The intracellular pathogen Legionella pneumophila hijacks the endoplasmic reticulum (ER)-derived vesicles to create an organelle designated Legionella-containing vacuole (LCV) required for bacterial replication. Maturation of the LCV involved acquisition of Rab1, which is mediated by the bacterial effector protein SidM/DrrA. SidM/DrrA is a bifunctional enzyme having the activity of both Rab1-specific GDP dissociation inhibitor (GDI) displacement factor (GDF) and guanine nucleotide exchange factor (GEF). LidA, another Rab1-interacting bacterial effector protein, was reported to promote SidM/DrrA-mediated recruitment of Rab1 to the LCV as well. Here we report the crystal structures of LidA complexes with GDP- and GTP-bound Rab1 respectively. Structural comparison revealed that GDP-Rab1 bound by LidA exhibits an active and nearly identical conformation with that of GTP-Rab1, suggesting that LidA can disrupt the switch function of Rab1 and render it persistently active. As with GTP, LidA maintains GDP-Rab1 in the active conformation through interaction with its two conserved switch regions. Consistent with the structural observations, biochemical assays showed that LidA binds to GDP- and GTP-Rab1 equally well with an affinity approximately 7.5 nM. We propose that the tight interaction with Rab1 allows LidA to facilitate SidM/DrrA-catalyzed release of Rab1 from GDIs. Taken together, our results support a unique mechanism by which a bacterial effector protein regulates Rab1 recycling.
Legionella pneumophila delivers 275 validated substrates into the host cytosol by its Dot/Icm type IV secretion system. Several substrates including SidM/DrrA and LidA directly interact with the host Rab GTPases and interfere with the vesicle secretion pathway. SidM/DrrA is necessary for Rab1 recruitment, function as a Rab1 specific GDI displacement factor and guanine nucleotide exchange factor. LidA has the auxiliary activity for Rab1 recruitment, whereas it is more important for the formation of the replication vacuole compared with SidM/DrrA. LidA is predicted to be the first substrate secreted by the Dot/Icm system and is critical for maintaining the integrity of the bacterial cell. Moreover, it expresses throughout the intracellular growth phase, localizes to early secretory compartments, and interacts with several members of Rab family. Here we present the crystal structures of LidA coiled-coil domain in complex with two different states of Rab1, GDP- and GTP-bound. The GDP-bound Rab1 in the complex surprisingly has the same conformation with the GTP-bound Rab1, revealing that LidA can retain Rab1 persistently in its active state. Our structures add a new insight into the regulation of the host Rab1 membrane cycle by pathogen-secreted coiled-coil effector.
Chlamydiae are intracellular bacteria that develop within a membrane-bound vacuole called an inclusion. To ensure that the inclusion is a safe niche for chlamydial replication, chlamydiae exploit a number of host cell processes, including membrane-trafficking pathways. Recently, several Rab GTPases were found to associate with the inclusions of various chlamydial species. Here we report that Cpn0585, a Chlamydia pneumoniae inclusion membrane protein (Inc), interacts with multiple Rab GTPases. The results from yeast two-hybrid experiments revealed that an amino-terminally truncated form of Cpn0585 (Cpn0585102-651) interacts with Rab1, Rab10, and Rab11 but not with Rab4 or Rab6. Cpn0585-Rab GTPase interactions are direct and GTP dependent as shown in glutathione S-transferase pull-down assays using native and recombinant Cpn0585. In C. pneumoniae-infected HEp-2 cells transfected with enhanced green fluorescent protein (EGFP)-tagged Rab GTPases, the colocalization with Cpn0585 at the inclusion membrane was partial for EGFP-Rab1 and EGFP-Rab10, but extensive for wild-type EGFP-Rab11A and the constitutively active GTPase-deficient EGFP-Rab11AQ70L. Moreover, Cpn0585 colocalized with EGFP-Rab11AQ70L as early as 2 h postinfection. Upon delivery into live C. pneumoniae-infected cells, Cpn0585628-651-specific antibodies bound to the inclusion membrane, demonstrating that the Rab GTPase-interacting domain of Cpn0585 faces the host cell cytosol. Finally, ectopic expression of Cpn0585102-651 partially inhibited the development of C. pneumoniae inclusions in EGFP. but not in EGFP-Rab11AQ70L-expressing HEp-2 cells. Collectively, these data suggest that Cpn0585 is involved in the recruitment of Rab GTPases to the inclusion membrane and that interfering with this function may adversely impact the fitness of the C. pneumoniae inclusion for chlamydial replication.
An important role in the evolution of intracellular trafficking machinery in eukaryotes played small GTPases belonging to the Rab family known as pivotal regulators of vesicle docking, fusion and transport. The Rab family is very diversified and divided into several specialized subfamilies. We focused on the VII functional group comprising Rab7 and Rab9, two related subfamilies, and analysed 210 sequences of these proteins. Rab7 regulates traffic from early to late endosomes and from late endosome to vacuole/lysosome, whereas Rab9 participates in transport from late endosomes to the trans-Golgi network.
