Phosphoprotein inhibition of calcium-stimulated exocytosis in sea urchin eggs

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Deciphering the molecular mechanisms underlying sea urchin reversible adhesion: A quantitative proteomics approach

Marine bioadhesives have unmatched performances in wet environments, being an inspiration for biomedical applications. In sea urchins specialized adhesive organs, tube feet, mediate reversible adhesion, being composed by a disc, producing adhesive and de-adhesive secretions, and a motile stem. After tube foot detachment, the secreted adhesive remains bound to the substratum as a footprint. Sea urchin adhesive is composed by proteins and sugars, but so far only one protein, Nectin, was shown to be over-expressed as a transcript in tube feet discs, suggesting its involvement in sea urchin adhesion. Here we use high-resolution quantitative mass-spectrometry to perform the first study combining the analysis of the differential proteome of an adhesive organ, with the proteome of its secreted adhesive. This strategy allowed us to identify 163 highly over-expressed disc proteins, specifically involved in sea urchin reversible adhesion; to find that 70% of the secreted adhesive components fall within five protein groups, involved in exocytosis and microbial protection; and to provide evidences that Nectin is not only highly expressed in tube feet discs but is an actual component of the adhesive. These results give an unprecedented insight into the molecular mechanisms underlying sea urchin adhesion, and opening new doors to develop wet-reliable, reversible, and ecological biomimetic adhesives.

SIGNIFICANCE

Sea urchins attach strongly but in a reversible manner to substratum, being a valuable source of inspiration for industrial and biomedical applications. Yet, the molecular mechanisms governing reversible adhesion are still poorly studied delaying the engineering of biomimetic adhesives. We used the latest mass spectrometry techniques to analyze the differential proteome of an adhesive organ and the proteome of its secreted adhesive, allowing us to uncover the key players in sea urchin reversible adhesion. We demonstrate, that Nectin, a protein previously pointed out as potentially involved in sea urchin adhesion, is not only highly expressed in tube feet discs, but is a genuine component of the secreted adhesive.

A cell-physiological description of GE, a GTP-binding protein that mediates exocytosis

Introduction of GTP gamma S or other non-metabolic analogues of GTP into permeabilized myeloid granulocytes (mast cells, eosinophils, neutrophils) constitutes a sufficient stimulus to induce exocytosis. We concentrate on mast cells. Exocytosis from cells permeabilized in isotonic glutamate solution proceeds in the absence of ATP and at exceedingly low levels (< 10(-9) M) of Ca2+. Mg2+ strongly promotes GTP gamma S-induced exocytosis but this requirement can be spared and then obliterated by lifting Ca2+ through 10(-7) to 10(-6) M. GTP provides only a modest support to exocytosis but becomes almost equipotent with GTP gamma S when Mg2+ is excluded. Ca2+ alone is unable to induce exocytosis. We envisage that the terminal stage of exocytosis (membrane fusion) requires activation of GE, a putative GTPase so far undefined as a molecular entity. Ca2+, presumed to act through a Ca(2+)-binding protein (CE, also undefined) supports exocytosis by promoting the exchange of guanine nucleotides on GE. In the absence of Mg2+ the onset of exocytosis is characterized by delays that have concentration-dependent (binding) and independent components. The latter are sensitive to the identity of the stimulating nucleotide (GTP < GTP gamma S < Gpp [NH]p) and may reflect activation of GE. The activation by Ca2+ and Mg2+ and the delays preceding onset of GTP gamma S-triggered exocytosis are reminiscent of the action of glucagon and Mg2+ in the activation of adenylate cyclase in hepatocyte membranes. The cell-physiological description predicts GE to be an alpha beta gamma heterotrimeric GTP-binding protein with functional similarity to GS.

