The roles of NSF, SNAPs and SNAREs during membrane fusion

Neurons, Glia, Extracellular Matrix and Neurovascular Unit: A Systems Biology Approach to the Complexity of Synaptic Plasticity in Health and Disease

The synaptic cleft has been vastly investigated in the last decades, leading to a novel and fascinating model of the functional and structural modifications linked to synaptic transmission and brain processing. The classic neurocentric model encompassing the neuronal pre- and post-synaptic terminals partly explains the fine-tuned plastic modifications under both pathological and physiological circumstances. Recent experimental evidence has incontrovertibly added oligodendrocytes, astrocytes, and microglia as pivotal elements for synapse formation and remodeling (tripartite synapse) in both the developing and adult brain. Moreover, synaptic plasticity and its pathological counterpart (maladaptive plasticity) have shown a deep connection with other molecular elements of the extracellular matrix (ECM), once considered as a mere extracellular structural scaffold altogether with the cellular glue (i.e., glia). The ECM adds another level of complexity to the modern model of the synapse, particularly, for the long-term plasticity and circuit maintenance. This model, called tetrapartite synapse, can be further implemented by including the neurovascular unit (NVU) and the immune system. Although they were considered so far as tightly separated from the central nervous system (CNS) plasticity, at least in physiological conditions, recent evidence endorsed these elements as structural and paramount actors in synaptic plasticity. This scenario is, as far as speculations and evidence have shown, a consistent model for both adaptive and maladaptive plasticity. However, a comprehensive understanding of brain processes and circuitry complexity is still lacking. Here we propose that a better interpretation of the CNS complexity can be granted by a systems biology approach through the construction of predictive molecular models that enable to enlighten the regulatory logic of the complex molecular networks underlying brain function in health and disease, thus opening the way to more effective treatments.

Perineuronal Nets and Their Role in Synaptic Homeostasis

Extracellular matrix (ECM) molecules that are released by neurons and glial cells form perineuronal nets (PNNs) and modulate many neuronal and glial functions. PNNs, whose structure is still not known in detail, surround cell bodies and dendrites, which leaves free space for synapses to come into contact. A reduction in the expression of many neuronal ECM components adversely affects processes that are associated with synaptic plasticity, learning, and memory. At the same time, increased ECM activity, e.g., as a result of astrogliosis following brain damage or in neuroinflammation, can also have harmful consequences. The therapeutic use of enzymes to attenuate elevated neuronal ECM expression after injury or in Alzheimer’s disease has proven to be beneficial by promoting axon growth and increasing synaptic plasticity. Yet, severe impairment of ECM function can also lead to neurodegeneration. Thus, it appears that to ensure healthy neuronal function a delicate balance of ECM components must be maintained. In this paper we review the structure of PNNs and their components, such as hyaluronan, proteoglycans, core proteins, chondroitin sulphate proteoglycans, tenascins, and Hapln proteins. We also characterize the role of ECM in the functioning of the blood-brain barrier, neuronal communication, as well as the participation of PNNs in synaptic plasticity and some clinical aspects of perineuronal net impairment. Furthermore, we discuss the participation of PNNs in brain signaling. Understanding the molecular foundations of the ways that PNNs participate in brain signaling and synaptic plasticity, as well as how they change in physiological and pathological conditions, may help in the development of new therapies for many degenerative and inflammatory diseases of the brain.

Gene encoding vesicle-associated membrane protein-associated protein from Triticum aestivum (TaVAP) confers tolerance to drought stress

Abiotic stresses like drought, salinity, high and low temperature, and submergence are major factors that limit the crop productivity. Hence, identification of genes associated with stress response in crops is a prerequisite for improving their tolerance to adverse environmental conditions. In an earlier study, we had identified a drought-inducible gene, vesicle-associated membrane protein-associated protein (TaVAP), in developing grains of wheat. In this study, we demonstrate that TaVAP is able to complement yeast and Arabidopsis mutants, which are impaired in their respective orthologs, signifying functional conservation. Constitutive expression of TaVAP in Arabidopsis imparted tolerance to water stress conditions without any apparent yield penalty. Enhanced tolerance to water stress was associated with maintenance of higher relative water content, photosynthetic efficiency, and antioxidant activities. Compared to wild type, the TaVAP-overexpressing plants showed enhanced lateral root proliferation that was attributed to higher endogenous levels of IAA. These studies are the first to demonstrate that TaVAP plays a critical role in growth and development in plants, and is a potential candidate for improving the abiotic stress tolerance in crop plants.

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors required during Trypanosoma cruzi parasitophorous vacuole development

Trypanosoma cruzi, the etiologic agent of Chagas disease, is an obligate intracellular parasite that exploits different host vesicular pathways to invade the target cells. Vesicular and target soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are key proteins of the intracellular membrane fusion machinery. During the early times of T. cruzi infection, several vesicles are attracted to the parasite contact sites in the plasma membrane. Fusion of these vesicles promotes the formation of the parasitic vacuole and parasite entry. In this work, we study the requirement and the nature of SNAREs involved in the fusion events that take place during T. cruzi infection. Our results show that inhibition of N-ethylmaleimide-sensitive factor protein, a protein required for SNARE complex disassembly, impairs T. cruzi infection. Both TI-VAMP/VAMP7 and cellubrevin/VAMP3, two v-SNAREs of the endocytic and exocytic pathways, are specifically recruited to the parasitophorous vacuole membrane in a synchronized manner but, although VAMP3 is acquired earlier than VAMP7, impairment of VAMP3 by tetanus neurotoxin fails to reduce T. cruzi infection. In contrast, reduction of VAMP7 activity by expression of VAMP7's longin domain, depletion by small interfering RNA or knockout, significantly decreases T. cruzi infection susceptibility as a result of a minor acquisition of lysosomal components to the parasitic vacuole. In addition, overexpression of the VAMP7 partner Vti1b increases the infection, whereas expression of a KIF5 kinesin mutant reduces VAMP7 recruitment to vacuole and, concomitantly, T. cruzi infection. Altogether, these data support a key role of TI-VAMP/VAMP7 in the fusion events that culminate in the T. cruzi parasitophorous vacuole development.