Although Rab7 and Rab9 proteins are quite small and show heterogeneous rates of substitution in different lineages, we found a phylogenetic signal and inferred evolutionary relationships between them. Rab7 proteins evolved before radiation of main eukaryotic supergroups while Rab9 GTPases diverged from Rab7 before split of choanoflagellates and metazoans. Additional duplication of Rab9 and Rab7 proteins resulting in several isoforms occurred in the early evolution of vertebrates and next in teleost fishes and tetrapods. Three Rab7 lineages emerged before divergence of monocots and eudicots and subsequent duplications of Rab7 genes occurred in particular angiosperm clades. Interestingly, several Rab7 copies were identified in some representatives of excavates, ciliates and amoebozoans. The presence of many Rab copies is correlated with significant differences in their expression level. The diversification of analysed Rab subfamilies is also manifested by non-conserved sequences and structural features, many of which are involved in the interaction with regulators and effectors. Individual sites discriminating different subgroups of Rab7 and Rab9 GTPases have been identified.
Phylogenetic reconstructions of Rab7 and Rab9 proteins were performed by a variety of methods. These Rab GTPases show diversification both at the phylogenetic, expression and structural levels. The presence of many Rab7 and Rab9 isoforms suggests their functional specialization and complexity of subcellular trafficking even in unicellular eukaryotes. The identified less conserved regions in analysed Rab sequences may directly contribute to such a differentiation.
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.
Rab proteins are small GTPases that act as essential regulators of vesicular trafficking. 44 subfamilies are known in humans, performing specific sets of functions at distinct subcellular localisations and tissues. Rab function is conserved even amongst distant orthologs. Hence, the annotation of Rabs yields functional predictions about the cell biology of trafficking. So far, annotating Rabs has been a laborious manual task not feasible for current and future genomic output of deep sequencing technologies. We developed, validated and benchmarked the Rabifier, an automated bioinformatic pipeline for the identification and classification of Rabs, which achieves up to 90% classification accuracy. We cataloged roughly 8.000 Rabs from 247 genomes covering the entire eukaryotic tree. The full Rab database and a web tool implementing the pipeline are publicly available at www.RabDB.org. For the first time, we describe and analyse the evolution of Rabs in a dataset covering the whole eukaryotic phylogeny. We found a highly dynamic family undergoing frequent taxon-specific expansions and losses. We dated the origin of human subfamilies using phylogenetic profiling, which enlarged the Rab repertoire of the Last Eukaryotic Common Ancestor with Rab14, 32 and RabL4. Furthermore, a detailed analysis of the Choanoflagellate Monosiga brevicollis Rab family pinpointed the changes that accompanied the emergence of Metazoan multicellularity, mainly an important expansion and specialisation of the secretory pathway. Lastly, we experimentally establish tissue specificity in expression of mouse Rabs and show that neo-functionalisation best explains the emergence of new human Rab subfamilies. With the Rabifier and RabDB, we provide tools that easily allows non-bioinformaticians to integrate thousands of Rabs in their analyses. RabDB is designed to enable the cell biology community to keep pace with the increasing number of fully-sequenced genomes and change the scale at which we perform comparative analysis in cell biology.
Intracellular compartmentalisation via membrane-delimited organelles is a fundamental feature of the eukaryotic cell. Understanding its origins and specialisation into functionally distinct compartments is a major challenge in evolutionary cell biology. We focus on the Rab enzymes, critical organisers of the trafficking pathways that link the endomembrane system. Rabs form a large family of evolutionarily related proteins, regulating distinct steps in vesicle transport. They mark pathways and organelles due to their specific subcellular and tissue localisation. We propose a solution to the problem of identifying and annotating Rabs in hundreds of sequenced genomes. We developed an accurate bioinformatics pipeline that is able to take into account pre-existing and often inconsistent, manual annotations. We made it available to the community in form of a web tool, as well as a database containing thousands of Rabs assigned to sub-families, which yields clear functional predictions. Thousands of Rabs allow for a new level of analysis. We illustrate this by characterising for the first time the global evolutionary dynamics of the Rab family. We dated the emergence of subfamilies and suggest that the Rab family expands by duplicates acquiring new functions.
