Oxysterol binding protein (OSBP) homologs comprise a family of 12 proteins in humans (Jaworski et al., 2001; Lehto et al., 2001). Two variants of OSBP-related protein (ORP) 1 have been identified: a short one that consists of the carboxy-terminal ligand binding domain only (ORP1S, 437 aa) and a longer N-terminally extended form (ORP1L, 950 aa) encompassing three ankyrin repeats and a pleckstrin homology domain (PHD). We now report that the two mRNAs show marked differences in tissue expression. ORP1S predominates in skeletal muscle and heart, whereas ORP1L is the most abundant form in brain and lung. On differentiation of primary human monocytes into macrophages, both ORP1S and ORP1L mRNAs were induced, the up-regulation of ORP1L being >100-fold. The intracellular localization of the two ORP1 variants was found to be different. Whereas ORP1S is largely cytosolic, the ORP1L variant localizes to late endosomes. A significant amount of ORP1S but only little ORP1L was found in the nucleus. The ORP1L ankyrin repeat region (aa 1–237) was found to localize to late endosomes such as the full-length protein. This localization was even more pronounced for a fragment that additionally includes the PHD (aa 1–408). The amino-terminal region of ORP1L consisting of the ankyrin repeat and PHDs is therefore likely to be responsible for the targeting of ORP1L to late endosomes. Interestingly, overexpression of ORP1L was found to enhance the LXRα-mediated transactivation of a reporter gene, whereas ORP1S failed to influence this process. The results suggest that the two forms of ORP1 are functionally distinct and that ORP1L is involved in control of cellular lipid metabolism.
The small GTPase Rab7 controls late endocytic transport by the minus end–directed motor protein complex dynein–dynactin, but how it does this is unclear. Rab7-interacting lysosomal protein (RILP) and oxysterol-binding protein–related protein 1L (ORP1L) are two effectors of Rab7. We show that GTP-bound Rab7 simultaneously binds RILP and ORP1L to form a RILP–Rab7–ORP1L complex. RILP interacts directly with the C-terminal 25-kD region of the dynactin projecting arm p150Glued, which is required for dynein motor recruitment to late endocytic compartments (LEs). Still, p150Glued recruitment by Rab7–RILP does not suffice to induce dynein-driven minus-end transport of LEs. ORP1L, as well as βIII spectrin, which is the general receptor for dynactin on vesicles, are essential for dynein motor activity. Our results illustrate that the assembly of microtubule motors on endosomes involves a cascade of linked events. First, Rab7 recruits two effectors, RILP and ORP1L, to form a tripartite complex. Next, RILP directly binds to the p150Glued dynactin subunit to recruit the dynein motor. Finally, the specific dynein motor receptor Rab7–RILP is transferred by ORP1L to βIII spectrin. Dynein will initiate translocation of late endosomes to microtubule minus ends only after interacting with βIII spectrin, which requires the activities of Rab7–RILP and ORP1L.
Oxysterol-binding protein (OSBP) homologues, ORPs, are implicated in lipid homeostatic control, vesicle transport, and cell signaling. We analyzed here the quantity of ORP mRNAs in human subcutaneous (s.c.) and visceral adipose depots, as well as in the Simpson-Golabi-Behmel syndrome (SGBS) adipocyte cell model. All of the ORP mRNAs were present in the s.c and visceral adipose tissues, and the two depots shared an almost identical ORP mRNA expression pattern. SGBS adipocytes displayed a similar pattern, suggesting that the adipose tissue ORP expression pattern mainly derives from adipocytes. During SGBS cell adipogenic differentiation, ORP2, ORP3, ORP4, ORP7, and ORP8 mRNAs were down-regulated, while ORP11 was induced. To assess the impacts of ORPs on adipocyte differentiation, ORP3 and ORP8, proteins down-regulated during adipogenesis, were overexpressed in differentiating SGBS adipocytes, while ORP11, a protein induced during adipogenesis, was silenced. ORP8 overexpression resulted in reduced expression of the aP2 mRNA, while down-regulation of adiponectin and aP2 was observed in ORP11 silenced cells. Furthermore, ORP8 overexpression or silencing of ORP11 markedly decreased cellular triglyceride storage. These data identify the patterns of ORP expression in human adipose depots and SGBS adipocytes, and provide the first evidence for a functional impact of ORPs on the adipocyte phenotype.
Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large gene family that differentially localize to organellar membranes, reflecting a functional role in sterol signaling and/or transport. OSBP partitions between the endoplasmic reticulum (ER) and Golgi apparatus where it imparts sterol-dependent regulation of ceramide transport and sphingomyelin synthesis. ORP9L also is localized to the ER–Golgi, but its role in secretion and lipid transport is unknown. Here we demonstrate that ORP9L partitioning between the trans-Golgi/trans-Golgi network (TGN), and the ER is mediated by a phosphatidylinositol 4-phosphate (PI-4P)-specific PH domain and VAMP-associated protein (VAP), respectively. In vitro, both OSBP and ORP9L mediated PI-4P–dependent cholesterol transport between liposomes, suggesting their primary in vivo function is sterol transfer between the Golgi and ER. Depletion of ORP9L by RNAi caused Golgi fragmentation, inhibition of vesicular somatitus virus glycoprotein transport from the ER and accumulation of cholesterol in endosomes/lysosomes. Complete cessation of protein transport and cell growth inhibition was achieved by inducible overexpression of ORP9S, a dominant negative variant lacking the PH domain. We conclude that ORP9 maintains the integrity of the early secretory pathway by mediating transport of sterols between the ER and trans-Golgi/TGN.
ORP5 works together with Niemann Pick C-1 to facilitate exit of cholesterol from endosomes and lysosomes.
Oxysterol-binding protein (OSBP) and its related proteins (ORPs) constitute a large and evolutionarily conserved family of lipid-binding proteins that target organelle membranes to mediate sterol signaling and/or transport. Here we characterize ORP5, a tail-anchored ORP protein that localizes to the endoplasmic reticulum. Knocking down ORP5 causes cholesterol accumulation in late endosomes and lysosomes, which is reminiscent of the cholesterol trafficking defect in Niemann Pick C (NPC) fibroblasts. Cholesterol appears to accumulate in the limiting membranes of endosomal compartments in ORP5-depleted cells, whereas depletion of NPC1 or both ORP5 and NPC1 results in luminal accumulation of cholesterol. Moreover, trans-Golgi resident proteins mislocalize to endosomal compartments upon ORP5 depletion, which depends on a functional NPC1. Our results establish the first link between NPC1 and a cytoplasmic sterol carrier, and suggest that ORP5 may cooperate with NPC1 to mediate the exit of cholesterol from endosomes/lysosomes.
Oxysterol binding protein (OSBP) and OSBP-related proteins (ORPS) have a conserved lipid-binding fold that accommodates cholesterol, oxysterols and/or phospholipids. The diversity of OSBP/ORPs and their potential ligands has complicated the analysis of transfer and signalling properties of this mammalian gene family. In this study we explored the use of the fluorescent sterol cholestatrienol (CTL) to measure sterol binding by ORP9 and competition by other putative ligands. Relative to cholesterol, CTL and dehydroergosterol (DHE) were poor ligands for OSBP. In contrast, both long (ORP9L) and short (ORP9S) variants of ORP9 rapidly extracted CTL, and to a lesser extent DHE, from liposomes. ORP9L and ORP9S also extracted [32P]phosphatidylinositol 4-phosphate (PI-4P) from liposomes, which was inhibited by mutating two conserved histidine residues (HH488,489AA) at the entrance to the binding pocket but not by a mutation in the lid region that inhibited cholesterol binding. Results of direct binding and competition assays showed that phosphatidylserine was poorly extracted from liposomes by ORP9 compared to CTL and PI-4P. ORP9L and PI-4P did not co-localize in the trans-Golgi/TGN of HeLa cells, and siRNA silencing of ORP9L expression did not affect PI-4P distribution in the Golgi apparatus. However, transient overexpression of ORP9L or ORP9S in CHO cells, but not the corresponding PI-4P binding mutants, prevented immunostaining of Golgi-associated PI-4P. The apparent sequestration of Golgi PI-4P by ORP9S was identified as a possible mechanism for its growth inhibitory effects. These studies identify ORP9 as a dual sterol/PI-4P binding protein that could regulate PI-4P in the Golgi apparatus.
In eukaryotes, different subcellular organelles have distinct cholesterol concentrations, which is thought to be critical for biological functions. Oxysterol-binding protein-related proteins (ORPs) have been assumed to mediate nonvesicular cholesterol trafficking in cells; however, their in vivo functions and therefore the biological significance of cholesterol in each organelle are not fully understood. Here, by generating deletion mutants of ORPs in Caenorhabditis elegans, we show that ORPs are required for the formation and function of multivesicular bodies (MVBs). In an RNAi enhancer screen using obr quadruple mutants (obr-1; -2; -3; -4), we found that MVB–related genes show strong genetic interactions with the obr genes. In obr quadruple mutants, late endosomes/lysosomes are enlarged and membrane protein degradation is retarded, although endocytosed soluble proteins are normally delivered to lysosomes and degraded. We also found that the cholesterol content of late endosomes/lysosomes is reduced in the mutants. In wild-type worms, cholesterol restriction induces the formation of enlarged late endosomes/lysosomes, as observed in obr quadruple mutants, and increases embryonic lethality upon knockdown of MVB–related genes. Finally, we show that knockdown of ORP1L, a mammalian ORP family member, induces the formation of enlarged MVBs in HeLa cells. Our in vivo findings suggest that the proper cholesterol level of late endosomes/lysosomes generated by ORPs is required for normal MVB formation and MVB–mediated membrane protein degradation.