Protein tyrosine and serine-threonine phosphatases in the sea urchin, Strongylocentrotus purpuratus: identification and potential functions

Protein phosphatases, in coordination with protein kinases, play crucial roles in regulation of signaling pathways. To identify protein tyrosine phosphatases (PTPs) and serine-threonine (ser-thr) phosphatases in the Strongylocentrotus purpuratus genome, 179 annotated sequences were studied (122 PTPs, 57 ser-thr phosphatases). Sequence analysis identified 91 phosphatases (33 conventional PTPs, 31 dual specificity phosphatases, 1 Class III Cysteine-based PTP, 1 Asp-based PTP, and 25 ser-thr phosphatases). Using catalytic sites, levels of conservation and constraint in amino acid sequence were examined. Nine of 25 receptor PTPs (RPTPs) corresponded to human, nematode, or fly homologues. Domain structure revealed that sea urchin-specific RPTPs including two, PTPRLec and PTPRscav, may act in immune defense. Embryonic transcription of each phosphatase was recorded from a high-density oligonucleotide tiling microarray experiment. Most RPTPs are expressed at very low levels, whereas nonreceptor PTPs (NRPTPs) are generally expressed at moderate levels. High expression was detected in MAP kinase phosphatases (MKPs) and numerous ser-thr phosphatases. For several expressed NRPTPs, MKPs, and ser-thr phosphatases, morpholino antisense-mediated knockdowns were performed and phenotypes obtained. Finally, to assess roles of annotated phosphatases in endomesoderm formation, a literature review of phosphatase functions in model organisms was superimposed on sea urchin developmental pathways to predict areas of functional activity.

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.

Effects of the phosphatase inhibitors, okadaic acid, ATPgammaS, and calyculin A on the dividing sand dollar egg

The effects of the phosphatase inhibitors, okadaic acid (OA), adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and calyculin A (CL-A) on anaphase chromosome movement, cytokinesis, and cytoskeletal structures at cell division were examined by being microinjected into mitotic sand dollar eggs. When OA was injected, chromosome movement was inhibited and, moreover, chromosomes were ejected from the polar regions of the mitotic apparatus. By immunofluorescence, microtubules were observed to be severed in the OA-injected eggs, causing the smooth cell surface to be changed to an irregular surface. When ATPgammaS and CL-A were injected, the effect on cell shape was remarkable: In dividing eggs, furrowing stopped within several seconds after injection, small blebs appeared on the cell surface and became large, spherical or dumbbell cell shapes then changed to irregular forms, and subsequently cytoplasmic flow occurred. Microfilament detection revealed that actin accumulation in the cortex, which was not limited to the furrow cortex, occurred shortly after injection. Cortical accumulation of actin is thought to induce force generation and random cortical contraction, and accordingly to result in bleb extrusion from the cortex. Consequently, the phosphatase inhibitors inhibited the transition from mitosis to interphase by mediating cortical accumulation of actin filaments and/or fragmentation of microtubules.

Membrane fusion

Although catalyzed by different proteins, the energy barriers for lipid bilayer fusion in exocytosis, viral fusion, and trafficking seem to be the same as those for the fusion of protein-free phospholipid membranes. To minimize this energy, fusion will proceed through a minimal number of lipid molecules, probably localized in bent non-bilayer intermediates. Experiments on phospholipid bilayer membrane fusion show the pathway of contact, hemifusion, flickering fusion pore formation, and fusion pore enlargement caused by swelling of the vesicle. Lipid curvature determines the barriers to hemifusion and fusion pore formation, while swelling-induced membrane tension drives fusion pore enlargement. Experiments on viral protein-induced cell-cell fusion and exocytosis show the same pathway with the same fundamental effects of lipid curvature and membrane tension. Thus while proteins control these reactions, lipid energetics determine the basic reaction scheme for membrane fusion.

