Membrane fusion

The Membrane Interaction of Alpha-Synuclein

A presynaptic protein closely related to Parkinson's disease (PD), α-synuclein (α-Syn), has been studied extensively regarding its pathogenic mechanisms. As a physiological protein in presynapses, however, α-Syn's physiological function remains unclear. Its location in nerve terminals and effects on membrane fusion also imply its functional role in synaptic transmission, including its possible interaction with high-curvature membranes via its N-terminus and amorphous C-terminus. PD-related mutants that disrupt the membrane interaction (e.g., A30P and G51D) additionally suggest a relationship between α-Syn's pathogenic mechanisms and physiological roles through the membrane binding. Here, we summarize recent research on how α-Syn and its variants interact with membranes and influence synaptic transmission. We list several membrane-related connections between the protein's physiological function and the pathological mechanisms that stand to expand current understandings of α-Syn.

Cholesterol stabilizes recombinant exocytic fusion pores by altering membrane bending rigidity

SNARE-mediated membrane fusion proceeds via the formation of a fusion pore. This intermediate structure is highly dynamic and can flicker between open and closed states. In cells, cholesterol has been reported to affect SNARE-mediated exocytosis and fusion pore dynamics. Here, we address the question of whether cholesterol directly affects the flickering rate of reconstituted fusion pores in vitro. These experiments were enabled by the recent development of a nanodisc⋅black lipid membrane recording system that monitors dynamic transitions between the open and closed states of nascent recombinant pores with submillisecond time resolution. The fusion pores formed between nanodiscs that bore the vesicular SNARE synaptobrevin 2 and black lipid membranes that harbored the target membrane SNAREs syntaxin 1A and SNAP-25B were markedly affected by cholesterol. These effects include strong reductions in flickering out of the open state, resulting in a significant increase in the open dwell-time. We attributed these effects to the known role of cholesterol in altering the elastic properties of lipid bilayers because manipulation of phospholipids to increase membrane stiffness mirrored the effects of cholesterol. In contrast to the observed effects on pore kinetics, cholesterol had no effect on the current that passed through individual pores and, hence, did not affect pore size. In conclusion, our results show that cholesterol dramatically stabilizes fusion pores in the open state by increasing membrane bending rigidity.

Dysfunction of the SNARE complex in neurological and psychiatric disorders

The communication between neurons constitutes the basis of all neural activities, and synaptic vesicle exocytosis is the fundamental biological event that mediates most communication between neurons in the central nervous system. The SNARE complex is the core component of the protein machinery that facilitates the fusion of synaptic vesicles with presynaptic terminals and thereby the release of neurotransmitters. In synapses, each release event is dependent on the assembly of the SNARE complex. In recent years, basic research on the SNARE complex has provided a clearer understanding of the mechanism underlying the formation of the SNARE complex and its role in vesicle formation. Emerging evidence indicates that abnormal expression or dysfunction of the SNARE complex in synapse physiology might contribute to abnormal neurotransmission and ultimately to synaptic dysfunction. Clinical research using postmortem tissues suggests that SNARE complex dysfunction is correlated with various neurological diseases, and some basic research has also confirmed the important role of the SNARE complex in the pathology of these diseases. Genetic and pharmacogenetic studies suggest that the SNARE complex and individual proteins might represent important molecular targets in neurological disease. In this review, we summarize the recent progress toward understanding the SNARE complex in regulating membrane fusion events and provide an update of the recent discoveries from clinical and basic research on the SNARE complex in neurodegenerative, neuropsychiatric, and neurodevelopmental diseases.

Programmed cell fusion in development and homeostasis

During multicellular organism development, complex structures are sculpted to form organs and tissues, which are maintained throughout adulthood. Many of these processes require cells to fuse with one another, or with themselves. These plasma membrane fusions merge endoplasmic cellular content across external, exoplasmic, space. In the nematode Caenorhabditis elegans, such cell fusions serve as a unique sculpting force, involved in the embryonic morphogenesis of the skin-like multinuclear hypodermal cells, but also in refining delicate structures, such as valve openings and the tip of the tail. During post-embryonic development, plasma membrane fusions continue to shape complex neuron structures and organs such as the vulva, while during adulthood fusion participates in cell and tissue repair. These processes rely on two fusion proteins (fusogens): EFF-1 and AFF-1, which are part of a broader family of structurally related membrane fusion proteins, encompassing sexual reproduction, viral infection, and tissue remodeling. The established capabilities of these exoplasmic fusogens are further expanded by new findings involving EFF-1 and AFF-1 in endocytic vesicle fission and phagosome sealing. Tight regulation by cell-autonomous and non-cell autonomous mechanisms orchestrates these diverse cell fusions at the correct place and time-these processes and their significance are discussed in this review.

Membrane packing defects in synaptic vesicles recruit complexin and synuclein

Complexin-1 (Cpx) and α-synuclein (α-Syn) are involved in neurotransmitter release through an interaction with synaptic vesicles (SVs). Recent studies demonstrated that Cpx and α-Syn preferentially associate with highly curved membranes, like SVs, to correctly position them for fusion. Here, based on recent experimental results, to further propose a possible explanation for this mechanism, we performed in silico simulations probing interactions between Cpx or α-Syn and membranes of varying curvature. We found that the preferential association is attributed to smaller, curved membranes containing more packing defects that expose hydrophobic acyl tails, which may favorably interact with hydrophobic residues of Cpx and α-Syn. The number of membrane defects is proportional to the curvature and the size can be regulated by cholesterol.