Recent studies indicate that lipid droplets isolated from a variety of different cells are rich in proteins known to regulate membrane traffic. Among these proteins are multiple Rab GTPases. Rabs are GTP switches that regulate intracellular membrane traffic through an ability to control membrane-membrane docking as well as vesicle motility. Here we present evidence that the multiple Rabs associated with droplets have a function in regulating membrane traffic. Droplet Rabs are removed by Rab GDP-dissociation inhibitor (RabGDI) in a GDP-dependent reaction, and are recruited to Rab-depleted droplets from cytosol in a GTP-dependent reaction. Rabs also control the recruitment of the early endosome (EE) marker EEA1 from cytosol. We use an in vitro reconstitution assay to show that transferrin receptor positive EEs bind to the droplet in a GTP/Rab-dependent reaction that appears not to lead to membrane fusion. This docking reaction is insensitive to ATPγs but is blocked by ATP. Finally, we show that when GTP bound active or GDP bound inactive Rab5 is targeted to the droplet, the active form recruits EEA1. We conclude that the Rabs associated with droplets may be capable of regulating the transient interaction of specific membrane systems, probably to transport lipids between membrane compartments.
The secretory pathway is a process characteristic of cells specialized in secretion such as endocrine cells and neurons. It consists of different stages that are dependent on specific transport of proteins in vesicular-tubular carriers. Biochemical analyses have unveiled a number of protein families that confer identity to carrier vesicles and specificity to their transport. Among them is the family of Rab proteins, Ras-like small GTPases that anchor to the surface of transport vesicles and participate in vesicle formation from the donor compartment, transport along cytoskeletal tracks, and docking and fusion with the acceptor compartment. All of these functions are accomplished through the recruitment of effector proteins, such as sorting adaptors, tethering factors, kinases, phosphatases, and motors. The numerous Rab proteins have distinct subcellular distributions throughout the endomembrane system, which ensures efficient cargo transfer. Rab proteins act as molecular switches that alternate between a cytosolic GDP-bound, inactive form and a membrane-associated GTP-bound, active conformation. Cycling between inactive and active states is a highly regulated process that enables Rabs to confer spatio-temporal precision to the different stages through which a vesicle passes during its lifespan. This review focuses on our current knowledge on Rab functioning, from their structural features to the multiple regulatory proteins and effectors that control Rab activity and translate Rab function. Furthermore, we also summarize the information available on a particular Rab protein, Rab18, which has been linked to the control of secretory granule traffic in neuroendocrine cells.
neuroendocrine cells; Rab proteins; Rab18; secretory pathway; vesicle traffic
Retroviruses take advantage of cellular trafficking machineries to assemble and release new infectious particles. Rab proteins regulate specific steps in intracellular membrane trafficking by recruiting tethering, docking and fusion factors, as well as the actin- and microtubule-based motor proteins that facilitate vesicle traffic. Using virological tests and RNA interference targeting Rab proteins, we demonstrate that the late endosome-associated Rab7A is required for HIV-1 propagation. Analysis of the late steps of the HIV infection cycle shows that Rab7A regulates Env processing, the incorporation of mature Env glycoproteins into viral particles and HIV-1 infectivity. We also show that siRNA-mediated Rab7A depletion induces a BST2/Tetherin phenotype on HIV-1 release. BST2/Tetherin is a restriction factor that impedes HIV-1 release by tethering mature virus particles to the plasma membrane. Our results suggest that Rab7A contributes to the mechanism by which Vpu counteracts the restriction factor BST2/Tetherin and rescues HIV-1 release. Altogether, our results highlight new roles for a major regulator of the late endocytic pathway, Rab7A, in the late stages of the HIV-1 replication cycle.
Human immunodeficiency virus (HIV) propagation requires the assistance of host cell factors at all stages of the infection cycle. HIV exploits components of the cellular membrane sorting machinery for its assembly, budding and release. Rab GTPases are key regulators of membrane-trafficking events, including exocytosis and endocytosis, in eukaryotic cells. Here we show that the late endosome associated Rab7A plays a major role in HIV-1 replication. We find that Rab7A regulates the production of infectious HIV-1 particles at two critical stages. First, Rab7A is required for efficient Env processing and, thus, for the incorporation of mature HIV-1 envelope glycoproteins into virions. Second, Rab7A contributes to the mechanism that counteracts the restriction imposed on HIV-1 release by the cellular restriction factor BST2/Tetherin that physically tethers viral particles to the plasma membrane of infected cells. Altogether these data highlight new roles for a major player of the late endocytic pathway, Rab7A, in the late stages of the HIV-1 replication cycle.
Rab GTPases regulate all stages of membrane trafficking, including vesicle budding, cargo sorting, transport, tethering, and fusion1, 2. In the inactive (GDP-bound) conformation, accessory factors facilitate the targeting of Rab GTPases to intracellular compartments3–8. Following nucleotide exchange to the active (GTP-bound) conformation, Rab GTPases interact with functionally diverse effectors including lipid kinases, motor proteins, and tethering complexes. How effectors distinguish between homologous Rab GTPases represents an unresolved problem with respect to the specificity of vesicular trafficking. Using a structural proteomic approach, we have determined the specificity and structural basis underlying the interaction of the multivalent effector Rabenosyn-5 with the Rab family. The results demonstrate that even structurally similar effectors can achieve highly selective recognition of distinct subsets of Rab GTPases exclusively through interactions with the switch and interswitch regions. The observed specificity is determined at a family-wide level by structural diversity in the active conformation, which governs the spatial disposition of critical conserved recognition determinants, and by a small number of both positive and negative sequence determinants that allow further discrimination between Rab GTPases with similar switch conformations.