Extracellular adenosine triphosphate affects neural cell adhesion molecule (NCAM)-mediated cell adhesion and neurite outgrowth

The neural cell adhesion molecule (NCAM) plays an important role in synaptic plasticity in embryonic and adult brain. Recently, it has been demonstrated that NCAM is capable of binding and hydrolyzing extracellular ATP. The purpose of the present study was to evaluate the role of extracellular ATP in NCAM-mediated cellular adhesion and neurite outgrowth. We here show that extracellularly added adenosine triphosphate (ATP) and its structural analogues, adenosine-5'-O-(3-thiothiophosphate), beta, gamma-methylenadenosine-5'-triphosphate, beta, gamma-imidoadenosine-5-triphosphate, and UTP, in varying degrees inhibited aggregation of hippocampal neurons. Rat glial BT4Cn cells are unable to aggregate when grown on agar but acquire this capacity when transfected with NCAM. However, addition of extracellular ATP to NCAM-transfected BT4Cn cells inhibited aggregation. Furthermore, neurite outgrowth from hippocampal neurons in cultures allowing NCAM-homophilic interactions was inhibited by addition of extracellular nucleotides. These findings indicate that NCAM-mediated adhesion may be modulated by extracellular ATP. Moreover, extracellularly added ATP stimulated neurite outgrowth from hippocampal neurons under conditions non-permissive for NCAM-homophilic interactions, and neurite outgrowth stimulated by extracellular ATP could be inhibited by a synthetic peptide corresponding to the so-called cell adhesion molecule homology domain (CHD) of the fibroblast growth factor receptor (FGFR) and by FGFR antibodies binding to this domain. Antibodies against the fibronectin type-III homology modules of NCAM, in which a putative site for ATP binding and hydrolysis is located, also abolished the neurite outgrowth-promoting effect of ATP. The non-hydrolyzable analogues of ATP all strongly inhibited neurite outgrowth. Our results indicate that extracellular ATP may be involved in synaptic plasticity through a modulation of NCAM-mediated adhesion and neurite outgrowth.

Apextrin, a novel extracellular protein associated with larval ectoderm evolution in Heliocidaris erythrogramma

During the evolution of direct development in the sea urchin Heliocidaris erythrogramma major modifications occurred, which allowed the precocious formation of adult-specific structures and led to a novel larval body that surrounds these structures. The HeET-1 gene was isolated in a differential screen for transcripts enriched in the early embryos of H. erythrogramma relative to those of its indirect-developing congener, H. tuberculata. HeET-1 was unique among the three genes found in that no homologous transcript was detected in H. tuberculata total embryonic RNA blots. To verify this apparently extreme differential expression of the HeET-1 genes in Heliocidaris, we isolated the HeET-1 homologue from H. tuberculata genomic DNA and used it to probe blots of poly(A)+ RNA prepared from H. tuberculata embryos. It is expressed in H. tuberculata embryos at levels undetectable by this technique. The predicted amino acid sequence of HeET-1 suggested that it encodes a novel secreted protein. To assess the function of HeET-1, we raised polyclonal antisera to the HeET-1-encoded protein. We find that it is present in eggs in a type of secretory vesicle and that this maternal pool is gradually secreted after fertilization. As cells acquire apical-basal polarity in the blastula the protein becomes localized to the apical extracellular matrix, leading us to name the protein apextrin. The apical extracellular localization of apextrin is maintained in the columnar cells of the larval ectoderm until their internalization at metamorphosis. Ingressing mesenchyme cells rapidly endocytose apextrin upon leaving the vegetal plate. Comparison with fibropellin III, an apical lamina component, suggests that apextrin is an extracellular protein that is in tighter association with the plasma membrane than is the hyalin layer or apical lamina. We propose that apextrin is involved in apical cell adhesion and that its high level of expression may represent an adaptive cooption necessary for strengthening the large H. erythrogramma embryo.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Regulated exocytosis and sequential construction of the extracellular matrix surrounding the sea urchin zygote