Altered mechanisms underlying the abnormal glutamate release in amyotrophic lateral sclerosis at a pre-symptomatic stage of the disease

Abnormal Glu release occurs in the spinal cord of SOD1(G93A) mice, a transgenic animal model for human ALS. Here we studied the mechanisms underlying Glu release in spinal cord nerve terminals of SOD1(G93A) mice at a pre-symptomatic disease stage (30days) and found that the basal release of Glu was more elevated in SOD1(G93A) with respect to SOD1 mice, and that the surplus of release relies on synaptic vesicle exocytosis. Exposure to high KCl or ionomycin provoked Ca(2+)-dependent Glu release that was likewise augmented in SOD1(G93A) mice. Equally, the Ca(2+)-independent hypertonic sucrose-induced Glu release was abnormally elevated in SOD1(G93A) mice. Also in this case, the surplus of Glu release was exocytotic in nature. We could determine elevated cytosolic Ca(2+) levels, increased phosphorylation of Synapsin-I, which was causally related to the abnormal Glu release measured in spinal cord synaptosomes of pre-symptomatic SOD1(G93A) mice, and increased phosphorylation of glycogen synthase kinase-3 at the inhibitory sites, an event that favours SNARE protein assembly. Western blot experiments revealed an increased number of SNARE protein complexes at the nerve terminal membrane, with no changes of the three SNARE proteins and increased expression of synaptotagmin-1 and β-Actin, but not of an array of other release-related presynaptic proteins. These results indicate that the abnormal exocytotic Glu release in spinal cord of pre-symptomatic SOD1(G93A) mice is mainly based on the increased size of the readily releasable pool of vesicles and release facilitation, supported by plastic changes of specific presynaptic mechanisms.

Differential Effects of Munc18s on Multiple Degranulation-Relevant Trans-SNARE Complexes

Mast cell exocytosis, which includes compound degranulation and vesicle-associated piecemeal degranulation, requires multiple Q- and R- SNAREs. It is not clear how these SNAREs pair to form functional trans-SNARE complexes and how these trans-SNARE complexes are selectively regulated for fusion. Here we undertake a comprehensive examination of the capacity of two Q-SNARE subcomplexes (syntaxin3/SNAP-23 and syntaxin4/SNAP-23) to form fusogenic trans-SNARE complexes with each of the four granule-borne R-SNAREs (VAMP2, 3, 7, 8). We report the identification of at least six distinct trans-SNARE complexes under enhanced tethering conditions: i) VAMP2/syntaxin3/SNAP-23, ii) VAMP2/syntaxin4/SNAP-23, iii) VAMP3/syntaxin3/SNAP-23, iv) VAMP3/syntaxin4/SNAP-23, v) VAMP8/syntaxin3/SNAP-23, and vi) VAMP8/syntaxin4/SNAP-23. We show for the first time that Munc18a operates synergistically with SNAP-23-based non-neuronal SNARE complexes (i to iv) in lipid mixing, in contrast to Munc18b and c, which exhibit no positive effect on any SNARE combination tested. Pre-incubation with Munc18a renders the SNARE-dependent fusion reactions insensitive to the otherwise inhibitory R-SNARE cytoplasmic domains, suggesting a protective role of Munc18a for its cognate SNAREs. Our findings substantiate the recently discovered but unexpected requirement for Munc18a in mast cell exocytosis, and implicate post-translational modifications in Munc18b/c activation.

Cortical Granule Exocytosis Is Mediated by Alpha-SNAP and N-Ethilmaleimide Sensitive Factor in Mouse Oocytes

Cortical granule exocytosis (CGE), also known as cortical reaction, is a calcium- regulated secretion that represents a membrane fusion process during meiotic cell division of oocytes. The molecular mechanism of membrane fusion during CGE is still poorly understood and is thought to be mediated by the SNARE pathway; nevertheless, it is unkown if SNAP (acronym for soluble NSF attachment protein) and NSF (acronym for N-ethilmaleimide sensitive factor), two key proteins in the SNARE pathway, mediate CGE in any oocyte model. In this paper, we documented the gene expression of α-SNAP, γ-SNAP and NSF in mouse oocytes. Western blot analysis showed that the expression of these proteins maintains a similar level during oocyte maturation and early activation. Their localization was mainly observed at the cortical region of metaphase II oocytes, which is enriched in cortical granules. To evaluate the function of these proteins in CGE we set up a functional assay based on the quantification of cortical granules metaphase II oocytes activated parthenogenetically with strontium. Endogenous α-SNAP and NSF proteins were perturbed by microinjection of recombinant proteins or antibodies prior to CGE activation. The microinjection of wild type α-SNAP and the negative mutant of α-SNAP L294A in metaphase II oocytes inhibited CGE stimulated by strontium. NEM, an irreversibly inhibitor of NSF, and the microinjection of the negative mutant NSF D1EQ inhibited cortical reaction. The microinjection of anti-α-SNAP and anti-NSF antibodies was able to abolish CGE in activated metaphase II oocytes. The microinjection of anti-γ SNAP antibody had no effect on CGE. Our findings indicate, for the first time in any oocyte model, that α-SNAP, γ-SNAP, and NSF are expressed in mouse oocytes. We demonstrate that α-SNAP and NSF have an active role in CGE and propose a working model.

Epimorphin is a novel regulator of the progesterone receptor isoform-a

Epimorphin/syntaxin-2 is a membrane-tethered protein localized extracellularly (Epim) and intracellularly (Stx-2). The extracellular form Epim stimulates morphogenic processes in a range of tissues, including in murine mammary glands where its overexpression in luminal epithelial cells is sufficient to drive hyperplasia and neoplasia. We analyzed WAP-Epim transgenic mice to gain insight into how Epim promotes malignancy. Ectopic overexpression of Epim during postnatal mammary gland development led to early side-branching onset, precocious bud formation, and increased proliferation of mammary epithelial cells. Conversely, peptide-based inhibition of Epim function reduced side branching. Because increased side branching and hyperplasia occurs similarly in mice upon overexpression of the progesterone receptor isoform-a (Pgr-a), we investigated whether Epim exhibits these phenotypes through Pgr modulation. Epim overexpression indeed led to a steep upregulation of both total Pgr mRNA and Pgr-a protein levels. Notably, the Pgr antagonist RU486 abrogated Epim-induced ductal side branching, mammary epithelial cell proliferation, and bud formation. Evaluation of Epim signaling in a three-dimensional ex vivo culture system showed that its action was dependent on binding to its extracellular receptor, integrin-αV, and on matrix metalloproteinase 3 activity downstream of Pgr-a. These findings elucidate a hitherto unknown transcriptional regulator of Pgr-a, and shed light on how overexpression of Epim leads to malignancy.

