Explore the vast range of titles available.

The mechanism by which nutrient status regulates the fusion of autophagosomes with endosomes/lysosomes is poorly understood. Here, we report that O -linked β- N -acetylglucosamine ( O -GlcNAc) transferase (OGT) mediates O -GlcNAcylation of the SNARE protein SNAP-29 and regulates autophagy in a nutrient-dependent manner. In mammalian cells, OGT knockdown, or mutating the O -GlcNAc sites in SNAP-29, promotes the formation of a SNAP-29-containing SNARE complex, increases fusion between autophagosomes and endosomes/lysosomes, and promotes autophagic flux. In Caenorhabditis elegans , depletion of ogt-1 has a similar effect on autophagy; moreover, expression of an O -GlcNAc-defective SNAP-29 mutant facilitates autophagic degradation of protein aggregates. O -GlcNAcylated SNAP-29 levels are reduced during starvation in mammalian cells and in C. elegans . Our study reveals a mechanism by which O -GlcNAc-modification integrates nutrient status with autophagosome maturation. Zhang and colleagues report that starvation reduces O -GlcNAcylation of the SNARE protein SNAP-29. This promotes formation of a competent SNARE complex that increases autophagosome–lysosome fusion, increasing autophagosome maturation and flux.

The essential autophagy mediator ATG14 promotes vesicle fusion by forming homo-oligomers, which bind to a component of the SNARE membrane fusion complex and stabilize this complex on autophagosomes. Autophagy-specific membrane fusion During the process of autophagy, cells break down their own components and sequester them in specialized vesicles called autophagosomes, which eventually fuse with lysosomes for degradation of their content. Membrane fusion is an important event not only during early biogenesis of autophagosomal vesicles but also when autophagosomes unite with lysosomes. Qing Zhong and colleagues show here that the essential autophagy mediator ATG14 promotes vesicle fusion by forming homo-oligomers, which bind to a component of the SNARE membrane fusion complex and stabilize this complex on autophagosomes. Autophagy, an important catabolic pathway implicated in a broad spectrum of human diseases, begins by forming double membrane autophagosomes that engulf cytosolic cargo and ends by fusing autophagosomes with lysosomes for degradation 1 , 2 . Membrane fusion activity is required for early biogenesis of autophagosomes and late degradation in lysosomes 3 , 4 , 5 , 6 , 7 . However, the key regulatory mechanisms of autophagic membrane tethering and fusion remain largely unknown. Here we report that ATG14 (also known as beclin-1-associated autophagy-related key regulator (Barkor) or ATG14L), an essential autophagy-specific regulator of the class III phosphatidylinositol 3-kinase complex 8 , 9 , 10 , 11 , promotes membrane tethering of protein-free liposomes, and enhances hemifusion and full fusion of proteoliposomes reconstituted with the target (t)-SNAREs (soluble N -ethylmaleimide-sensitive factor attachment protein receptors) syntaxin 17 (STX17) and SNAP29, and the vesicle (v)-SNARE VAMP8 (vesicle-associated membrane protein 8). ATG14 binds to the SNARE core domain of STX17 through its coiled-coil domain, and stabilizes the STX17–SNAP29 binary t-SNARE complex on autophagosomes. The STX17 binding, membrane tethering and fusion-enhancing activities of ATG14 require its homo-oligomerization by cysteine repeats. In ATG14 homo-oligomerization-defective cells, autophagosomes still efficiently form but their fusion with endolysosomes is blocked. Recombinant ATG14 homo-oligomerization mutants also completely lose their ability to promote membrane tethering and to enhance SNARE-mediated fusion in vitro . Taken together, our data suggest an autophagy-specific membrane fusion mechanism in which oligomeric ATG14 directly binds to STX17–SNAP29 binary t-SNARE complex on autophagosomes and primes it for VAMP8 interaction to promote autophagosome–endolysosome fusion.

Tumour cells secrete exosomes that are involved in the remodelling of the tumour–stromal environment and promoting malignancy. The mechanisms governing tumour exosome release, however, remain incompletely understood. Here we show that tumour cell exosomes secretion is controlled by pyruvate kinase type M2 (PKM2), which is upregulated and phosphorylated in tumours. During exosome secretion, phosphorylated PKM2 serves as a protein kinase to phosphorylate synaptosome-associated protein 23 (SNAP-23), which in turn enables the formation of the SNARE complex to allow exosomes release. Direct phosphorylation assay and mass spectrometry confirm that PKM2 phosphorylates SNAP-23 at Ser95. Ectopic expression of non-phosphorylated SNAP-23 mutant (Ser95→Ala95) significantly reduces PKM2-mediated exosomes release whereas expression of selective phosphomimetic SNAP-23 mutants (Ser95→Glu95 but not Ser20→Glu20) rescues the impaired exosomes release induced by PKM2 knockdown. Our findings reveal a non-metabolic function of PKM2, an enzyme associated with tumour cell reliance on aerobic glycolysis, in promoting tumour cell exosome release. Exosomes, vesicles secreted by cancer cells, have a role in cancer progression but the mechanisms regulating their biogenesis are mostly unknown. Here the authors show that PKM2, a rate-limiting glycolytic enzyme overexpressed in cancer cells, mediates exosomes exocytosis by phosphorylating SNAP-23.