The multivesicular body (MVB) sorting pathway provides a mechanism for the lysosomal degradation of membrane proteins, such as growth factor receptors. The formation of MVBs is unique in that the curvature is directed toward the lumen of the compartment rather than the cytosol. During MVB formation, the curvature-inducing proteins, such as clathrins, could not be involved in the inward invagination of the endosomal membrane. Under these circumstances, lipids have been assumed to play a role in the membrane invagination step by creating local membrane environments; however, the lipids involved in this step have not been fully elucidated. Here we demonstrate that cholesterol, an essential membrane component in animals, is critical for MVB formation and function. We found that disruption of OSBP–related proteins (ORPs), which have been proposed to function in cellular cholesterol distribution and metabolism, reduces the cholesterol content in late endosomes/lysosomes, leading to impaired MVB function. MVB sorting pathway is known to be involved in many processes, including growth factor receptor down-regulation, exosome secretion, antigen presentation, the budding of enveloped viruses, and cytokinesis. Our findings provide a novel link between cholesterol and these biologically important functions.
Expression of the adenovirus protein RIDα rescues the cholesterol storage phenotype in NPC1-deficient cells by inducing formation of lipid droplets. The function of RIDα is independent of NPC1 but dependent on NPC2 and the oxysterol-binding protein ORP1L. This study provides the first evidence that ORP1L plays a role in sterol transport and LD formation.
Niemann–Pick disease type C (NPC) is caused by mutations in NPC1 or NPC2, which coordinate egress of low-density-lipoprotein (LDL)-cholesterol from late endosomes. We previously reported that the adenovirus-encoded protein RIDα rescues the cholesterol storage phenotype in NPC1-mutant fibroblasts. We show here that RIDα reconstitutes deficient endosome-to-endoplasmic reticulum (ER) transport, allowing excess LDL-cholesterol to be esterified by acyl-CoA:cholesterol acyltransferase and stored in lipid droplets (LDs) in NPC1-deficient cells. Furthermore, the RIDα pathway is regulated by the oxysterol-binding protein ORP1L. Studies have classified ORP1L as a sterol sensor involved in LE positioning downstream of GTP-Rab7. Our data, however, suggest that ORP1L may play a role in transport of LDL-cholesterol to a specific ER pool designated for LD formation. In contrast to NPC1, which is dispensable, the RIDα/ORP1L-dependent route requires functional NPC2. Although NPC1/NPC2 constitutes the major pathway, therapies that amplify minor egress routes for LDL-cholesterol could significantly improve clinical management of patients with loss-of-function NPC1 mutations. The molecular identity of putative alternative pathways, however, is poorly characterized. We propose RIDα as a model system for understanding physiological egress routes that use ORP1L to activate ER feedback responses involved in LD formation.
The small guanosine triphosphatase Rab7 regulates late endocytic trafficking. Rab7-interacting lysosomal protein (RILP) and oxysterol-binding protein–related protein 1L (ORP1L) are guanosine triphosphate (GTP)–Rab7 effectors that instigate minus end–directed microtubule transport. We demonstrate that RILP and ORP1L both interact with the group C adenovirus protein known as receptor internalization and degradation α (RIDα), which was previously shown to clear the cell surface of several membrane proteins, including the epidermal growth factor receptor and Fas (Carlin, C.R., A.E. Tollefson, H.A. Brady, B.L. Hoffman, and W.S. Wold. 1989. Cell. 57:135–144; Shisler, J., C. Yang, B. Walter, C.F. Ware, and L.R. Gooding. 1997. J. Virol. 71:8299–8306). RIDα localizes to endocytic vesicles but is not homologous to Rab7 and is not catalytically active. We show that RIDα compensates for reduced Rab7 or dominant-negative (DN) Rab7(T22N) expression. In vitro, Cu2+ binding to RIDα residues His75 and His76 facilitates the RILP interaction. Site-directed mutagenesis of these His residues results in the loss of RIDα–RILP interaction and RIDα activity in cells. Additionally, expression of the RILP DN C-terminal region hinders RIDα activity during an acute adenovirus infection. We conclude that RIDα coordinates recruitment of these GTP-Rab7 effectors to compartments that would ordinarily be perceived as early endosomes, thereby promoting the degradation of selected cargo.