The pathology causing stages of the human malaria parasite Plasmodium falciparum reside within red blood cells that are devoid of any regulated transport system. The parasite, therefore, is entirely responsible for mediating vesicular transport within itself and in the infected erythrocyte cytoplasm, and it does so in part via its family of 11 Rab GTPases. Putative functions have been ascribed to Plasmodium Rabs due to their homology with Rabs of yeast, particularly with Saccharomyces that has an equivalent number of rab/ypt genes and where analyses of Ypt function is well characterized.
Rabs are important regulators of vesicular traffic due to their capacity to recruit specific effectors. In order to identify P. falciparum Rab (PfRab) effectors, we first built a Ypt-interactome by exploiting genetic and physical binding data available at the Saccharomyces genome database (SGD). We then constructed a PfRab-interactome using putative parasite Rab-effectors identified by homology to Ypt-effectors. We demonstrate its potential by wet-bench testing three predictions; that casein kinase-1 (PfCK1) is a specific Rab5B interacting protein and that the catalytic subunit of cAMP-dependent protein kinase A (PfPKA-C) is a PfRab5A and PfRab7 effector.
The establishment of a shared set of physical Ypt/PfRab-effector proteins sheds light on a core set Plasmodium Rab-interactants shared with yeast. The PfRab-interactome should benefit vesicular trafficking studies in malaria parasites. The recruitment of PfCK1 to PfRab5B+ and PfPKA-C to PfRab5A+ and PfRab7+ vesicles, respectively, suggests that PfRab-recruited kinases potentially play a role in early and late endosome function in malaria parasites.
Interactom; Kinase; Plasmodium; Rab; Yeast; Ypt
Dense core vesicles (DCVs) are thought to be generated at the late Golgi apparatus as immature DCVs, which subsequently undergo a maturation process through clathrin-mediated membrane remodeling events. This maturation process is required for efficient processing of neuropeptides within DCVs and for removal of factors that would otherwise interfere with DCV release. Previously, we have shown that the GTPase, RAB-2, and its effector, RIC-19, are involved in DCV maturation in Caenorhabditis elegans motoneurons. In rab-2 mutants, specific cargo is lost from maturing DCVs and missorted into the endosomal/lysosomal degradation route. Cargo loss could be prevented by blocking endosomal delivery. This suggests that RAB-2 is involved in retention of DCV components during the sorting process at the Golgi-endosomal interface. To understand how RAB-2 activity is regulated at the Golgi, we screened for RAB-2–specific GTPase activating proteins (GAPs). We identified a potential RAB-2 GAP, TBC-8, which is exclusively expressed in neurons and which, when depleted, shows similar DCV maturation defects as rab-2 mutants. We could demonstrate that RAB-2 binds to its putative GAP, TBC-8. Interestingly, TBC-8 also binds to the RAB-2 effector, RIC-19. This interaction appears to be conserved as TBC-8 also interacted with the human ortholog of RIC-19, ICA69. Therefore, we propose that a dynamic ON/OFF cycling of RAB-2 at the Golgi induced by the GAP/effector complex is required for proper DCV maturation.
Synaptic transmission is mainly mediated by the triggered release of neurotransmitters from synaptic vesicles (SVs). To regulate synaptic transmission and neuronal activity, neurons also release neuropeptides and hormones from dense core vesicles (DCVs). While SVs can be recycled locally at the synapse, DCVs have to be newly synthesized in the cell body after release. The formation of new DCVs requires a multi-step maturation process. During this maturation, the neuropeptides are processed into their active form and factors that would disturb DCV release are removed. Only properly matured DCVs are able to undergo efficient release after stimulation. Since DCV biogenesis mainly uses the normal secretory pathway, an elaborate machinery must exist that guarantees efficient sorting and retention of DCV cargo. Previously, we identified the small GTPase RAB-2 and its effector RIC-19/ICA69 to be involved in the retention of soluble cargo in DCVs. In a screen for molecules that regulate RAB-2 activity during DCV maturation, we identified the evolutionarily conserved TBC domain-containing protein, TBC-8. We demonstrate that TBC-8 is a putative RAB-2 GAP, which also binds to RIC-19/ICA69. Thus, RAB-2 might recruit its own GAP via its effector RIC-19, which suggests that a highly dynamic cycling of RAB-2 is required for DCV maturation.