After fertilization most eggs become surrounded by a complex extracellular matrix. This study examines those matrix assembly processes that are triggered by fertilization of the sea urchin egg. The study uses antibodies that identify five different storage compartments in the egg. These compartments release their protein contents in a highly regulated fashion to assemble and modify the extraembryonic layers. The exocytosis sequence begins with a fertilization wave that progresses from the site of sperm entry and elevates the fertilization envelope above a water-filled perivitelline space. The immediate surface of the zygote then becomes covered by a newly secreted hyaline layer. Prior to fertilization some of the antigens are localized to cortical granules. Others are found in "basal laminar vesicles" that are released in a wave beginning at about 30 sec, or roughly at the same time as cortical granule exocytosis. The remaining antigens are exocytosed with a rather precise timing, but with a delay of several to tens of minutes relative to the first wave of exocytosis. "Apical vesicles," so named because antigens from this class are preferentially exocytosed toward the apical cell surface of polarized cells, include antigens that are exocytosed beginning at about 5 min postfertilization. The fourth compartment, named "echinonectin vesicles" release echinonectin, a protein that is deposited to the inner side of the hyaline layer. Surface staining of echinonectin is first detected about 10-15 min following sperm contact. Finally, maternal cadherin, which is stored in yet a fifth distinct compartment, is not detected on the surface until at least 30 min following fertilization. The data are also consistent with the notion that the tightly regulated timing of exocytosis contributes to the ordered assembly of the hyaline layer and elevation of the fertilization envelope. Finally, two of the vesicle classes continue to exocytose after the cells become polarized. In polarized cells apical and basal laminar antigens are trafficked toward opposite sides of the same cell after passing through the same trans-Golgi network-like compartment.

ATP is required in platelet serotonin exocytosis for protein phosphorylation and priming of secretory vesicles docked on the plasma membrane

Calcium-evoked secretion generally requires the presence of millimolar concentrations of Mg-ATP. We investigated the role of Mg-ATP in the secretion of serotonin from electropermeabilized bovine platelets. The secretion of serotonin was lost within 5 minutes when the Mg-ATP concentration was diluted to less than 0.1 mM, but was maintained when ATP-gamma S (adenosine 5′-O-3-thiotriphosphate) was used instead of ATP. Okadaic acid, a potent inhibitor of protein phosphatase, could also maintain the exocytotic activity even when ATP was diluted. Decrease in the secretory activity was paralleled by a decrease in phosphorylation level of four proteins after dilution of ATP, but the activity was maintained when the thiophosphorylation level of these proteins was maintained. Two of these proteins were digested by a protease, calpain, which has been shown to lead to a loss in the exocytotic activity. Electron microscopic studies showed that calcium did not induce the formation of distinct bridge-like structures between the granule membrane and the plasma membrane in Mg-ATP-diluted cells, previously shown as the structure transiently formed prior to fusion of the two membranes. Anchorage of the secretory dense granules to the plasma membrane and the presence of the amorphous structures between the granules and the plasma membrane were unchanged by dilution of ATP. These results indicate that ATP is not required for the anchorage itself, but is required to prime anchored granules for calcium-triggered secretion. Maintenance of the phosphorylated state of proteins by ATP enables the calcium trigger to form the bridge-like structures preceding membrane fusion events.

Cytoplasm calcium-binding proteins of germ cells and embryos of the sea urchin

Synchronous, demonstrative, easily reproducible fertilization with the following embryonic development makes the process in the sea urchin extremely attractive for studying many biological enigmas. In particular, germ and embryonic cells of the sea urchin present a wide opportunity for investigating different associated phenomena launched by an increase in concentration of Ca2+ in cells ([Ca2+]i). Ca2+ ions participate in the activation of diverse processes of respiration and sperm motility (Shapiro et al., 1990; Brokaw, 1991), chemotaxis of spermatozoa to components of the egg jelly (Ward et al., 1985), acrosomal reaction (Trimmer et al., 1986; Shapiro et al., 1990), cortical reaction, formation of the fertilization membrane (Sasaki, 1984; Sardet and Chang, 1987), cellular division in the embryo (Poenie et al., 1985; Silver, 1986; Whitaker and Patel, 1990), their adhesion (McClay and Matranga, 1986), differentiation and formation of spicules (Mitsunaga et al., 1988) and metamorphosis (Carpenter et al., 1984). The present review combines information on the function of calcium-binding proteins and their targets, calmodulin regulation of NAD-kinase, exocytosis of cortical granules, Ca(2+)- and calmodulin-dependent protein phosphatase, Ca(2+)-dependent protein phosphorylation, regulation of ion-exchanger in the germ and embryonic cells as well as Ca(2+)- and calmodulin control of sperm motility in sea urchins.