Late endosomes (LEs) have characteristic intracellular distributions determined by their interactions with various motor proteins. Motor proteins associated to the dynactin subunit p150Glued bind to LEs via the Rab7 effector Rab7-interacting lysosomal protein (RILP) in association with the oxysterol-binding protein ORP1L. We found that cholesterol levels in LEs are sensed by ORP1L and are lower in peripheral vesicles. Under low cholesterol conditions, ORP1L conformation induces the formation of endoplasmic reticulum (ER)–LE membrane contact sites. At these sites, the ER protein VAP (VAMP [vesicle-associated membrane protein]-associated ER protein) can interact in trans with the Rab7–RILP complex to remove p150Glued and associated motors. LEs then move to the microtubule plus end. Under high cholesterol conditions, as in Niemann-Pick type C disease, this process is prevented, and LEs accumulate at the microtubule minus end as the result of dynein motor activity. These data explain how the ER and cholesterol control the association of LEs with motor proteins and their positioning in cells.
We earlier identified OSBP-related protein 8 (ORP8) as an endoplasmic reticulum oxysterol-binding protein implicated in cellular lipid homeostasis. We now investigated its action in hepatic cells in vivo and in vitro. Adenoviral overexpression of ORP8 in mouse liver induced a decrease of cholesterol, phospholipids, and triglycerides in serum (−34%, −26%, −37%, respectively) and liver tissue (−40%, −12%, −24%), coinciding with reduction of nuclear (n)SREBP-1 and -2 and mRNA levels of their target genes. Consistently, excess ORP8 reduced nSREBPs in HuH7 cells, and ORP8 overexpression or silencing by RNA interference moderately suppressed or induced the expression of SREBP-1 and SREBP-2 target genes, respectively. In accordance, cholesterol biosynthesis was reduced by ORP8 overexpression and enhanced by ORP8 silencing in [3H]acetate pulse-labeling experiments. ORP8, previously shown to bind 25-hydroxycholesterol, was now shown to bind also cholesterol in vitro. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation analyses revealed the nuclear pore component Nup62 as an interaction partner of ORP8. Co-localization of ORP8 and Nup62 at the nuclear envelope was demonstrated by BiFC and confocal immunofluorescence microscopy. Furthermore, the impact of overexpressed ORP8 on nSREBPs and their target mRNAs was inhibited in cells depleted of Nup62. Our results reveal that ORP8 has the capacity to modulate lipid homeostasis and SREBP activity, probably through an indirect mechanism, and provide clues of an entirely new mode of ORP action.
Rab7, a member of the Rab family small G proteins, has been shown to regulate intracellular vesicle traffic to late endosome/lysosome and lysosome biogenesis, but the exact roles of Rab7 are still undetermined. Accumulating evidence suggests that each Rab protein has multiple target proteins that function in the exocytic/endocytic pathway. We have isolated a new Rab7 target protein, Rabring7 (Rab7-interacting RING finger protein), using a CytoTrap system. It contains an H2 type RING finger motif at the C termini. Rabring7 shows no homology with RILP, which has been reported as another Rab7 target protein. GST pull-down and coimmunoprecipitation assays demonstrate that Rabring7 specifically binds the GTP-bound form of Rab7 at the N-terminal portion. Rabring7 is found mainly in the cytosol and is recruited efficiently to late endosomes/lysosomes by the GTP-bound form of Rab7 in BHK cells. Overexpression of Rabring7 not only affects epidermal growth factor degradation but also causes the perinuclear aggregation of lysosomes, in which the accumulation of the acidotropic probe LysoTracker is remarkably enhanced. These results suggest that Rabring7 plays crucial roles as a Rab7 target protein in vesicle traffic to late endosome/lysosome and lysosome biogenesis.
The oxysterol-binding protein (OSBP) and related proteins (ORPs) are sterol-binding proteins that may be involved in cellular sterol transportation, sterol metabolism and signal transduction pathways. Four ORP genes were cloned from Aedes aegypti. Based on amino acid sequence homology to human proteins, they are AeOSBP, AeORP1, AeORP8 and AeORP9. Splicing variants of AeOSBP and AeORP8 were identified. The temporal and spatial transcription patterns of members of the AeOSBP gene family through developmental stages and the gonotrophic cycle were profiled. AeORP1 transcription seemed to be head tissue-specific, whereas AeOSBP and AeORP9 expressions were induced by a blood meal. Furthermore, over-expression of AeORPs facilitated [3H]-cholesterol uptake in Aedes aegypti cultured Aag-2 cells.