Brain-selective kinase 2 (BRSK2) phosphorylation on PCTAIRE1 negatively regulates glucose-stimulated insulin secretion in pancreatic β-cells

Brain-selective kinase 2 (BRSK2) has been shown to play an essential role in neuronal polarization. In the present study, we show that BRSK2 is also abundantly expressed in pancreatic islets and MIN6 β-cell line. Yeast two-hybrid screening, GST fusion protein pull-down, and co-immunoprecipitation assays reveal that BRSK2 interacts with CDK-related protein kinase PCTAIRE1, a kinase involved in neurite outgrowth and neurotransmitter release. In MIN6 cells, BRSK2 co-localizes with PCTAIRE1 in the cytoplasm and phosphorylates one of its serine residues, Ser-12. Phosphorylation of PCTAIRE1 by BRSK2 reduces glucose-stimulated insulin secretion (GSIS) in MIN6 cells. Conversely, knockdown of BRSK2 by siRNA increases serum insulin levels in mice. Our results reveal a novel function of BRSK2 in the regulation of GSIS in β-cells via a PCTAIRE1-dependent mechanism and suggest that BRSK2 is an attractive target for developing novel diabetic drugs.

Requirements for the catalytic cycle of the N-ethylmaleimide-Sensitive Factor (NSF)

The N-ethylmaleimide-Sensitive Factor (NSF) was one of the initial members of the ATPases Associated with various cellular Activities Plus (AAA(+)) family. In this review, we discuss what is known about the mechanism of NSF action and how that relates to the mechanisms of other AAA(+) proteins. Like other family members, NSF binds to a protein complex (i.e., SNAP-SNARE complex) and utilizes ATP hydrolysis to affect the conformations of that complex. SNAP-SNARE complex disassembly is essential for SNARE recycling and sustained membrane trafficking. NSF is a homo-hexamer; each protomer is composed of an N-terminal domain, NSF-N, and two adjacent AAA-domains, NSF-D1 and NSF-D2. Mutagenesis analysis has established specific roles for many of the structural elements of NSF-D1, the catalytic ATPase domain, and NSF-N, the SNAP-SNARE binding domain. Hydrodynamic analysis of NSF, labeled with (Ni(2+)-NTA)(2)-Cy3, detected conformational differences in NSF, in which the ATP-bound conformation appears more compact than the ADP-bound form. This indicates that NSF undergoes significant conformational changes as it progresses through its ATP-hydrolysis cycle. Incorporating these data, we propose a sequential mechanism by which NSF uses NSF-N and NSF-D1 to disassemble SNAP-SNARE complexes. We also illustrate how analytical centrifugation might be used to study other AAA(+) proteins.

New ependymal cells are born postnatally in two discrete regions of the mouse brain and support ventricular enlargement in hydrocephalus

A heterogeneous population of ependymal cells lines the brain ventricles. The evidence about the origin and birth dates of these cell populations is scarce. Furthermore, the possibility that mature ependymal cells are born (ependymogenesis) or self-renewed (ependymal proliferation) postnatally is controversial. The present study was designed to investigate both phenomena in wild-type (wt) and hydrocephalic α-SNAP mutant (hyh) mice at different postnatal stages. In wt mice, proliferating cells in the ventricular zone (VZ) were only found in two distinct regions: the dorsal walls of the third ventricle and Sylvian aqueduct (SA). Most proliferating cells were monociliated and nestin+, likely corresponding to radial glial cells. Postnatal cumulative BrdU-labeling showed that most daughter cells remained in the VZ of both regions and they lost nestin-immunoreactivity. Furthermore, some labeled cells became multiciliated and GLUT-1+, indicating they were ependymal cells born postnatally. Postnatal pulse BrdU-labeling and Ki-67 immunostaining further demonstrated the presence of cycling multiciliated ependymal cells. In hydrocephalic mutants, the dorsal walls of the third ventricle and SA expanded enormously and showed neither ependymal disruption nor ventriculostomies. This phenomenon was sustained by an increased ependymogenesis. Consequently, in addition to the physical and geometrical mechanisms traditionally explaining ventricular enlargement in fetal-onset hydrocephalus, we propose that postnatal ependymogenesis could also play a role. Furthermore, as generation of new ependymal cells during postnatal stages was observed in distinct regions of the ventricular walls, such as the roof of the third ventricle, it may be a key mechanism involved in the development of human type 1 interhemispheric cysts.

A simple PCR-based genotyping method for M105I mutation of alpha-SNAP enhances the study of early pathological changes in hyh phenotype

alpha-SNAP is an essential component of the protein machinery responsible for membrane fusion events in different cell types. The hyh (hydrocephalus with hop gait) mouse carries a missense mutation in Napa gene that results in a point mutation (M105I) in alpha-SNAP protein. Homozygous animals for the mutant allele have been identified by the clinical and/or neuropathological phenotype, or by direct sequencing of PCR products. The aims of the present study were (i) to develop a high-throughput technique to genotype hyh mice, (ii) to correlate genotype-phenotype, and (iii) to analyze the earliest pathological changes of hyh mutant mice. As no restriction sites are affected by the hyh mutation, we resolved this problem by creating a BspHI restriction site with a modified (mismatch) polymerase chain reaction (PCR) primer in wild-type allele. This artificially created restriction site (ACRS)-PCR technique is a simple, rapid and reliable method to genotype hyh mice in a day-work procedure. Biochemical and histological analysis of genotyped hyh embryos at different developmental stages allowed us to identify and characterize the earliest brain pathological changes of the hyh phenotype, including the first signs of neuroepithelial disruption and neuronal ectopia. In addition, genotype-phenotype analysis of 327 animals confirmed that (i) hyh is a single-gene autosomal recessive disorder, and (ii) the disorder has 100% penetrance (i.e., the mutation was only present in affected mice). The genotyping method described here enhances the potentiality of hyh mouse as a unique in vivo model to study the role of membrane trafficking in different developmental and physiological processes.

Methods for functional analysis of macroautophagy in filamentous fungi

Autophagy is a bulk degradative process responsible for the turnover of membranes, organelles, and proteins in eukaryotic cells. Genetic and molecular regulation of autophagy has been independently elucidated in budding yeast and mammalian cells. In filamentous fungi, autophagy is required for several important physiological functions, such as asexual and sexual differentiation, pathogenic development, starvation stress and programmed cell death during heteroincompatibility. Here, we detail biochemical and microscopy methods useful for measuring the rate of induction of autophagy in filamentous fungi, and we summarize the methods that have been routinely used for monitoring macroautophagy in both yeast and filamentous fungi. The role of autophagy in carbohydrate catabolism and cell survival is discussed along with the specific functions of macroautophagy in fungal development and pathogenesis.