Cellular sophistication is not exclusive to multicellular organisms, and unicellular eukaryotes can resemble differentiated animal cells in their complex network of membrane-bound structures. These comparisons can be illuminated by genome-wide surveys of key gene families. We report a systematic analysis of Rabs in a complex unicellular Ciliate, including gene prediction and phylogenetic clustering, expression profiling based on public data, and Green Fluorescent Protein (GFP) tagging. Rabs are monomeric GTPases that regulate membrane traffic. Because Rabs act as compartment-specific determinants, the number of Rabs in an organism reflects intracellular complexity. The Tetrahymena Rab family is similar in size to that in humans and includes both expansions in conserved Rab clades as well as many divergent Rabs. Importantly, more than 90% of Rabs are expressed concurrently in growing cells, while only a small subset appears specialized for other conditions. By localizing most Rabs in living cells, we could assign the majority to specific compartments. These results validated most phylogenetic assignments, but also indicated that some sequence-conserved Rabs were co-opted for novel functions. Our survey uncovered a rare example of a nuclear Rab and substantiated the existence of a previously unrecognized core Rab clade in eukaryotes. Strikingly, several functionally conserved pathways or structures were found to be associated entirely with divergent Rabs. These pathways may have permitted rapid evolution of the associated Rabs or may have arisen independently in diverse lineages and then converged. Thus, characterizing entire gene families can provide insight into the evolutionary flexibility of fundamental cellular pathways.
Single-celled organisms appear simple compared to multicellular organisms, but this may not be true at the level of the individual cell. In fact, microscopic observations suggest that protists can possess networks of organelles just as elaborate as those in animal cells. Consistent with this idea, recent analysis has identified large families of genes in protists that are predicted to act as determinants for complex membrane networks. To test these predictions and to probe relationships between cellular structures across a wide swath of evolution, we focused on one gene family in the single-celled organism Tetrahymena. These genes control the traffic between organelles, with each gene controlling a single step in this traffic. We asked three questions about each of 56 genes in the family. First, what is the gene related to in humans? Second, under what conditions is the gene being used in Tetrahymena? Third, what is the role of each gene? The results provide insights into both the dynamics and evolution of membrane traffic, including the finding that some pathways appearing both structurally and functionally similar in protists and animals are likely to have arisen independently in the two lineages.
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.
The expression of the recently identified dermokine (Dmkn) gene leads to four families of proteins with as yet unknown functions. The secreted α, β and γ isoforms share an epidermis-restricted expression pattern, whereas the δ isoform is intracellular and ubiquitous. To get an insight into Dmknδ function, we performed yeast two-hybrid screening and identified the small GTPases Rab5 as partners for Dmknδ. The Rab5 proteins are known to regulate membrane docking and fusion in the early endocytic pathway. GST pull-down assays confirmed the direct interaction between Rab5 and Dmknδ. Transient expression of Dmknδ in HeLa cells led to the formation of punctate structures colocalized with endogenous Rab5 and clathrin, indicating Dmknδ involvement in the early steps of endocytosis. Dmknδ indeed colocalized with transferrin at early stages of endocytosis, but did not modulate its endocytosis or recycling kinetics. We also showed that Dmknδ was able to bind both inactive (GDP-bound) and active (GTP-bound) forms of Rab5 in vitro but preferentially targeted GDP-bound form in HeLa cells. Interestingly, Dmknδ expression rescued the Rab5S34N-mediated inhibition of endosome fusion. Moreover, Dmknδ caused the enlargement of vesicles positive for Rab5 by promoting GTP loading onto the small GTPase. Together our data reveal that Dmknδ activates Rab5 function and thus is involved in the early endosomal trafficking.
The Rab GTPase Ypt7p and its effector complex HOPS participate in catalyzing the fusion of yeast vacuoles. The role of the vacuolar kinase Yck3p in this relation is examined. It is shown how the regulatory ability of the Rab GTPase cycle is enforced only by posttranslational modification of the effector complex HOPS.
The homotypic fusion of yeast vacuoles requires the Rab-family GTPase Ypt7p and its effector complex, homotypic fusion and vacuole protein sorting complex (HOPS). Although the vacuolar kinase Yck3p is required for the sensitivity of vacuole fusion to proteins that regulate the Rab GTPase cycle—Gdi1p (GDP-dissociation inhibitor [GDI]) or Gyp1p/Gyp7p (GTPase-activating protein)—this kinase phosphorylates HOPS rather than Ypt7p. We addressed this puzzle in reconstituted proteoliposome fusion reactions with all-purified components. In the presence of HOPS and Sec17p/Sec18p, there is comparable fusion of 4-SNARE (soluble N-ethylmaleimide–sensitive factor attachment protein receptor) proteoliposomes when they have Ypt7p bearing either GDP or GTP, a striking exception to the rule that only GTP-bound forms of Ras-superfamily GTPases have active conformations. However, the phosphorylation of HOPS by recombinant Yck3p confers a strict requirement for GTP-bound Ypt7p for binding phosphorylated HOPS, for optimal membrane tethering, and for proteoliposome fusion. Added GTPase-activating protein promotes GTP hydrolysis by Ypt7p, and added GDI captures Ypt7p in its GDP-bound state during nucleotide cycling. In either case, the net conversion of Ypt7:GTP to Ypt7:GDP has no effect on HOPS binding or activity but blocks fusion mediated by phosphorylated HOPS. Thus guanine nucleotide specificity of the vacuolar fusion Rab Ypt7p is conferred through downstream posttranslational modification of its effector complex.
