Reconstitution of Rab- and SNARE-dependent membrane fusion by synthetic endosomes

Local changes in protein filament properties drive large-scale membrane transformations involved in endosome tethering and fusion

Large-scale cellular transformations are triggered by subtle physical and structural changes to individual biomacromolecular and membrane components. A prototypical example of such an event is the orchestrated fusion of membranes within an endosome that enables transport of cargo and processing of biochemical moieties. In this work, we demonstrate how protein filaments on the endosomal membrane surface can leverage a rigid-to-flexible transformation to elicit a large-scale change in membrane flexibility to enable membrane fusion. We develop a polymer field-theoretic model that captures molecular alignment arising from nematic interactions with varying surface density and fraction of flexible filaments, which are biologically controlled within the endosomal membrane. We then predict the collective elasticity of the filament brush in response to changes in the filament alignment, predicting a greater than 20-fold increase of the effective membrane elasticity over the bare membrane elasticity that is triggered by filament alignment. These results show that the endosome can modulate the filament properties to orchestrate membrane fluidization that facilitates vesicle fusion, providing an example of how active processes that modulate local molecular properties can result in large-scale transformations that are essential to cellular survival.

Function and regulation of Rab GTPases in cancers

The Rab small GTPases are characterized by the distinct intracellular localization and modulate various endocytic, transcytic and exocytic transport pathways. Rab proteins function as scaffolds that connect signaling pathways and intracellular membrane trafficking processes through the recruitment of effectors, such as tethering factors, phosphatases, motors and kinases. In different cancers, Rabs play as either an onco-protein or a tumor suppressor role, highly dependending on the context. The molecular mechanistic research has revealed that Rab proteins are involved in cancer progression through influences on migration, invasion, metabolism, exosome secretion, autophagy, and drug resistance of cancer cells. Therefore, targeting Rab GTPases to recover the dysregulated vesicle transport systems may provide potential strategy to restrain cancer progression. In this review, we discuss the regulation of Rab protein level and activity in modulating pathways involved in tumor progression, and propose that Rab proteins may serve as a prognostic factor in different cancers.

Rab GTPases and phosphoinositides fine-tune SNAREs dependent targeting specificity of intracellular vesicle traffic

In the secretory pathway the destination of trafficking vesicles is determined by specific proteins that, with the notable exception of SNAREs, are recruited from soluble pools. Previously we have shown that microinjected proteoliposomes containing early or late endosomal SNAREs, respectively, are targeted to the corresponding endogenous compartments, with targeting specificity being dependent on the recruitment of tethering factors by some of the SNAREs. Here, we show that targeting of SNARE-containing liposomes is refined upon inclusion of polyphosphoinositides and Rab5. Intriguingly, targeting specificity is dependent on the concentration of PtdIns(3)P, and on the recruitment of PtdIns(3)P binding proteins such as rabenosyn-5 and PIKfyve, with conversion of PtdIns(3)P into PtdIns(3,5)P2 re-routing the liposomes towards late endosomes despite the presence of GTP-Rab5 and early endosomal SNAREs. Our data reveal a complex interplay between permissive and inhibitory targeting signals that sharpen a basic targeting and fusion machinery for conveying selectivity in intracellular membrane traffic.

Coordination of RAB-8 and RAB-11 during unconventional protein secretion

Multiple physiology-pertinent transmembrane proteins reach the cell surface via the Golgi-bypassing unconventional protein secretion (UcPS) pathway. By employing C. elegans-polarized intestine epithelia, we recently have revealed that the small GTPase RAB-8/Rab8 serves as an important player in the process. Nonetheless, its function and the relevant UcPS itinerary remain poorly understood. Here, we show that deregulated RAB-8 activity resulted in impaired apical UcPS, which increased sensitivity to infection and environmental stress. We also identified the SNARE VTI-1/Vti1a/b as a new RAB-8-interacting factor involved in the apical UcPS. Besides, RAB-11/Rab11 was capable of recruiting RABI-8/Rabin8 to reduce the guanine nucleotide exchange activity of SMGL-1/GEF toward RAB-8, indicating the necessity of a finely tuned RAB-8/RAB-11 network. Populations of RAB-8- and RAB-11-positive endosomal structures containing the apical UcPS cargo moved toward the apical side. In the absence of RAB-11 or its effectors, the cargo was retained in RAB-8- and RAB-11-positive endosomes, respectively, suggesting that these endosomes are utilized as intermediate carriers for the UcPS.

StRAB4 gene is required for filamentous growth, conidial development, and pathogenicity in Setosphaeria turcica

Setosphaeria turcica, the fungal pathogen responsible for northern corn leaf blight in maize, forms specialized infectious structures called appressoria that are critical for fungal penetration of maize epidermal cells. The Rab family of proteins play a crucial role in the growth, development, and pathogenesis of many eukaryotic species. Rab4, in particular, is a key regulator of endocytosis and vesicle trafficking, essential for filamentous growth and successful infection by other fungal pathogens. In this study, we silenced StRAB4 in S. turcica to gain a better understanding the function of Rab4 in this plant pathogen. Phenotypically, the mutants exhibited a reduced growth rate, a significant decline in conidia production, and an abnormal conidial morphology. These phenotypes indicate that StRab4 plays an instrumental role in regulating mycelial growth and conidial development in S. turcica. Further investigations revealed that StRab4 is a positive regulator of cell wall integrity and melanin secretion. Functional enrichment analysis of differentially expressed genes highlighted primary enrichments in peroxisome pathways, oxidoreductase and catalytic activities, membrane components, and cell wall organization processes. Collectively, our findings emphasize the significant role of StRab4 in S. turcica infection and pathogenicity in maize and provide valuable insights into fungal behavior and disease mechanisms.

Orchestrating vesicular and nonvesicular membrane dynamics by intrinsically disordered proteins

Compartmentalization by membranes is a common feature of eukaryotic cells and serves to spatiotemporally confine biochemical reactions to control physiology. Membrane-bound organelles such as the endoplasmic reticulum (ER), the Golgi complex, endosomes and lysosomes, and the plasma membrane, continuously exchange material via vesicular carriers. In addition to vesicular trafficking entailing budding, fission, and fusion processes, organelles can form membrane contact sites (MCSs) that enable the nonvesicular exchange of lipids, ions, and metabolites, or the secretion of neurotransmitters via subsequent membrane fusion. Recent data suggest that biomolecule and information transfer via vesicular carriers and via MCSs share common organizational principles and are often mediated by proteins with intrinsically disordered regions (IDRs). Intrinsically disordered proteins (IDPs) can assemble via low-affinity, multivalent interactions to facilitate membrane tethering, deformation, fission, or fusion. Here, we review our current understanding of how IDPs drive the formation of multivalent protein assemblies and protein condensates to orchestrate vesicular and nonvesicular transport with a special focus on presynaptic neurotransmission. We further discuss how dysfunction of IDPs causes disease and outline perspectives for future research.