Structural analysis of SNARE motifs from sea perch, Lateolabrax japonicus by computerized approaches

Three cDNA sequences encoding four SNARE (N-ethylmaleimide-sensitive fusion protein attachment protein receptors) motifs were cloned from sea perch, and the deduced peptide sequences were analyzed for structural prediction by using 14 different web servers and softwares. The "ionic layer" structure, the three dimensional extension and conformational characters of the SNARE 7S core complex by using bioinformatics approaches were compared respectively with those from mammalian X-ray crystallographic investigations. The result suggested that the formation and stabilization of fish SNARE core complex might be driven by hydrophobic association, hydrogen bond among R group of core amino acids and electrostatic attraction at molecular level. This revealed that the SNARE proteins interaction of the fish may share the same molecular mechanism with that of mammal, indicating the universality and solidity of SNARE core complex theory. This work is also an attempt to get the protein 3D structural information which appears to be similar to that obtained through X-ray crystallography, only by using computerized approaches.

Structure of the N-terminal domain of PEX1 AAA-ATPase. Characterization of a putative adaptor-binding domain

Peroxisomes are responsible for several pathways in primary metabolism, including beta-oxidation and lipid biosynthesis. PEX1 and PEX6 are hexameric AAA-type ATPases, both of which are indispensable in targeting over 50 peroxisomal resident proteins from the cytosol to the peroxisomes. Although the tandem AAA-ATPase domains in the central region of PEX1 and PEX6 are highly similar, the N-terminal sequences are unique. To better understand the distinct molecular function of these two proteins, we analyzed the unique N-terminal domain (NTD) of PEX1. Extensive computational analysis revealed weak similarity (<10% identity) of PEX1 NTD to the N-terminal domains of other membrane-related type II AAA-ATPases, such as VCP (p97) and NSF. We have determined the crystal structure of mouse PEX1 NTD at 2.05-A resolution, which clearly demonstrated that the domain belongs to the double-psi-barrel fold family found in the other AAA-ATPases. The N-domains of both VCP and NSF are structural neighbors of PEX1 NTD with a 2.7- and 2.1-A root mean square deviation of backbone atoms, respectively. Our findings suggest that the supradomain architecture, which is composed of a single N-terminal domain followed by tandem AAA domains, is a common feature of organellar membrane-associating AAA-ATPases. We propose that PEX1 functions as a protein unfoldase in peroxisomal biogenesis, using its N-terminal putative adaptor-binding domain.

Characterization of alpha-soluble N-ethylmaleimide-sensitive fusion attachment protein in alveolar type II cells: implications in lung surfactant secretion

N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (alpha-SNAP) are thought to be soluble factors that transiently bind and disassemble SNAP receptor complex during exocytosis in neuronal and endocrine cells. Lung surfactant is secreted via exocytosis of lamellar bodies from alveolar epithelial type II cells. However, the secretion of lung surfactant is a relatively slow process, and involvement of SNAP receptor and its cofactors (NSF and alpha-SNAP) in this process has not been demonstrated. In this study, we investigated a possible role of alpha-SNAP in surfactant secretion. alpha-SNAP was predominantly associated with the membranes in alveolar type II cells as determined by Western blot and immunocytochemical analysis using confocal microscope. Membrane-associated alpha-SNAP was not released from the membrane fraction when the cells were lyzed in the presence of Ca2+ or Mg2+ATP. The alkaline condition (0.1 M Na2CO3, pH 12), known to extract peripheral membrane proteins also failed to release it from the membrane. Phase separation using Triton X-114 showed that alpha-SNAP partitioned into both aqueous and detergent phases. NSF had membrane-bound characteristics similar to alpha-SNAP in type II cells. Permeabilization of type II cells with beta-escin resulted in a partial loss of alpha-SNAP from the cells, but cellular NSF was relatively unchanged. Addition of exogenous alpha-SNAP to the permeabilized cells increased surfactant secretion in a dose-dependent manner, whereas exogenous NSF has much less effects. An alpha-SNAP antisense oligonucleotide decreased its protein level and inhibited surfactant secretion. Our results suggest a role of alpha-SNAP in lung surfactant secretion.

A novel assay to study autophagy: regulation of autophagosome vacuole size by amino acid deprivation

Autophagy is a normal degradative pathway that involves the sequestration of cytoplasmic portions and intracellular organelles in a membrane vacuole called the autophagosome. These vesicles fuse with lysosomes and the sequestered material is degraded. Owing to the complexity of the autophagic pathway and to its inaccessibility to external probes, little is known about the molecular mechanisms that regulate autophagy in higher eukaryotic cells. We used the autofluorescent drug monodansylcadaverine (MDC), a specific autophagolysosome marker to analyze at the molecular level the machinery involved in the autophagic process. We have developed a morphological and biochemical assay to study authophagy in living cells based on the incorporation of MDC. With this assay we observed that the accumulation of MDC was specifically induced by amino acid deprivation and was inhibited by 3-methlyadenine, a classical inhibitor of the autophagic pathway. Additionally, wortmannin, an inhibitor of PI3-kinases that blocks autophagy at an early stage, inhibited the accumulation of MDC in autophagic vacuoles. We also found that treatment of the cells with N-ethylmaleimide (NEM), an agent known to inhibit several vesicular transport events, completely blocked the incorporation of MDC, suggesting that an NEM-sensitive protein is required for the formation of autophagic vacuoles. Conversely, vinblastine, a microtubule depolymerizing agent that induces the accumulation of autophagic vacuoles by preventing their degradation, increased the accumulation of MDC and altered the distribution and size of the autophagic vacuoles. Our results indicate that in the presence of vinblastine very large MDC-vacuoles accumulated mainly under starvation conditions, indicating that the expansion of autophagosomes is upregulated by amino acid deprivation. Furthermore, these MDC-vacuoles were labeled with LC3, one of the mammalian homologues of the yeast protein Apg8/Aut7 that plays an important role in autophagosome formation.

Synaptic pathology in the anterior cingulate cortex in schizophrenia and mood disorders. A review and a Western blot study of synaptophysin, GAP-43 and the complexins

There are several reports of ultrastructural and protein changes affecting synapses in the anterior cingulate cortex in schizophrenia. Altered cytoarchitecture has also been described in this region in schizophrenia as well as in mood disorders. In this paper we review the literature and present a new study investigating synaptic abnormalities in the anterior cingulate cortex (area 24) in the Stanley Foundation brain series. We used Western blotting to assess four synaptic proteins: synaptophysin, growth-associated protein-43 (GAP-43), complexin I and complexin II, which inform about somewhat different aspects of the synaptic circuitry. Synaptophysin, complexin II and GAP-43 were reduced in bipolar disorder. The decreases correlated with the duration of illness and tended to be greater in subjects without a family history. Complexin II was also reduced in major depression. Complexin I and the housekeeping protein beta-actin did not differ between groups. None of the proteins changed significantly in schizophrenia. The results indicate the presence of a synaptic pathology in the anterior cingulate cortex in mood disorders, especially bipolar disorder. The abnormalities may contribute to the dysfunction of cingulate neural circuits. The loss of synaptophysin is suggestive of decreased synaptic density whilst the decrease in GAP-43 may denote impaired synaptic plasticity and the reduction of complexin II but not complexin I implies that the alterations particularly affect excitatory connections. The reductions may be progressive.