Oxysterol-binding protein; cholesterol; gene expression; sterol transport
Oxysterol-binding protein (OSBP)-related proteins (ORPs) are lipid-binding proteins that are conserved from yeast to humans. They are implicated in many cellular processes including signaling, vesicular trafficking, lipid metabolism, and nonvesicular sterol transport. All ORPs contain an OSBP-related domain (ORD) that has a hydrophobic pocket that binds a single sterol. ORDs also contain additional membrane binding surfaces, some of which bind phosphoinositides and may regulate sterol binding. Studies in yeast suggest that ORPs function as sterol transporters, perhaps in regions where organelle membranes are closely apposed. Yeast ORPs also participate in vesicular trafficking, although their role is unclear. In mammalian cells, some ORPs function as sterol sensors that regulate the assembly of protein complexes in response to changes in cholesterol levels. This review will summarize recent advances in our understanding of how ORPs bind lipids and membranes and how they function in diverse cellular processes.
cholesterol; sterol; phosphoinositides; signaling; lipid transport; membranes; membrane contact sites; lipid transport proteins
Hepatitis C virus (HCV) RNA replication involves complex interactions among the 3’x RNA element within the HCV 3’ untranslated region, viral and host proteins. However, many of the host proteins remain unknown. In this study, we devised an RNA affinity chromatography /2D/MASS proteomics strategy and identified nine putative 3’ X-associated host proteins; among them is oxysterol-binding protein-related protein 4 (ORP4), a cytoplasmic receptor for oxysterols. We determined the relationship between ORP4 expression and HCV replication. A very low level of constitutive ORP4 expression was detected in hepatocytes. Ectopically expressed ORP4 was detected in the endoplasmic reticulum and inhibited luciferase reporter gene expression in HCV subgenomic replicon cells and HCV core expression in JFH-1-infected cells. Expression of ORP4S, an ORP4 variant that lacked the N-terminal pleckstrin-homology domain but contained the C-terminal oxysterol-binding domain also inhibited HCV replication, pointing to an important role of the oxysterol-binding domain in ORP4-mediated inhibition of HCV replication. ORP4 was found to associate with HCV NS5B and its expression led to inhibition of the NS5B activity. ORP4 expression had little effect on intracellular lipid synthesis and secretion, but it induced lipid droplet formation in the context of HCV replication. Taken together, these results demonstrate that ORP4 is a negative regulator of HCV replication, likely via interaction with HCV NS5B in the replication complex and regulation of intracellular lipid homeostasis. This work supports the important role of lipids and their metabolism in HCV replication and pathogenesis.
Rab21, a member of the Rab GTPase family, is known to be involved in membrane trafficking, but its implication in macropinocytosis is unclear. We analyzed the spatiotemporal localization of Rab21 in M-CSF-stimulated RAW264 macrophages by the live-cell imaging of fluorescent protein-fused Rab21. It was demonstrated that wild-type Rab21 was transiently associated with macropinosomes. Rab21 was recruited to the macropinosomes after a decrease in PI(4,5)P2 and PI(3,4,5)P3 levels. Although Rab21 was largely colocalized with Rab5, the recruitment of Rab21 to the macropinosomes lagged a minute behind that of Rab5, and preceded that of Rab7. Then, Rab21 was dissociated from the macropinosomes prior to the accumulation of Lamp1, a late endosomal/lysosomal marker. Our analysis of Rab21 mutants revealed that the GTP-bound mutant, Rab21-Q78L, was recruited to the macropinosomes, similarly to wild-type Rab21. However, the GDP-bound mutant, Rab21-T33N, did not localize on the formed macropinosomes, suggesting that the binding of GTP to Rab21 is required for the proper recruitment of Rab21 onto the macropinosomes. However, neither mutation of Rab21 significantly affected the rate of macropinosome formation. These data indicate that Rab21 is a transient component of early and intermediate stages of macropinocytosis, and probably functions in macropinosome maturation before fusing with lysosomal compartments.