Cellular Targeting of Oligonucleotides by Conjugation with Small Molecules

Drug candidates derived from oligonucleotides (ON) are receiving increased attention that is supported by the clinical approval of several ON drugs. Such therapeutic ON are designed to alter the expression levels of specific disease-related proteins, e.g., by displaying antigene, antisense, and RNA interference mechanisms. However, the high polarity of the polyanionic ON and their relatively rapid nuclease-mediated cleavage represent two major pharmacokinetic hurdles for their application in vivo. This has led to a range of non-natural modifications of ON structures that are routinely applied in the design of therapeutic ON. The polyanionic architecture of ON often hampers their penetration of target cells or tissues, and ON usually show no inherent specificity for certain cell types. These limitations can be overcome by conjugation of ON with molecular entities mediating cellular 'targeting', i.e., enhanced accumulation at and/or penetration of a specific cell type. In this context, the use of small molecules as targeting units appears particularly attractive and promising. This review provides an overview of advances in the emerging field of cellular targeting of ON via their conjugation with small-molecule targeting structures.

From cartoons to quantitative models in Golgi transport

BACKGROUND

Cell biology is evolving to become a more formal and quantitative science. In particular, several mathematical models have been proposed to address Golgi self-organisation and protein and lipid transport. However, most scientific articles about the Golgi apparatus are still using static cartoons that miss the dynamism of this organelle.

RESULTS

In this report, we show that schematic drawings of Golgi trafficking can be easily translated into an agent-based model using the Repast platform. The simulations generate an active interplay among cisternae and vesicles rendering quantitative predictions about Golgi stability and transport of soluble and membrane-associated cargoes. The models can incorporate complex networks of molecular interactions and chemical reactions by association with COPASI, a software that handles ordinary differential equations.

CONCLUSIONS

The strategy described provides a simple, flexible and multiscale support to analyse Golgi transport. The simulations can be used to address issues directly linked to the mechanism of transport or as a way to incorporate the complexity of trafficking to other cellular processes that occur in dynamic organelles.

SIGNIFICANCE

We show that the rules implicitly present in most schematic representations of intracellular trafficking can be used to build dynamic models with quantitative outputs that can be compared with experimental results.

The Habc domain of syntaxin 3 is a ubiquitin binding domain

Syntaxins are a family of membrane-anchored SNARE proteins that are essential components required for membrane fusion in eukaryotic intracellular membrane trafficking pathways. Syntaxins contain an N-terminal regulatory domain, termed the Habc domain that is not highly conserved at the primary sequence level but folds into a three-helix bundle that is structurally conserved among family members. The syntaxin Habc domain has previously been found to be structurally very similar to the GAT domain present in GGA family members and related proteins that are otherwise completely unrelated to syntaxins. Because the GAT domain has been found to be a ubiquitin binding domain we hypothesized that the Habc domain of syntaxins may also bind to ubiquitin. Here, we report that the Habc domain of syntaxin 3 (Stx3) indeed binds to monomeric ubiquitin with low affinity. This domain binds efficiently to K63-linked poly-ubiquitin chains within a narrow range of chain lengths but not to K48-linked poly-ubiquitin chains. Other syntaxin family members also bind to K63-linked poly-ubiquitin chains but with different chain length specificities. Molecular modeling suggests that residues of the GGA3-GAT domain known to be important for ionic and hydrophobic interactions with ubiquitin may have equivalent, conserved residues within the Habc domain of Stx3. We conclude that the syntaxin Habc domain and the GAT domain are both structurally and functionally related, and likely share a common ancestry despite sequence divergence. Binding of Ubiquitin to the Habc domain may regulate the function of syntaxins in membrane fusion or may suggest additional functions of this protein family.

Current Methods for Detecting Cell Membrane Transient Interactions

Short-lived cell membrane complexes play a key role in regulating cell signaling and communication. Many of these complexes are formed based on low-affinity and transient interactions among various lipids and proteins. New techniques have emerged to study these previously overlooked membrane transient interactions. Exciting functions of these transient interactions have been discovered in cellular events such as immune signaling, host-pathogen interactions, and diseases such as cancer. In this review, we have summarized current experimental methods that allow us to detect and analyze short-lived cell membrane protein-protein, lipid-protein, and lipid-lipid interactions. These methods can provide useful information about the strengths, kinetics, and/or spatial patterns of membrane transient interactions. However, each method also has its own limitations. We hope this review can be used as a guideline to help the audience to choose proper approaches for studying membrane transient interactions in different membrane trafficking and cell signaling events.

AtNSF regulates leaf serration by modulating intracellular trafficking of PIN1 in Arabidopsis thaliana

In eukaryotes, N-ethylmaleimide-sensitive factor (NSF) is a conserved AAA+ ATPase and a key component of the membrane trafficking machinery that promotes the fusion of secretory vesicles with target membranes. Here, we demonstrate that the Arabidopsis thaliana genome contains a single copy of NSF, AtNSF, which plays an essential role in the regulation of leaf serration. The AtNSF knock-down mutant, atnsf-1, exhibited more serrations in the leaf margin. Moreover, polar localization of the PIN-FORMED1 (PIN1) auxin efflux transporter was diffuse around the margins of atnsf-1 leaves and root growth was inhibited in the atnsf-1 mutant. More PIN1-GFP accumulated in the intracellular compartments of atnsf-1 plants, suggesting that AtNSF is required for intracellular trafficking of PIN between the endosome and plasma membrane. Furthermore, the serration phenotype was suppressed in the atnsf-1 pin1-8 double mutant, suggesting that AtNSF is required for PIN1-mediated polar auxin transport to regulate leaf serration. The CUP-SHAPED COTYLEDON2 (CUC2) transcription factor gene is up-regulated in atnsf-1 plants and the cuc2-3 single mutant exhibits smooth leaf margins, demonstrating that AtNSF also functions in the CUC2 pathway. Our results reveal that AtNSF regulates the PIN1-generated auxin maxima with a CUC2-mediated feedback loop to control leaf serration. This article is protected by copyright. All rights reserved.

