Examination of Sec22 Homodimer Formation and Role in SNARE-dependent Membrane Fusion

SNARE Protein AoSec22 Orchestrates Mycelial Growth, Vacuole Assembly, Trap Formation, Stress Response, and Secondary Metabolism in Arthrobotrys oligospora

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) facilitate intracellular vesicle trafficking and membrane fusion in eukaryotes and play a vital role in fungal growth, development, and pathogenicity. However, the functions of SNAREs are still largely unknown in nematode-trapping fungi. Arthrobotrys oligospora is a representative species of nematode-trapping fungi that can produce adhesive networks (traps) for nematode predation. In this study, we characterized AoSec22 in A. oligospora, a homolog of the yeast SNARE protein Sec22. Deletion of Aosec22 resulted in remarkable reductions in mycelial growth, the number of nuclei, conidia yield, and trap formation, especially for traps that failed to develop mature three-dimensional networks. Further, absence of Aosec22 impaired fatty acid utilization, autophagy, and stress tolerance; in addition, the vacuoles became small and fragmented in the hyphal cells of the ∆Aosec22 mutant, and large vacuoles failed to form. The reduced sporulation capacity correlated with the transcriptional repression of several sporulation-related genes, and the impaired accumulation of lipid droplets is in line with the transcriptional repression of several genes involved in fatty acid oxidation. Moreover, absence of Aosec22 remarkably impaired secondary metabolism, resulting in 4717 and 1230 compounds upregulated and downregulated in the ∆Aosec22 mutant, respectively. Collectively, our data highlighted that the SNARE protein AoSec22 plays a pleiotropic role in mycelial growth and development, vacuole assembly, lipid metabolism, stress response, and secondary metabolism; in particular, it is required for the proper development of traps in A. oligospora.

Synthetic antibody-derived immunopeptide provides neuroprotection in glaucoma through molecular interaction with retinal protein histone H3.1

Glaucoma is a group of optic neuropathies characterized by the progressive degeneration of retinal ganglion cells (RGCs) as well as their axons leading to irreversible loss of sight. Medical management of the intraocular pressure (IOP) still represents the gold standard in glaucoma therapy, which only manages a single risk factor and does not directly address the neurodegenerative component of this eye disease. Recently, our group showed that antibody-derived immunopeptides (encoding complementarity-determining regions, CDRs) provide attractive glaucoma medication candidates and directly interfere its pathogenic mechanisms by different modes of action. In accordance with these findings, the present study showed the synthetic complementary-determining region 2 (CDR2) peptide (INSDGSSTSYADSVK) significantly increased RGC viability in vitro in a concentration-dependent manner (p < 0.05 using a CDR2 concentration of 50 μg/mL). Employing state-of the-art immunoprecipitation experiments, we confirmed that synthetic CDR2 exhibited a high affinity toward the retinal target protein histone H3.1 (HIST1H3A) (p < 0.001 and log2-fold change > 3). Furthermore, molecular dynamics (MD) simulations along with virtual docking analyses predicted potential CDR2-specific binding regions of HIST1H3A, which might represent essential post-translational modification (PTM) sites for epigenetic regulations. Quantitative mass spectrometry (MS) analysis of retinas demonstrated 39 proteins significantly affected by CDR2 treatment (p < 0.05). An up-regulation of proteins involved in the energy production (e.g., ATP5F1B and MT-CO2) as well as the regulatory ubiquitin proteasome system (e.g., PSMC5) was induced by the synthetic CDR2 peptide. On the other hand, CDR2 reduced metabolic key enzymes (e.g., DDAH1 and MAOB) as well as ER stress-related proteins (e.g., SEC22B and VCP) and these data were partially confirmed by microarray technology. Our outcome measurements indicate that specific protein-peptide interactions influence the regulatory epigenetic function of HIST1H3A promoting the neuroprotective mechanism on RGCs in vitro. In addition to IOP management, such synthetic peptides as CDR2 might serve as a synergistic immunotherapy for glaucoma in the future.