Sec22b and Stx4 Depletion Has No Major Effect on Cross-Presentation of PLGA Microsphere-Encapsulated Antigen and a Synthetic Long Peptide In Vitro

The induction of CTL responses by vaccines is important to combat infectious diseases and cancer. Biodegradable poly(lactic-co-glycolic acid) (PLGA) microspheres and synthetic long peptides are efficiently internalized by professional APCs and prime CTL responses after cross-presentation of Ags on MHC class I molecules. Specifically, they mainly use the cytosolic pathway of cross-presentation that requires endosomal escape, proteasomal processing, and subsequent MHC class I loading of Ags in the endoplasmic reticulum (ER) and/or the endosome. The vesicle SNARE protein Sec22b has been described as important for this pathway by mediating vesical trafficking for the delivery of ER-derived proteins to the endosome. As this function has also been challenged, we investigated the role of Sec22b in cross-presentation of the PLGA microsphere-encapsulated model Ag OVA and a related synthetic long peptide. Using CRISPR/Cas9-mediated genome editing, we generated Sec22b knockouts in two murine C57BL/6-derived APC lines and found no evidence for an essential role of Sec22b. Although pending experimental evidence, the target SNARE protein syntaxin 4 (Stx4) has been suggested to promote cross-presentation by interacting with Sec22b for the fusion of ER-derived vesicles with the endosome. In the current study, we show that, similar to Sec22b, Stx4 knockout in murine APCs had very limited effects on cross-presentation under the conditions tested. This study contributes to characterizing cross-presentation of two promising Ag delivery systems and adds to the discussion about the role of Sec22b/Stx4 in related pathways. Our data point toward SNARE protein redundancy in the cytosolic pathway of cross-presentation.

W546 stacking disruption traps the human porphyrin transporter ABCB6 in an outward-facing transient state

Human ATP-binding cassette transporter subfamily B6 (ABCB6) is a mitochondrial ATP-driven pump that translocates porphyrins from the cytoplasm into mitochondria for heme biosynthesis. Within the transport pathway, a conserved aromatic residue W546 located in each monomer plays a pivotal role in stabilizing the occluded conformation via π-stacking interactions. Herein, we employed cryo-electron microscopy to investigate the structural consequences of a single W546A mutation in ABCB6, both in detergent micelles and nanodiscs. The results demonstrate that the W546A mutation alters the conformational dynamics of detergent-purified ABCB6, leading to entrapment of the transporter in an outward-facing transient state. However, in the nanodisc system, we observed a direct interaction between the transporter and a phospholipid molecule that compensates for the absence of the W546 residue, thereby facilitating the normal conformational transition of the transporter toward the occluded state following ATP hydrolysis. The findings also reveal that adoption of the outward-facing conformation causes charge repulsion between ABCB6 and the bound substrate, and rearrangement of key interacting residues at the substrate-binding site. Consequently, the affinity for the substrate is significantly reduced, facilitating its release from the transporter.

Membrane tethers at a glance

Cargo delivery from one compartment to the next relies on the fusion of vesicles with different cellular organelles in a process that requires the concerted action of tethering factors. Although all tethers act to bridge vesicle membranes to mediate fusion, they form very diverse groups as they differ in composition, and in their overall architecture and size, as well as their protein interactome. However, their conserved function relies on a common design. Recent data on class C Vps complexes indicates that tethers play a significant role in membrane fusion beyond vesicle capturing. Furthermore, these studies provide additional mechanistic insights into membrane fusion events and reveal that tethers should be considered as key players of the fusion machinery. Moreover, the discovery of the novel tether FERARI complex has changed our understanding of cargo transport in the endosomal system as it has been shown to mediate 'kiss-and-run' vesicle-target membrane interactions. In this Cell Science at a Glance and the accompanying poster, we compare the structure of the coiled-coil and the multisubunit CATCHR and class C Vps tether families on the basis of their functional analogy. We discuss the mechanism of membrane fusion, and summarize how tethers capture vesicles, mediate membrane fusion at different cellular compartments and regulate cargo traffic.

Rab GTPases, tethers, and SNAREs work together to regulate Arabidopsis cell plate formation

Cell plates are transient structures formed by the fusion of vesicles at the center of the dividing plane; furthermore, these are precursors to new cell walls and are essential for cytokinesis. Cell plate formation requires a highly coordinated process of cytoskeletal rearrangement, vesicle accumulation and fusion, and membrane maturation. Tethering factors have been shown to interact with the Ras superfamily of small GTP binding proteins (Rab GTPases) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are essential for cell plate formation during cytokinesis and are fundamental for maintaining normal plant growth and development. In Arabidopsis thaliana, members of the Rab GTPases, tethers, and SNAREs are localized in cell plates, and mutations in the genes encoding these proteins result in typical cytokinesis-defective phenotypes, such as the formation of abnormal cell plates, multinucleated cells, and incomplete cell walls. This review highlights recent findings on vesicle trafficking during cell plate formation mediated by Rab GTPases, tethers, and SNAREs.

The function of VAMP2 in mediating membrane fusion: An overview

Vesicle-associated membrane protein 2 (VAMP2, also known as synaptobrevin-2), encoded by VAMP2 in humans, is a key component of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. VAMP2 combined with syntaxin-1A (SYX-1A) and synaptosome-associated protein 25 (SNAP-25) produces a force that induces the formation of fusion pores, thereby mediating the fusion of synaptic vesicles and the release of neurotransmitters. VAMP2 is largely unstructured in the absence of interaction partners. Upon interaction with other SNAREs, the structure of VAMP2 stabilizes, resulting in the formation of four structural domains. In this review, we highlight the current knowledge of the roles of the VAMP2 domains and the interaction between VAMP2 and various fusion-related proteins in the presynaptic cytoplasm during the fusion process. Our summary will contribute to a better understanding of the roles of the VAMP2 protein in membrane fusion.