Identification and partial sequence analysis of novel annexins in Lytechinus pictus oocytes

The annexins are a major class of calcium-binding proteins with unknown functions. In an attempt to define novel model systems in which to study members of the annexin family, we have investigated the expression of annexins in eggs from the sea urchin Lytechinus pictus. Western blot analysis of L. pictus eggs using antisera raised against human annexins I, V and VI revealed the presence of immunoreactive proteins of approximately 34 kDa, 35 kDa and 68 kDa respectively. The sea urchin annexins behaved similarly to their mammalian counterparts, both during purification and in their ability to bind calcium-dependently to anionic phospholipids. Of the three sea urchin annexins, the 34 kDa form was most abundant, yielding sufficient quantities for peptide microsequencing. The amino acid sequences derived in this way showed the L. pictus annexin to be closely related both to mammalian annexin I and to annexins IX, X and XII from Drosophila and Hydra. However, N-terminal sequence from the L. pictus annexin showed it to be a novel member of the annexin super-gene family. The results are interesting in view of the complex evolution of the annexin gene family, and also point to the potential usefulness of echinoderm eggs as a model system in which to study annexin function.

Oocyte reaction to penetrating sperm

Before fertilisation the egg is metabolically quiescent and its nucleus is arrested at metaphase of the second meiosis. After sperm-egg fusion, the arrested nucleus resumes meiosis, and then changes into the female pronucleus. Such a sequence of morphological and biochemical events is called “activation”. The inital responses of the egg to activation by the sperm include cortical granule exocytosis and resumption of meiosis.

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+

It is known that sea-urchin egg cortical-granule exocytosis is inhibited by agents such as N-ethylmaleimide (NEM) which modify thiol groups. The fusion-related proteins modified by these agents have yet to be identified, nor is there information regarding the topography of these thiol groups. Furthermore, the step in cortical-granule exocytosis at which these thiol groups participate is unknown. In this study we have investigated the topological properties of, and the temporal requirement for the function of, the fusion-related thiol groups by treating the isolated exocytotic apparatus with high-molecular-mass dextrans and BSA carrying thiol-reactive 3-(2-pyridyldithio)propionate groups. The dextran derivatives inhibited exocytosis. The BSA derivative was much less inhibitory. Inhibition was reversed by treatment with dithiothreitol. When NEM was added to the dextran-derivative-treated exocytotic apparatus, treatment with dithiothreitol completely reversed inhibition, indicating that the dextran derivatives inhibit by reacting at the NEM-sensitive sites. A pulse of Ca2+ applied in the presence of inhibitors did not trigger any fusion following the removal of the inhibitor by dithiothreitol. These data show that the thiol groups, the modification of which by NEM inhibits exocytosis, are exposed to the medium in terms of their accessibility to macromolecules. They also show that the fusion-related thiol groups are required during the Ca(2+)-dependent stage of exocytosis.

Okadaic acid treatment leads to a fragmentation of the trans-Golgi network and an increase in expression of TGN38 at the cell surface

Okadaic acid (OA) is a protein phosphatase inhibitor which has, among other properties, previously been shown to induce a fragmentation of the cisternae of the Golgi stack [for review, see Lucocq (1992) J. Cell Sci. 103, 875-880]. The effects of OA an reversible and mimic intracellular events which occur during mitosis. To date, due to a lack of endogenous marker proteins, the effects of OA on the trans-Golgi network (TGN) has not been studied. Certain drugs, e.g. Brefeldin A (BFA), have different effects on the morphology of the Golgi stack and the TGN; it is therefore relevant to ask what effect(s) OA has on the TGN. We now present data from a study in which we have used antibodies to TGN38, an integral membrane protein predominantly localized to the TGN of rat NRK cells [Luzio, Brake, Banting, Howell, Braghetta and Stanley (1990) Biochem. J. 270, 97-102], to investigate the effects of OA on this organelle. OA induces a reversible fragmentation of the TGN. This fragmentation occurs with similar kinetics to that observed within the Golgi stack, and is independent of protein synthesis. The sensitivity of the TGN to OA is similar to that of the Golgi stack. The fragmentation of the TGN induced by OA also leads to a 10-fold increase in the level of TGN38 expressed at the plasma membrane.