Phosphoinositides containing stearic acid are required for interaction between Rho GTPases and the exocyst to control the late steps of polarised exocytosis

Cell polarity is achieved by regulators such as small G proteins, exocyst members and phosphoinositides, with the latter playing a key role when bound to the exocyst proteins Sec3p and Exo70p, and Rho GTPases. This ensures asymmetric growth via the routing of proteins and lipids to the cell surface using actin cables. Previously, using a yeast mutant for a lysophosphatidylinositol acyl transferase encoded by the PSI1 gene, we demonstrated the role of stearic acid in the acyl chain of phosphoinositides in cytoskeletal organisation and secretion. Here, we use a genetic approach to characterise the effect on late steps of the secretory pathway. The constitutive overexpression of PSI1 in mutants affecting kinases involved in the phosphoinositide pathway demonstrated the role of molecular species containing stearic acid in bypassing a lack of phosphatidylinositol-4-phosphate PI(4)P at the plasma membrane, which is essential for the function of the Cdc42p module. Decreasing the levels of stearic acid-containing phosphoinositides modifies the environment of the actors involved in the control of late steps in the secretory pathway. This leads to decreased interactions between Exo70p and Sec3p, with Cdc42p, Rho1p and Rho3p, due to disruption of the GTP/GDP ratio of at least Rho1p and Rho3p GTPases, thereby preventing activation of the exocyst.

AtSEC22 Regulates Cell Morphogenesis via Affecting Cytoskeleton Organization and Stabilities

The plant cytoskeleton forms a stereoscopic network that regulates cell morphogenesis. The cytoskeleton also provides tracks for trafficking of vesicles to the target membrane. Fusion of vesicles with the target membrane is promoted by SNARE proteins, etc. The vesicle-SNARE, Sec22, regulates membrane trafficking between the ER and Golgi in yeast and mammals. Arabidopsis AtSEC22 might also regulate early secretion and is essential for gametophyte development. However, the role of AtSEC22 in plant development is unclear. To clarify the role of AtSEC22 in the regulation of plant development, we isolated an AtSEC22 knock-down mutant, atsec22-4, and found that cell morphogenesis and development were seriously disturbed. atsec22-4 exhibited shorter primary roots (PRs), dwarf plants, and partial abortion. More interestingly, the atsec22-4 mutant had less trichomes with altered morphology, irregular stomata, and pavement cells, suggesting that cell morphogenesis was perturbed. Further analyses revealed that in atsec22-4, vesicle trafficking was blocked, resulting in the trapping of proteins in the ER and collapse of structures of the ER and Golgi apparatus. Furthermore, AtSEC22 defects resulted in impaired organization and stability of the cytoskeleton in atsec22-4. Our findings revealed essential roles of AtSEC22 in membrane trafficking and cytoskeleton dynamics during plant development.

Interorganelle Tethering to Endocytic Organelles Determines Directional Cytokine Transport in CD4+ T Cells

Delivery of vesicles to their desired destinations plays a central role in maintaining proper cell functionality. In certain scenarios, depending on loaded cargos, the vesicles have spatially distinct destinations. For example, in T cells, some cytokines (e.g., IL-2) are polarized to the T cell-target cell interface, whereas the other cytokines are delivered multidirectionally (e.g., TNF-α). In this study, we show that in primary human CD4⁺ T cells, both TNF-α⁺ and IL-2⁺ vesicles can tether with endocytic organelles (lysosomes/late endosomes) by forming membrane contact sites. Tethered cytokine-containing vesicle (CytV)-endocytic organelle pairs are released sequentially. Only endocytic organelle-tethered CytVs are preferentially transported to their desired destination. Mathematical models suggest that endocytic organelle tethering could regulate the direction of cytokine transport by selectively attaching different microtubule motor proteins (such as kinesin and dynein) to the corresponding CytVs. These findings establish the previously unknown interorganelle tethering to endocytic organelles as a universal solution for directional cytokine transport in CD4⁺ T cells. Modulating tethering to endocytic organelles can, therefore, coordinately control directionally distinct cytokine transport.

The function of SEC22B and its role in human diseases

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are a large protein complex that is involved in the membrane fusion in vesicle trafficking, cell growth, cytokinesis, membrane repair, and synaptic transmission. As one of the SNARE proteins, SEC22B functions in membrane fusion of vesicle trafficking between the endoplasmic reticulum and the Golgi apparatus, antigen cross-presentation, secretory autophagy, and other biological processes. However, apart from not being SNARE proteins, there is little knowledge known about its two homologs (SEC22A and SEC22C). SEC22B alterations have been reported in many human diseases, especially, many mutations of SEC22B in human cancers have been detected. In this review, we will introduce the specific functions of SEC22B, and summarize the researches about SEC22B in human cancers and other diseases. These findings have laid the foundation for further studies to clarify the exact mechanism of SEC22B in the pathological process and to seek new therapeutic targets and better treatment strategies.