Rab GTPases regulate vesicular traffic in eukaryotic cells by cycling between the active GTP-bound and inactive GDP-bound states. Their functions are modulated by the diverse selection of effector proteins that bind to specific Rabs in their activated state. We previously described the expression of Rab13 in bone cells. To search for novel Rab13 interaction partners, we screened a newborn rat bone marrow cDNA library for Rab13 effectors with a bacterial two-hybrid system. We found that Rab13 binds to the C-terminus of Endospanin-2, a small transmembrane protein. In addition to Rab13 also Rab8 bound to Endospanin-2, while no binding of Rab7, Rab10, Rab11 or Rab32 was observed. Rab13 and Rab8 also interacted with Endospanin-1, a close homolog of Endospanin-2. Rab13 and Endospanin-2 colocalised in perinuclear vesicular structures in Cos1 cells suggesting direct binding also in vivo. Endospanin-2 is implicated in the regulation of the cell surface growth hormone receptor (GHR), but the inhibition of Rab13 expression did not affect GHR cell surface expression. This suggests that the Rab13–Endospanin-2 interaction may have functions other than GHR regulation. In conclusion, we have identified a novel interaction for Rab13 and Rab8 with Endospanin-2 and Endospanin-1. The role of this interaction in cell physiology, however, remains to be elucidated.
▸ Rab13 and Rab8 both interact with Endospanin-2 and Endospanin-1. ▸ Rab13 and Rab8 binding to endospanins is specific; Rabs 7, 10, 11 and 32 do not bind. ▸ Rab13 binding to Endospanin-2 is nucleotide-dependent. ▸ Rab13 and Endospanin-2 colocalise in perinuclear vesicles and at the cell periphery.
Vesicle trafficking; Rab13; Rab effector; Protein interaction; Endospanin; Osteoclast; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GFP, green fluorescent protein; GHR, growth hormone receptor; GST, glutathione-S-transferase; HA, human influenza hemagglutinin; MBP, maltose binding protein; OB-R, leptin receptor; VPS55, vacuolar protein sorting 55.
Cephalostatin 1, OSW-1, ritterazine B and schweinfurthin A are natural products that potently, and in some cases selectively, inhibit the growth of cultured human cancer cell lines. The cellular targets of these small molecules have yet to be identified. We have discovered that these molecules target oxysterol binding protein (OSBP) and its closest paralog, OSBP-related protein 4L (ORP4L)—proteins not known to be involved in cancer cell survival. OSBP and the ORPs constitute an evolutionarily conserved protein superfamily, members of which have been implicated in signal transduction, lipid transport and lipid metabolism. The functions of OSBP and the ORPs, however, remain largely enigmatic. Based on our findings, we have named the aforementioned natural products ORPphilins. Here we used ORPphilins to reveal new cellular activities of OSBP. The ORPphilins are powerful probes of OSBP and ORP4L that will be useful in uncovering their cellular functions and their roles in human diseases.
The Rab7 GTPase promotes membrane fusion reactions between late endosomes and lysosomes. In previous studies, we demonstrated that Rab7 inactivation blocks growth factor withdrawal-induced cell death. These results led us to hypothesize that growth factor withdrawal activates Rab7. Here, we show that growth factor deprivation increased both the fraction of Rab7 that was associated with cellular membranes and the percentage of Rab7 bound to guanosine triphosphate (GTP). Moreover, expressing a constitutively GTP-bound mutant of Rab7, Rab7-Q67L, was sufficient to trigger cell death even in the presence of growth factors. This activated Rab7 mutant was also able to reverse the growth factor-independent cell survival conferred by protein kinase C (PKC) δ inhibition. PKCδ is one of the most highly induced proteins after growth factor withdrawal and contributes to the induction of apoptosis. To evaluate whether PKCδ regulates Rab7, we first examined lysosomal morphology in cells with reduced PKCδ activity. Consistent with a potential role as a Rab7 activator, blocking PKCδ function caused profound lysosomal fragmentation comparable to that observed when Rab7 was directly inhibited. Interestingly, PKCδ inhibition fragmented the lysosome without decreasing Rab7-GTP levels. Taken together, these results suggest that Rab7 activation by growth factor withdrawal contributes to the induction of apoptosis and that Rab7-dependent fusion reactions may be targeted by signaling pathways that limit growth factor-independent cell survival.
Legionella pneumophila is an intracellular pathogen that resides within a membrane-bound compartment that is derived from vesicles exiting the endoplasmic reticulum (ER). To create this compartment, these bacteria use a type IV secretion system to deliver effector proteins that subvert host cell functions. Several Legionella effector proteins modulate the function of the host protein Rab1, which is a GTPase that is recruited to the Legionella-containing vacuole (LCV). Here, we examined which of the Rab1-directed enzymatic activities displayed by Legionella effectors are important for localizing the Rab1 protein to the LCV membrane. The guanine nucleotide exchange factor (GEF) domain in the effector protein DrrA (SidM) was essential for Rab1 recruitment to the LCV and Rab1 AMPylation by the nucleotidyltransferase domain in DrrA was important for Rab1 retention. Legionella organisms producing mutant DrrA proteins that were severely attenuated for GEF activity in vitro retained the ability to localize Rab1 to the LCV. Rab1 localization to the LCV mediated by these GEF-defective mutants required AMPylation. Importantly, we found that efficient localization of Rab1 to the LCV occurred when Rab1 GEF activity and Rab1 AMPylation activity were provided by separate proteins. Rab1 phosphocholination (PCylation) by the effector protein AnkX, however, was unable to substitute for Rab1 AMPylation. Lastly, the defect in Rab1 localization to the LCV in AMPylation-deficient strains of Legionella was partially suppressed if the GTPase-activating protein (GAP) LepB was eliminated. Thus, our data indicate that AMPylation of Rab1 is an effective strategy to maintain this GTPase on the LCV membrane.