Ectodomain Pulling Combines with Fusion Peptide Inserting to Provide Cooperative Fusion for Influenza Virus and HIV

Enveloped viruses include the most dangerous human and animal pathogens, in particular coronavirus, influenza virus, and human immunodeficiency virus (HIV). For these viruses, receptor binding and entry are accomplished by a single viral envelope protein (termed the fusion protein), the structural changes of which trigger the remodeling and merger of the viral and target cellular membranes. The number of fusion proteins required for fusion activity is still under debate, and several studies report this value to range from 1 to 9 for type I fusion proteins. Here, we consider the earliest stage of viral fusion based on the continuum theory of membrane elasticity. We demonstrate that membrane deformations induced by the oblique insertion of amphipathic fusion peptides mediate the lateral interaction of these peptides and drive them to form into a symmetric fusion rosette. The pulling force produced by the structural rearrangements of the fusion protein ectodomains gives additional torque, which deforms the membrane and additionally stabilizes the symmetric fusion rosette, thus allowing a reduction in the number of fusion peptides needed for fusion. These findings can resolve the large range of published cooperativity indices for HIV, influenza, and other type I fusion proteins.

The Arabidopsis exocyst subunits EXO70B1 and EXO70B2 regulate FLS2 homeostasis at the plasma membrane

The plasma membrane (PM)-localized receptor kinase FLAGELLIN SENSING 2 (FLS2) recognizes bacterial flagellin or its immunogenic epitope flg22, and initiates microbe-associated molecular pattern-triggered immunity, which inhibits infection by bacterial pathogens. The localization, abundance and activity of FLS2 are under dynamic control. Here, we demonstrate that Arabidopsis thaliana EXO70B1, a subunit of the exocyst complex, plays a critical role in FLS2 signaling that is independent of the truncated Toll/interleukin-1 receptor-nucleotide binding sequence protein TIR-NBS2 (TN2). In the exo70B1-3 mutant, the abundance of FLS2 protein at the PM is diminished, consistent with the impaired flg22 response of this mutant. EXO70B1-GFP plants showed increased FLS2 accumulation at the PM and therefore enhanced FLS2 signaling. The EXO70B1-mediated trafficking of FLS2 to the PM is partially independent of the PENETRATION 1 (PEN1)-containing secretory pathway. In addition, EXO70B1 interacts with EXO70B2, a close homolog of EXO70B1, and both proteins associate with FLS2 and contribute to the accumulation of FLS2 at the PM. Taken together, our data suggest that the exocyst complex subunits EXO70B1 and EXO70B2 regulate the trafficking of FLS2 to the PM, which represents a new layer of regulation of FLS2 function in plant immunity.

Analysis of Mitochondrial Membrane Fusion GTPase OPA1 Expressed by the Silkworm Expression System

Mitochondria are highly dynamic organelles, which move and fuse to regulate their shape, size, and fundamental function. The dynamin-related GTPases play a critical role in mitochondrial membrane fusion. In vitro reconstitution of membrane fusion using recombinant proteins and model membranes is quite useful in elucidating the molecular mechanisms underlying membrane fusion and to identify the essential elements involved in fusion. However, only a few reconstituting approaches have been reported for mitochondrial fusion machinery due to the difficulty of preparing active recombinant mitochondrial fusion GTPases. Recently, we succeeded in preparing a sufficient amount of recombinant OPA1 involved in mitochondrial inner membrane fusion using a BmNPV bacmid-silkworm expression system. In this chapter, we describe the method for the expression and purification of a membrane-anchored form of OPA1 and liposome-based in vitro reconstitution of membrane fusion.

RAB7 GTPase regulates actin dynamics for DRP1-mediated mitochondria function and spindle migration in mouse oocyte meiosis

RAB7 is a small GTPase that belongs to the Rab family, and as a vesicle trafficking factor it is shown to regulate the transport to late endocytic compartments, autophagosome maturation and organelle function. In present study, we showed the critical roles of RAB7 GTPase on actin dynamics and mitochondria function in oocyte meiosis. RAB7 mainly accumulated at cortex and spindle periphery during oocyte maturation. RAB7 depletion caused the failure of polar body extrusion and asymmetric division, and Rab7 exogenous mRNA supplement could rescue the defects caused by RAB7 RNAi. Based on mass spectrometry analysis, we found that RAB7 associated with several actin nucleation factors and mitochondria-related proteins in oocytes. The depletion of RAB7 caused the decrease of actin dynamics, which further affected meiotic spindle migration to the oocyte cortex. In addition, we found that RAB7 could maintain mitochondrial membrane potential and the mitochondrial distribution in mouse oocytes, and this might be due to its effects on the phosphorylation of DRP1 at Ser616 domain. Taken together, our data indicated that RAB7 transported actin nucleation factor for actin polarization, which further affected the phosphorylation of DRP1 for mitochondria dynamics and the meiotic spindle migration in mouse oocytes.

Continuum Models of Membrane Fusion: Evolution of the Theory

Starting from fertilization, through tissue growth, hormone secretion, synaptic transmission, and sometimes morbid events of carcinogenesis and viral infections, membrane fusion regulates the whole life of high organisms. Despite that, a lot of fusion processes still lack well-established models and even a list of main actors. A merger of membranes requires their topological rearrangements controlled by elastic properties of a lipid bilayer. That is why continuum models based on theories of membrane elasticity are actively applied for the construction of physical models of membrane fusion. Started from the view on the membrane as a structureless film with postulated geometry of fusion intermediates, they developed along with experimental and computational techniques to a powerful tool for prediction of the whole process with molecular accuracy. In the present review, focusing on fusion processes occurring in eukaryotic cells, we scrutinize the history of these models, their evolution and complication, as well as open questions and remaining theoretical problems. We show that modern approaches in this field allow continuum models of membrane fusion to stand shoulder to shoulder with molecular dynamics simulations, and provide the deepest understanding of this process in multiple biological systems.

Structural analysis of a trimeric assembly of the mitochondrial dynamin-like GTPase Mgm1

Mitochondria are dynamic double-membraned organelles, which undergo cycles of division and fusion mediated by dynamin-related GTPases. How dynamins mediate mitochondrial inner membrane fusion is not understood. In humans, defective inner membrane fusion causes a degenerative form of blindness termed dominant optic atrophy. Here, we solve the structure of a trimer assembly of an inner membrane fusion dynamin. Our observations underscore the structural plasticity of dynamins and suggest a previously unidentified mode of how they might mediate membrane fusion events.