The Unc-51 like autophagy activating kinase 1 (ULK1) complex plays a central role in the initiation stage of autophagy. However, the function of ULK1 in the late stage of autophagy is unknown. Here, we report that ULK1, a central kinase of the ULK1 complex involved in autophagy initiation, promotes autophagosome–lysosome fusion. PKCα phosphorylates ULK1 and prevents autolysosome formation. PKCα phosphorylation of ULK1 does not change its kinase activity; however, it decreases autophagosome–lysosome fusion by reducing the affinity of ULK1 for syntaxin 17 (STX17). Unphosphorylated ULK1 recruited STX17 and increased STX17′s affinity towards synaptosomal-associated protein 29 (SNAP29). Additionally, phosphorylation of ULK1 enhances its interaction with heat shock cognate 70 kDa protein (HSC70) and increases its degradation through chaperone-mediated autophagy (CMA). Our study unearths a key mechanism underlying autolysosome formation, a process in which the kinase activity of PKCα plays an instrumental role, and reveals the significance of the mutual regulation of macroautophagy and CMA in maintaining the balance of autophagy. The ULK complex plays a well-known role in initiating autophagy, to recycle cellular components in response to nutritional stress. Here, the authors demonstrate a late role for ULK in auotophagosome–lysosome fusion and provide a direct link between macroautophagy and chaperone mediated autophagy.

The Arabidopsis thaliana Qa-SNARE SYP121 (=SYR1/PEN1) drives vesicle traffic at the plasma membrane of cells throughout the vegetative plant. It facilitates responses to drought, to the water stress hormone abscisic acid, and to pathogen attack, and it is essential for recovery from so-called programmed stomatal closure. How SYP121-mediated traffic is regulated is largely unknown, although it is thought to depend on formation of a fusion-competent SNARE core complex with the cognate partners VAMP721 and SNAP33. Like SYP121, the Arabidopsis Sec1/Munc18 protein SEC11 (=KEULE) is expressed throughout the vegetative plant. We find that SEC11 binds directly with SYP121 both in vitro and in vivo to affect secretory traffic. Binding occurs through two distinct modes, one requiring only SEC11 and SYP121 and the second dependent on assembly of a complex with VAMP721 and SNAP33. SEC11 competes dynamically for SYP121 binding with SNAP33 and VAMP721, and this competition is predicated by SEC11 association with the N terminus of SYP121. These and additional data are consistent with a model in which SYP121-mediated vesicle fusion is regulated by an unusual \"handshaking\" mechanism of concerted SEC11 debinding and rebinding. They also implicate one or more factors that alter or disrupt SEC11 association with the SYP121 N terminus as an early step initiating SNARE complex formation.

Growing knowledge of the key molecular components involved in biological processes such as endocytosis, exocytosis, and motility has enabled direct testing of proposed mechanistic models by reconstitution. However, current techniques for building increasingly complex cellular structures and functions from purified components are limited in their ability to create conditions that emulate the physical and biochemical constraints of real cells. Here we present an integrated method for forming giant unilamellar vesicles with simultaneous control over (i) lipid composition and asymmetry, (ii) oriented membrane protein incorporation, and (iii) internal contents. As an application of this method, we constructed a synthetic system in which membrane proteins were delivered to the outside of giant vesicles, mimicking aspects of exocytosis. Using confocal fluorescence microscopy, we visualized small encapsulated vesicles docking and mixing membrane components with the giant vesicle membrane, resulting in exposure of previously encapsulated membrane proteins to the external environment. This method for creating giant vesicles can be used to test models of biological processes that depend on confined volume and complex membrane composition, and it may be useful in constructing functional systems for therapeutic and biomaterials applications.

The homotypic fusion of yeast vacuoles, each with 3Q‐ and 1R‐SNARE, requires SNARE chaperones (Sec17p/Sec18p and HOPS) and regulatory lipids (sterol, diacylglycerol and phosphoinositides). Pairs of liposomes of phosphatidylcholine/phosphatidylserine, bearing three vacuolar Q‐SNAREs on one and the R‐SNARE on the other, undergo slow lipid mixing, but this is unaffected by HOPS and inhibited by Sec17p/Sec18p. To study these essential fusion components, we reconstituted proteoliposomes of a more physiological composition, bearing vacuolar lipids and all four vacuolar SNAREs. Their fusion requires Sec17p/Sec18p and HOPS, and each regulatory lipid is important for rapid fusion. Although SNAREs can cause both fusion and lysis, fusion of these proteoliposomes with Sec17p/Sec18p and HOPS is not accompanied by lysis. Sec17p/Sec18p, which disassemble SNARE complexes, and HOPS, which promotes and proofreads SNARE assembly, act synergistically to form fusion‐competent SNARE complexes, and this synergy requires phosphoinositides. This is the first chemically defined model of the physiological interactions of these conserved fusion catalysts.

