here we report the genetic mapping of Rab17 and Rab18 on mouse chromosomes 1 and 18, respectively. We also discuss some implications of Rab17 and Rab18 mapping, including their candidacy for the mouse mutations ln (leaden), Tw (twirler), and ax (ataxia).

Vacuoles induced by Helicobacter pylori toxin contain both late endosomal and lysosomal markers

Intoxication of mammalian cells with the vacuolating toxin (VacA) released by Helicobacter pylori causes the formation of large acidic vacuoles containing the vacuolar ATPase proton pump and Rab7, a late endosome marker. Here, we describe a novel subcellular fractionation procedure, and we show that nanomolar concentrations of VacA induce a clear redistribution of lysosomal membrane glycoproteins among endocytic compartments. This redistribution is an early event in the process of cellular intoxication by VacA and precedes the formation of macroscopic vacuoles. The absence of the cation independent mannose 6-P receptor and the presence of Rab7 and of lysosomal membrane proteins in the newly formed compartment suggest that the vacuolating toxin induces the accumulation of a post-endosomal hybrid compartment presenting both late endosomal and lysosomal features.

The diversity of Rab proteins in vesicle transport

Rab proteins have been primarily implicated in vesicle docking as regulators of SNARE pairing. Recent findings, however, indicate that their function in vesicle trafficking can go beyond this role, and a number of proteins, unrelated to each other, have been identified as putative Rab effectors. Although the GTPase switch of Rab proteins is highly conserved, functional mechanisms may be highly diversified among members of the Rab family.

Evidence for a recycling role for Rab7 in regulating a late step in endocytosis and in retention of lysosomal enzymes in Dictyostelium discoideum

The mammalian small molecular weight GTPase Rab7 (Ypt7 in yeast) has been implicated in regulating membrane traffic at postinternalization steps along the endosomal pathway. A cDNA encoding a protein 85% identical at the amino acid level to mammalian Rab7 has been cloned from Dictyostelium discoideum. Subcellular fractionation and immunofluorescence microscopy indicated that Rab7 was enriched in lysosomes, postlysosomes, and maturing phagosomes. Cell lines were generated that overexposed Rab7 wild-type (WT), Rab7 Q67L (constitutively active form), and Rab7 T22N (dominant negative form) proteins. The Rab7 T22N cell line internalized fluid phase markers and latex beads (phagocytosis) at one-third the rate of control cells, whereas Rab7 WT and Rab7 Q67L cell lines were normal in uptake rates but exocytosed fluid phase faster than control cells. In contrast, fluid phase markers resided in acidic compartments for longer periods of time and were more slowly exocytosed from Rab7 T22N cells as compared with control cells. Light microscopy indicated that Rab7-expressing cell lines contained morphologically altered endosomal compartments. Compared with control cells, Rab7 WT- and Rab7 Q67L-expressing cells contained a reduced number of vesicles, the size of postlysosomes (> 2.5 microns) and an increased number of smaller vesicles, many of which were nonacidic; in control cells, > 90% of the smaller vesicles were acidic. In contrast, Rab7 T22N cells contained an increased proportion of large acidic vesicles relative to nonacidic vesicles. Radiolabel pulse-chase experiments indicated that all of the cell lines processed and targeted lysosomal alpha-mannosidase normally, indicating the lack of a significant role for Rab7 in the targeting pathway; however, retention of mature lysosomal hydrolases was affected in Rab7 WT and Rab7 T22N cell lines. Contrary to the results observed for the fluid phase efflux experiments, Rab7 T22N cells oversecreted alpha-mannosidase, whereas Rab7 WT cells retained this hydrolase as compared with control cells. These data support a model that Rab7 may regulate retrograde transport of lysosomal enzymes and the V-type H(+)-ATPase from postlysosomes to lysosomes coupled with the efficient release of fluid phase from cells.

Reconstitution of phagosome-lysosome fusion in streptolysin O-permeabilized cells

We have reconstituted fusion between phagosomes and lysosomes in streptolysin O-permeabilized J774-E macrophages. Fusion was assessed by measuring the delivery of avidin-conjugated horseradish peroxidase pre-internalized into lysosomes to phagosomes containing biotinylated beta-glucuronidase-conjugated paramagnetic beads (1-2 microm). Fusion was dependent on energy and exogenously supplied cytosol. Phagosome-lysosome fusion was greatly inhibited when microtubules were depolymerized by nocodazole treatment, suggesting that fusion occurs via microtubule-dependent transport. Furthermore, fusion was inhibited by GTPgammaS and Rab GDP dissociation inhibitor. These results suggest that rab proteins are involved in the regulation of fusion. Lastly, anti-NEM-sensitive factor (NSF) antibodies inhibited fusion, and addition of recombinant NSF wild type partially restored the fusogenic activity, indicating that NSF is required for fusion between phagosomes and lysosomes.