Self-association of the H3 region of syntaxin 1A. Implications for intermediates in SNARE complex assembly

Intracellular membrane fusion requires SNARE proteins found on the vesicle and target membranes. SNAREs associate by formation of a parallel four-helix bundle, and it has been suggested that formation of this complex promotes membrane fusion. The membrane proximal region of the cytoplasmic domain of the SNARE syntaxin 1A, designated H3, contributes one of the four helices to the SNARE complex. In the crystal structure of syntaxin 1A H3, four molecules associate as a homotetramer composed of two pairs of parallel helices that are anti-parallel to each other. The H3 oligomer observed in the crystals is also found in solution, as assessed by gel filtration and chemical cross-linking studies. The crystal structure reveals that the highly conserved Phe-216 packs against conserved Gln-226 residues present on the anti-parallel pair of helices. Modeling indicates that Phe-216 prevents parallel tetramer formation. Mutation of Phe-216 to Leu appears to allow formation of parallel tetramers, whereas mutation to Ala destabilizes the protein. These results indicate that Phe-216 has a role in preventing formation of stable parallel helical bundles, thus favoring the interaction of the H3 region of syntaxin 1a with other proteins involved in membrane fusion.

Gaf-1, a gamma -SNAP-binding protein associated with the mitochondria

The role of alpha/beta-SNAP (Soluble NSF Attachment Protein) in vesicular trafficking is well established; however, the function of the ubiquitously expressed gamma-SNAP remains unclear. To further characterize the cellular role of this enigmatic protein, a two-hybrid screen was used to identify new, gamma-SNAP-binding proteins and to uncover potentially novel functions for gamma-SNAP. One such SNAP-binding protein, termed Gaf-1 (gamma-SNAP associate factor-1) specifically binds gamma- but not alpha-SNAP. The full-length Gaf-1 (75 kDa) is ubiquitously expressed and is found stoichiometrically associated with gamma-SNAP in cellular extracts. This binding is distinct from other SNAP interactions since no alpha-SNAP or NSF coprecipitated with Gaf-1. Subcellular fractionation and immunofluorescence analysis show that Gaf-1 is peripherally associated with the outer mitochondrial membrane. Only a fraction of gamma-SNAP was mitochondrial with the balance being either cytosolic or associated with other membrane fractions. GFP-gamma-SNAP and the C-terminal domain of Gaf-1 both show a reticular distribution in HEK-293 cells. This reticular structure colocalizes with Gaf-1 and mitochondria as well as with microtubules but not with other cytoskeletal elements. These data identify a class of gamma-SNAP interactions that is distinct from other members of the SNAP family and point to a potential role for gamma-SNAP in mitochondrial dynamics.

Role of p97 and syntaxin 5 in the assembly of transitional endoplasmic reticulum

Transitional endoplasmic reticulum (tER) consists of confluent rough and smooth endoplasmic reticulum (ER) domains. In a cell-free incubation system, low-density microsomes (1.17 g cc(-1)) isolated from rat liver homogenates reconstitute tER by Mg(2+)GTP- and Mg(2+)ATP-hydrolysis-dependent membrane fusion. The ATPases associated with different cellular activities protein p97 has been identified as the relevant ATPase. The ATP depletion by hexokinase or treatment with either N-ethylmaleimide or anti-p97 prevented assembly of the smooth ER domain of tER. High-salt washing of low-density microsomes inhibited assembly of the smooth ER domain of tER, whereas the readdition of purified p97 with associated p47 promoted reconstitution. The t-SNARE syntaxin 5 was observed within the smooth ER domain of tER, and antisyntaxin 5 abrogated formation of this same membrane compartment. Thus, p97 and syntaxin 5 regulate assembly of the smooth ER domain of tER and hence one of the earliest membrane differentiated components of the secretory pathway.

Expression of complexin I and II mRNAs and their regulation by antipsychotic drugs in the rat forebrain

Complexin (cx) I and II are homologous synaptic protein genes which are differentially expressed in mouse and human brain and differentially affected in schizophrenia. We characterized the distribution of cx I and II mRNAs in rat forebrain and examined whether their abundance, or the transcript of the synaptic marker synaptophysin, is affected by 14 days' administration of antipsychotic drugs (haloperidol, chlorpromazine, risperidone, olanzapine, or clozapine). Cx I mRNA predominated in medial habenula, medial septum-diagonal band complex, and thalamus, whereas cx II mRNA was more abundant in most other regions, including isocortex and hippocampus. Within the hippocampus, cx I mRNA was primarily expressed by interneurons and cx II mRNA by granule cells and pyramidal neurons. Localized cx II mRNA signal was seen in the dentate gyrus molecular layer, suggestive of its transport into granule cell dendrites. Antipsychotic treatment produced selective, modest effects on cx mRNA expression. Cx I mRNA was elevated by olanzapine in dorsolateral striatum and frontoparietal cortex, while the abundance of cx II mRNA relative to cx I mRNA was decreased in both areas by olanzapine and haloperidol. Chlorpromazine increased cx II mRNA in frontoparietal cortex and synaptophysin mRNA in dorsolateral striatum. In summary, the data have implications both for understanding the effects of antipsychotic medication on synaptic organization, and for synaptic protein expression studies in patients treated with the drugs.

Mutational analysis of the functional motifs of RuvB, an AAA+ class helicase and motor protein for holliday junction branch migration

Escherichia coli RuvB protein, together with RuvA, promotes branch migration of Holliday junctions during homologous recombination and recombination repair. The RuvB molecular motor is an intrinsic ATP-dependent DNA helicase with a hexameric ring structure and its architecture has been suggested to be related to those of the members of the AAA+ protein class. In this study, we isolated a large number of plasmids carrying ruvB mutant genes and identified amino acid residues important for the RuvB functions by examining the in vivo DNA repair activities of the mutant proteins. Based on these mutational studies and amino acid conservation among various RuvBs, we identified 10 RuvB motifs that agreed well with the features of the AAA+ protein class and that distinguished the primary structure of RuvB from that of typical DNA/RNA helicases with seven conserved helicase motifs.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

A screen for dominant negative mutants of SEC18 reveals a role for the AAA protein consensus sequence in ATP hydrolysis