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.
An engineered deletion of a nonconserved loop in the RUN domain of Rab6-interacting protein 1 enhances the solubility of the recombinant protein when expressed in E. coli. Crystals of the engineered effector in complex with Rab6a diffracted X-rays to 3.25 Å resolution.
Effectors of the Rab small GTPases are large multi-domain proteins which have proved difficult to express in soluble form in Escherichia coli. Generally, effectors are recruited to a distinct subcellular compartment by active (GTP-bound) Rabs, which are linked to membranes by one or two prenylated Cys residues at their C-termini. Following recruitment via their Rab-binding domain (RBD), effectors carry out various aspects of vesicle formation, transport, tethering and fusion through their other domains. Previously, successful purification of the RUN–PLAT tandem domains (residues 683–1061) of the 1263-residue Rab6-interacting protein 1 (R6IP1) required co-expression with Rab6, as attempts to solubly express the effector alone were unsuccessful. R6IP1 is also known as DENN domain-containing protein 5 (DENND5) and is expressed as two isoforms, R6IP1A/B (DENND5A/B), which differ by 24 amino acids at the N-terminus. Here, a deletion in R6IP1 was engineered to enable soluble expression and to improve the quality of the crystals grown in complex with Rab6. A large 23-residue loop linking two α-helices in the RUN1 domain was removed and replaced with a short linker. This loop resides on the opposite face to the Rab6-binding site and is not conserved in the RUN-domain family. In contrast to wild-type R6IP1–Rab6 crystals, which took several weeks to grow to full size, the engineered R6IP1 (RPdel)–Rab6 crystals could be grown in a matter of days.
Rab6-interacting protein 1; DENN domain-containing protein 5; RUN domain
The Rab11-Family Interacting Protein (Rab11-FIP) group of effector proteins contain a highly conserved region in their C-termini that bind the GTPase, Rab11. Rab11 belongs to the largest family of small GTPases and is believed to regulate vesicle docking with target membranes and vesicle fusion. The amino acid sequence of the Rab11-FIP proteins predicts coiled-coil formation in the conserved C-terminal domain. In this study on Rab11-FIP2, we found experimental evidence for the coiled-coil and then defined the minimal structured core using limited proteolysis. We also showed that the Rab11-FIP2 coiled-coil domain forms a parallel homodimer in solution using cross-linking and mutagenesis and sedimentation equilibrium experiments. Various constructs representing the C-terminal domain of Rab11-FIP2 were characterized by circular dichroism and their affinity with Rab11 measured using isothermal titration calorimetry. The longest construct was both well-structured and bound Rab11. A construct truncated at the N-terminus was poorly structured, but retained the same affinity for binding to Rab11. Conformational changes were also demonstrated upon complex formation between Rab11 and Rab11-FIP2. A construct truncated at the C-terminus, which was the minimal coiled-coil domain defined by limited proteolysis, did not retain the ability to interact with Rab11 although it was as well-structured as the longer peptide. These data show that coiled-coil formation and Rab11 binding are separable functions of the C-terminal domain of Rab11-FIP2. The dissection of Rab11 binding from the formation of defined structure in a coiled-coil provides a potential mechanism for regulating Rab11-dependent endosomal trafficking.
Rab11-FIP2; Coiled-coil domain; ITC; CD; Limited proteolysis
When the bacterium Legionella pneumophila, the causative agent of Legionnaires' disease, is phagocytosed by alveolar macrophages, it delivers a large number of effector proteins through its Dot/Icm type IV secretion system into the host cell cytosol. Among those proteins is LidA, an effector that interacts with several host GTPases of the Rab family, including Rab6A′, a regulator of retrograde vesicle trafficking within eukaryotic cells. The effect of LidA on Rab6A′ function and the role of Rab6A′ for L. pneumophila growth within host cells has been unclear. Here, we show that LidA preferentially binds Rab6A′ in the active GTP-bound conformation. Rab6 binding occurred through the central region of LidA and followed a stoichiometry for LidA and Rab6A′ of 1:2. LidA maintained Rab6A′ in the active conformation by efficiently blocking the hydrolysis of GTP by Rab6A′, even in the presence of cellular GTPase-activating proteins, suggesting that the function of Rab6A′ must be important for efficient intracellular replication of L. pneumophila. Accordingly, we found that production of constitutively inactive Rab6A′(T27N) but not constitutively active Rab6A′(Q72L) significantly reduced the ability of L. pneumophila to initiate intracellular replication in human macrophages. Thus, the presence of an active pool of Rab6 within host cells early during infection is required to support efficient intracellular growth of L. pneumophila.