Rab7 regulates transport from early to late endocytic compartments in Xenopus oocytes

Rab7 has been shown to localize to late endosomes and to mediate transport from early to late endosome/lysosome in mammalian cells and in yeast. We developed a novel assay to quantify transport from early to late endosomes using the Xenopus oocyte. Oocytes were pulsed with avidin after which the oocytes were incubated to allow avidin transport to a late compartment. The oocytes were then allowed to internalize biotin-horseradish peroxidase (HRP). The oocytes were then injected with test proteins and incubated further to allow transport of biotin-HRP from early endosomes to late endosomal/lysosomal compartments. Transport was quantified by assessing the formation of HRP-biotin-avidin complexes. Injection of Rab7:wild-type (WT) and Rab7:Q67L, a GTPase defective mutant, stimulated transport. Rab5:WT had no effect. Rab7:WT-stimulated transport was inhibited by nocodazole, suggesting a role for intact microtubules. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocked Rab7:WT-stimulated transport, but Rab7:Q67L-stimulated transport was unaffected by the drug. Rab7:Q67L is constitutively activated and may not require phosphatidylinositol 3-kinase activity for activation. Rab7-stimulated transport requires N-ethylmaleimide-sensitive factor (NSF) activity as transport was blocked by N-ethylmaleimide and ATPase defective NSF mutants. Our results indicate that sequentially acting endocytic Rab GTPases utilize similar factors although their modes of action may be different.

Arrest of mycobacterial phagosome maturation is caused by a block in vesicle fusion between stages controlled by rab5 and rab7

Mycobacterium tuberculosis and the closely related organism Mycobacterium bovis can survive and replicate inside macrophages. Intracellular survival is at least in part attributed to the failure of mycobacterial phagosomes to undergo fusion with lysosomes. The transformation of phagosomes into phagolysosomes involves gradual acquisition of markers from the endosomal compartment. Members of the rab family of small GTPases which confer fusion competence in the endocytic pathway are exchanged sequentially onto the phagosomal membranes in the course of their maturation. To identify the step at which the fusion capability of phagosomes containing mycobacteria is compromised, we purified green fluorescent protein-labeled M. bovis BCG phagosomal compartments (MPC) and compared GTP-binding protein profiles of these vesicles with latex bead phagosomal compartments (LBC). We report that the MPC do not acquire rab7, specific for late endosomes, even 7 days postinfection, whereas this GTP-binding protein is present on the LBC within hours after phagocytosis. By contrast, rab5 is retained and enriched with time on the MPC, suggesting fusion competence with an early endosomal compartment. Prior infection of macrophages with M. bovis BCG also affected the dynamics of rab5 and rab7 acquisition by subsequently formed LBC. Selective exclusion of rab7, coupled with the retention of rab5 on the mycobacterial phagosome, may allow organisms from the M. tuberculosis complex to avert the usual physiological destination of phagocytosed material.

Cloning and mapping of human Rab7 and Rab9 cDNA sequences and identification of a Rab9 pseudogene

Rab GTPases reside in specific intracellular compartments and are key regulators of vesicular transport. To facilitate studies of the mechanism of lysosomal integral membrane protein (LAMP-1) transport, cDNAs for human Rab7 and Rab9 were isolated, and their nucleotide sequences were determined. During isolation and characterization of these cDNAs a Rab9 pseudogene was identified. The sequences are highly homologous to other mammalian Rab proteins and also share homology with proteins of the Rab GTPase family. Rab7 and the Rab9 pseudogene were mapped to chromosomes 3 and 5, respectively, by amplification of their sequences from human monochromosomal somatic cell hybrids. In addition, preliminary studies using antisense expression indicate that down-regulation of either Rab7 or Rab9 proteins induces severe cell vacuolation that resembles the phenotype seen in fibroblasts from patients with Chediak-Higashi syndrome.