Hydrolase compartmentalization limits rate of digestion in Acanthamoeba

The kinetics of lysosomal enzyme acquisition by newly formed phagosomes was studied by following the rate of digestion of radiolabeled yeast fed to Acanthamoeba. The distribution of hydrolases among phagosomes was assessed by electron microscopic acid phosphatase cytochemistry and by measurement of three glycosidases in isolated early and late phagosomes. The results show that compartmentalization of hydrolases limit the digestion of large phagocytic loads. The hydrolases appear to be sequestered into the early phagosomes and not to be distributed either by small vesicle transport or phagosome-phagosome fusion to those formed later. We infer from these results that newly internalized surface membrane in phagosomes is not rapidly randomized with internal pools, but is recycled to the surface as a function of the digestive process.

Calcium and GTP-gamma-S as single effectors of secretion from permeabilized rat mast cells: requirements for ATP

Treatment with metabolic inhibitors and addition of exogenous MgATP exerted different effects on secretion from streptolysin-O-permeabilized mast cells, responding to calcium and GTP-gamma-S as single effectors (i.e., independently of each other). These effects were also strongly dependent on the experimental conditions. Thus cells, triggered by Ca2+ at the time of permeabilization, did not require MgATP, but after metabolic inhibition rapidly became absolutely dependent on its provision, requiring high (> mM) concentrations. AMP-PNP was not effective. After longer treatment with metabolic inhibitors, the absolute dependence on MgATP was also exhibited by cells responding to dual effectors (i.e., Ca2+ and GTP-gamma-S applied together). In contrast, calcium independent secretion due to GTP-gamma-S was more resistant to metabolic inhibition, exhibiting no absolute requirements for MgATP. Once the responsiveness to GTP-gamma-S had been lost, it could not be restored by addition of MgATP. MgATP, in fact, inhibited the response of permeabilized cells to GTP-gamma-S. This effect could be mimicked by AMP-PNP. When permeabilized cells were washed before triggering, MgATP (0.1-1 mM concentration range) was no longer inhibitory but stimulatory. These differences between Ca(2+)- and GTP-gamma-S-induced responses indicate that ATP utilization is essential to the calcium, but not to the guanine nucleotide, pathway to secretion. The rate of the response to calcium/MgATP was much slower in the absence than in the presence of GTP-gamma-S. The onset of secretion occurred after an initial delay. This lag phase was abolished by addition of GTP-gamma-S, suggesting that a GTP-binding protein may control a reaction which constitutes a rate-limiting step in the secretory process.

Effects of okadaic acid indicate a role for dephosphorylation in pancreatic stimulus-secretion coupling

Okadaic acid completely inhibits phosphatase 2A at nanomolar concentrations, while complete inhibition of type 1 phosphatases occurs at 1 microM. Phosphatase 2B is significantly inhibited only at concentrations > 1 microM. In rat pancreatic acini, 1 microM okadaic acid shifted the cholecystokinin (CCK) dose-response curve for stimulating amylase release to the right without reducing maximal secretion. At 3 microM, okadaic acid inhibited maximal CCK-induced amylase release to 78 +/- 7% of control, whereas the inactive analogue 1-Nor-okadaone had no effect. Three lines of evidence indicate that this inhibition by okadaic acid occurs at a late step in stimulus-secretion coupling: 1) intracellular Ca2+ signaling in response to agonist stimulation was not appreciably altered by okadaic acid; 2) stimulation with phorbol ester plus thapsigargin (thus by-passing receptor activation), which gave 85 +/- 4% of maximal CCK-induced amylase release, was inhibited 66 +/- 4% by 3 microM okadaic acid; and 3) Ca(2+)-induced amylase secretion in streptolysin O-permeabilized cells was also reduced by 85 +/- 7%. Two-dimensional polyacrylamide gel electrophoresis of 32P-labeled acini and autoradiography demonstrated that okadaic acid dose dependently increased overall protein phosphorylation. Correspondingly, okadaic acid also led to an inhibition of CCK-induced dephosphorylation. These results show that okadaic acid inhibits pancreatic acinar secretion at a step after generation of intracellular messengers and indicate a role for protein dephosphorylation in stimulus-secretion coupling.

