NSF-independent fusion mechanisms

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.

NSF independent fusion of Salmonella-containing late phagosomes with early endosomes

Initial characterizations of live-Salmonella-containing early (LSEP) and late phagosomes (LSLP) in macrophages show that both phagosomes retain Rab5 and EEA1. In addition, LSEP specifically contain transferrin receptor whereas LSLP possess relatively more rabaptin-5. In contrast to LSLP, late-Salmonella-containing vacuoles in epithelial cells show significantly reduced levels of Rab5 and EEA1. Subsequent results demonstrate that both phagosomes efficiently fuse with early endosomes (EE). In contrast to LSEP, fusion between LSLP and EE is insensitive to ATPgammaS treatment. Furthermore, LSLP fuses with EE in absence of NEM-sensitive fusion factor (NSF) as well as in the presence of NSF:D1EQ mutant demonstrating that LSLP fusion with EE is NSF independent.

Distinct roles for the AAA ATPases NSF and p97 in the secretory pathway

NSF and p97 are related AAA proteins implicated in membrane trafficking and organelle biogenesis. p97 is also involved in pathways that lead to ubiquitin-dependent proteolysis, including ER-associated degradation (ERAD). In this study, we have used dominant interfering ATP-hydrolysis deficient mutants (NSF(E329Q) and p97(E578Q)) to compare the function of these AAA proteins in the secretory pathway of mammalian cells. Expressing NSF(E329Q) promotes disassembly of Golgi stacks into dispersed vesicular structures. It also rapidly inhibits glycosaminoglycan sulfation, reflecting disruption of intra-Golgi transport. In contrast, expressing p97(E578Q) does not affect Golgi structure or function; glycosaminoglycans are normally sulfated and secreted, as is the VSV-G ts045 protein. Instead, expression of p97(E578Q) causes ubiquitinated proteins to accumulate on ER membranes and slows degradation of the ERAD substrate cystic-fibrosis transmembrane-conductance regulator. In addition, expression of p97(E578Q) eventually causes the ER to swell. More specific assessment of effects of p97(E578Q) on organelle assembly shows that the Golgi apparatus disperses and reassembles normally after treatment with brefeldin A and during mitosis. These findings demonstrate that ATP-hydrolysis-dependent activities of NSF and p97 in the cell are not equivalent and suggest that only NSF is directly involved in regulating membrane fusion.

In vivo membrane trafficking role for an insect N-ethylmaleimide-sensitive factor which is developmentally regulated in endocrine cells

The hexameric ATPase, N-ethylmaleimide-sensitive factor (NSF) is implicated in the release of neurotransmitters and in mediating fusion between intracellular membranes. Due to the conservation of proteins in constitutive and regulated membrane fusion reactions, NSF and its downstream targets have been predicted also to participate in fusion reactions underlying endocrine function, but there is little experimental evidence to support such a role for NSF in insect neuroendocrine secretion. Here we have characterized the NSF orthologue (MsNSF) from the endocrine model for development Manduca sexta. MsNSF is developmentally regulated in endocrine organs of the protocerebral complex. Enrichment of MsNSF in corpora cardiaca (CC) and not in corpora allata (CA) indicates that it might play a preferential role in releasing hormones produced in CC. Endocrine/paracrine cells of the enteric system in M. sexta exhibit selective MsNSF enrichment. Together the data point to a more selective participation of MsNSF in development of M. sexta by its involvement in a subset of factors, whereas other as-yet-unidentified homolog(s) might regulate secretion from CA and a large set of endocrine/paracrine cells. We further characterized the in vivo role of MsNSF by heterologous expression. In contrast to vertebrate NSF, MsNSF is functional in yeast membrane fusion in vivo. MsNSF rectifies defects in SEC18 (yeast NSF homologue) at nearly all discernible steps where Sec18p has been implicated in the biosynthetic route. This underscores the utility of our approach to delineate functional roles for proteins from systems that are not currently amenable to in vitroreconstitution.

Apical macropinocytosis in polarized MDCK cells: regulation by N-ethylmaleimide-sensitive proteins

In cells tested so far endocytosis seems to be dependent on N-ethylmaleimide (NEM)-sensitive proteins, and treatment with NEM results in a complete block of endocytosis. We here demonstrate that treatment of polarized MDCK I cells with NEM strongly increased endocytosis of ricin and horseradish peroxidase at the apical side, and electron microscopy revealed NEM-induced formation of large macropinosomes at the apical pole. The NEM-stimulated apical endocytosis seemed to involve phosphatidylinositol-3 kinase, protein kinase C and phospholipase D and it was dependent on ATP. Moreover, in contrast to endocytosis in nonpolarized cells ricin endocytosis at the basolateral side continued in the presence of NEM whereas endocytosis of transferrin was blocked. Furthermore, recycling of ricin endocytosed in the absence of NEM was not inhibited on either side upon addition of NEM demonstrating the existence of a NEM-resistant fusion machinery. The results suggest that the fusogenic property of both the apical and the basolateral plasma membrane of MDCK cells differs from that typically observed in cells unable to polarize.