Molecular insights into endolysosomal microcompartment formation and maintenance

The endolysosomal system of eukaryotic cells has a key role in the homeostasis of the plasma membrane, in signaling and nutrient uptake, and is abused by viruses and pathogens for entry. Endocytosis of plasma membrane proteins results in vesicles, which fuse with the early endosome. If destined for lysosomal degradation, these proteins are packaged into intraluminal vesicles, converting an early endosome to a late endosome, which finally fuses with the lysosome. Each of these organelles has a unique membrane surface composition, which can form segmented membrane microcompartments by membrane contact sites or fission proteins. Furthermore, these organelles are in continuous exchange due to fission and fusion events. The underlying machinery, which maintains organelle identity along the pathway, is regulated by signaling processes. Here, we will focus on the Rab5 and Rab7 GTPases of early and late endosomes. As molecular switches, Rabs depend on activating guanine nucleotide exchange factors (GEFs). Over the last years, we characterized the Rab7 GEF, the Mon1-Ccz1 (MC1) complex, and key Rab7 effectors, the HOPS complex and retromer. Structural and functional analyses of these complexes lead to a molecular understanding of their function in the context of organelle biogenesis.

Subcellular trafficking and post-translational modification regulate PIN polarity in plants

Auxin regulates plant growth and tropism responses. As a phytohormone, auxin is transported between its synthesis sites and action sites. Most natural auxin moves between cells via a polar transport system that is mediated by PIN-FORMED (PIN) auxin exporters. The asymmetrically localized PINs usually determine the directionality of intercellular auxin flow. Different internal cues and external stimuli modulate PIN polar distribution and activity at multiple levels, including transcription, protein stability, subcellular trafficking, and post-translational modification, and thereby regulate auxin-distribution-dependent development. Thus, the different regulation levels of PIN polarity constitute a complex network. For example, the post-translational modification of PINs can affect the subcellular trafficking of PINs. In this review, we focus on subcellular trafficking and post-translational modification of PINs to summarize recent progress in understanding PIN polarity.

Non-coding RNAs and macrophage interaction in tumor progression

The macrophages are abundantly found in TME and their M2 polarization is in favor of tumor malignancy. On the other hand, non-coding RNAs (ncRNAs) can modulate macrophage polarization in TME to affect cancer progression. The miRNAs can dually induce/suppress M2 polarization of macrophages and by affecting various molecular pathways, they modulate tumor progression and therapy response. The lncRNAs can affect miRNAs via sponging and other molecular pathways to modulate macrophage polarization. A few experiments have also examined role of circRNAs in targeting signaling networks and affecting macrophages. The therapeutic targeting of these ncRNAs can mediate TME remodeling and affect macrophage polarization. Furthermore, exosomal ncRNAs derived from tumor cells or macrophages can modulate polarization and TME remodeling. Suppressing biogenesis and secretion of exosomes can inhibit ncRNA-mediated M2 polarization of macrophages and prevent tumor progression. The ncRNAs, especially exosomal ncRNAs can be considered as non-invasive biomarkers for tumor diagnosis.

Calcium Enabled Remote Loading of a Weak Acid Into pH-sensitive Liposomes and Augmented Cytosolic Delivery to Cancer Cells via the Proton Sponge Effect

While delivery of chemotherapeutics to cancer cells by nanomedicines can improve therapeutic outcomes, many fail due to the low drug loading (DL), poor cellular uptake and endosomal entrapment. This study investigated the potential to overcome these limitations using pH-sensitive liposomes (PSL) empowered by the use of calcium acetate. An acidic dinitrobenzamide mustard prodrug SN25860 was used as a model drug, with non pH-sensitive liposomes (NPSL) as a reference. Calcium acetate as a remote loading agent allowed to engineer PSL- and NPSL-SN25860 with DL of > 31.1% (w/w). The IC₅₀ of PSL-SN25860 was 21- and 141-fold lower than NPSL and free drug, respectively. At 48 h following injection of PSL-SN25860, NPSL-SN25860 and the free drug, drug concentrations in EMT6-nfsB murine breast tumors were 56.3 µg/g, 6.76 µg/g and undetectable (< 0.015 µg/g), respectively (n = 3). Meanwhile, the ex vivo tumor clonogenic assay showed 9.1%, 19.4% and 42.7% cell survival in the respective tumors. Live-cell imaging and co-localization analysis suggested endosomal escape was accomplished by destabilization of PSL followed by release of Ca²⁺ in endosomes allowing induction of a proton sponge effect. Subsequent endosomal rupture was observed approximately 30 min following endocytosis of PSL containing Ca²⁺. Additionally, calcium in liposomes promoted internalization of both PSL and NPSL. Taken together, this study demonstrated multifaceted functions of calcium acetate in promoting drug loading into liposomes, cellular uptake, and endosomal escape of PSL for efficient cytoplasmic drug delivery. The results shed light on designing nano-platforms for cytoplasmic delivery of various therapeutics.

Quantitative correlative microscopy reveals the ultrastructural distribution of endogenous endosomal proteins

The key endosomal regulators Rab5, EEA1, and APPL1 are frequently applied in fluorescence microscopy to mark early endosomes, whereas Rab7 is used as a marker for late endosomes and lysosomes. However, endogenous levels of these proteins localize poorly in immuno-EM, and systematic studies on their native ultrastructural distributions are lacking. To address this gap, we here present a quantitative, on-section correlative light and electron microscopy (CLEM) approach. Using the sensitivity of fluorescence microscopy, we label hundreds of organelles that are subsequently visualized by EM and classified by ultrastructure. We show that Rab5 predominantly marks small, endocytic vesicles and early endosomes. EEA1 colocalizes with Rab5 on early endosomes, but unexpectedly also labels Rab5-negative late endosomes, which are positive for PI(3)P but lack Rab7. APPL1 is restricted to small Rab5-positive, tubulo-vesicular profiles. Rab7 primarily labels late endosomes and lysosomes. These data increase our understanding of the structural-functional organization of the endosomal system and introduce quantitative CLEM as a sensitive alternative for immuno-EM.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.