Membrane fusion

Common themes are emerging from the study of viral, cell-cell, intracellular, and liposome fusion. Viral and cellular membrane fusion events are mediated by fusion proteins or fusion machines. Viral fusion proteins share important characteristics, notably a fusion peptide within a transmembrane-anchored polypeptide chain. At least one protein involved in a cell-cell fusion reaction resembles viral fusion proteins. Components of intracellular fusion machines are utilized in multiple membrane trafficking events and are conserved through evolution. Fusion pores develop during and intracellular fusion events suggesting similar mechanisms for many, if not all, fusion events.

A cortical phosphoprotein ('PP63') sensitive to exocytosis triggering in Paramecium cells. Immunolocalization and quenched-flow correlation of time course of dephosphorylation with membrane fusion

We had previously shown that a phosphoprotein of 63 kDa ('PP63') is rapidly and selectively dephosphorylated during synchronous (less than or equal to 1 s) trichocyst exocytosis in Paramecium cells and then rephosphorylated within less than or equal to 1 min [Zieseniss & Plattner (1985) J. Cell Biol. 101, 2028-2035]. Using a new quenched-flow device, we now find a strict correlation between PP63 dephosphorylation and the process of membrane fusion, both occurring within 80 ms. Uptake of 32P over 90 min, followed by exocytosis and rephosphorylation for 1 min, results in a rather selective phosphorylation of the dephosphorylated form, P63, to PP63. Solubilization by repeated freezing and thawing allows isolations of P63 and PP63. On isoelectric focusing autoradiograms they have pI values of 6.05, 5.95 (major spots), 5.85 and 5.75. All spots are sensitive to alkaline, but not to acidic, hydrolysis (except for the pI-6.05 spot). On two-dimensional-gel autoradiograms the most prominent spot, of pI 5.95, is most extensively de- and re-phosphorylated. This spot, from de- and re-phosphorylated samples, was used to produce monospecific antibodies. A cortical localization of PP63 was revealed by producing Western blots from isolated cell-surface fragments ('cortices') and by immunofluorescence labelling. We assume that both P63 and PP63 are attached to cortical structures, e.g. around trichocysts, though they are partly soluble. This localization and the strict correlation of PP63 dephosphorylation with exocytotic membrane fusion suggests a role in fusion regulation.

Repetitive calcium transients and the role of calcium in exocytosis and cell cycle activation in the mouse egg

The role of calcium in cortical granule exocytosis and activation of the cell cycle at fertilization was examined in the mouse egg using the calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) and the fluorescent calcium indicator fluo-3. BAPTA and fluo-3 were introduced into zona-free mouse eggs by a 30-min incubation with 0.01-50 microM BAPTA acetoxymethyl ester (AM) and/or 1-20 microM fluo-3 AM prior to in vitro fertilization. Incubation of eggs in greater than or equal to 5.0 microM BAPTA AM inhibited cortical granule exocytosis in all cases. Introduction of the calcium chelator into the egg blocked second polar body formation at greater than or equal to 1.0 microM BAPTA AM. Sperm entry occurred in all eggs regardless of the BAPTA AM concentration. Sperm induce a large transient increase in calcium lasting 2.3 +/- 0.6 min, followed by repetitive transients lasting 0.5 +/- 0.1 min and occurring at 3.4 +/- 1.4-min intervals. Incubation with greater than or equal to 5.0 microM BAPTA AM inhibited all calcium transients. Introduction of BAPTA also inhibited calcium transients, exocytosis, and the resumption of meiosis following application of the calcium ionophore A23187 or SrCl2, which activate eggs. These results demonstrate that the calcium increase at fertilization is required for cortical granule exocytosis and resumption of the cell cycle in a mammalian egg.