Q-SNARE protein FgSyn8 plays important role in growth, DON production and pathogenicity of Fusarium graminearum

SNAREs (Soluble N-ethylmaleimide-sensitive factor attachment protein receptors) help intracellular vesicle trafficking and membrane fusion among eukaryotes. They are vital for growth and development of phyto-pathogenic fungi such as Fusarium graminearum which causes Fusarium Head Blight (FHB) of wheat and barley. The SNARE protein Syn8 and its homologues play many roles among different organisms. Here, we have characterized FgSyn8 in F. graminearum as a homologue of Syn8. We have integrated biochemical, microbiological and molecular genetic approaches to investigate the roles of this protein. Our results reveal that FgSyn8 is indispensable for normal vegetative growth, conidiation, conidial morphology and pathogenicity of F. graminearum. Deoxynivalenol (DON) biochemical assay reveals active participation of this protein in DON production of F. graminearum. This has further been confirmed by the production of bulbous structures among the intercalary hyphae. FgSyn8 mutant strain produced defects in perithecia formation which portrays its role in sexual reproduction. In summary, our results support that the SNARE protein FgSyn8 is required for vegetative growth, sexual reproduction, DON production and pathogenicity of F. graminearum.

Diverse Role of SNARE Protein Sec22 in Vesicle Trafficking, Membrane Fusion, and Autophagy

Protein synthesis begins at free ribosomes or ribosomes attached with the endoplasmic reticulum (ER). Newly synthesized proteins are transported to the plasma membrane for secretion through conventional or unconventional pathways. In conventional protein secretion, proteins are transported from the ER lumen to Golgi lumen and through various other compartments to be secreted at the plasma membrane, while unconventional protein secretion bypasses the Golgi apparatus. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are involved in cargo vesicle trafficking and membrane fusion. The ER localized vesicle associated SNARE (v-SNARE) protein Sec22 plays a major role during anterograde and retrograde transport by promoting efficient membrane fusion and assisting in the assembly of higher order complexes by homodimer formation. Sec22 is not only confined to ER-Golgi intermediate compartments (ERGIC) but also facilitates formation of contact sites between ER and plasma membranes. Sec22 mutation is responsible for the development of atherosclerosis and symptoms in the brain in Alzheimer's disease and aging in humans. In the fruit fly Drosophila melanogaster, Sec22 is essential for photoreceptor morphogenesis, the wingless signaling pathway, and normal ER, Golgi, and endosome morphology. In the plant Arabidopsis thaliana, it is involved in development, and in the nematode Caenorhabditis elegans, it is in involved in the RNA interference (RNAi) pathway. In filamentous fungi, it affects cell wall integrity, growth, reproduction, pathogenicity, regulation of reactive oxygen species (ROS), expression of extracellular enzymes, and transcriptional regulation of many development related genes. This review provides a detailed account of Sec22 function, summarizes its domain structure, discusses its genetic redundancy with Ykt6, discusses what is known about its localization to discrete membranes, its contributions in conventional and unconventional autophagy, and a variety of other roles across different cellular systems ranging from higher to lower eukaryotes, and highlights some of the surprises that have originated from research on Sec22.

Salmonella SipA mimics a cognate SNARE for host Syntaxin8 to promote fusion with early endosomes

Intracellular pathogens can modulate host Rabs and SNAREs to support their replication and immune evasion. Singh et al. show that the Salmonella effector SipA functionally mimics an R-SNARE and recruits host Q-SNAREs to promote membrane fusion. Thus, SNARE mimicry by this intracellular pathogen effector modulates the host trafficking machinery for Salmonella survival.

SipA is a major effector of Salmonella, which causes gastroenteritis and enteric fever. Caspase-3 cleaves SipA into two domains: the C-terminal domain regulates actin polymerization, whereas the function of the N terminus is unknown. We show that the cleaved SipA N terminus binds and recruits host Syntaxin8 (Syn8) to Salmonella-containing vacuoles (SCVs). The SipA N terminus contains a SNARE motif with a conserved arginine residue like mammalian R-SNAREs. SipA^R204Q and SipA^1–435R204Q do not bind Syn8, demonstrating that SipA mimics a cognate R-SNARE for Syn8. Consequently, Salmonella lacking SipA or that express the SipA^1–435R204Q SNARE mutant are unable to recruit Syn8 to SCVs. Finally, we show that SipA mimicking an R-SNARE recruits Syn8, Syn13, and Syn7 to the SCV and promotes its fusion with early endosomes to potentially arrest its maturation. Our results reveal that SipA functionally substitutes endogenous SNAREs in order to hijack the host trafficking pathway and promote Salmonella survival.