Exosomes in triple negative breast cancer: From bench to bedside

Exosomes are lipid bilayer extracellular vesicles with a size of 30-150 nm, which can be released by various types of cells including breast cancer cells. Exosomes are enriched with multiple nucleic acids, lipids, proteins and play critical biological roles by binding to recipient cells and transmitting various biological cargos. Studies have reported that tumor-derived exosomes are involved in cancer initiation and progression, such as promoting cancer invasion and metastasis, accelerating angiogenesis, contributing to epithelial-mesenchymal transition, and enhancing drug resistance in tumors. Recently the dysregulating of exosomes has been found in triple-negative breast cancer (TNBC), relating to the clinicopathological characteristics and prognosis of TNBC patients. Considering the poor prognosis and lack of adequate response to conventional therapy of TNBC, the discovery of certain exosomes as a new target for diagnosis and treatment of TNBC may be a good choice that provides new opportunities for the early diagnosis, clinical treatment of TNBC. Here, we first discuss the innovative prognostic and predictive effects of exosomes on TNBC, as well as the practical clinical problems. Secondly, we focus on the new therapeutic areas represented by exosomes, especially the impact of introducing exosomes in TNBC treatment in the future.

Who's in control? Principles of Rab GTPase activation in endolysosomal membrane trafficking and beyond

The eukaryotic endomembrane system consists of multiple interconnected organelles. Rab GTPases are organelle-specific markers that give identity to these membranes by recruiting transport and trafficking proteins. During transport processes or along organelle maturation, one Rab is replaced by another, a process termed Rab cascade, which requires at its center a Rab-specific guanine nucleotide exchange factor (GEF). The endolysosomal system serves here as a prime example for a Rab cascade. Along with endosomal maturation, the endosomal Rab5 recruits and activates the Rab7-specific GEF Mon1-Ccz1, resulting in Rab7 activation on endosomes and subsequent fusion of endosomes with lysosomes. In this review, we focus on the current idea of Mon1-Ccz1 recruitment and activation in the endolysosomal and autophagic pathway. We compare identified principles to other GTPase cascades on endomembranes, highlight the importance of regulation, and evaluate in this context the strength and relevance of recent developments in in vitro analyses to understand the underlying foundation of organelle biogenesis and maturation.

Mammalian Neuronal mRNA Transport Complexes: The Few Knowns and the Many Unknowns

Hundreds of messenger RNAs (mRNAs) are transported into neurites to provide templates for the assembly of local protein networks. These networks enable a neuron to configure different cellular domains for specialized functions. According to current evidence, mRNAs are mostly transported in rather small packages of one to three copies, rarely containing different transcripts. This opens up fascinating logistic problems: how are hundreds of different mRNA cargoes sorted into distinct packages and how are they coupled to and released from motor proteins to produce the observed mRNA distributions? Are all mRNAs transported by the same transport machinery, or are there different adaptors or motors for different transcripts or classes of mRNAs? A variety of often indirect evidence exists for the involvement of proteins in mRNA localization, but relatively little is known about the essential activities required for the actual transport process. Here, we summarize the different types of available evidence for interactions that connect mammalian mRNAs to motor proteins to highlight at which point further research is needed to uncover critical missing links. We further argue that a combination of discovery approaches reporting direct interactions, in vitro reconstitution, and fast perturbations in cells is an ideal future strategy to unravel essential interactions and specific functions of proteins in mRNA transport processes.

Sec17/Sec18 can support membrane fusion without help from completion of SNARE zippering

Membrane fusion requires R-, Qa-, Qb-, and Qc-family SNAREs that zipper into RQaQbQc coiled coils, driven by the sequestration of apolar amino acids. Zippering has been thought to provide all the force driving fusion. Sec17/αSNAP can form an oligomeric assembly with SNAREs with the Sec17 C-terminus bound to Sec18/NSF, the central region bound to SNAREs, and a crucial apolar loop near the N-terminus poised to insert into membranes. We now report that Sec17 and Sec18 can drive robust fusion without requiring zippering completion. Zippering-driven fusion is blocked by deleting the C-terminal quarter of any Q-SNARE domain or by replacing the apolar amino acids of the Qa-SNARE that face the center of the 4-SNARE coiled coils with polar residues. These blocks, singly or combined, are bypassed by Sec17 and Sec18, and SNARE-dependent fusion is restored without help from completing zippering.

The role of host cell Rab GTPases in influenza A virus infections

Influenza A virus (IAV) is a crucial cause of respiratory infections in humans worldwide. Therefore, studies should clarify adaptation mechanisms of IAV and critical factors of the viral pathogenesis in human hosts. GTPases of the Rab family are the largest branch of the Ras-like small GTPase superfamily, and they regulate almost every step during vesicle-mediated trafficking. Evidence has shown that Rab proteins participate in the lifecycle of IAV. In this mini-review, we outline the regulatory mechanisms of different Rab proteins in the lifecycle of IAV. Understanding the role of Rab proteins in IAV infections is important to develop broad-spectrum host-targeted antiviral strategies.

The Arabidopsis R-SNARE VAMP714 is essential for polarisation of PIN proteins and auxin responses

The plant hormone auxin and its directional intercellular transport play a major role in diverse aspects of plant growth and development. The establishment of auxin gradients requires the asymmetric distribution of members of the auxin efflux carrier PIN-FORMED (PIN) protein family to the plasma membrane. An endocytic pathway regulates the recycling of PIN proteins between the plasma membrane and endosomes, providing a mechanism for dynamic localisation. N-Ethylmaleimide-sensitive factor adaptor protein receptors (SNAP receptors, SNAREs) mediate fusion between vesicles and target membranes and are classed as Q- or R-SNAREs based on their sequence. We analysed gain- and loss-of-function mutants, dominant-negative transgenics and localisation of the Arabidopsis R-SNARE VAMP714 protein to understand its function. We demonstrate that VAMP714 is essential for the insertion of PINs into the plasma membrane, for polar auxin transport, root gravitropism and morphogenesis. VAMP714 gene expression is upregulated by auxin, and the VAMP714 protein co-localises with endoplasmic reticulum and Golgi vesicles and with PIN proteins at the plasma membrane. It is proposed that VAMP714 mediates the delivery of PIN-carrying vesicles to the plasma membrane, and that this forms part of a positive regulatory loop in which auxin activates a VAMP714-dependent PIN/auxin transport system to control development.