Skeletal muscle is the main site of insulin-dependent glucose uptake. Insulin inactivation of the Akt substrate Rab-GAP AS160 leads to Rab8A activation in myocytes. The molecular motor myosin Va is a Rab8A effector in this pathway, leading to GLUT4 translocation to the myocyte surface, linking signal transduction to vesicle traffic.
Rab-GTPases are important molecular switches regulating intracellular vesicle traffic, and we recently showed that Rab8A and Rab13 are activated by insulin in muscle to mobilize GLUT4-containing vesicles to the muscle cell surface. Here we show that the unconventional motor protein myosin Va (MyoVa) is an effector of Rab8A in this process. In CHO-IR cell lysates, a glutathione S-transferase chimera of the cargo-binding COOH tail (CT) of MyoVa binds Rab8A and the related Rab10, but not Rab13. Binding to Rab8A is stimulated by insulin in a phosphatidylinositol 3-kinase–dependent manner, whereas Rab10 binding is insulin insensitive. MyoVa-CT preferentially binds GTP-locked Rab8A. Full-length green fluorescent protein (GFP)–MyoVa colocalizes with mCherry-Rab8A in perinuclear small puncta, whereas GFP–MyoVa-CT collapses the GTPase into enlarged perinuclear depots. Further, GFP–MyoVa-CT blocks insulin-stimulated translocation of exofacially myc-tagged GLUT4 to the surface of muscle cells. Mutation of amino acids in MyoVa-CT predicted to bind Rab8A abrogates both interaction with Rab8A (not Rab10) and inhibition of insulin-stimulated GLUT4myc translocation. Of importance, small interfering RNA–mediated MyoVa silencing reduces insulin-stimulated GLUT4myc translocation. Rab8A colocalizes with GLUT4 in perinuclear but not submembrane regions visualized by confocal total internal reflection fluorescence microscopy. Hence insulin signaling to the molecular switch Rab8A connects with the motor protein MyoVa to mobilize GLUT4 vesicles toward the muscle cell plasma membrane.
The Ras-superfamily of small G proteins is a family of GTP hydrolases that is regulated by GTP/GDP binding states. One member of the Ras-superfamily, Rab, is involved in the regulation of vesicle trafficking, which is critical to endocytosis, biosynthesis, secretion, cell differentiation and cell growth. The active form of the Rab proteins, which contains GTP, can recruit specific binding partners, such as sorting adaptors, tethering factors, kinases, phosphatases and motor proteins, thereby influencing vesicle formation, transport, and tethering. Many Rab proteins share the same interacting partners and perform unique roles in specific locations. Because functional loss of the Rab pathways has been implicated in a variety of diseases, the Rab GTPase family has been extensively investigated. In this review, we summarize Rab GTPase- mediated membrane trafficking while focusing on the structures of Rab protein and Rab-effector complexes. This review provides detailed information that helps explain how the Rab GTPase family is involved in membrane trafficking.
membrane trafficking; ras-superfamily; small G protein; rab GTPase; protein structure
Chlamydia species are obligate intracellular bacteria that replicate within a membrane-bound vacuole, the inclusion, which is trafficked to the peri-Golgi region by processes that are dependent on early chlamydial gene expression. Although neither the host nor the chlamydial proteins that regulate the intracellular trafficking have been clearly defined, several enhanced green fluorescent protein (EGFP)-tagged Rab GTPases, including Rab6, are recruited to Chlamydia trachomatis inclusions. To further characterize the association of Rab6 with C. trachomatis inclusions, we examined the intracellular localization of guanine nucleotide-binding mutants of Rab6 and demonstrated that only active GTP-bound and not inactive GDP-bound EGFP-Rab6 mutants were recruited to the inclusion, suggesting that EGFP-Rab6 interacts with the inclusion via a host Rab6 effector or a chlamydial protein that mimics a Rab6 effector. Using EGFP-tagged fusion proteins, we also demonstrated that the Rab6 effector Bicaudal D1 (BICD1) localized to C. trachomatis inclusions in a biovar-specific manner. In addition, we demonstrated that EGFP-Rab6 and its effector EGFP-BICD1 are recruited to the inclusion in a microtubule- and Golgi apparatus-independent but chlamydial gene expression-dependent mechanism. Finally, in contrast to the Rab6-dependent Golgi apparatus localization of endogenous BICD1, EGFP-BICD1 was recruited to the inclusion by a Rab6-independent mechanism. Collectively, these data demonstrate that neither Rab6 nor BICD1 is trafficked to the inclusion via a Golgi apparatus-localized intermediate, suggesting that each protein is trafficked to the C. trachomatis serovar L2 inclusion by a unique, but as-yet-undefined, mechanism.