Activities that enable the intracellular pathogen Legionella pneumophila to subvert the function of the host protein Rab1 were investigated. Our data show that a posttranslational modification called AMPylation is critical for maintaining a pool of Rab1 on the LCV membrane. AMPylation of Rab1 led to the accumulation of GTP-bound Rab1 on the LCV membrane by protecting the protein from inactivation by GAPs. Importantly, PCylation of Rab1 by the Legionella effector protein AnkX was neither necessary nor sufficient to maintain Rab1 on the LCV, indicating that AMPylation and PCylation represent functionally distinct activities. We conclude that modification of Rab1 by AMPylation is an effective strategy to spatially and temporally regulate the function of this GTPase on a membrane-bound organelle.
The 60+ members of the mammalian Rab protein family group into subfamilies postulated to share common functionality. The Rab VI subfamily contains 5 Rab proteins, Rab6a/a’, Rab6b, Rab6c and Rab41. High-level knockdown of Rab6a/a’ has little effect on the tightly organized Golgi ribbon in HeLa cells as seen by fluorescence microscopy. In striking contrast, we found Rab41 was strongly required for normal Golgi ribbon organization.
Treatment of HeLa cells with Rab41 siRNAs scattered the Golgi ribbon into clustered, punctate Golgi elements. Overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a similar Golgi phenotype. By electron microscopy, Rab41 depletion produced short, isolated Golgi stacks. Golgi-associated vesicles accumulated. At low expression levels, wild type and GTP-locked Rab41 showed little concentration in the Golgi region, but puncta were observed and most were in ruffled regions at the cell periphery. There was 25% co-localization of GTP-locked Rab41 with the ER marker, Sec61p. GDP-locked Rab41, as expected, displayed an entirely diffuse cytoplasmic distribution. Depletion of Rab41 or overexpression of GDP-locked Rab41 partially inhibited ER-to-Golgi transport of VSV-G protein. However, Rab41 knockdown had little, if any, effect on endosome-to-Golgi transport of SLTB. Additionally, after a 2-day delay, treatment with Rab41 siRNA inhibited cell growth, while overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a rapid, progressive cell loss. In double knockdown experiments with Rab6, the Golgi ribbon was fragmented, a result consistent with Rab41 and Rab6 acting in parallel.
We provide the first evidence for distinctive Rab41 effects on Golgi organization, ER-to-Golgi trafficking and cell growth. When combined with the evidence that Rab6a/a’ and Rab6b have diverse roles in Golgi function, while Rab6c regulates mitotic function, our data indicate that Rab VI subfamily members, although related by homology and structure, share limited functional conservation.
DIAPH1, the RhoA effector protein, forms a complex in adrenocortical cells and is phosphorylated by a cAMP/PKA-dependent pathway. Phosphorylation differentially modulates protein–protein interactions, regulates the stability of the protein, and facilitates sumoylation.
Diaphanous homologue 1 (DIAPH1) is a Rho effector protein that coordinates cellular dynamics by regulating microfilament and microtubule function. We previously showed that DIAPH1 plays an integral role in regulating the production of cortisol by controlling the rate of mitochondrial movement, by which activation of the adrenocorticotropin (ACTH)/cAMP signaling pathway stimulates mitochondrial trafficking and promotes the interaction between RhoA and DIAPH1. In the present study we use mass spectrometry to identify DIAPH1 binding partners and find that DIAPH1 interacts with several proteins, including RhoA, dynamin-1, kinesin, β-tubulin, β-actin, oxysterol-binding protein (OSBP)–related protein 2 (ORP2), and ORP10. Moreover, DIAPH1 is phosphorylated in response to dibutyryl cAMP (Bt2cAMP) at Thr-759 via a pathway that requires extracellular signal-related kinase (ERK). Alanine substitution of Thr-759 renders DIAPH1 more stable and attenuates the interaction between DIAPH1 and kinesin, ORP2, and actin but has no effect on the ability of the protein to interact with RhoA or β-tubulin. Finally, overexpression of a DIAPH1 T759A mutant significantly decreases the rate of Bt2cAMP-stimulated mitochondrial movement. Taken together, our findings establish a key role for phosphorylation in regulating the stability and function of DIAPH1.