SNAP-23 requirement for transferrin recycling in Streptolysin-O-permeabilized Madin-Darby canine kidney cells

Fusion of recycling and transcytotic vesicles with the apical and basolateral plasma membrane domains of Madin-Darby canine kidney (MDCK) cells requires the N-ethylmaleimide-sensitive factor and is sensitive to botulinum neurotoxin serotype E (BoNT/E). BoNT/E is thought to selectively proteolyze the 25,000-dalton synaptosomal associated protein (SNAP-25), a protein found in neurons or cells of neuroendocrine origin. However, SNAP-25 is not found in MDCK cells. One possible target for BoNT/E in MDCK cells is SNAP-23, a newly described SNAP-25 homolog that is found in several organs including kidney. Currently, the function of SNAP-23 is unknown. We have reconstituted transferrin recycling in permeabilized MDCK cells to assess the role of SNAP-23 in the endocytic traffic of this protein. We find that: (i) SNAP-23 is expressed in MDCK cells and is found both at the basolateral plasma membrane and associated with apical and basolateral vesicles, (ii) canine SNAP-23 is cleaved by BoNT/E, (iii) transferrin recycling is N-ethylmaleimide-sensitive factor-dependent and BoNT/E-sensitive, and (iv) addition of either exogenous SNAP-23 or anti-SNAP-23 antibodies inhibits ligand recycling. Our observations suggest that SNAP-23 may be required for fusion of recycling vesicles with the basolateral membrane of polarized MDCK cells.

The SNARE machinery is involved in apical plasma membrane trafficking in MDCK cells

We have investigated the controversial involvement of components of the SNARE (soluble N-ethyl maleimide-sensitive factor [NSF] attachment protein [SNAP] receptor) machinery in membrane traffic to the apical plasma membrane of polarized epithelial (MDCK) cells. Overexpression of syntaxin 3, but not of syntaxins 2 or 4, caused an inhibition of TGN to apical transport and apical recycling, and leads to an accumulation of small vesicles underneath the apical plasma membrane. All other tested transport steps were unaffected by syntaxin 3 overexpression. Botulinum neurotoxin E, which cleaves SNAP-23, and antibodies against alpha-SNAP inhibit both TGN to apical and basolateral transport in a reconstituted in vitro system. In contrast, we find no evidence for an involvement of N-ethyl maleimide-sensitive factor in TGN to apical transport, whereas basolateral transport is NSF-dependent. We conclude that syntaxin 3, SNAP-23, and alpha-SNAP are involved in apical membrane fusion. These results demonstrate that vesicle fusion with the apical plasma membrane does not use a mechanism that is entirely unrelated to other cellular membrane fusion events, but uses isoforms of components of the SNARE machinery, which suggests that they play a role in providing specificity to polarized membrane traffic.

Soluble NSF-attachment proteins

Soluble NSF-attachment proteins (SNAPs) are highly conserved proteins that participate in intracellular membrane fusion and vesicular trafficking. In mammals, there are three different isoforms of SNAPs, alpha-, beta- and gamma-SNAP. alpha- and gamma-SNAP are ubiquitously expressed, whereas beta-SNAP is the brain-specific isoform. SNAPs recruit NSF to the membrane after being bound to specific membrane receptors termed SNAREs, NSF, SNAPs and SNAREs form a heterooligomeric complex that is disrupted upon ATP hydrolysis by NSF, which is a prerequisite of membrane fusion. In addition, beta-SNAP interacts with the putative synaptic calcium sensor protein, synaptotagmin, and may be involved in calcium-regulated exocytosis. In the future, this property might be exploited for the development of new therapeutics for certain CNS pathologies.

