The N-ethylmaleimide-sensitive protein thiol groups necessary for sea-urchin egg cortical-granule exocytosis are highly exposed to the medium and are required for triggering by Ca2+

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Dissecting the mechanism of Ca2+-triggered membrane fusion: probing protein function using thiol reactivity

1. Ca(2+)-triggered membrane fusion involves the coordinated actions of both lipids and proteins, but the specific mechanisms remain poorly understood. The urchin cortical vesicle model is a stage-specific native preparation fully enabling the directly coupled functional-molecular analyses necessary to identify critical components of fast triggered membrane fusion. 2. Recent work on lipidic components has established a direct role for cholesterol in the fusion mechanism via local contribution of negative curvature to readily enable the formation of transient lipidic fusion intermediates. In addition, cholesterol- and sphingomyelin-enriched domains regulate the efficiency of fusion by focally organizing other components to ensure an optimized response to the triggering Ca(2+) transient. 3. There is less known about the identity of proteins involved in the Ca(2+)-triggering steps of membrane fusion. Thiol reagents can be used as unbiased tools to probe protein functions. Comparisons of several thiol-reactive reagents have identified different effects on Ca(2+) sensitivity and the extent of fusion, suggesting that there are at least two distinct thiol sites that participate in the fusion mechanism: one that regulates the efficiency of Ca(2+) sensing/triggering and one that may function during the membrane merger event itself. 4. To identify the proteins that regulate Ca(2+) sensitivity, the fluorescent thiol reagent Lucifer yellow iodoacetamide was used to potentiate fusion and simultaneously tag the proteins involved. Ongoing work involves the isolation of cholesterol-enriched membrane fractions to reduce the complexity of the labelled proteome, narrowing the number of candidate proteins.

Identifying critical components of native Ca2+-triggered membrane fusion. Integrating studies of proteins and lipids

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Despite realization that the fusion machinery must include lipids and proteins working in concert, only of late has work in the field focused more equally on both these components. Here we use isolated sea urchin egg cortical vesicles (CV), a stage-specific preparation of Ca(2+)-sensitive release-ready vesicles that enables the tight coupling of molecular and functional analyses necessary to dissect molecular mechanisms. The stalk-pore hypothesis proposes that bilayer merger proceeds rapidly via transient, high-negative curvature, intermediate membrane structures. Consistent with this, cholesterol, a major component of the CV membrane, contributes to a critical local negative curvature that supports formation of lipidic fusion intermediates. Following cholesterol depletion, structurally dissimilar lipids having intrinsic negative curvature greater than or equal to cholesterol recover the ability of CV to fuse but do not recover fusion efficiency (Ca(2+) sensitivity and kinetics). Conversely, cholesterol- and sphingomyelin-enriched microdomains regulate the efficiency of the fusion mechanism, presumably by contributing spatial and functional organization of other critical lipids and proteins at the fusion site. Critical proteins are thought to participate in Ca(2+) sensing, initiating membrane deformations, and facilitating fusion pore expansion. Capitalizing on a novel effect of the thiol-reactive reagent iodoacetamide (IA), potentiation of the Ca(2+) sensitivity and kinetics, a fluorescently tagged IA has been used to enhance fusion efficiency and simultaneously label the proteins involved. Isolation of cholesterol-enriched CV membrane fractions, using density gradient centrifugation, is being used to narrow the list of protein candidates potentially critical to the mechanism of fast Ca(2+)-triggered membrane fusion.

Enhancement of the Ca(2+)-triggering steps of native membrane fusion via thiol-reactivity

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Isolated urchin cortical vesicles (CV) provide a stage-specific preparation to study the mechanisms by which Ca(2+) triggers the merger of two apposed native membranes. Thiol-reactive reagents that alkylate free sulfhydryl groups on proteins have been consistently shown to inhibit triggered fusion. Here, we characterize a novel effect of the alkylating reagent iodoacetamide (IA). IA was found to enhance the kinetics and Ca(2+) sensitivity of both CV-plasma membrane and CV-CV fusion. If Sr(2+), a weak Ca(2+) mimetic, was used to trigger fusion, the potentiation was even greater than that observed for Ca(2+), suggesting that IA acts at the Ca(2+)-sensing step of triggered fusion. Comparison of IA to other reagents indicates that there are at least two distinct thiol sites involved in the underlying fusion mechanism: one that regulates the efficiency of fusion and one that interferes with fusion competency.