The fusion of inner mitochondrial membranes requires dynamin-like GTPases, Mgm1 in yeast and OPA1 in mammals, but how they mediate membrane fusion is poorly understood. Here, we determined the crystal structure of Saccharomyces cerevisiae short Mgm1 (s-Mgm1) in complex with GDP. It revealed an N-terminal GTPase (G) domain followed by two helix bundles (HB1 and HB2) and a unique C-terminal lipid-interacting stalk (LIS). Dimers can form through antiparallel HB interactions. Head-to-tail trimers are built by intermolecular interactions between the G domain and HB2-LIS. Biochemical and in vivo analyses support the idea that the assembly interfaces observed here are native and critical for Mgm1 function. We also found that s-Mgm1 interacts with negatively charged lipids via both the G domain and LIS. Based on these observations, we propose that membrane targeting via the G domain and LIS facilitates the in cis assembly of Mgm1, potentially generating a highly curved membrane tip to allow inner membrane fusion.

Polymerization-Induced Polymersome Fusion

The dynamic interactions of membranes, particularly their fusion and fission, are critical for the transmission of chemical information between cells. Fusion is primarily driven by membrane tension built up through membrane deformation. For artificial polymersomes, fusion is commonly induced via the external application of a force field. Herein, fusion-promoted development of anisotropic tubular polymersomes (tubesomes) was achieved in the absence of an external force by exploiting the unique features of aqueous ring-opening metathesis polymerization-induced self-assembly (ROMPISA). The out-of-equilibrium tubesome morphology was found to arise spontaneously during polymerization, and the composition of each tubesome sample (purity and length distribution) could be manipulated simply by targeting different core-block degrees of polymerization (DPs). The evolution of tubesomes was shown to occur via fusion of "monomeric" spherical polymersomes, evidenced most notably by a step-growth-like relationship between the fraction of tubular to spherical nano-objects and the average number of fused particles per tubesome (analogous to monomer conversion and DP, respectively). Fusion was also confirmed by Förster resonance energy transfer (FRET) studies to show membrane blending and confocal microscopy imaging to show mixing of the polymersome lumens. We term this unique phenomenon polymerization-induced polymersome fusion, which operates via the buildup of membrane tension exerted by the growing polymer chains. Given the growing body of evidence demonstrating the importance of nanoparticle shape on biological activity, our methodology provides a facile route to reproducibly obtain samples containing mixtures of spherical and tubular polymersomes, or pure samples of tubesomes, of programmed length. Moreover, the capability to mix the interior aqueous compartments of polymersomes during polymerization-induced fusion also presents opportunities for its application in catalysis, small molecule trafficking, and drug delivery.

A protocell with fusion and division

A protocell is a synthetic form of cellular life that is constructed from phospholipid vesicles and used to understand the emergence of life from a nonliving chemical network. To be considered 'living', a protocell should be capable of self-proliferation, which includes successive growth and division processes. The growth of protocells can be achieved via vesicle fusion approaches. In this review, we provide a brief overview of recent research on the formation of a protocell, fusion and division processes of the protocell, and encapsulation of a defined chemical network such as the genetic material. We also provide some perspectives on the challenges and future developments of synthetic protocell research.

Identifying SNAREs by Incorporating Deep Learning Architecture and Amino Acid Embedding Representation

SNAREs (soluble N-ethylmaleimide-sensitive factor activating protein receptors) are a group of proteins that are crucial for membrane fusion and exocytosis of neurotransmitters from the cell. They play an important role in a broad range of cell processes, including cell growth, cytokinesis, and synaptic transmission, to promote cell membrane integration in eukaryotes. Many studies determined that SNARE proteins have been associated with a lot of human diseases, especially in cancer. Therefore, identifying their functions is a challenging problem for scientists to better understand the cancer disease as well as design the drug targets for treatment. We described each protein sequence based on the amino acid embeddings using fastText, which is a natural language processing model performing well in its field. Because each protein sequence is similar to a sentence with different words, applying language model into protein sequence is challenging and promising. After generating, the amino acid embedding features were fed into a deep learning algorithm for prediction. Our model which combines fastText model and deep convolutional neural networks could identify SNARE proteins with an independent test accuracy of 92.8%, sensitivity of 88.5%, specificity of 97%, and Matthews correlation coefficient (MCC) of 0.86. Our performance results were superior to the state-of-the-art predictor (SNARE-CNN). We suggest this study as a reliable method for biologists for SNARE identification and it serves a basis for applying fastText word embedding model into bioinformatics, especially in protein sequencing prediction.

Sec20 is Required for Autophagic and Endocytic Degradation Independent of Golgi-ER Retrograde Transport

Endocytosis and autophagy are evolutionarily conserved degradative processes in all eukaryotes. Both pathways converge to the lysosome where cargo is degraded. Improper lysosomal degradation is observed in many human pathologies, so its regulatory mechanisms are important to understand. Sec20/BNIP1 (BCL2/adenovirus E1B 19 kDa protein-interacting protein 1) is a BH3 (Bcl-2 homology 3) domain-containing SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors) protein that has been suggested to promote Golgi-ER retrograde transport, mitochondrial fission, apoptosis and mitophagy in yeast and vertebrates. Here, we show that loss of Sec20 in Drosophila fat cells causes the accumulation of autophagic vesicles and prevents proper lysosomal acidification and degradation during bulk, starvation-induced autophagy. Furthermore, Sec20 knockdown leads to the enlargement of late endosomes and accumulation of defective endolysosomes in larval Drosophila nephrocytes. Importantly, the loss of Syx18 (Syntaxin 18), one of the known partners of Sec20, led to similar changes in nephrocytes and fat cells. Interestingly. Sec20 appears to function independent of its role in Golgi-ER retrograde transport in regulating lysosomal degradation, as the loss of its other partner SNAREs Use1 (Unconventional SNARE In The ER 1) and Sec22 or tethering factor Zw10 (Zeste white 10), which function together in the Golgi-ER pathway, does not cause defects in autophagy or endocytosis. Thus, our data identify a potential new transport route specific to lysosome biogenesis and function.