Sequential actions of Rab5 and Rab7 regulate endocytosis in the Xenopus oocyte

To explore the role of GTPases in endocytosis, we developed an assay using Xenopus oocytes injected with recombinant proteins to follow the uptake of the fluid phase marker HRP. HRP uptake was inhibited in cells injected with GTPgammaS or incubated with aluminum fluoride, suggesting a general role for GTPases in endocytosis. Injection of Rab5 into oocytes, as well as Rab5:Q79L, a mutant with decreased GTPase activity, increased HRP uptake. Injection of Rab5:S34N, the dominant-negative mutant, inhibited HRP uptake. Injection of N-ethylmaleimide-sensitive factor (NSF) stimulated HRP uptake, and ATPase-defective NSF mutants inhibited HRP uptake when coinjected with Rab5:Q79L, confirming a requirement for NSF in endocytosis. Surprisingly, injection of Rab7:WT stimulated both uptake and degradation/activation of HRP. The latter appears to be due to enhanced transport to a late endosomal/prelysosomal degradative compartment that is monensin sensitive. Enhancement of uptake by Rab7 appears to function via an Rab5-sensitive pathway in oocytes since the stimulatory effect of Rab7 was blocked by coinjection of Rab5:S34N. Stimulation of uptake by Rab5 was blocked by Rab5:S34N but not by Rab7:T22N. Our results suggest that Rab7, while functioning downstream of Rab5, may be rate limiting for endocytosis in oocytes.

Endocytic trafficking of megalin/RAP complexes: dissociation of the complexes in late endosomes

Megalin (gp330) is a member of the low-density lipoprotein receptor gene family. Like other members of the family, it is an endocytic receptor that binds a number of specific ligands. Megalin also binds the receptor-associated protein (RAP) that serves as an exocytic traffic chaperone and inhibits ligand binding to the receptor. To investigate the fate of megalin/RAP complexes, we bound RAP glutathione-S-transferase fusion protein (RAP-GST) to megalin at the surface of L2 yolk sac carcinoma cells and followed the trafficking of the complexes by immunofluorescence and immunogold labeling and by their distribution on Percoll gradients. We show that megalin/RAP-GST complexes, which are internalized via clathrin-coated pits, are delivered to early endosomes where they accumulate during an 18 degrees C temperature block and colocalize with transferrin and transferrin receptor. Upon release from the temperature block, the complexes travel to late endosomes where they colocalize with rab7 and can be coprecipitated with anti-RAP-GST antibodies. Dissociation of the complex occurs in late endosomes and is most likely triggered by the low pH (approximately 5.5) of this compartment. RAP is then rapidly delivered to lysosomes and degraded whereas megalin is recycled to the cell surface. When the ligand, lipoprotein lipase, was bound to megalin, the receptor was found to recycle through early endosomes. We conclude that in contrast to receptor/ligand complexes, megalin/RAP complexes traffic through late endosomes, which is a novelty for members of the low-density lipoprotein receptor gene family.

Role of the small GTPase Rab7 in the late endocytic pathway

Rab7 is a small GTPase localized to the late endosomal compartment. Its function was investigated by overexpressing dominant negative or constitutively active mutants in BHK-21 cells. The effects of such overexpression on the internalization and/or degradation of different endocytic markers and on the morphology of the late endosomal compartment were analyzed. We observed a marked inhibition of the degradation of 125I-low density lipoproteins in cells transfected with the Rab7 dominant negative mutants while the rate of internalization was not affected. Moreover in these cells there was an accumulation of many small vesicles scattered throughout the cytoplasm. In contrast, overexpression of the activating mutants led to the appearance of atypically large endocytic structures and caused a dramatic change in the distribution of the cation-independent mannose 6-phosphate receptor. Our data indicate that the Rab7 protein in mammalian cells is present on a late endosomal compartment much larger than the compartment labeled by the cation-independent mannose 6-phosphate receptor. Rab7 also appears to play a fundamental role in controlling late endocytic membrane traffic.

The small GTP binding protein rab7 is essential for cellular vacuolation induced by Helicobacter pylori cytotoxin

The VacA cytotoxin, produced by toxigenic strains of Helicobacter pylori, induces the formation of large vacuoles highly enriched in the small GTPase rab7. To probe the role of rab7 in vacuolization, HeLa cells were transfected with a series of rab mutants and exposed to VacA. Dominant-negative mutants of rab7 effectively prevented vacuolization, whereas homologous rab5 and rab9 mutants were only partially inhibitory or ineffective, respectively. Expression of wild-type or GTPase-deficient rab mutants synergized with VacA in inducing vacuolization. In vitro fusion of late endosomes was enhanced by active rab7 and inhibited by inactive rab7, consistent with vacuole formation by merging of late endosomes in a process that requires functional rab7. Taken together, the effects of overexpressed rab proteins described here indicate that continuous membrane flow along the endocytic pathway is necessary for vacuole growth.