The small GTPase rab1a and its isoform rab1b are essential regulating components in the vesicle transport between the ER and the Golgi apparatus. Rab1 is thought to act as a molecular switch and can change between an active GTP-bound and an inactive GDP-bound conformation. To elucidate the function of rab1, several approaches have been established to isolate effector proteins, which interact with the activated conformation of rab1. To date p115, GM130, golgin-84 and MICAL have been identified as direct interacting partners. Together with rab1, these molecules are components of a protein complex, which mediates and regulates intracellular vesicle transport.
Here, we report the characterization of Iporin, which is similar to KIAA0375 as a novel rab1-interacting protein. It was initially identified by yeast two-hybrid screening experiments with the active mutant of rab1b (rab1b Q67R) as bait. Iporin contains a SH3 domain and two polyproline stretches, which are known to play a role in protein/protein interactions. In addition, Iporin encloses a RUN domain, which seems to be a major part of the rab1binding domain (R1BD). Iporin is ubiquitously expressed and immunofluorescence staining displays a cytosolic punctual distribution. Interestingly, we also show that Iporin interacts with another rab1 interacting partner, the GM130 protein.
Our results demonstrate that Iporin is a potential new interacting partner of rab1. Iporin is different from already identified rab1 interacting proteins concerning protein structure and cellular localization. We conclude that Iporin might function as a link between the targeting of ER derived vesicles, triggered by the rab1 GTPase and a signaling pathway regulated by molecules containing SH3 and/or poly-proline regions. The characterization of this novel intermolecular relation could help to elucidate how vesicles find their way from ER to the Golgi apparatus.
The RAB-5 and RAB-7 GTPases regulate endosome to lysosome trafficking. Here, we show that Caenorhabditis elegans TBC-2 functions as a RAB-5 GAP. TBC-2 colocalizes with RAB-7 on late endosomes, and requires RAB-7 for membrane localization where TBC-2 could function to antagonize RAB-5 activity during early to late endosome maturation.
During endosome maturation the early endosomal Rab5 GTPase is replaced with the late endosomal Rab7 GTPase. It has been proposed that active Rab5 can recruit and activate Rab7, which in turn could inactivate and remove Rab5. However, many of the Rab5 and Rab7 regulators that mediate endosome maturation are not known. Here, we identify Caenorhabditis elegans TBC-2, a conserved putative Rab GTPase-activating protein (GAP), as a regulator of endosome to lysosome trafficking in several tissues. We show that tbc-2 mutant animals accumulate enormous RAB-7–positive late endosomes in the intestine containing refractile material. RAB-5, RAB-7, and components of the homotypic fusion and vacuole protein sorting (HOPS) complex, a RAB-7 effector/putative guanine nucleotide exchange factor (GEF), are required for the tbc-2(−) intestinal phenotype. Expression of activated RAB-5 Q78L in the intestine phenocopies the tbc-2(−) large late endosome phenotype in a RAB-7 and HOPS complex-dependent manner. TBC-2 requires the catalytic arginine-finger for function in vivo and displays the strongest GAP activity on RAB-5 in vitro. However, TBC-2 colocalizes primarily with RAB-7 on late endosomes and requires RAB-7 for membrane localization. Our data suggest that TBC-2 functions on late endosomes to inactivate RAB-5 during endosome maturation.
A systematic screen of the entire human Rab GTPase family for interactions with myosin Va identified 10 novel Rab partners for myosin Va, all of which belong to the endocytic recycling and post-Golgi secretory membrane network. However, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes.
Myosin Va is a widely expressed actin-based motor protein that binds members of the Rab GTPase family (3A, 8A, 10, 11A, 27A) and is implicated in many intracellular trafficking processes. To our knowledge, myosin Va has not been tested in a systematic screen for interactions with the entire Rab GTPase family. To that end, we report a yeast two-hybrid screen of all human Rabs for myosin Va-binding ability and reveal 10 novel interactions (3B, 3C, 3D, 6A, 6A′, 6B, 11B, 14, 25, 39B), which include interactions with three new Rab subfamilies (Rab6, Rab14, Rab39B). Of interest, myosin Va interacts with only a subset of the Rabs associated with the endocytic recycling and post-Golgi secretory systems. We demonstrate that myosin Va has three distinct Rab-binding domains on disparate regions of the motor (central stalk, an alternatively spliced exon, and the globular tail). Although the total pool of myosin Va is shared by several Rabs, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes. We also present evidence that myosin Va is necessary for maintaining a peripheral distribution of Rab11- and Rab14-positive endosomes.