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We have investigated the in vivo functional role of rab5, a small GTPase associated with the plasma membrane and early endosomes. Wild-type rab5 or rab5-ile133, a mutant protein defective in GTP binding, was overexpressed in baby hamster kidney cells. In cells expressing the rab5ile 133 protein, the rate of endocytosis was decreased by 50% compared with normal, while the rate of recycling was not significantly affected. The morphology of early endosomes was also drastically changed by the mutant protein, which induced accumulation of small tubules and vesicles at the periphery of the cell. Surprisingly, overexpression of wild-type rab5 accelerated the uptake of endocytic markers and led to the appearance of atypically large early endosomes. We conclude that rab5 is a rate-limiting component of the machinery regulating the kinetics of membrane traffic in the early endocytic pathway.

The molecular machinery behind lysosome biogenesis and the maintenance of the perinuclear aggregate of late endocytic structures is not well understood. A likely candidate for being part of this machinery is the small GTPase Rab7, but it is unclear whether this protein is associated with lysosomes or plays any role in the regulation of the perinuclear lysosome compartment. Previously, Rab7 has mainly been implicated in transport from early to late endosomes. We have now used a new approach to analyze the role of Rab7: transient expression of Enhanced Green Fluorescent Protein (EGFP)-tagged Rab7 wt and mutant proteins in HeLa cells. EGFP-Rab7 wt was associated with late endocytic structures, mainly lysosomes, which aggregated and fused in the perinuclear region. The size of the individual lysosomes as well as the degree of perinuclear aggregation increased with the expression levels of EGFP-Rab7 wt and, more dramatically, the active EGFP-Rab7Q67L mutant. In contrast, upon expression of the dominant-negative mutants EGFP-Rab7T22N and EGFP-Rab7N125I, which localized mainly to the cytosol, the perinuclear lysosome aggregate disappeared and lysosomes, identified by colocalization of cathepsin D and lysosome-associated membrane protein-1, became dispersed throughout the cytoplasm, they were inaccessible to endocytosed molecules such as low-density lipoprotein, and their acidity was strongly reduced, as determined by decreased accumulation of the acidotropic probe LysoTracker Red. In contrast, early endosomes associated with Rab5 and the transferrin receptor, late endosomes enriched in the cation-independent mannose 6-phosphate receptor, and the trans-Golgi network, identified by its enrichment in TGN-38, were unchanged. These data demonstrate for the first time that Rab7, controlling aggregation and fusion of late endocytic structures/lysosomes, is essential for maintenance of the perinuclear lysosome compartment. INTRODUCTIONThe "kiss-and-run" model for lysosome biogenesis proposes that maintenance of the lysosomal compartment depends on continuous fusions of late endocytic structures accompanied by fission events (Storrie and Desjardins, 1996). This model implies that, in addition to heterotypic fusions between late endosomes and lysosomes in the perinuclear region, there could also be continuous exchange within the lysosomal vesicle population (homotypic fusion). Indeed, evidence for fusions and bidirectional traffic of soluble material between lysosomes and late endosomes has been reported (Jahraus et al., 1994;Mullock et al., 1994;van Deurs et al., 1995;Futter et al., 1996;Storrie and Desjardins, 1996;Bright et al., 1997;Mullock et al., 1998).Several different molecules are part of the machinery responsible for vesicle docking and fusion, Rab GTPases and SNAREs being among the best studied (Rothman and Warren, 1994;Denesvre and Malhotra, 1996;Pfeffer, 1996Pfeffer, , 1999Olkkonen and Stenmark, 1997;Mayer, 1999;Waters and Pfeffer, 1999). Rab proteins are important regulators of membrane traffic on the biosynthetic ...

The small GTP binding protein Rab7 has a role in the late endocytic pathway and lysosome biogenesis. The role of mammalian Rab7 in autophagy is, however, unknown. We have addressed this by inhibiting Rab7 function with RNA interference and overexpression of dominant negative Rab7. We show here that Rab7 was needed for the formation of preferably perinuclear, large aggregates, where the autophagosome marker LC3 colocalised with Rab7 and late endosomal and lysosomal markers. By electron microscopy we showed that these large aggregates corresponded to autophagic vacuoles surrounding late endosomal or lysosomal vesicles. Our experiments with quantitative electron microscopy showed that Rab7 was not needed for the initial maturation of early autophagosomes to late autophagic vacuoles, but that it participated in the final maturation of late autophagic vacuoles. Finally, we showed that the recruitment of Rab7 to autophagic vacuoles was retarded in cells deficient in the lysosomal membrane proteins Lamp1 and Lamp2, which we have recently shown to accumulate late autophagic vacuoles during starvation. In conclusion, our results showed a role for Rab7 in the final maturation of late autophagic vacuoles.

Vesicular pathways coupling the neuromuscular junction with the motor neuron soma are essential for neuronal function and survival. To characterize the organelles responsible for this long-distance crosstalk, we developed a purification strategy based on a fragment of tetanus neurotoxin (TeNT H(C)) conjugated to paramagnetic beads. This approach enabled us to identify, among other factors, the small GTPase Rab7 as a functional marker of a specific pool of axonal retrograde carriers, which transport neurotrophins and their receptors. Furthermore, Rab5 is essential for an early step in TeNT H(C) sorting but is absent from axonally transported vesicles. Our data demonstrate that TeNT H(C) uses a retrograde transport pathway shared with p75(NTR), TrkB, and BDNF, which is strictly dependent on the activities of both Rab5 and Rab7. Therefore, Rab7 plays an essential role in axonal retrograde transport by controlling a vesicular compartment implicated in neurotrophin traffic.