Transport through the yeast endocytic pathway occurs through morphologically distinct compartments and requires an active secretory pathway and Sec18p/N-ethylmaleimide-sensitive fusion protein

Molecules travel through the yeast endocytic pathway from the cell surface to the lysosome-like vacuole by passing through two sequential intermediates. Immunofluorescent detection of an endocytosed pheromone receptor was used to morphologically identify these intermediates, the early and late endosomes. The early endosome is a peripheral organelle that is heterogeneous in appearance, whereas the late endosome is a large perivacuolar compartment that corresponds to the prevacuolar compartment previously shown to be an endocytic intermediate. We demonstrate that inhibiting transport through the early secretory pathway in sec mutants quickly impedes transport from the early endosome. Treatment of sensitive cells with brefeldin A also blocks transport from this compartment. We provide evidence that Sec18p/N-ethylmaleimide-sensitive fusion protein, a protein required for membrane fusion, is directly required in vivo for forward transport early in the endocytic pathway. Inhibiting protein synthesis does not affect transport from the early endosome but causes endocytosed proteins to accumulate in the late endosome. As newly synthesized proteins and the late steps of secretion are not required for early to late endosome transport, but endoplasmic reticulum through Golgi traffic is, we propose that efficient forward transport in the early endocytic pathway requires delivery of lipid from secretory organelles to endosomes.

Rab1a and multiple other Rab proteins are associated with the transcytotic pathway in rat liver

To better understand the function of Rab1a, we have immunoisolated Rab1a-associated transport vesicles from rat liver using affinity-purified anti-Rab1a-coated magnetic beads. A fraction enriched in endoplasmic reticulum (ER) to Golgi transport vesicles (CV2, rho = 1.158) was subjected to immunoisolation, and proteins of the bound and non-bound subfractions were analyzed by Western blotting. To our surprise, we found that immunoisolated vesicles contained not only ER markers (105-kDa form of the polymeric IgA receptor (pIgAR)) but also transcytotic markers (dIgA and the 120-kDa form of pIgAR), suggesting that Rab1a is associated with transcytotic vesicles in rat liver. To investigate this possibility, we used an antibody to the cytoplasmic domain of pIgAR to immunoisolate transcytotic vesicles from a fraction (CV1, rho = 1. 146) known to be enriched in these vesicles. Rab1a was detected in the immunoadsorbed subfractions. The composition of the vesicles immunoisolated from the CV1 fraction on anti-Rab1a was similar to that of transcytotic vesicles immunoisolated from the same fraction on pIgAR. Both were enriched in transcytotic markers and depleted in ER and Golgi markers. The main difference between the two was that those isolated on anti-Rab1a appeared to be enriched in postendosomal transcytotic vesicles, whereas those isolated on pIgAR contained both pre- and postendosomal elements. Analysis of anti-Rab1a isolated vesicles using [alpha-32P]GTP overlay demonstrated the presence of multiple GTP-binding proteins. Some of these were identified by immunoblotting as epithelia-specific Rab17 and ubiquitous Rabs1b, -2, and -6. Taken together, these results indicate that: 1) Rab1a is associated with both ER to Golgi and postendosomal transcytotic vesicles, and 2) multiple GTP-binding proteins are associated with each class of isolated vesicle.

Rab4 and Rab7 define distinct nonoverlapping endosomal compartments

Several Rab GTPases have been localized to distinct compartments of the endocytic pathway. Rab4 is associated with early endosomes and recycling vesicles and regulates membrane recycling from early endosomes. Rab7 is localized to late endosomes and is involved in the regulation of membrane transport between late endosomes and lysosomes. Although Rab4 and Rab7 appear to regulate distinct transport events in endocytosis, it is not clear whether they perform their activities in related or entirely distinct intracellular compartments. To address this question, we generated stable cell lines expressing Rab4 tagged with a novel X31 influenza hemagglutinin (NH) epitope tag. These antibodies are characterized in this paper and were used to immunoisolate endocytic vesicles with cytoplasmically exposed NHRab4. Immunoisolated membranes contain internalized 125I-transferrin, but are devoid of Rab7. Confocal immunofluorescence microscopy showed that the early endosomal GTPases Rab4 and Rab5 both do not codistribute with Rab7 within the same cell. These observations suggest that each of the three Rab GTPases operationally defines a distinct station of the endocytic pathway.