Membrane fusion of secretory vesicles of the sea urchin egg in the absence of NSF

The role of cytosolic ATPases such as N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) in membrane fusion is controversial. We examined the physiology and biochemistry of ATP and NSF in the cortical system of the echinoderm egg to determine if NSF is an essential factor in membrane fusion during Ca(2+)-triggered exocytosis. Neither exocytosis in vitro, nor homotypic cortical vesicle (CV) fusion required soluble proteins or nucleotides, and both occurred in the presence of non-hydrolyzable analogs of ATP. While sensitive to thiol-specific reagents, CV exocytosis is not restored by the addition of cytosolic NSF, and fusion and NSF function are differentially sensitive to thiol-specific agents. To test participation of tightly bound, non-exchangeable NSF in CV-CV fusion, we cloned the sea urchin homolog and developed a species-specific antibody for western blots and physiological analysis. This antibody was without effect on CV exocytosis or homotypic fusion, despite being functionally inhibitory. NSF is detectable in intact cortices, cortices from which CVs had been removed and isolated CVs treated with ATP-gamma-S and egg cytosol to reveal NSF binding sites. In contrast, isolated CVs, though all capable of Ca(2+)-triggered homotypic fusion, contain less than one hexamer of NSF per CV. Thus NSF is not a required component of the CV fusion machinery.

Influenza hemagglutinins outside of the contact zone are necessary for fusion pore expansion

Current models for membrane fusion in diverse biological processes are focused on the local action of fusion proteins present in the contact zone where the proteins anchored in one membrane might interact directly with the other membrane. Are the fusion proteins outside of the contact zone just bystanders? Here we assess the role of these "outsider" proteins in influenza virus hemagglutinin-mediated fusion between red blood cells and either hemagglutinin-expressing cells or viral particles. To selectively inhibit or enhance the actions of hemagglutinin outsiders, the antibodies that bind to hemagglutinin and proteases that cleave it were conjugated to polystyrene microspheres too large to enter the contact zone. We also involved hemagglutinin outsiders into interactions with additional red blood cells. We find the hemagglutinin outsiders to be necessary and sufficient for fusion. Interfering with the activity of the hemagglutinin outsiders inhibited fusion. Selective conversion of hemagglutinin outsiders alone into fusion-competent conformation was sufficient to achieve fusion. The discovered functional role of fusion proteins located outside of the contact zone suggests a tempting analogy to mechanisms by which proteins mediate membrane fission from outside of the fission site.

Plasma membrane resident 'fusion complexes' mediate reconstituted exocytosis

Calcium-triggered exocytosis is thought to be mediated by membrane-associated protein complexes. In sea urchin eggs, high concentrations of calcium activate multiple 'fusion complexes' per cortical vesicle-plasma membrane docking site. Some of these fusion complexes are known to reside in the vesicle membrane. It is not known if fusion complexes also reside in the plasma membrane, or if plasma membrane-resident fusion complexes require cognate partners in the vesicle membrane. Using reconstitution, we show that N-ethylmaleimide treatment of either vesicles or plasma membrane fragments prior to reconstitution does not completely inhibit exocytosis. Treatment of both components did result in complete inhibition. Upon reconstitution, cortical vesicles and the early endosomes formed by compensatory endocytosis both contributed, on average, two fusion complexes per reconstituted docking site. The plasma membrane contributed, on average, two fusion complexes per docking site when assembled with cortical vesicles, but only one complex when reconstituted with endosomes. We conclude that there are at least two types of plasma membrane-resident fusion complexes that participate in reconstituted cortical vesicle-plasma membrane fusion. The activity of one of these fusion complexes is target-specific for cortical vesicles, while the second type also supports fusion with endosomes.

An analysis of the lamellar structure of sea urchin egg cortical granules using X-ray scattering

Cortical granules (CGs) are secretory vesicles associated with egg and oocyte plasma membranes that undergo exocytosis at fertilisation. In the sea urchin Strongylocentrotus purpuratus, the internal organisation of these CGs exhibits a lamellar-type morphology. The different lamellar layers correspond to proteoglycans, structural proteins and enzymes required for fertilisation envelope assembly and modification of the post-fertilisation egg surface. We have studied the lamellar structure of CGs using X-ray scattering and reveal the contrast density variation of the lamellae in the native state. The structure of functionally competent CGs in situ differs significantly from that determined by electron microscopic studies. We observed a strong periodicity of the lamellar structure of 280 A as opposed to the 590 A repeat observed previously. Fusion of the CGs produced a loss of the lamellar repeat and the development of a broad peak corresponding to a 20 A periodicity that may be indicative of the molecular packing in the resulting hydrated gel structure.

Biochemical and functional studies of cortical vesicle fusion: the SNARE complex and Ca2+ sensitivity

Cortical vesicles (CV) possess components critical to the mechanism of exocytosis. The homotypic fusion of CV centrifuged or settled into contact has a sigmoidal Ca2+ activity curve comparable to exocytosis (CV-PM fusion). Here we show that Sr2+ and Ba2+ also trigger CV-CV fusion, and agents affecting different steps of exocytotic fusion block Ca2+, Sr2+, and Ba2+-triggered CV-CV fusion. The maximal number of active fusion complexes per vesicle, <n\>Max, was quantified by NEM inhibition of fusion, showing that CV-CV fusion satisfies many criteria of a mathematical analysis developed for exocytosis. Both <n\>Max and the Ca2+ sensitivity of fusion complex activation were comparable to that determined for CV-PM fusion. Using Ca2+-induced SNARE complex disruption, we have analyzed the relationship between membrane fusion (CV-CV and CV-PM) and the SNARE complex. Fusion and complex disruption have different sensitivities to Ca2+, Sr2+, and Ba2+, the complex remains Ca2+- sensitive on fusion-incompetent CV, and disruption does not correlate with the quantified activation of fusion complexes. Under conditions which disrupt the SNARE complex, CV on the PM remain docked and fusion competent, and isolated CV still dock and fuse, but with a markedly reduced Ca2+ sensitivity. Thus, in this system, neither the formation, presence, nor disruption of the SNARE complex is essential to the Ca2+-triggered fusion of exocytotic membranes. Therefore the SNARE complex alone cannot be the universal minimal fusion machine for intracellular fusion. We suggest that this complex modulates the Ca2+ sensitivity of fusion.