Endogenous syntaxins 2, 3 and 4 exhibit distinct but overlapping patterns of expression at the hepatocyte plasma membrane

To investigate the mechanisms regulating polarized vesicle delivery to the cell surface in hepatocytes, we have characterized the endogenous plasma membrane (PM)-associated syntaxins. These integral membrane proteins are components of the membrane docking/fusion apparatus and are thought to function as vesicle receptors at the PM. In hepatocytes, the PM is divided into two domains, the apical and basolateral. If syntaxins are mediating the specific recognition of vesicles delivered to either membrane surface, the simple prediction is that each domain expresses one syntaxin isoform. However, we report that rat hepatocytes express three endogenous PM-associated syntaxin isoforms, syntaxins 2, 3 and 4. By biochemical subfractionation, we determined that the syntaxins exhibit distinct, but overlapping patterns of expression among the PM domains. Syntaxin 4 is primarily expressed at the basolateral surface while syntaxins 2 and 3 are enriched at the apical PM. The immunolocalization of syntaxins 2 and 4 in rat hepatocytes and PM sheets revealed similarly complex patterns of PM expression with enhanced apical staining for both. A significant proportion of syntaxin 3 (25%) was detected in subcellular fractions containing transport vesicles. We have used quantitative immunoblotting to determine that the syntaxins are relatively abundant PM molecules (11-260 nM) in rat liver, spleen and kidney. Also, we determined that the syntaxin binding protein, Munc-18, is present at concentrations from 1.5-20 nM in the same tissues. Although this fundamental quantitative and morphological information is lacking in other systems, it is critical not only for defining syntaxin function, but also for predicting the specific mechanisms that regulate vesicle targeting in hepatocytes and other tissues.

Proteolytic cleavage of cellubrevin and vesicle-associated membrane protein (VAMP) by tetanus toxin does not impair insulin-stimulated glucose transport or GLUT4 translocation in rat adipocytes

Acute insulin stimulation of glucose transport in fat and skeletal muscle occurs principally as a result of the hormonal induced translocation of the GLUT4 glucose transporter from intracellular vesicular stores to the plasma membrane. The precise mechanisms governing the fusion of GLUT4 vesicles with the plasma membrane are very poorly understood at present but may share some similarities with synaptic vesicle fusion, as vesicle-associated membrane protein (VAMP) and cellubrevin, two proteins implicated in the process of membrane fusion, are resident in GLUT4-containing vesicles isolated from rat and murine 3T3-L1 adipocytes respectively. In this study we show that proteolysis of both cellubrevin and VAMP, induced by electroporation of isolated rat adipocytes with tetanus toxin, does not impair insulin-stimulated glucose transport or GLUT4 translocation. The hormone was found to stimulate glucose uptake by approx. 16-fold in freshly isolated rat adipocytes. After a single electroporating pulse, the ability of insulin to activate glucose uptake was lowered, but the observed stimulation was nevertheless nearly 5-fold higher than the basal rate of glucose uptake. Electroporation of adipocytes with 600 nM tetanus toxin resulted in a complete loss of both cellubrevin and VAMP expression within 60 min. However, toxin-mediated proteolysis of both these proteins had no effect on the ability of insulin to stimulate glucose transport which was elevated approx. 5-fold, an activation of comparable magnitude to that observed in cells electroporated without tetanus toxin. The lack of any significant change in insulin-stimulated glucose transport was consistent with the finding that toxin-mediated proteolysis of both cellubrevin and VAMP had no detectable effect on insulin-induced translocation of GLUT4 in adipocytes. Our findings indicate that, although cellubrevin and VAMP are resident proteins in adipocyte GLUT4-containing vesicles, they are not required for the acute insulin-induced delivery of GLUT4 to the plasma membrane.

Differential localization of syntaxin isoforms in polarized Madin-Darby canine kidney cells

Syntaxins, integral membrane proteins that are part of the ubiquitous membrane fusion machinery, are thought to act as target membrane receptors during the process of vesicle docking and fusion. Several isoforms of the syntaxin family have been previously identified in mammalian cells, some of which are localized to the plasma membrane. We investigated the subcellular localization of these putative plasma membrane syntaxins in polarized epithelial cells, which are characterized by the presence of distinct apical and basolateral plasma membrane domains. Syntaxins 2, 3, and 4 were found to be endogenously present in Madin-Darby canine kidney cells. The localization of syntaxins 1A, 1B, 2, 3, and 4 in stably transfected Madin-Darby canine kidney cell lines was studied with confocal immunofluorescence microscopy. Each syntaxin isoform was found to have a unique pattern of localization. Syntaxins 1A and 1B were present only in intracellular structures, with little or no apparent plasma membrane staining. In contrast, syntaxin 2 was found on both the apical and basolateral surface, whereas the plasma membrane localization of syntaxins 3 and 4 were restricted to the apical or basolateral domains, respectively. Syntaxins are therefore the first known components of the plasma membrane fusion machinery that are differentially localized in polarized cells, suggesting that they may play a central role in targeting specificity.