Phosphatidylinositol 4,5 Bisphosphate Controls the cis and trans Interactions of Synaptotagmin 1

Synaptotagmin 1 acts as the Ca²⁺ sensor for synchronous neurotransmitter release; however, the mechanism by which it functions is not understood and is presently a topic of considerable interest. Here, we describe measurements on full-length membrane-reconstituted synaptotagmin 1 using site-directed spin labeling in which we characterize the linker region as well as the cis (vesicle membrane) and trans (cytoplasmic membrane) binding of its two C2 domains. In the full-length protein, the C2A domain does not undergo membrane insertion in the absence of Ca²⁺; however, the C2B domain will bind to and penetrate in trans to a membrane containing phosphatidylinositol 4,5 bisphosphate, even if phosphatidylserine (PS) is present in the cis membrane. In the presence of Ca²⁺, the Ca²⁺ binding loops of C2A and C2B both insert into the membrane interface; moreover, C2A preferentially inserts into PS-containing bilayers and will bind in a cis configuration to membranes containing PS even if a phosphatidylinositol 4,5 bisphosphate membrane is presented in trans. The data are consistent with a bridging activity for synaptotagmin 1 in which the two domains bind to opposing vesicle and plasma membranes. The failure of C2A to bind membranes in the absence of Ca²⁺ and the long unstructured segment linking C2A to the vesicle membrane indicates that synaptotagmin 1 could act to significantly shorten the vesicle-plasma membrane distance with increasing levels of Ca²⁺.

Vps8 overexpression inhibits HOPS-dependent trafficking routes by outcompeting Vps41/Lt

Two related multisubunit tethering complexes promote endolysosomal trafficking in all eukaryotes: Rab5-binding CORVET that was suggested to transform into Rab7-binding HOPS. We have previously identified miniCORVET, containing Drosophila Vps8 and three shared core proteins, which are required for endosome maturation upstream of HOPS in highly endocytic cells (Lőrincz et al., 2016a). Here, we show that Vps8 overexpression inhibits HOPS-dependent trafficking routes including late endosome maturation, autophagosome-lysosome fusion, crinophagy and lysosome-related organelle formation. Mechanistically, Vps8 overexpression abolishes the late endosomal localization of HOPS-specific Vps41/Lt and prevents HOPS assembly. Proper ratio of Vps8 to Vps41 is thus critical because Vps8 negatively regulates HOPS by outcompeting Vps41. Endosomal recruitment of miniCORVET- or HOPS-specific subunits requires proper complex assembly, and Vps8/miniCORVET is dispensable for autophagy, crinophagy and lysosomal biogenesis. These data together indicate the recruitment of these complexes to target membranes independent of each other in Drosophila, rather than their transformation during vesicle maturation.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Glycan-decorated protocells: novel features for rebuilding cellular processes

In synthetic biology approaches, lipid vesicles are widely used as protocell models. While many compounds have been encapsulated in vesicles (e.g. DNA, cytoskeleton and enzymes), the incorporation of glycocalyx components in the lipid bilayer has attracted much less attention so far. In recent years, glycoconjugates have been integrated in the membrane of giant unilamellar vesicles (GUVs). These minimal membrane systems have largely contributed to shed light on the molecular mechanisms of cellular processes. In this review, we first introduce several preparation and biophysical characterization methods of GUVs. Then, we highlight specific applications of protocells investigating glycolipid-mediated endocytosis of toxins, viruses and bacteria. In addition, we delineate how prototissues have been assembled from glycan-decorated protocells by using lectin-mediated cross-linking of opposed glycoreceptors (e.g. glycolipids and glycopeptides). In future applications, glycan-decorated protocells might be useful for investigating cell-cell interactions (e.g. adhesion and communication). We also speculate about the implication of lectin-glycoreceptor interactions in membrane fusion processes.

ELKS active zone proteins as multitasking scaffolds for secretion

Synaptic vesicle exocytosis relies on the tethering of release ready vesicles close to voltage-gated Ca²⁺ channels and specific lipids at the future site of fusion. This enables rapid and efficient neurotransmitter secretion during presynaptic depolarization by an action potential. Extensive research has revealed that this tethering is mediated by an active zone, a protein dense structure that is attached to the presynaptic plasma membrane and opposed to postsynaptic receptors. Although roles of individual active zone proteins in exocytosis are in part understood, the molecular mechanisms that hold the protein scaffold at the active zone together and link it to the presynaptic plasma membrane have remained unknown. This is largely due to redundancy within and across scaffolding protein families at the active zone. Recent studies, however, have uncovered that ELKS proteins, also called ERC, Rab6IP2 or CAST, act as active zone scaffolds redundant with RIMs. This redundancy has led to diverse synaptic phenotypes in studies of ELKS knockout mice, perhaps because different synapses rely to a variable extent on scaffolding redundancy. In this review, we first evaluate the need for presynaptic scaffolding, and we then discuss how the diverse synaptic and non-synaptic functional roles of ELKS support the hypothesis that ELKS provides molecular scaffolding for organizing vesicle traffic at the presynaptic active zone and in other cellular compartments.