Legionella hijacks the host Golgi-to-ER retrograde pathway for the association of Legionella-containing vacuole with the ER

Legionella pneumophila (L. pneumophila) is a gram-negative bacterium that replicates in a compartment that resembles the host endoplasmic reticulum (ER). To create its replicative niche, L. pneumophila manipulates host membrane traffic and fusion machineries. Bacterial proteins called Legionella effectors are translocated into the host cytosol and play a crucial role in these processes. In an early stage of infection, Legionella subverts ER-derived vesicles (ERDVs) by manipulating GTPase Rab1 to facilitate remodeling of the Legionella-containing vacuole (LCV). Subsequently, the LCV associates with the ER in a mechanism that remains elusive. In this study, we show that L. pneumophila recruits GTPases Rab33B and Rab6A, which regulate vesicle trafficking from the Golgi to the ER, to the LCV to promote the association of LCV with the ER. We found that recruitment of Rab6A to the LCV depends on Rab33B. Legionella effector SidE family proteins, which phosphoribosyl-ubiquitinate Rab33B, were found to be necessary for the recruitment of Rab33B to the LCV. Immunoprecipitation experiments revealed that L. pneumophila facilitates the interaction of Rab6 with ER-resident SNAREs comprising syntaxin 18, p31, and BNIP1, but not tethering factors including NAG, RINT-1, and ZW10, which are normally required for syntaxin 18-mediated fusion of Golgi-derived vesicles with the ER. Our results identified a Rab33B-Rab6A cascade on the LCV and the interaction of Rab6 with ER-resident SNARE proteins for the association of LCV with the ER and disclosed the unidentified physiological role of SidE family proteins.

Legionella pneumophila causes a sever pneumonia called Legionnaires’ disease and a threat of this disease has increased on a world-wide scale. As a feature of L. pneumophila, it secrets over 300 bacterial effectors to adapt and survive inside the host and many of effectors modify the host proteins in a unique manner. L. pneumophila is known to travel inside the host and final destination of this pathogens is the host ER. In the initial step of this travel, L. pneumophila subverts host early vesicular trafficking to remodel the membrane composition of Legionella-containing vacuole (LCV). Although this remodeling process has been well characterized, the molecular machinery of association of remodeled vacuoles with the ER is still obscure. This paper shows that the host GTPases Rab6A and Rab33B, both of which control Golgi-to-ER traffic, are recruited to the LCV in a cascade manner and are required for the association of LCVs with the ER through the interaction between Rab6A and ER-resident t-SNARE proteins. Of note, we demonstrate that a bacteria-specific Rab33B modification called phosphoribosyl-ubiquitination by Legionella effectors proteins of the SidE family is essential for the recruitment of Rab33B to the LCV.

Di-arginine and FFAT-like motifs retain a subpopulation of PRA1 at ER-mitochondria membrane contact sites

Prenylated Rab Acceptor 1 (PRA1/Rabac1) is a four-pass transmembrane protein that has been found to localize to the Golgi and promiscuously associate with a diverse array of Rab GTPases. We have previously identified PRA1 to be among the earliest significantly down-regulated genes in the rd1 mouse model of retinitis pigmentosa, a retinal degenerative disease. Here, we show that an endogenous subpopulation of PRA1 resides within the endoplasmic reticulum (ER) at ER-mitochondria membrane contact sites in cultured mammalian cells. We also demonstrate that PRA1 contains two previously unidentified ER retention/retrieval amino acid sequences on its cytosolic N-terminal region: a membrane distal di-arginine motif and a novel membrane proximal FFAT-like motif. Using a truncation construct that lacks complete Golgi targeting information, we show that mutation of either motif leads to an increase in cell surface localization, while mutation of both motifs exhibits an additive effect. We also present evidence that illustrates that N- or C- terminal addition of a tag to full-length PRA1 leads to differential localization to either the Golgi or reticular ER, phenotypes that do not completely mirror endogenous protein localization. The presence of multiple ER retention motifs on the PRA1 N-terminal region further suggests that it has a functional role within the ER.

FgVps9, a Rab5 GEF, Is Critical for DON Biosynthesis and Pathogenicity in Fusarium graminearum

Rab GTPases play an important role in vesicle-mediated membrane trafficking in eukaryotes. Previous studies have demonstrated that deletion of RAB5/VPS21 reduces endocytosis and virulence of fungal phytopathogens in their host plants. However, Rab5 GTPase cycle regulators have not been characterized in Fusarium graminearum, the causal agent of Fusarium head blight (FHB) or head scab disease in cereal crops. In this study, we have identified and characterized a Rab5 guanine nucleotide exchange factor (GEF), the Vps9 homolog FgVps9, in F. graminearum. Yeast two hybrid (Y2H) assays have shown that FgVps9 specifically interacts with the guanosine diphosphate (GDP)-bound (inactive) forms of FgRab51 and FgRab52, the Rab5 isoforms in F. graminearum. Deletion of FgVPS9 shows impaired fungal growth and conidiation. Pathogenicity assays indicate that deletion of FgVPS9 can significantly decrease the virulence of F. graminearum in wheat. Cytological analyses have indicated that FgVps9 colocalizes with FgRab51 and FgRab52 on early endosomes and regulates endocytosis and autophagy processes. Gene expression and cytological examination have shown that FgVps9 and FgRab51 or FgRab52 function in concert to control deoxynivalenol (DON) biosynthesis by regulating the expression of trichothecene biosynthesis-related genes and toxisome biogenesis. Taken together, FgVps9 functions as a GEF for FgRab51 and FgRab52 to regulate endocytosis, which, as a basic cellular function, has significant impact on the vegetative growth, asexual development, autophagy, DON production, and plant infection in F. graminearum.

Stochastic activation and bistability in a Rab GTPase regulatory network

The eukaryotic endomembrane system is controlled by small GTPases of the Rab family, which are activated at defined times and locations in a switch-like manner. While this switch is well understood for an individual protein, how regulatory networks produce intracellular activity patterns is currently not known. Here, we combine in vitro reconstitution experiments with computational modeling to study a minimal Rab5 activation network. We find that the molecular interactions in this system give rise to a positive feedback and bistable collective switching of Rab5. Furthermore, we find that switching near the critical point is intrinsically stochastic and provide evidence that controlling the inactive population of Rab5 on the membrane can shape the network response. Notably, we demonstrate that collective switching can spread on the membrane surface as a traveling wave of Rab5 activation. Together, our findings reveal how biochemical signaling networks control vesicle trafficking pathways and how their nonequilibrium properties define the spatiotemporal organization of the cell.