An evolutionarily ancient mechanism is used for intracellular membrane fusion events ranging from endoplasmic reticulum-Golgi traffic in yeast to synaptic vesicle exocytosis in the human brain. At the heart of this mechanism is the core complex of N-ethylmaleimide-sensitive fusion protein (NSF), soluble NSF attachment proteins (SNAPs), and SNAP receptors (SNAREs). Although these proteins are accepted as key players in vesicular traffic, their molecular mechanisms of action remain unclear. To illuminate important structure-function relationships in NSF, a screen for dominant negative mutants of yeast NSF (Sec18p) was undertaken. This involved random mutagenesis of a GAL1-regulated SEC18 yeast expression plasmid. Several dominant negative alleles were identified on the basis of galactose-inducible growth arrest, of which one, sec18-109, was characterized in detail. The sec18-109 phenotype (abnormal membrane trafficking through the biosynthetic pathway, accumulation of a membranous tubular network, growth suppression, increased cell density) is due to a single A-G substitution in SEC18 resulting in a missense mutation in Sec18p (Thr(394)-->Pro). Thr(394) is conserved in most AAA proteins and indeed forms part of the minimal AAA consensus sequence that serves as a signature of this large protein family. Analysis of recombinant Sec18-109p indicates that the mutation does not prevent hexamerization or interaction with yeast alpha-SNAP (Sec17p), but instead results in undetectable ATPase activity that cannot be stimulated by Sec17p. This suggests a role for the AAA protein consensus sequence in regulating ATP hydrolysis. Furthermore, this approach of screening for dominant negative mutants in yeast can be applied to other conserved proteins so as to highlight important functional domains in their mammalian counterparts.

Molecular mechanisms of platelet exocytosis: requirements for alpha-granule release

Platelets function by secreting components necessary for primary clot formation. This report describes an in vitro assay that measures alpha-granule secretion. Using permeabilized platelets, it is possible to recreate Ca(2+)-stimulated release of platelet factor 4 (PF4) that is ATP- and temperature-dependent. Though other divalent cations can replace Ca(2+) (i.e., Sr(2+), Mn(2+), Zn(2+)), there is no effect of Ba(2+). Analysis by electron microscopy indicates that the in vitro assay also mimics the cytoskeletal rearrangements and granule centralization that occurs upon platelet activation in vivo. Antibody inhibition studies show that PF4 release requires the general membrane fusion protein N-ethylmaleimide-sensitive factor (NSF) and well as the target membrane SNAP receptors (t-SNAREs), syntaxin 2, 4, and SNAP-23. As shown by electron microscopy, the anti-t-SNARE antibodies block granule to target membrane fusion. This finding is unique in that it is the first report of a role for two syntaxins in the same exocytosis event.

The lipolytic stimulation of 3T3-L1 adipocytes promotes the translocation of hormone-sensitive lipase to the surfaces of lipid storage droplets

Hormone-sensitive lipase catalyzes the rate-limiting step in the release of fatty acids from triacylglycerol-rich lipid storage droplets of adipocytes, which contain the body's major energy reserves. Hormonal stimulation of cAMP formation and the activation of cAMP-dependent protein kinase leads to the phosphorylation of hormone-sensitive lipase and a large increase in lipolysis in adipocytes. By contrast, phosphorylation of hormone-sensitive lipase by the kinase in vitro results in a comparatively minor increase in catalytic activity. In this study, we investigate the basis for this discrepancy by using immunofluorescence microscopy to locate hormone-sensitive lipase in lipolytically stimulated and unstimulated 3T3-L1 adipocytes. In unstimulated cells, hormone-sensitive lipase is diffusely distributed throughout the cytosol. Upon stimulation of cells with the beta-adrenergic receptor agonist, isoproterenol, hormone-sensitive lipase translocates from the cytosol to the surfaces of intracellular lipid droplets concomitant with the onset of lipolysis, as measured by the release of glycerol to the culture medium. Both hormone-sensitive lipase translocation and lipolysis are reversed by the incubation of cells with the beta-adrenergic receptor antagonist, propranolol. The treatment of cells with cycloheximide fails to inhibit lipase translocation or lipolysis, indicating that the synthesis of nascent proteins is not required. Cytochalasin D and nocodazole used singly and in combination also failed to have a major effect, thus suggesting that the polymerization of microfilaments and microtubules and the formation of intermediate filament networks is unnecessary. Hormone-sensitive lipase translocation and lipolysis were inhibited by N-ethylmaleimide and a combination of deoxyglucose and sodium azide. We propose that the major consequence of the phosphorylation of hormone-sensitive lipase following the lipolytic stimulation of adipocytes is the translocation of the lipase from the cytosol to the surfaces of lipid storage droplets.

Spectrin (betaSpIIsigma1) is an essential component of synaptic transmission

The cellular mechanism that underlies the regulated release of synaptic vesicles during neurotransmission is not fully known. Our previous data has shown that brain spectrin (alphaSpIIsigma1/betaSpIIsigma1)2 is localized in axons and nerve terminals and we have shown that the beta subunit (betaSpIIsigma1) contains a synapsin-binding domain capable of interacting with synapsin and small synaptic vesicles in vitro and in vivo. These findings suggested a role for brain beta-spectrin in synaptic neurotransmission. To examine this possibility further, peptide-specific antibodies directed against epitopes within the synapsin-binding domain of brain beta-spectrin, or against flanking regions, were injected into the presynaptic neuron of synaptically paired rat hippocampal neurons in culture. Here, we show that the antibodies directed against the synapsin-binding domain specifically blocked synaptic neurotransmission.

Hepatitis C virus RNA polymerase and NS5A complex with a SNARE-like protein

Hepatitis C virus (HCV) NS5A is a phosphoprotein that possesses a cryptic trans-activation activity. To investigate its potential role in viral replication, we searched for the cellular proteins interacting with NS5A protein by yeast two-hybrid screening of a human hepatocyte cDNA library. We identified a newly discovered soluble N-ethylmaleimide-sensitive factor attachment protein receptor-like protein termed human vesicle-associated membrane protein-associated protein of 33 kDa (hVAP-33). In vitro binding assay and in vivo coimmunoprecipitation studies confirmed the interaction between hVAP-33 and NS5A. Interestingly, hVAP-33 was also shown to interact with NS5B, the viral RNA-dependent RNA polymerase. NS5A and NS5B bind to different domains of hVAP-33: NS5A binds to the C-terminus, whereas NS5B binds to the N-terminus of hVAP-33. Immunofluorescent staining showed a significant colocalization of hVAP-33 with both NS5A and NS5B proteins. hVAP-33 contains a coiled-coil domain followed by a membrane-spanning domain at its C-terminus. Cell fractionation analysis revealed that hVAP-33 is predominantly associated with the ER, the Golgi complex, and the prelysosomal membrane, consistent with its potential role in intracellular membrane trafficking. These interactions provide a mechanism for membrane association of the HCV RNA replication complex and further suggest that NS5A is a part of the viral RNA replication complex.