Blocking skeletal muscle DHPRs/Ryr1 prevents neuromuscular synapse loss in mutant mice deficient in type III Neuregulin 1 (CRD-Nrg1)

Schwann cells are integral components of vertebrate neuromuscular synapses; in their absence, pre-synaptic nerve terminals withdraw from post-synaptic muscles, leading to muscle denervation and synapse loss at the developing neuromuscular junction (NMJ). Here, we report a rescue of muscle denervation and neuromuscular synapses loss in type III Neuregulin 1 mutant mice (CRD-Nrg1-/-), which lack Schwann cells. We found that muscle denervation and neuromuscular synapse loss were prevented in CRD-Nrg1-/-mice when presynaptic activity was blocked by ablating a specific gene, such as Snap25 (synaptosomal-associated 25 kDa protein) or Chat (choline acetyltransferase). Further, these effects were mediated by a pathway that requires postsynaptic acetylcholine receptors (AChRs), because ablating Chrna1 (acetylcholine receptor α1 subunit), which encodes muscle-specific AChRs in CRD-Nrg1-/-mice also rescued muscle denervation. Moreover, genetically ablating muscle dihydropyridine receptor (DHPR) β1 subunit (Cacnb1) or ryanodine receptor 1 (Ryr1) also rescued muscle denervation and neuromuscular synapse loss in CRD-Nrg1-/-mice. Thus, these genetic manipulations follow a pathway-from presynaptic to postsynaptic, and, ultimately to muscle activity mediated by DHPRs and Ryr1. Importantly, electrophysiological analyses reveal robust synaptic activity in the rescued, Schwann-cell deficient NMJs in CRD-Nrg1-/-Cacnb1-/-or CRD-Nrg1-/-Ryr1-/-mutant mice. Thus, a blockade of synaptic activity, although sufficient, is not necessary to preserve NMJs that lack Schwann cells. Instead, a blockade of muscle activity mediated by DHRPs and Ryr1 is both necessary and sufficient for preserving NMJs that lack Schwann cells. These findings suggest that muscle activity mediated by DHPRs/Ryr1 may destabilize developing NMJs and that Schwann cells play crucial roles in counteracting such a destabilizing activity to preserve neuromuscular synapses during development.

SNARE-CNN: a 2D convolutional neural network architecture to identify SNARE proteins from high-throughput sequencing data

Deep learning has been increasingly and widely used to solve numerous problems in various fields with state-of-the-art performance. It can also be applied in bioinformatics to reduce the requirement for feature extraction and reach high performance. This study attempts to use deep learning to predict SNARE proteins, which is one of the most vital molecular functions in life science. A functional loss of SNARE proteins has been implicated in a variety of human diseases (e.g., neurodegenerative, mental illness, cancer, and so on). Therefore, creating a precise model to identify their functions is a crucial problem for understanding these diseases, and designing the drug targets. Our SNARE-CNN model which uses two-dimensional convolutional neural networks and position-specific scoring matrix profiles could identify SNARE proteins with achieved sensitivity of 76.6%, specificity of 93.5%, accuracy of 89.7%, and MCC of 0.7 in cross-validation dataset. We also evaluate the performance of our model via an independent dataset and the result shows that we are able to solve the overfitting problem. Compared with other state-of-the-art methods, this approach achieved significant improvement in all of the metrics. Throughout the proposed study, we provide an effective model for identifying SNARE proteins and a basis for further research that can apply deep learning in bioinformatics, especially in protein function prediction. SNARE-CNN are freely available at https://github.com/khanhlee/snare-cnn.

High efficiency liposome fusion induced by reducing undesired membrane peptides interaction

A full membrane fusion model which attains both complete lipid mixing and content mixing liposomal membranes mediated by coiled-coil forming lipopeptides LPK [L-PEG12-(KIAALKE)3] and LPE [L-PEG12-(EIAALEK)3] is presented. The electrostatic effects of lipid anchored peptides on fusion efficiency was investigated. For this, the original amino acid sequence of the membrane bound LPK was varied at its ‘f’-position of the helical structure, i.e. via mutating the anionic glutamate residues by either neutral serines or cationic lysines. Both CD and fluorescence measurements showed that replacing the negatively charged glutamate did not significantly alter the peptide ability to form a coiled coil, but lipid mixing and content mixing assays showed more efficient liposome-liposome fusion resulting in almost quantitative content mixing for the lysine mutated analogue (LPKK) in conjunction with LPE. A mechanism is proposed for a fusion model triggered by membrane destabilizing effects mediated by the membrane destabilizing activety of LPK in cooperation with the electrostatic activity of LPE. This new insight may enlightens the further development of a promising nano carrier tool for biomedical applications.

Exocyst subunit Sec6 is positioned by microtubule overlaps in the moss phragmoplast prior to cell plate membrane arrival

During plant cytokinesis a radially expanding membrane-enclosed cell plate is formed from fusing vesicles that compartmentalizes the cell in two. How fusion is spatially restricted to the site of cell plate formation is unknown. Aggregation of cell-plate membrane starts near regions of microtubule overlap within the bipolar phragmoplast apparatus of the moss Physcomitrella patens Since vesicle fusion generally requires coordination of vesicle tethering and subsequent fusion activity, we analyzed the subcellular localization of several subunits of the exocyst, a tethering complex active during plant cytokinesis. We found that the exocyst complex subunit Sec6 but not the Sec3 or Sec5 subunits localized to microtubule overlap regions in advance of cell plate construction in moss. Moreover, Sec6 exhibited a conserved physical interaction with an ortholog of the Sec1/Munc18 protein KEULE, an important regulator for cell-plate membrane vesicle fusion in Arabidopsis Recruitment of the P. patens protein KEULE and vesicles to the early cell plate was delayed upon Sec6 gene silencing. Our findings, thus, suggest that vesicle-vesicle fusion is, in part, enabled by a pool of exocyst subunits at microtubule overlaps, which is recruited independently of vesicle delivery.

A Golgi-Released Subpopulation of the Trans-Golgi Network Mediates Protein Secretion in Arabidopsis

Spatiotemporal coordination of protein trafficking among organelles is essential for eukaryotic cells. The post-Golgi interface, including the trans-Golgi network (TGN), is a pivotal hub for multiple trafficking pathways. The Golgi-released independent TGN (GI-TGN) is a compartment described only in plant cells, and its cellular and physiological roles remain elusive. In Arabidopsis (Arabidopsis thaliana), the SYNTAXIN OF PLANTS (SYP) 4 group Qa-SNARE (soluble N-ethylmaleimide) membrane fusion proteins are shared components of TGN and GI-TGN and regulate secretory and vacuolar transport. Here we reveal that GI-TGNs mediate the transport of the R-SNARE VESICLE-ASSOCIATED MEMBRANE PROTEIN (VAMP) 721 to the plasma membrane. In interactions with a nonadapted powdery mildew pathogen, the SYP4 group of SNAREs is required for the dynamic relocation of VAMP721 to plant-fungus contact sites via GI-TGNs, thereby facilitating complex formation with its cognate SNARE partner PENETRATION1 to restrict pathogen entry. Furthermore, quantitative proteomic analysis of leaf apoplastic fluid revealed constitutive and pathogen-inducible secretion of cell wall-modification enzymes in a SYP4- and VAMP721-dependent manner. Hence, the GI-TGN acts as a transit compartment between the Golgi apparatus and the plasma membrane. We propose a model in which the GA-TGN matures into the GI-TGN and then into secretory vesicles by increasing the abundance of VAMP721-dependent secretory pathway components.