Asymmetric Rab activation of vacuolar HOPS to catalyze SNARE complex assembly

Intracellular membrane fusion requires Rab-family GTPases, their effector tethers, soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, and SNARE chaperones of the Sec1/Munc18 (SM), Sec17/α-SNAP, and Sec18/NSF families. We have developed an assay using fluorescence resonance energy transfer to measure SNARE complex formation in real time. We now show that yeast vacuolar SNAREs assemble spontaneously into RQaQbQc complexes when the R- and Qa-SNAREs are concentrated in the same micelles or in cis on the same membrane. When SNAREs are free in solution or are tethered to distinct membranes, assembly requires catalysis by HOPS, the vacuolar SM and tethering complex. The Rab Ypt7 and vacuole lipids together allosterically activate the bound HOPS for catalyzing SNARE assembly, even if none of the SNAREs are membrane bound. HOPS-dependent fusion between proteoliposomes bearing R- or Qa-SNAREs shows a strict requirement for Ypt7 on the R-SNARE proteoliposomes but not on the Qa-SNARE proteoliposomes. This asymmetry is reflected in the strikingly different capacity of Ypt7 in cis to either the R- or Qa-SNARE to stimulate SNARE complex assembly. Membrane-bound Ypt7 activates HOPS to catalyze 4-SNARE complex assembly when it is on the same membrane as the R-SNARE but not the Qa-SNARE, thus explaining the asymmetric need for Ypt7 for fusion.

Intracellular Vesicle Fusion Requires a Membrane-Destabilizing Peptide Located at the Juxtamembrane Region of the v-SNARE

Intracellular vesicle fusion is mediated by soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) and Sec1/Munc18 (SM) proteins. It is generally accepted that membrane fusion occurs when the vesicle and target membranes are brought into close proximity by SNAREs and SM proteins. In this work, we demonstrate that, for fusion to occur, membrane bilayers must be destabilized by a conserved membrane-embedded motif located at the juxtamembrane region of the vesicle-anchored v-SNARE. Comprised of basic and hydrophobic residues, the juxtamembrane motif perturbs the lipid bilayer structure and promotes SNARE-SM-mediated membrane fusion. The juxtamembrane motif can be functionally substituted with an unrelated membrane-disrupting peptide in the membrane fusion reaction. These findings establish the juxtamembrane motif of the v-SNARE as a membrane-destabilizing peptide. Requirement of membrane-destabilizing peptides is likely a common feature of biological membrane fusion.

Exosomal long non-coding RNAs: biological properties and therapeutic potential in cancer treatment

Exosomes and long non-coding RNAs (lncRNAs) are emerging as important elements contributing to a more comprehensive understanding of cancer development and progression. The discovery of lncRNAs in exosomes further indicates their bona fide biological functional roles in cancer development and drug resistance. In this review, we describe the biogenesis of exosomes and summarize the function of exosomal lncRNAs in the field of cancer research. These findings strikingly advance current knowledge of exosomal lncRNAs and suggest that they may be promising diagnostic biomarkers and therapeutic targets for cancer.

Toward long-lasting artificial cells that better mimic natural living cells

Chemical communication is ubiquitous in biology, and so efforts in building convincing cellular mimics must consider how cells behave on a population level. Simple model systems have been built in the laboratory that show communication between different artificial cells and artificial cells with natural, living cells. Examples include artificial cells that depend on purely abiological components and artificial cells built from biological components and are driven by biological mechanisms. However, an artificial cell solely built to communicate chemically without carrying the machinery needed for self-preservation cannot remain active for long periods of time. What is needed is to begin integrating the pathways required for chemical communication with metabolic-like chemistry so that robust artificial systems can be built that better inform biology and aid in the generation of new technologies.

The NAE Pathway: Autobahn to the Nucleus for Cell Surface Receptors

Various growth factors and full-length cell surface receptors such as EGFR are translocated from the cell surface to the nucleoplasm, baffling cell biologists to the mechanisms and functions of this process. Elevated levels of nuclear EGFR correlate with poor prognosis in various cancers. In recent years, nuclear EGFR has been implicated in regulating gene transcription, cell proliferation and DNA damage repair. Different models have been proposed to explain how the receptors are transported into the nucleus. However, a clear consensus has yet to be reached. Recently, we described the nuclear envelope associated endosomes (NAE) pathway, which delivers EGFR from the cell surface to the nucleus. This pathway involves transport, docking and fusion of NAEs with the outer membrane of the nuclear envelope. EGFR is then presumed to be transported through the nuclear pore complex, extracted from membranes and solubilised. The SUN1/2 nuclear envelope proteins, Importin-beta, nuclear pore complex proteins and the Sec61 translocon have been implicated in the process. While this framework can explain the cell surface to nucleus traffic of EGFR and other cell surface receptors, it raises several questions that we consider in this review, together with implications for health and disease.

CORVET, CHEVI and HOPS – multisubunit tethers of the endo-lysosomal system in health and disease

Multisubunit tethering complexes (MTCs) are multitasking hubs that form a link between membrane fusion, organelle motility and signaling. CORVET, CHEVI and HOPS are MTCs of the endo-lysosomal system. They regulate the major membrane flows required for endocytosis, lysosome biogenesis, autophagy and phagocytosis. In addition, individual subunits control complex-independent transport of specific cargoes and exert functions beyond tethering, such as attachment to microtubules and SNARE activation. Mutations in CHEVI subunits lead to arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, while defects in CORVET and, particularly, HOPS are associated with neurodegeneration, pigmentation disorders, liver malfunction and various forms of cancer. Diseases and phenotypes, however, vary per affected subunit and a concise overview of MTC protein function and associated human pathologies is currently lacking. Here, we provide an integrated overview on the cellular functions and pathological defects associated with CORVET, CHEVI or HOPS proteins, both with regard to their complexes and as individual subunits. The combination of these data provides novel insights into how mutations in endo-lysosomal proteins lead to human pathologies.

SNAREs define targeting specificity of trafficking vesicles by combinatorial interaction with tethering factors

Membrane traffic operates by vesicles that bud from precursor organelles and are transported to their target compartment where they dock and fuse. Targeting requires tethering factors recruited by small GTPases and phosphoinositides whereas fusion is carried out by SNARE proteins. Here we report that vesicles containing the Q-SNAREs syntaxin 13 (Stx13) and syntaxin 6 (Stx6) together are targeted to a different endosomal compartment than vesicles containing only Stx6 using injection of artificial vesicles. Targeting by Stx6 requires Vps51, a component of the GARP/EARP tethering complexes. In contrast, targeting by both Stx6 and Stx13 is governed by Vps13B identified here as tethering factor functioning in transport from early endosomes to recycling endosomes. Vps13B specifically binds to Stx13/Stx6 as well as to Rab14, Rab6, and PtdIns(3)P. We conclude that SNAREs use a combinatorial code for recruiting tethering factors, revealing a key function in targeting that is independent of SNARE pairing during fusion.