Analyses of proteins involved in vesicular trafficking in platelets of mouse models of Hermansky Pudlak syndrome

Hermansky Pudlak syndrome (HPS) is an autosomal recessive inherited disorder characterized by defects in synthesis and/or secretion of three related subcellular organelles: melanosomes, platelet-dense granules, and lysosomes. In the mouse, mutant forms of any of 14 separate genes result in an HPS-like phenotype. The mouse pearl and mocha genes encode subunits of the AP3 adaptor protein complex, confirming that HPS mutations involve proteins regulating intracellular vesicular trafficking. Therefore, expression of several additional proteins involved in vesicular transport was examined by immunoblotting of platelet extracts from HPS mutant and control mice. Platelet levels of SCAMPS (secretory carrier membrane proteins), Rab11, Rab31, NSF (N-ethylmaleimide-sensitive fusion protein), syntaxin 2, syntaxin 4, munc18c, and p115/TAP (p115/transcytosis-associated protein) were not significantly altered in several different HPS mutants. However, gunmetal (gm/gm) platelets contained decreased amounts of SNAP-23. The Snap23 gene was mapped to mouse chromosome 5, demonstrating it cannot encode the gm gene, which maps to chromosome 14. It is likely therefore that the gm gene functions upstream of SNAP-23 in vesicular trafficking.

Crystal structure of the amino-terminal domain of N-ethylmaleimide-sensitive fusion protein

The cytosolic ATPase N-ethylmaleimide-sensitive fusion protein (NSF) disassembles complexes of membrane-bound proteins known as SNAREs, an activity essential for vesicular trafficking. The amino-terminal domain of NSF (NSF-N) is required for the interaction of NSF with the SNARE complex through the adaptor protein alpha-SNAP. The crystal structure of NSF-N reveals two subdomains linked by a single stretch of polypeptide. A polar interface between the two subdomains indicates that they can move with respect to one another during the catalytic cycle of NSF. Structure-based sequence alignments indicate that in addition to NSF orthologues, the p97 family of ATPases contain an amino-terminal domain of similar structure.

Synaptic-like microvesicles in mammalian pinealocytes

The recent deciphering of the protein composition of the synaptic vesicle membrane has led to the unexpected identification of a compartment of electron-lucent microvesicles in neuroendocrine cells which resemble neuronal synaptic vesicles in terms of molecular structure and function. These vesicles are generally referred to as synaptic-like microvesicles (SLMVs) and have been most intensively studied in pancreatic beta-cells, chromaffin cells of the adrenal medulla, and pinealocytes of the pineal gland. This chapter focuses on the present knowledge of SLMVs as now well-established constituents of mammalian pinealocytes. I review the results of morphological, immunocytochemical, and biochemical studies that were important for the characterization of this novel population of secretory vesicles in the pineal organ. The emerging concept that SLMVs serve as a device for intercellular communication within the pineal gland is outlined, and unanswered questions such as those pertaining to the physiological function and regulation of pineal SLMVs are discussed.

Phosphorylation of p97(VCP) and p47 in vitro by p34cdc2 kinase

The hexameric ATPase p97/yeast Cdc48p has been implicated in a number of cellular events that are regulated during mitosis, including homotypic membrane fusion, spindle pole body function, and ubiquitin-dependent protein degradation. p97/Cdc48p contains two conserved consensus p34cdc2 kinase phosphorylation sites within its second ATP binding domain. This domain is likely to play a role in stabilising the hexameric form of the protein. We therefore investigated whether p97 could be phosphorylated by p34cdc2 kinase in vitro, and whether phosphorylation might influence the oligomeric status of p97. Monomeric, but not hexameric, p97 was phosphorylated by p34cdc2 kinase, as was the p97-associated protein p47. However, phosphorylation by p34cdc2 kinase did not impair subsequent re-hexamerisation of p97, implying that the phosphorylated residue(s) are not critical for interaction between p97 monomers. Moreover, p97 within both interphase and mitotic cytosols was almost exclusively hexameric, suggesting that the activity of p97 is not regulated during mitosis by influencing the extent of oligomerisation.

Characterization of the nucleoside triphosphatase activity of poliovirus protein 2C reveals a mechanism by which guanidine inhibits poliovirus replication

The highly conserved non-structural protein 2C of picornaviruses is involved in viral genome replication and encapsidation and in the rearrangement of intracellular structures. 2C binds RNA, has nucleoside triphosphatase activity, and shares three motifs with superfamily III helicases. Motifs "A" and "B" are involved in nucleotide triphosphate (NTP) binding and hydrolysis, whereas a function for motif "C" has not yet been demonstrated. Poliovirus RNA replication is inhibited by millimolar concentrations of guanidine hydrochloride (GdnHCl). Resistance and dependence to GdnHCl map to 2C. To characterize the nucleoside triphosphatase activity of 2C, we purified poliovirus recombinant 2C fused to glutathione S-transferase (GST-2C) from Escherichia coli. GST-2C hydrolyzed ATP with a Km of 0.7 mM. Other NTPs, including GTP, competed with ATP for binding to 2C but were poor substrates for hydrolysis. Mutation of conserved residues in motif A and B abolished ATPase activity, as did mutation of the conserved asparagine residue in motif C, an observation indicating the involvement of this motif in ATP hydrolysis. GdnHCl at millimolar concentrations inhibited ATP hydrolysis. Mutations in 2C that confer poliovirus resistant to or dependent on GdnHCl increased the tolerance to GdnHCl up to 100-fold.

Red bell pepper chromoplasts exhibit in vitro import competency and membrane targeting of passenger proteins from the thylakoidal sec and DeltapH pathways but not the chloroplast signal recognition particle pathway

Chloroplast to chromoplast development involves new synthesis and plastid localization of nuclear-encoded proteins, as well as changes in the organization of internal plastid membrane compartments. We have demonstrated that isolated red bell pepper (Capsicum annuum) chromoplasts contain the 75-kD component of the chloroplast outer envelope translocon (Toc75) and are capable of importing chloroplast precursors in an ATP-dependent fashion, indicating a functional general import apparatus. The isolated chromoplasts were able to further localize the 33- and 17-kD subunits of the photosystem II O2-evolution complex (OE33 and OE17, respectively), lumen-targeted precursors that utilize the thylakoidal Sec and DeltapH pathways, respectively, to the lumen of an internal membrane compartment. Chromoplasts contained the thylakoid Sec component protein, cpSecA, at levels comparable to chloroplasts. Routing of OE17 to the lumen was abolished by ionophores, suggesting that routing is dependent on a transmembrane DeltapH. The chloroplast signal recognition particle pathway precursor major photosystem II light-harvesting chlorophyll a/b protein failed to associate with chromoplast membranes and instead accumulated in the stroma following import. The Pftf (plastid fusion/translocation factor), a chromoplast protein, integrated into the internal membranes of chromoplasts during in vitro assays, and immunoblot analysis indicated that endogenous plastid fusion/translocation factor was also an integral membrane protein of chromoplasts. These data demonstrate that the internal membranes of chromoplasts are functional with respect to protein translocation on the thylakoid Sec and DeltapH pathways.