Rab7 is a small GTPase that controls transport to endocytic degradative compartments. Here we report the identi®cation of a novel 45 kDa protein that specifically binds Rab7GTP at its C-terminus. This protein contains a domain comprising two coiled-coil regions typical of myosin-like proteins and is found mainly in the cytosol. We named it RILP (Rab-interacting lysosomal protein) since it can be recruited ef®ciently on late endosomal and lysosomal membranes by Rab7GTP. RILP-C33 (a truncated form of the protein lacking the N-terminal half) strongly inhibits epidermal growth factor and low-density lipoprotein degradation, and causes dispersion of lysosomes similarly to Rab7 dominant-negative mutants. More importantly, expression of RILP reverses/prevents the effects of Rab7 dominant-negative mutants. All these data are consistent with a model in which RILP represents a downstream effector for Rab7 and both proteins act together in the regulation of late endocytic traf®c.

The main organelles of the secretory and endocytic pathways--the endoplasmic reticulum (ER) and endosomes, respectively--are connected through contact sites whose numbers increase as endosomes mature. One function of such sites is to enable dephosphorylation of the cytosolic tails of endosomal signalling receptors by an ER-associated phosphatase, whereas others serve to negatively control the association of endosomes with the minus-end-directed microtubule motor dynein or mediate endosome fission. Cholesterol transfer and Ca(2+) exchange have been proposed as additional functions of such sites. However, the compositions, activities and regulations of ER-endosome contact sites remain incompletely understood. Here we show in human and rat cell lines that protrudin, an ER protein that promotes protrusion and neurite outgrowth, forms contact sites with late endosomes (LEs) via coincident detection of the small GTPase RAB7 and phosphatidylinositol 3-phosphate (PtdIns(3)P). These contact sites mediate transfer of the microtubule motor kinesin 1 from protrudin to the motor adaptor FYCO1 on LEs. Repeated LE-ER contacts promote microtubule-dependent translocation of LEs to the cell periphery and subsequent synaptotagmin-VII-dependent fusion with the plasma membrane. Such fusion induces outgrowth of protrusions and neurites, which requires the abilities of protrudin and FYCO1 to interact with LEs and kinesin 1. Thus, protrudin-containing ER-LE contact sites are platforms for kinesin-1 loading onto LEs, and kinesin-1-mediated translocation of LEs to the plasma membrane, fuelled by repeated ER contacts, promotes protrusion and neurite outgrowth.

Nascent phagosomes must undergo a series of fusion and fission reactions to acquire the microbicidal properties required for the innate immune response. Here we demonstrate that this maturation process involves the GTPase Rab7. Rab7 recruitment to phagosomes was found to precede and to be essential for their fusion with late endosomes and/or lysosomes. Active Rab7 on the phagosomal membrane associates with the effector protein RILP (Rab7-interacting lysosomal protein), which in turn bridges phagosomes with dyneindynactin, a microtubule-associated motor complex. The motors not only displace phagosomes in the centripetal direction but, strikingly, promote the extension of phagosomal tubules toward late endocytic compartments. Fusion of tubules with these organelles was documented by fluorescence and electron microscopy. Tubule extension and fusion with late endosomes and/or lysosomes were prevented by expression of a truncated form of RILP lacking the dynein-dynactin-recruiting domain. We conclude that full maturation of phagosomes requires the retrograde emission of tubular extensions, which are generated by activation of Rab7, recruitment of RILP, and consequent association of phagosomes with microtubule-associated motors.Leukocytes eliminate pathogens and apoptotic cells by initially engulfing them into a phagocytic vacuole. The vacuole, which is derived from the plasmalemma, needs to undergo extensive remodeling to acquire microbicidal and lytic capabilities (28). Such remodeling, also known as maturation, involves sequential fusion with various components of the endolysosomal pathway and concomitant fission events that maintain the vacuolar size nearly constant (1, 28).The molecular machinery underlying maturation, particularly the process of phagolysosome formation, is poorly understood. It is generally thought that phagosomal maturation shares some features with the progression of endosomes to lysosomes, a complex process that is orchestrated by Rab GTPases (33). Accordingly, Rab5 and Rab7 have been detected on the membranes of early and intermediate phagosomes, respectively (12, 13, 23). Moreover, using an in vitro reconstitution system, Funato and colleagues (15) found that inhibition of Rab function by addition of excess Rab-GDP dissociation inhibitor impaired phagosome maturation. Inhibition of Rab5 may be at least partly responsible for this effect, since immunodepletion of this protein precluded the fusion of phagosomes with endosomes (2, 3). The mode of action of Rab5 and the possible role of other Rabs, particularly Rab7, in phagosomal maturation remain obscure.The objective of the present studies was to analyze the involvement of Rab7 and its only known effector, RILP (Rab7-interacting lysosomal protein), in phagolysosome formation by mammalian macrophages. To this end, we transfected cells of the murine monocyte/macrophage line RAW 264.7 with chimeric constructs of enhanced green fluorescent protein (EGFP) and either normal or mutated forms of Rab7 and RILP, and we monitored their distribution, dy...