Intracellular vesicle targeting is mediated by Rab GTPases that cooperate with phosphatidylinositides and SNARE proteins, which then facilitate membrane fusion. Here, the authors microinject artificial vesicles into HeLa cells and find that SNAREs play a more prominent role in targeting specificity of trafficking vesicles than previously known.

Novel binding partners for Prenylated Rab Acceptor 1 identified by a split-ubiquitin yeast two-hybrid screen

OBJECTIVE

Prenylated Rab Acceptor 1 (PRA1) is a transmembrane protein localized to the early secretory pathway. It has been found to interact with an array of Rab GTPases, leading to its hypothesized function in the recycling of Rab GTPases. However, all previous strategies used to screen for novel interacting partners have utilized a classic yeast two-hybrid approach that requires both bait and its potential binding partners to be cytosolic proteins. In the split-ubiquitin yeast two-hybrid screen, a protein interaction leads to the re-constitution of ubiquitin, which is followed by proteolytic release of a transcription activator that migrates to the nucleus alone. This allows for bait and/or prey to be integral membrane protein(s). To better understand the in vivo function of PRA1, we took an unbiased approach that screened PRA1 against a normalized mouse neuronal cDNA library using this variant of the classic screening strategy.

RESULTS

We report 41 previously unidentified potential PRA1 binding partners revealed by this screen and validate the screen by confirming three of these interactions using a bi-molecular fluorescence complementation assay in mammalian cells. The identified proteins reside throughout the secretory pathway and are both membrane-bound and cytosolic in their identity, suggesting alternative functions for PRA1.

Role of Rab GTPases in Alzheimer's Disease

Alzheimer's disease (AD) comprises two major pathological hallmarks: extraneuronal deposition of β-amyloid (Aβ) peptides ("senile plaques") and intraneuronal aggregation of the microtubule-associated protein tau ("neurofibrillary tangles"). Aβ is derived from sequential cleavage of the β-amyloid precursor protein by β- and γ-secretases, while aggregated tau is hyperphosphorylated in AD. Mounting evidence suggests that dysregulated trafficking of these AD-related proteins contributes to AD pathogenesis. Rab proteins are small GTPases that function as master regulators of vesicular transport and membrane trafficking. Multiple Rab GTPases have been implicated in AD-related protein trafficking, and their expression has been observed to be altered in postmortem AD brain. Here we review current implicated roles of Rab GTPase dysregulation in AD pathogenesis. Further elucidation of the pathophysiological role of Rab GTPases will likely reveal novel targets for AD therapeutics.

An interaction network between the SNARE VAMP7 and Rab GTPases within a ciliary membrane-targeting complex

The Arf4-rhodopsin complex (mediated by the VxPx motif in rhodopsin) initiates expansion of vertebrate rod photoreceptor cilia-derived light-sensing organelles through stepwise assembly of a conserved trafficking network. Here, we examine its role in the sorting of VAMP7 (also known as TI-VAMP) - an R-SNARE possessing a regulatory longin domain (LD) - into rhodopsin transport carriers (RTCs). During RTC formation and trafficking, VAMP7 colocalizes with the ciliary cargo rhodopsin and interacts with the Rab11-Rabin8-Rab8 trafficking module. Rab11 and Rab8 bind the VAMP7 LD, whereas Rabin8 (also known as RAB3IP) interacts with the SNARE domain. The Arf/Rab11 effector FIP3 (also known as RAB11FIP3) regulates VAMP7 access to Rab11. At the ciliary base, VAMP7 forms a complex with the cognate SNAREs syntaxin 3 and SNAP-25. When expressed in transgenic animals, a GFP-VAMP7ΔLD fusion protein and a Y45E phosphomimetic mutant colocalize with endogenous VAMP7. The GFP-VAMP7-R150E mutant displays considerable localization defects that imply an important role of the R-SNARE motif in intracellular trafficking, rather than cognate SNARE pairing. Our study defines the link between VAMP7 and the ciliary targeting nexus that is conserved across diverse cell types, and contributes to general understanding of how functional Arf and Rab networks assemble SNAREs in membrane trafficking.

Rab GTPase Function in Endosome and Lysosome Biogenesis

Eukaryotic cells maintain a highly organized endolysosomal system. This system regulates the protein and lipid content of the plasma membrane, it participates in the intracellular quality control machinery and is needed for the efficient removal of damaged organelles. This complex network comprises an endosomal membrane system that feeds into the lysosomes, yet also allows recycling of membrane proteins, and probably lipids. Moreover, lysosomal degradation provides the cell with macromolecules for further growth. In this review, we focus primarily on the role of the small Rab GTPases Rab5 and Rab7 as organelle markers and interactors of multiple effectors on endosomes and lysosomes and highlight their role in membrane dynamics, particularly fusion along the endolysosomal pathway.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Rab5 and Alsin regulate stress-activated cytoprotective signaling on mitochondria

Mitochondrial stress response is essential for cell survival, and damaged mitochondria are a hallmark of neurodegenerative diseases. Thus, it is fundamental to understand how mitochondria relay information within the cell. Here, by investigating mitochondrial-endosomal contact sites we made the surprising observation that the small GTPase Rab5 translocates from early endosomes to mitochondria upon oxidative stress. This process is reversible and accompanied by an increase in Rab5-positive endosomes in contact with mitochondria. Interestingly, activation of Rab5 on mitochondria depends on the Rab5-GEF ALS2/Alsin, encoded by a gene mutated in amyotrophic lateral sclerosis (ALS). Alsin-deficient human-induced pluripotent stem cell-derived spinal motor neurons are defective in relocating Rab5 to mitochondria and display increased susceptibility to oxidative stress. These findings define a novel pathway whereby Alsin catalyzes the assembly of the Rab5 endocytic machinery on mitochondria. Defects in stress-sensing by endosomes could be crucial for mitochondrial quality control during the onset of ALS.

The inside of a human cell is divided into compartments called organelles, which are surrounded by membranes. Each organelle plays a specific role in keeping the cell healthy and also has unique mix of molecular markers on its surface. These markers allow other molecules to identify the different organelles, meaning that specific organelles can communicate with each other and coordinate their activities. One way that organelles can do this is via so-called membrane contact sites, which are small areas where the compartments’ outer membranes come close together.