Calcium can disrupt the SNARE protein complex on sea urchin egg secretory vesicles without irreversibly blocking fusion

The homotypic fusion of sea urchin egg cortical vesicles (CV) is a system in which to correlate the biochemistry and physiology of membrane fusion. Homologues of vesicle-associated membrane protein (VAMP), syntaxin, and SNAP-25 were identified in CV membranes. A VAMP and syntaxin immunoreactive band at a higher apparent molecular mass (approximately 70 kDa) was detected; extraction and analysis confirmed that the band contained VAMP, SNAP-25, and syntaxin. This complex was also identified by immunoprecipitation and by sucrose gradient analysis. VAMP in the complex was insensitive to proteolysis by tetanus toxin. All criteria identify the SNARE complex as that described in other secretory systems. Complexes exist pre-formed on individual CV membranes and form between contacting CV. Most notably, CV SNARE complexes are disrupted in response to [Ca2+]free that trigger maximal fusion. N-Ethylmaleimide, which blocks fusion at or before the Ca2+-triggering step, blocks complex disruption by Ca2+. However, disruption is not blocked by lysophosphatidylcholine, which transiently arrests a late stage of fusion. Since removal of lysophosphatidylcholine from Ca2+-treated CV is known to allow fusion, complex disruption occurs independently from the membrane fusion step. As Ca2+ disrupts rather than stabilizes the complex, the presumably coiled-coil SNARE interactions are not needed at the time of fusion. These findings rule out models of fusion in which SNARE complex formation goes to completion ("zippers-up") after Ca2+ binding removes a "fusion-clamp."

Poisson-distributed active fusion complexes underlie the control of the rate and extent of exocytosis by calcium

We have investigated the consequences of having multiple fusion complexes on exocytotic granules, and have identified a new principle for interpreting the calcium dependence of calcium-triggered exocytosis. Strikingly different physiological responses to calcium are expected when active fusion complexes are distributed between granules in a deterministic or probabilistic manner. We have modeled these differences, and compared them with the calcium dependence of sea urchin egg cortical granule exocytosis. From the calcium dependence of cortical granule exocytosis, and from the exposure time and concentration dependence of N-ethylmaleimide inhibition, we determined that cortical granules do have spare active fusion complexes that are randomly distributed as a Poisson process among the population of granules. At high calcium concentrations, docking sites have on average nine active fusion complexes.

Characterization of N-ethylmaleimide-sensitive thiol groups required for the GTP-dependent fusion of endoplasmic reticulum membranes

The GTP-dependent fusion activity of endoplasmic reticulum membranes is thought to be required for the structural maintenance and post-mitotic regeneration of the endoplasmic reticulum. This fusion is sensitive to the thiol-alkylating agent N-ethylmaleimide. In many intracellular fusion events N-ethylmaleimide-sensitivity is associated with a homotrimeric ATPase called N-ethylmaleimide-sensitive fusion protein or NSF. The addition of cytosol containing NSF is known to restore fusion activity to N-ethylmaleimide-treated membranes. We found that the inhibition of fusion of rat liver endoplasmic reticulum membranes (microsomes) by N-ethylmaleimide was not reversed by the addition of untreated cytosol. Fusion was also unaffected by treatment with a buffer known to remove NSF from membranes. Accordingly, no membrane-associated NSF was detected by immunoblot analysis. These data suggest that microsome fusion requires an N-ethylmaleimide-sensitive component distinct from NSF. This component was tightly associated with the membranes, so we used a number of chemical probes to characterize it in situ. Its thiol groups did not appear to be part of a GTP-binding site. They showed relatively low reactivity with sodium periodate, which induces the formation of disulphide bonds between proximate thiol groups. The thiols were not protected against N-ethylmaleimide by Zn2+, a potent inhibitor of fusion which is known to efficiently co-ordinate thiol groups. To characterize the topology of the fusion-related thiol groups we used bulky thiol-specific reagents prepared by conjugating BSA or 10 kDa aminodextran to the bifunctional reagent N-succinimidyl 3-(2-pyridyldithio)propionate. The inhibition of fusion by these reagents indicated that these thiols are highly exposed on the membranes. This exposure might be important for the function of these groups during GTP-triggered fusion.