Robust CRISPR/Cas9-Mediated Tissue-Specific Mutagenesis Reveals Gene Redundancy and Perdurance in Drosophila

Tissue-specific loss-of-function (LOF) analysis is essential for characterizing gene function. Here, we present a simple, yet highly efficient, clustered regularly interspaced short palindromic repeats (CRISPR)-mediated tissue-restricted mutagenesis (CRISPR-TRiM) method for ablating gene function in Drosophila This binary system consists of a tissue-specific Cas9 and a ubiquitously expressed multi-guide RNA (gRNA) transgene. We describe convenient toolkits for making enhancer-driven Cas9 lines and multi-gRNAs that are optimized for mutagenizing somatic cells. We demonstrate that insertions or deletions in coding sequences more reliably cause somatic mutations than DNA excisions induced by two gRNAs. We further show that enhancer-driven Cas9 is less cytotoxic yet results in more complete LOF than Gal4-driven Cas9 in larval sensory neurons. Finally, CRISPR-TRiM efficiently unmasks redundant soluble N-ethylmaleimide-sensitive factor attachment protein receptor gene functions in neurons and epidermal cells. Importantly, Cas9 transgenes expressed at different times in the neuronal lineage reveal the extent to which gene products persist in cells after tissue-specific gene knockout. These CRISPR tools can be applied to analyze tissue-specific gene function in many biological processes.

Highly Efficient Protein-free Membrane Fusion: A Giant Vesicle Study

Membrane fusion is a ubiquitous process in biology and is a prerequisite for many intracellular delivery protocols relying on the use of liposomes as drug carriers. Here, we investigate in detail the process of membrane fusion and the role of opposite charges in a protein-free lipid system based on cationic liposomes (LUVs, large unilamellar vesicles) and anionic giant unilamellar vesicles (GUVs) composed of different palmitoyloleoylphosphatidylcholine (POPC)/palmitoyloleoylphosphatidylglycerol (POPG) molar ratios. By using a set of optical-microscopy- and microfluidics-based methods, we show that liposomes strongly dock to GUVs of pure POPC or low POPG fraction (up to 10 mol%) in a process mainly associated with hemifusion and membrane tension increase, commonly leading to GUV rupture. On the other hand, docked LUVs quickly and very efficiently fuse with negative GUVs of POPG fractions at or above 20 mol%, resulting in dramatic GUV area increase in a charge-dependent manner; the vesicle area increase is deduced from GUV electrodeformation. Importantly, both hemifusion and full fusion are leakage-free. Fusion efficiency is quantified by the lipid transfer from liposomes to GUVs using fluorescence resonance energy transfer (FRET), which leads to consistent results when compared to fluorescence-lifetime-based FRET. We develop an approach to deduce the final composition of single GUVs after fusion based on the FRET efficiency. The results suggest that fusion is driven by membrane charge and appears to proceed up to charge neutralization of the acceptor GUV.

Biochemical studies of membrane fusion at the single-particle level

To study membrane fusion mediated by synaptic proteins, proteoliposomes have been widely used for in vitro ensemble measurements with limited insights into the fusion mechanism. Single-particle techniques have proven to be powerful in overcoming the limitations of traditional ensemble methods. Here, we summarize current single-particle methods in biophysical and biochemical studies of fusion mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and other synaptic proteins, together with their advantages and limitations.

An In Vitro Assay of Trans-SNARE Complex Formation During Yeast Vacuole Fusion Using Epitope Tag-Free SNAREs

SNARE complexes assembled between fusing membranes (in trans) are the core machinery driving lipid bilayer merger. Thus, an assay monitoring the formation of these trans-SNARE complexes is essential for SNARE-mediated membrane fusion studies. Homotypic yeast vacuole fusion is an important model system for such studies. Although several assays measuring trans-SNARE complex formation are available to study yeast vacuole fusion, most use SNAREs conjugated with epitope tags, which may affect the function of SNAREs or even the formation of trans-SNARE complexes. Here, I describe an assay for trans-SNARE complex formation during yeast vacuole fusion that does not require epitope-tagged SNAREs.

Reconstituted Proteoliposome Fusion Mediated by Yeast SNARE-Family Proteins

Membrane fusion mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-family proteins is an essential process for intracellular membrane trafficking in all eukaryotic cells, which delivers proteins and lipids to their appropriate subcellular membrane compartments such as organelles and plasma membrane. The molecular basis of SNARE-mediated membrane fusion has been revealed by studying fusion of reconstituted proteoliposomes bearing purified SNARE-family proteins and chemically defined lipid species. This chapter describes the detailed experimental protocols for (1) purification of recombinant SNARE-family and SM (Sec1/Munc18-family) proteins in the yeast Saccharomyces cerevisiae; (2) preparation of reconstituted proteoliposomes bearing purified yeast SNARE proteins; and (3) developing an assay to monitor lipid mixing between reconstituted SNARE-bearing proteoliposomes. Lipid mixing assays for reconstituted SNARE-bearing proteoliposomes are useful for evaluating the intrinsic capacity of SNARE-family proteins to directly catalyze membrane fusion and to determine the specificity of membrane fusion.