Mitochondria are organelles that release energy inside human cells. These compartments also work to keep the levels of toxic chemicals called reactive oxygen species in the cell within a safe range. This is important because cells can die if these levels become too high – a state known as oxidative stress. Mitochondria also communicate with other organelles called endosomes, which receive materials from the cell surface, sort and direct them to different destinations throughout the cell.

In many diseases affecting the nervous system, the mitochondria and endosomes in nerve cells do not work properly. These cells also have higher than normal levels of oxidative stress. Hsu et al. therefore wanted to find out if mitochondria and endosomes worked together to help cells to cope with this kind of stress.

Hsu et al. triggered oxidative stress in human cancer cells by exposing them first to a dye that stained the mitochondria and then to intense light. In stressed cells, a subset of endosomes called early endosomes formed many more membrane contact sites with mitochondria than in non-stressed cells. At the same time, the protein Rab5, usually found on early endosomes, relocated to the surface of mitochondria. Human cells previously engineered to produce larger than normal amounts of Rab5 were also more likely to survive oxidative stress. These experiments suggested that early endosomes cooperate with mitochondria, via Rab5, to protect cells from oxidative stress.

So, how is Rab5 relocated to mitochondria? Hsu et al. searched for activators of Rab5 and found that Alsin also migrated to mitochondria in stressed cells. The gene for Alsin is also mutated in amyotrophic lateral sclerosis (ALS), a degenerative nerve disorder that remains poorly understood. Next, Hsu et al. deleted the gene for Alsin from human stem cells growing in the laboratory and coaxed these cells into becoming nerve cells. Experiments with these cells showed that the absence of Alsin prevented Rab5 from moving to the mitochondria. Nerve cells lacking Alsin were also more susceptible to oxidative stress than normal cells.

Together, these findings show that early endosomes work with mitochondria to sense and ward off oxidative stress. They also reveal an unexpected connection between this process and a gene mutated in ALS. Further experiments are now needed to explore if problems with endosomes or mitochondria, and specifically with molecules like Alsin and Rab5, are responsible for other neurodegenerative disorders, like Parkinson’s disease and Huntington’s disease.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

A guanine nucleotide exchange factor (GEF) limits Rab GTPase-driven membrane fusion

The identity of organelles in the endomembrane system of any eukaryotic cell critically depends on the correctly localized Rab GTPase, which binds effectors and thus promotes membrane remodeling or fusion. However, it is still unresolved which factors are required and therefore define the localization of the correct fusion machinery. Using SNARE-decorated proteoliposomes that cannot fuse on their own, we now demonstrate that full fusion activity can be achieved by just four soluble factors: a soluble SNARE (Vam7), a guanine nucleotide exchange factor (GEF, Mon1-Ccz1), a Rab-GDP dissociation inhibitor (GDI) complex (prenylated Ypt7-GDI), and a Rab effector complex (HOPS). Our findings reveal that the GEF Mon1-Ccz1 is necessary and sufficient for stabilizing prenylated Ypt7 on membranes. HOPS binding to Ypt7-GTP then drives SNARE-mediated fusion, which is fully GTP-dependent. We conclude that an entire fusion cascade can be controlled by a GEF.

Probing and manipulating intracellular membrane traffic by microinjection of artificial vesicles

There is still a large gap in our understanding between the functional complexity of cells and the reconstruction of partial cellular functions in vitro from purified or engineered parts. Here we have introduced artificial vesicles of defined composition into living cells to probe the capacity of the cellular cytoplasm in dealing with foreign material and to develop tools for the directed manipulation of cellular functions. Our data show that protein-free liposomes, after variable delay times, are captured by the Golgi apparatus that is reached either by random diffusion or, in the case of large unilamellar vesicles, by microtubule-dependent transport via a dynactin/dynein motor complex. However, insertion of early endosomal SNARE proteins suffices to convert liposomes into trafficking vesicles that dock and fuse with early endosomes, thus overriding the default pathway to the Golgi. Moreover, such liposomes can be directed to mitochondria expressing simple artificial affinity tags, which can also be employed to divert endogenous trafficking vesicles. In addition, fusion or subsequent acidification of liposomes can be monitored by incorporation of appropriate chemical sensors. This approach provides an opportunity for probing and manipulating cellular functions that cannot be addressed by conventional genetic approaches. We conclude that the cellular cytoplasm has a remarkable capacity for self-organization and that introduction of such macromolecular complexes may advance nanoengineering of eukaryotic cells.

Rab GTPases: master regulators that establish the secretory and endocytic pathways

Several of the most important discoveries in the field of membrane traffic have come from studies of Rab GTPases by Marino Zerial and Peter Novick and their colleagues. Zerial was the first to discover that Rab GTPases represent identity markers for different membrane-bound compartments, and each Rab organizes a collection of specific effectors into function-specifying membrane microdomains to carry out receptor trafficking. Novick discovered that the order (and thus polarity) of Rab GTPases along the secretory and endocytic pathways are established by their specific, cognate guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), which partner with one Rab to regulate the subsequent- and prior-acting Rabs. Such so-called Rab cascades have evolved to establish domains that contain unique Rab proteins and their cognate effectors, which drive all steps of membrane trafficking. These findings deserve much broader recognition by the biomedical research community and are highlighted here, along with open questions that require serious attention for full understanding of the molecular basis of Rab GTPase-regulated membrane trafficking in eukaryotic cells.

Rab25 GTPase: Functional roles in cancer

Rab25, a small GTPase belongs to the Rab protein family, has a pivotal role in cancer pathophysiology. Rab25 governs cell-surface receptors recycling and cellular signaling pathways activation, allowing it to control a diverse range of cellular functions, including cell proliferation, cell motility and cell death. Aberrant expression of Rab25 was linked to cancer development. Majority of research findings revealed that Rab25 is an oncogene. Elevated expression of Rab25 was correlated with poor prognosis and aggressiveness of renal, lung, breast, ovarian and other cancers. However, tumor suppressor function of Rab25 was reported in several cancers, such as colorectal cancer, indicating the tumor type-specific function of Rab25. In this review, we recapitulate the current knowledge of Rab25 in cancer development and therapy.

Meeting report - Emerging Concepts in Cell Organization

New concepts in cell organization emerged in a medieval castle during a snowy week in January 2017 in the middle of the Austrian Alps. The occasion was the 10th Annaberg EMBO workshop in Goldegg am See; organized by Gabriele Seethaler, Catherine Rabouille and Marino Zerial. There were 95 participants, including many who gave talks and presented posters, enjoying a familial and trusting atmosphere that stimulated lively exchange of (unpublished) results, new ideas and thoughts.