Distinct cellular locations of the syntaxin family of proteins in rat pancreatic acinar cells

Syntaxins are cytoplasmically oriented integral membrane soluble NEM-sensitive factor receptors (SNAREs; soluble NEM-sensitive factor attachment protein receptors) thought to serve as targets for the assembly of protein complexes important in regulating membrane fusion. The SNARE hypothesis predicts that the fidelity of vesicle traffic is controlled in part by the correct recognition of vesicle SNAREs with their cognate target SNARE partner. Here, we show that in the exocrine acinar cell of the pancreas, multiple syntaxin isoforms are expressed and that they appear to reside in distinct membrane compartments. Syntaxin 2 is restricted to the apical plasma membrane whereas syntaxin 4 is found most abundantly on the basolateral membranes. Surprisingly, syntaxin 3 was found to be localized to a vesicular compartment, the zymogen granule membrane. In addition, we show that these proteins are capable of specific interaction with vesicle SNARE proteins. Their nonoverlapping locations support the general principle of the SNARE hypothesis and provide new insights into the mechanisms of polarized secretion in epithelial cells.

Annexin II in exocytosis: catecholamine secretion requires the translocation of p36 to the subplasmalemmal region in chromaffin cells

Annexin II is a Ca(2+)-dependent membrane-binding protein present in a wide variety of cells and tissues. Within cells, annexin II is found either as a 36-kD monomer (p36) or as a heterotetrameric complex (p90) coupled with the S-100-related protein, p11. Annexin II has been suggested to be involved in exocytosis as it can restore the secretory responsiveness of permeabilized chromaffin cells. By quantitative confocal immunofluorescence, immunoreplica analysis and immunoprecipitation, we show here the translocation of p36 from the cytosol to a subplasmalemmal Triton X-100 insoluble fraction in chromaffin cells following nicotinic stimulation. A synthetic peptide corresponding to the NH2-terminal domain of p36 which contains the phosphorylation sites was microinjected into individual chromaffin cells and catecholamine secretion was monitored by amperometry. This peptide blocked completely the nicotine-induced recruitment of p36 to the cell periphery and strongly inhibited exocytosis evoked by either nicotine or high K+. The light chain of annexin II, p11, was selectively expressed by adrenergic chromaffin cells, and was only present in the subplasmalemmal Triton X-100 insoluble protein fraction of both resting and stimulated cells. p11 can modify the Ca(2+)- and/or the phospholipid-binding properties of p36. We found that loss Ca2+ was required to stimulate the translocation of p36 and to trigger exocytosis in adrenergic chromaffin cells. Our findings suggest that the translocation of p36 to the subplasmalemmal region is an essential event in regulated exocytosis and support the idea that the presence of p11 in adrenergic cells may confer a higher Ca2+ affinity to the exocytotic pathway in these cells.

Exchanging the extracellular domain of amyloid precursor protein for horseradish peroxidase does not interfere with alpha-secretase cleavage of the beta-amyloid region, but randomizes secretion in Madin-Darby canine kidney cells

Secretory processing and polarized sorting of horseradish peroxidase fused to the amyloid precursor protein transmembrane domain were compared with those of wild-type amyloid precursor protein in COS and polarized Madin-Darby canine kidney (MDCK) cells. The cellular and secreted forms of the chimeric protein were enzymatically active in colorimetric and cytochemical assays after reconstitution with hemin and Ca2+. The peroxidase enzyme was secreted by a proteolytic process, similar to the parent amyloid precursor protein. In polarized MDCK cells, amyloid precursor protein was secreted exclusively in the basolateral compartment, while the peroxidase chimeric protein was secreted in both compartments. The basolateral sorting determinant for secretion must therefore be located in the extracellular domain of amyloid precursor protein. On the other hand, cell surface-associated peroxidase chimeric protein was similar to cell surface-associated wild-type amyloid precursor protein, mainly expressed at the basolateral side. The basolateral cell-surface expression, in contrast to the basolateral secretion, is therefore controlled by determinants in the cytoplasmic domain. Methylamine inhibited and bafilomycin slightly increased the basolateral secretion of both proteins, but both drugs strongly increased apical secretion. The default secretory pathway of COS cells and the basolateral (but not the apical) secretory pathway of MDCK cells are therefore comparably sensitive to methylamine and not to bafilomycin.