Single-Molecule Fluorescence Measurement of SNARE-Mediated Vesicle Fusion

This chapter expounds the single vesicle-vesicle fluorescence resonance energy transfer (FRET) measurement to study the membrane fusion mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. The formation of a four-α-helix bundle of SNARE proteins can drive two membranes to a close proximity for fusion. Through single-molecule FRET-based microscopy, the lipid-mixing process at the single-vesicle level can be tracked in real time. This reconstitution system is applicable to study the molecular mechanism of SNAREs during different membrane fusion stages, such as docking, hemifusion, and full fusion. Four main parts are described in this chapter, including SNARE reconstitution, imaging preparation, data collection, and analysis.

Functional Reconstitution of Intracellular Vesicle Fusion Using Purified SNAREs and Sec1/Munc18 (SM) Proteins

The fusion of intracellular vesicles with target membranes is mediated by two classes of conserved molecules-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAP receptors or SNAREs) and Sec1/Munc18 (SM) proteins. A conserved function of SM proteins is to recognize their cognate trans-SNARE complexes and accelerate fusion kinetics. Here, we describe a physiologically relevant reconstitution system in which macromolecular crowding agents are included to recapitulate the crowded intracellular environment. Through this system, we elucidate the molecular mechanisms by which SNAREs and SM proteins drive vesicle fusion.

Species-Specific Functional Regions of the Green Alga Gamete Fusion Protein HAP2 Revealed by Structural Studies

The cellular fusion protein HAP2, which is structurally homologous to viral class II fusion proteins, drives gamete fusion across several eukaryotic kingdoms. Gamete fusion is a highly controlled process in eukaryotes, and is allowed only between same species gametes. In spite of a conserved architecture, HAP2 displays several species-specific functional regions that were not resolved in the available X-ray structure of the green alga Chlamydomonas reinhardtii HAP2 ectodomain. Here we present an X-ray structure resolving these regions, showing a target membrane interaction surface made by three amphipathic helices in a horseshoe-shaped arrangement. HAP2 from green algae also features additional species-specific motifs inserted in regions that in viral class II proteins are critical for the fusogenic conformational change. Such insertions include a cystine ladder-like module evocative of EGF-like motifs responsible for extracellular protein-protein interactions in animals, and a mucin-like region. These features suggest potential HAP2 interaction sites involved in gamete fusion control.

•

Unprecedented organization of amphipathic α helices in the algal HAP2 fusion loops

•

An inserted EGF-like motif suggests a potential algal-specific fusion control site

•

An adjacent mucin-like region potentially modulates algal-specific interactions

•

Inter-chain stem/domain II interactions stabilize the post-fusion hairpin conformation

Membrane Destabilization Induced by Lipid Species Increases Activity of Phosphorothioate-Antisense Oligonucleotides

Chemically modified antisense oligonucleotides with phosphorothioate linkages (PS-ASOs) mediate site-specific cleavage of RNA by RNase H1 and are broadly used as research and therapeutic tools. PS-ASOs can enter cells via endocytic pathways and escape from membrane-enclosed endocytic organelles to reach target RNAs. We recently found that lysobisphosphatidic acid is required for release of PS-ASOs from late endosomes. Here, we evaluated the effects of other lipids on PS-ASO intracellular trafficking and activities. We show that free fatty acids, ceramide, and cholesterol increase PS-ASO activities. Free fatty acids induced formation of lipid droplets without changing the intracellular localization of PS-ASOs in early or late endosomes. Ceramide and cholesterol did not obviously induce the formation of lipid droplets, but cholesterol caused enlargement of endosome size and volume. Although none of those lipids appeared to influence PS-ASO internalization or intracellular trafficking processes, all led to an increase in leakiness of late endosomes. Thus, the membrane destabilization induced by these lipids likely contributes to PS-ASO release from late endosomes, which, in turn, increases PS-ASO activity.

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.

Fluorescence Activated Cell Sorting (FACS) in Genome-Wide Genetic Screening of Membrane Trafficking

About one-third of cellular proteins in eukaryotic cells are localized to membrane-enclosed organelles in the endomembrane system. Trafficking of these membrane proteins (including soluble lumenal proteins) among the organelles is mediated by small sac-like vesicles. Vesicle-mediated membrane trafficking regulates a broad range of biological processes, many of which are still poorly understood at the molecular level. A powerful approach to dissect a vesicle-mediated membrane trafficking pathway is unbiased genome-wide genetic screening, which only recently became possible in mammalian cells with the isolation of haploid human cell lines and the development of CRISPR-Cas9 genome editing. Here, we describe a FACS-based method to select populations of live mutant cells based on the surface levels of endogenous proteins or engineered reporters. Collection of these mutant populations enables subsequent deep sequencing and bioinformatics analysis to identify genes that regulate the trafficking pathway. This method can be readily adapted to genetically dissect a broad range of mammalian membrane trafficking processes using haploid genetics or CRISPR-Cas9 screens. © 2018 by John Wiley & Sons, Inc.

Acute depletion of diacylglycerol from the cis-Golgi affects localized nuclear envelope morphology during mitosis

Dysregulation of nuclear envelope (NE) assembly results in various cancers; for example, renal and some lung carcinomas ensue due to NE malformation. The NE is a dynamic membrane compartment and its completion during mitosis is a highly regulated process, but the detailed mechanism still remains incompletely understood. Previous studies have found that isolated diacylglycerol (DAG)-containing vesicles are essential for completing the fusion of the NE in nonsomatic cells. We investigated the impact of DAG depletion from the cis-Golgi in mammalian cells on NE reassembly. Using advanced electron microscopy, we observed an enriched DAG population of vesicles at the vicinity of the NE gaps of telophase mammalian cells. We applied a mini singlet oxygen generator-C1-domain tag that localized DAG-enriched vesicles at the perinuclear region, which suggested the existence of NE fusogenic vesicles. We quantified the impact of Golgi-DAG depletion by measuring the in situ NE rim curvature of the reforming NE. The rim curvature in these cells was significantly reduced compared with controls, which indicated a localized defect in NE morphology. Our novel results demonstrate the significance of the role of DAG from the cis-Golgi for the regulation of NE assembly.