SNARE-Encoding Genes VdSec22 and VdSso1 Mediate Protein Secretion Required for Full Virulence in Verticillium dahliae

Proteins that mediate cellular and subcellular membrane fusion are key factors in vesicular trafficking in all eukaryotic cells, including the secretion and transport of plant pathogen virulence factors. In this study, we identified vesicle-fusion components that included 22 soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), four Sec1/Munc18 (SM) family proteins, and 10 Rab GTPases encoded in the genome of the vascular wilt pathogen Verticillium dahliae Vd991. Targeted deletion of two SNARE-encoding genes in V. dahliae, VdSec22 and VdSso1, significantly reduced virulence of both mutants on cotton, relative to the wild-type Vd991 strain. Comparative analyses of the secreted protein content (exoproteome) revealed that many enzymes involved in carbohydrate hydrolysis were regulated by VdSec22 or VdSso1. Consistent with a role of these enzymes in plant cell-wall degradation, pectin, cellulose, and xylan utilization were reduced in the VdSec22 or VdSso1 mutant strains along with a loss of exoproteome cytotoxic activity on cotton leaves. Comparisons with a pathogenicity-related exoproteome revealed that several known virulence factors were not regulated by VdSec22 or VdSso1, but some of the proteins regulated by VdSec22 or VdSso1 displayed different characteristics, including the lack of a typical signal peptide, suggesting that V. dahliae employs more than one secretory route to transport proteins to extracellular sites during infection.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

The Multifaceted Role of SNARE Proteins in Membrane Fusion

Membrane fusion is a key process in all living organisms that contributes to a variety of biological processes including viral infection, cell fertilization, as well as intracellular transport, and neurotransmitter release. In particular, the various membrane-enclosed compartments in eukaryotic cells need to exchange their contents and communicate across membranes. Efficient and controllable fusion of biological membranes is known to be driven by cooperative action of SNARE proteins, which constitute the central components of the eukaryotic fusion machinery responsible for fusion of synaptic vesicles with the plasma membrane. During exocytosis, vesicle-associated v-SNARE (synaptobrevin) and target cell-associated t-SNAREs (syntaxin and SNAP-25) assemble into a core trans-SNARE complex. This complex plays a versatile role at various stages of exocytosis ranging from the priming to fusion pore formation and expansion, finally resulting in the release or exchange of the vesicle content. This review summarizes current knowledge on the intricate molecular mechanisms underlying exocytosis triggered and catalyzed by SNARE proteins. Particular attention is given to the function of the peptidic SNARE membrane anchors and the role of SNARE-lipid interactions in fusion. Moreover, the regulatory mechanisms by synaptic auxiliary proteins in SNARE-driven membrane fusion are briefly outlined.

Overexpression of Sly41 suppresses COPII vesicle-tethering deficiencies by elevating intracellular calcium levels

SLY41 is a multicopy suppressor of mutations in the essential Ypt1 GTPase. Overexpression of Sly41 elevates cytosolic Ca²⁺ concentration, which stimulates SNARE-dependent fusion of COPII vesicles with Golgi membranes and suppresses deficiencies in Ypt1-dependent vesicle tethering. Thus Ca²⁺ positively regulates vesicle fusion with Golgi membranes.

SLY41 was identified as a multicopy suppressor of loss of Ypt1, a Rab GTPase essential for COPII vesicle tethering at the Golgi complex. SLY41 encodes a polytopic membrane protein with homology to a class of solute transporter proteins, but how overexpression suppresses vesicle-tethering deficiencies is not known. Here we show that Sly41 is efficiently packaged into COPII vesicles and actively cycles between the ER and Golgi compartments. SLY41 displays synthetic negative genetic interactions with PMR1, which encodes the major Golgi-localized Ca²⁺/Mn²⁺ transporter and suggests that Sly41 influences cellular Ca²⁺ and Mn²⁺ homeostasis. Experiments using the calcium probe aequorin to measure intracellular Ca²⁺ concentrations in live cells reveal that Sly41 overexpression significantly increases cytosolic calcium levels. Although specific substrates of the Sly41 transporter were not identified, our findings indicate that localized overexpression of Sly41 to the early secretory pathway elevates cytosolic calcium levels to suppress vesicle-tethering mutants. In vitro SNARE cross-linking assays were used to directly monitor the influence of Ca²⁺ on tethering and fusion of COPII vesicles with Golgi membranes. Strikingly, calcium at suppressive concentrations stimulated SNARE-dependent membrane fusion when vesicle-tethering activity was reduced. These results show that calcium positively regulates the SNARE-dependent fusion stage of ER–Golgi transport.