Autophagy is an evolutionarily conserved process that enables catabolic and degradative pathways. These pathways commonly depend on vesicular transport controlled by Rabs, small GTPases inactivated by TBC/RabGAPs. The Rac1 effector TBC/RabGAP Armus (TBC1D2A) is known to inhibit Rab7, a key regulator of lysosomal function. However, the precise coordination of signaling and intracellular trafficking that regulates autophagy is poorly understood. We find that overexpression of Armus induces the accumulation of enlarged autophagosomes, while Armus depletion significantly delays autophagic flux. Upon starvation-induced autophagy, Rab7 is transiently activated. This spatiotemporal regulation of Rab7 guanosine triphosphate/guanosine diphosphate cycling occurs by Armus recruitment to autophagosomes via interaction with LC3, a core autophagy regulator. Interestingly, autophagy potently inactivates Rac1. Active Rac1 competes with LC3 for interaction with Armus and thus prevents its appropriate recruitment to autophagosomes. The precise coordination between Rac1 and Rab7 activities during starvation suggests that Armus integrates autophagy with signaling and endocytic trafficking.
► Autophagy strongly inactivates Rac1 GTPase and recruits the TBC/RabGAP Armus ► Appropriate Armus localization at autophagosomes requires binding to LC3 ► Armus inhibition delays autophagic flux and increases levels of Rab7·GTP ► Rac1, Armus, and Rab7 coordinate efficient lysosome fusion with autophagosomes
The Rac1 effector Armus is a GTPase-activating protein that regulates Rab7 function and thus endosomal trafficking to lysosomes. Carroll et al. find that Armus binds directly to the core autophagy factor LC3, facilitates fusion of autophagosomes with lysosomes, and integrates local Rac1 activity into the control of autophagy.
Rab14 binds in a GTP-dependent manner to RUFY1/Rabip4, which had been originally identified as a Rab4 effector. We suggest that Rab14 and Rab4 act sequentially; Rab14 is required for recruitment of RUFY1 onto endosomes and subsequent RUFY1 interaction with Rab4 may allow endosomal tethering and fusion.
The small GTPase Rab14 localizes to early endosomes and the trans-Golgi network, but its cellular functions on endosomes and its functional relationship with other endosomal Rab proteins are poorly understood. Here, we report that Rab14 binds in a GTP-dependent manner to RUFY1/Rabip4, which had been originally identified as a Rab4 effector. Rab14 colocalizes well with Rab4 on peripheral endosomes. Depletion of Rab14, but not Rab4, causes dissociation of RUFY1 from endosomal membranes. Coexpression of RUFY1 with either Rab14 or Rab4 induces clustering and enlargement of endosomes, whereas a RUFY1 mutant lacking the Rab4-binding region does not induce a significant morphological change in the endosomal structures even when coexpressed with Rab14 or Rab4. These findings suggest that Rab14 and Rab4 act sequentially, together with RUFY1; Rab14 is required for recruitment of RUFY1 onto endosomal membranes, and subsequent RUFY1 interaction with Rab4 may allow endosomal tethering and fusion. Depletion of Rab14 or RUFY1, as well as Rab4, inhibits efficient recycling of endocytosed transferrin, suggesting that Rab14 and Rab4 regulate endosomal functions through cooperative interactions with their dual effector, RUFY1.
Small GTPases of the rab family are crucial elements of the machinery
that controls membrane traffic. In the present study, we examined the
distribution and function of rab11. Rab11 was shown by confocal
immunofluorescence microscopy and EM to colocalize with internalized
transferrin in the pericentriolar recycling compartment of CHO and BHK
cells. Expression of rab11 mutants that are preferentially in the GTP- or
GDP-bound state caused opposite effects on the distribution of
transferrin-containing elements; rab11-GTP expression caused accumulation
of labeled elements in the perinuclear area of the cell, whereas rab11-GDP
caused a dispersion of the transferrin labeling. Functional studies showed
that the early steps of uptake and recycling for transferrin were not
affected by overexpression of rab11 proteins. However, recycling from the
later recycling endosome was inhibited in cells overexpressing the
rab11-GDP mutant. Rab5, which regulates early endocytic trafficking, acted
before rab11 in the transferrin-recycling pathway as expression of rab5-GTP
prevented transport to the rab11- positive recycling endosome. These
results suggest a novel role for rab11 in controlling traffic through the