Membrane fusion in vesicles of oligomerizable lipids

Membrane fusion has been examined in a model system of small unilamellar vesicles of synthetic lipids that can be oligomerized through the lipid headgroups. The oligomerization can be induced either in both bilayer leaflets or in the inner leaflet exclusively. Oligomerization leads to denser lipid headgroup packing, with concomitant reduction of lipid lateral diffusion and membrane permeability. As evidenced by lipid mixing assays, electron microscopy, and light scattering, calcium-induced fusion of the bilayer vesicles is strongly retarded and inhibited by oligomerization. Remarkably, oligomerization of only the inner leaflet of the bilayer is already sufficient to affect fusion. The efficiency of inhibition and retardation of fusion critically depend on the relative amount of oligomeric lipid present, on the concentration of calcium ions, and on temperature. Implications for the mechanism of bilayer membrane fusion are discussed in terms of lipid lateral diffusion and membrane curvature effects.

Homotypic fusion of immature secretory granules during maturation in a cell-free assay

The biogenesis of secretory granules embodies several morphological and biochemical changes. In particular, in neuroendocrine cells maturation of secretory granules is characterized by an increase in size which has been proposed to reflect homotypic fusion of immature secretory granules (ISGs). Here we describe an assay that provides the first biochemical evidence for such a fusion event and allows us to analyze its regulation. The assay reconstitutes homotypic fusion between one population of ISGs containing a [35S]sulfate-labeled substrate, secretogranin II (SgII), and a second population containing the prohormone convertase PC2. Both substrate and enzyme are targeted exclusively to ISGs. Fusion is measured by quantification of a cleavage product of SgII produced by PC2. With this assay we show that fusion only occurs between ISGs and not between ISGs and MSGs, is temperature dependent, and requires ATP and GTP and cytosolic proteins. NSF (N-ethylmaleimide-sensitive fusion protein) is amongst the cytosolic proteins required, whereas we could not detect a requirement for p97. The ability to reconstitute ISG fusion in a cell-free assay is an important advance towards the identification of molecules involved in the maturation of secretory granules and will increase our understanding of this process.

The effects of SNAP/SNARE complexes on the ATPase of NSF

The ATPase of the N-ethylmaleimide sensitive factor (NSF) appears to be central to the events that culminate in vesicle-target membrane fusion. Complexes containing different combinations of NSF, alpha-SNAP, Vamp-2 (synaptobrevin 2), syntaxin 1, and SNAP-25 were reconstituted and then tested for their effect on the ATPase of NSF. While NSF interacts with all alpha-SNAP-containing complexes, only the alpha-SNAP/t-SNARE complex significantly stimulated ATPase activity. This stimulation was dependent on increasing SNAP/t-SNARE complex and was saturable. The apparent stimulation of ATPase activity is due to a 10-fold increase in initial hydrolysis rate. Complex containing both v- and t-SNAREs bound significantly more alpha-SNAP but did not stimulate the ATPase of NSF.

A sequential view of neurotransmitter release

Only a few years ago it was thought that a single Ca2+-dependent membrane binding protein might control regulated exocytosis, but it is now clear that the coordinated actions of a large number of proteins and lipids are required for the precise targeting, docking and fusion of vesicles to the plasma membrane. Thinking was focused in 1993 by the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) hypothesis, which proposed that certain synaptic vesicle membrane proteins combined specifically with particular proteins in the synaptic membrane active zone to form a complex that interacted with synaptoplasmic proteins, ATP and calcium ions to fuse the vesicles with the presynaptic membrane. Much research that has followed has verified the basic predictions of the SNARE hypothesis. However, recent research indicates that SNARE proteins are more widely distributed in secretory systems and that the sequence in which the proteins function may not occur as was originally proposed. That has recently produced a period of deconstruction and reinterpretation of the SNARE hypothesis. Our present state of knowledge is briefly summarized in this review.

Crystal structure of the hexamerization domain of N-ethylmaleimide-sensitive fusion protein

N-ethylmaleimide-sensitive fusion protein (NSF) is a cytosolic ATPase required for many intracellular vesicle fusion reactions. NSF consists of an amino-terminal region that interacts with other components of the vesicle trafficking machinery, followed by two homologous ATP-binding cassettes, designated D1 and D2, that possess essential ATPase and hexamerization activities, respectively. The crystal structure of D2 bound to Mg2+-AMPPNP has been determined at 1.75 A resolution. The structure consists of a nucleotide-binding and a helical domain, and it is unexpectedly similar to the first two domains of the clamp-loading subunit delta' of E. coli DNA polymerase III. The structure suggests several regions responsible for coupling of ATP hydrolysis to structural changes in full-length NSF.

Formation and turnover of NSF- and SNAP-containing "fusion" complexes occur on undocked, clathrin-coated vesicle-derived membranes

Specificity of vesicular transport is determined by pair-wise interaction between receptors (SNAP receptors or SNAREs) associated with a transport vesicle and its target membrane. Two additional factors, N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (SNAP) are ubiquitous components of vesicular transport pathways. However, the precise role they play is not known. On the basis that NSF and SNAP can be recruited to preformed SNARE complexes, it has been proposed that NSF- and SNAP-containing complexes are formed after SNARE-dependent docking of transport vesicles. This would enable ATPase-dependent complex disassembly to be coupled directly to membrane fusion. Alternatively, binding and release of NSF/SNAP may occur before vesicle docking, and perhaps be involved in the activation of SNAREs. To gain more information about the point at which so-called 20S complexes form during the transport vesicle cycle, we have examined NSF/SNAP/SNARE complex turnover on clathrin-coated vesicle-derived membranes in situ. This has been achieved under conditions in which the extent of membrane docking can be precisely monitored. We demonstrate by UV-dependent cross-linking experiments, coupled to laser light-scattering analysis of membranes, that complexes containing NSF, SNAP, and SNAREs will form and dissociate on the surface of undocked transport vesicles.