Autophagy, a conserved catabolic process implicated in a diverse array of human diseases, requires efficient fusion between autophagosomes and lysosomes to function effectively. Recently, SNAP47 has been identified as a key component of the dual-purpose SNARE complex mediating autophagosome-lysosome fusion in both bulk and selective autophagy. However, the spatiotemporal regulatory mechanisms of this SNARE complex remain unknown. In this study, we found that SNAP47 undergoes acetylation followed by deacetylation during bulk autophagy and mitophagy. The acetylation status of SNAP47 is regulated by the acetyltransferase CBP and the deacetylase HDAC2. Notably, the spatiotemporal regulatory dynamics of SNAP47 acetylation differ between bulk autophagy and mitophagy due to distinct regulation on the activity of acetyltransferase and deacetylase. Acetylated SNAP47 inhibits autophagosome-lysosome fusion by indirectly impeding SNARE complex assembly. Mechanistically, deacetylated SNAP47 recruits HOPS components to autophagic vacuoles independently of STX17 and STX17-SNAP47 interaction, while acetylated SNAP47 inhibits this recruitment, consequently leading to the failure of SNARE complex assembly. Taken together, our study uncovers a SNAP47 acetylation-dependent regulatory mechanism governing autophagosome-lysosome fusion by modulating the recruitment of HOPS to autophagic vacuoles without involving STX17, SNAP47-STX17 interaction and ternary SNARE complex formation. Autophagy involves autophagosome-lysosome fusion, regulated by the SNAP47-containing SNARE complex. This study reveals how acetylation and deacetylation of SNAP47 control fusion by modulating HOPS recruitment.

Human melanoma cells express various tumour antigens that are recognized by CD8 + cytotoxic T lymphocytes (CTLs) and elicit tumour-specific responses in vivo . However, natural and therapeutically enhanced CTL responses in melanoma patients are of limited efficacy. The mechanisms underlying CTL effector phase failure when facing melanomas are still largely elusive. Here we show that, on conjugation with CTL, human melanoma cells undergo an active late endosome/lysosome trafficking, which is intensified at the lytic synapse and is paralleled by cathepsin-mediated perforin degradation and deficient granzyme B penetration. Abortion of SNAP-23-dependent lysosomal trafficking, pH perturbation or impairment of lysosomal proteolytic activity restores susceptibility to CTL attack. Inside the arsenal of melanoma cell strategies to escape immune surveillance, we identify a self-defence mechanism based on exacerbated lysosome secretion and perforin degradation at the lytic synapse. Interfering with this synaptic self-defence mechanism might be useful in potentiating CTL-mediated therapies in melanoma patients. Cytotoxic T lymphocytes recognise and eliminate tumour cells. Here, the authors show that on contact with these immune cells melanoma cells can resist T cell cytotoxicity by modulating the trafficking of their lysosomal compartment, this results in the degradation of the T cell protein perforin by the protease cathepsin B.

The autophagosomal SNARE Syntaxin17 (Syx17) forms a complex with Snap29 and Vamp7/8 to promote autophagosome-lysosome fusion via multiple interactions with the tethering complex HOPS. Here we demonstrate that, unexpectedly, one more SNARE (Ykt6) is also required for autophagosome clearance in Drosophila. We find that loss of Ykt6 leads to large-scale accumulation of autophagosomes that are unable to fuse with lysosomes to form autolysosomes. Of note, loss of Syx5, the partner of Ykt6 in ER-Golgi trafficking does not prevent autolysosome formation, pointing to a more direct role of Ykt6 in fusion. Indeed, Ykt6 localizes to lysosomes and autolysosomes, and forms a SNARE complex with Syx17 and Snap29. Interestingly, Ykt6 can be outcompeted from this SNARE complex by Vamp7, and we demonstrate that overexpression of Vamp7 rescues the fusion defect of ykt6 loss of function cells. Finally, a point mutant form with an RQ amino acid change in the zero ionic layer of Ykt6 protein that is thought to be important for fusion-competent SNARE complex assembly retains normal autophagic activity and restores full viability in mutant animals, unlike palmitoylation or farnesylation site mutant Ykt6 forms. As Ykt6 and Vamp7 are both required for autophagosome-lysosome fusion and are mutually exclusive subunits in a Syx17-Snap29 complex, these data suggest that Vamp7 is directly involved in membrane fusion and Ykt6 acts as a non-conventional, regulatory SNARE in this process.