Endocytosis and molecular sorting

Endocytosis in eukaryotic cells is characterized by the continuous and regulated formation of prolific numbers of membrane vesicles at the plasma membrane. These vesicles come in several different varieties, ranging from the actin-dependent formation of phagosomes involved in particle uptake, to smaller clathrin-coated vesicles responsible for the internalization of extracellular fluid and receptor-bound ligands. In general, each of these vesicle types results in the delivery of their contents to lysosomes for degradation. The membrane components of endocytic vesicles, on the other hand, are subject to a series of highly complex and iterative molecular sorting events resulting in their targeting to specific destinations. In recent years, much has been learned about the function of the endocytic pathway and the mechanisms responsible for the molecular sorting of proteins and lipids. This review attempts to integrate these new concepts with long-established views of endocytosis to present a more coherent picture of how the endocytic pathway is organized and how the intracellular transport of internalized membrane components is controlled. Of particular importance are emerging concepts concerning the protein-based signals responsible for molecular sorting and the cytosolic complexes responsible for the decoding of these signals.

The biogenesis of lysosomes: is it a kiss and run, continuous fusion and fission process?

Molecules are transferred to lysosomes, the major, acid pH, digestive compartment in eukaryotic cells, by a complex series of pathways that converge at a late endosome/prelysosomal compartment. Here, we discuss the relationship between this compartment and the lysosome. We propose that lysosomes are maintained within cells by a repeated series of kiss and run, transient fusion and fission processes with the late endosome/prelysosome compartment. Directionality to these processes may be conferred by pH gradients and retrieval mechanisms. The future challenge in testing this and any other proposed hypothesis for lysosomal biogenesis will be the establishment of molecular mechanisms.

Direct vesicular transport of MHC class II molecules from lysosomal structures to the cell surface

Newly synthesized MHC class II molecules are sorted to lysosomal structures where peptide loading can occur. Beyond this point in biosynthesis, no MHC class II molecules have been detected at locations other than the cell surface. We studied this step in intracellular transport by visualizing MHC class II molecules in living cells. For this purpose we stably expressed a modified HLA-DR1 beta chain with the Green Fluorescent Protein (GFP) coupled to its cytoplasmic tail (beta-GFP) in class II-expressing Mel JuSo cells. This modification of the class II beta chain does not affect assembly, intracellular distribution, and peptide loading of the MHC class II complex. Transport of the class II/ beta-GFP chimera was studied in living cells at 37 degrees C. We visualize rapid movement of acidic class II/beta-GFP containing vesicles from lysosomal compartments to the plasma membrane and show that fusion of these vesicles with the plasma membrane occurs. Furthermore, we show that this transport route does not intersect the earlier endosomal pathway.

A role for a Rab4-like GTPase in endocytosis and in regulation of contractile vacuole structure and function in Dictyostelium discoideum

The small Mr Rab4-like GTPase, RabD, localizes to the endosomal pathway and the contractile vacuole membrane system in Dictyostelium discoideum. Stably transformed cell lines overexpressing a dominant negative functioning RabD internalized fluid phase marker at 50% of the rate of wild-type cells. Mutant cells were also slower at recycling internalized fluid. Microscopic and biochemical approaches indicated that the transport of fluid to large postlysosome vacuoles was delayed in mutant cells, resulting in an accumulation in acidic smaller vesicles, probably lysosomes. Also, RabD N121I-expressing cell lines missorted a small but significant percentage of newly synthesized lysosomal alpha-mannosidase precursor polypeptides. However, the majority of the newly synthesized alpha-mannosidase was transported with normal kinetics and correctly delivered to lysosomes. Subcellular fractionation and immunofluorescent microscopy indicated that in mutant cells contractile vacuole membrane proteins were associated with compartments morphologically distinct from the normal reticular network. Osmotic tests revealed that the contractile vacuole functioned inefficiently in mutant cells. Our results suggest that RabD regulates membrane traffic along the endosomal pathway, and that this GTPase may play a role in regulating the structure and function of the contractile vacuole system by facilitating communication with the endosomal pathway.