Exocytosis In Chromaffin Cells Research Articles

Tyrosine kinases are required for catecholamine secretion and mitogen-activated protein kinase activation in bovine adrenal chromaffin cells.

Nicotine-induced catecholamine secretion in bovine adrenomedullary chromaffin cells is accompanied by rapid tyrosine phosphorylation of multiple cellular proteins, most notably the mitogen-activated protein kinases (MAPKs). The requirement for activation of tyrosine kinases and MAPKs in chromaffin cell exocytosis was investigated using a panel of tyrosine kinase inhibitors. Genistein and tyrphostin 23, two compounds that inhibit tyrosine kinases by distinct mechanisms, were found to inhibit secretion by > 90% in cells stimulated by nicotine, 55 mM KCl, or the Ca2+ ionophore A23187. Inhibition of secretion induced by all three secretagogues correlated with a block in both protein tyrosine phosphorylation and activation of the MAPKs and their activators (MEKs) in situ. However, neither genistein nor tyrphostin 23 inhibited the activities of the MAPKs or MEKs in vitro. These results indicate that the target(s) of inhibition lie downstream of Ca2+ influx and upstream of MEK activation. This Ca(2+)-activated tyrosine kinase activity could not be accounted for entirely by c-Src or Fyn (two nonreceptor tyrosine kinases that are expressed abundantly in chromaffin cells), because their in vitro kinase activities were not inhibited by tyrphostin 23 and only partially inhibited by genistein. These results demonstrate that an unidentified Ca(2+)-activated tyrosine kinase(s) is required for MAPK activation and exocytosis in chromaffin cells and suggest that MAPK participates in the regulation of secretion.

Ba2+ ions evoke two kinetically distinct patterns of exocytosis in chromaffin cells, but not in neurohypophysial nerve terminals

The coupling between divalent cations and exocytosis of large dense-cored vesicles (LDCV) was studied with capacitance-detection techniques in nerve terminals of the rat neurohypophysis (NHP) and bovine chromaffin cells. Ba2+ substitution for Ca2+ produced kinetically distinct responses in the two preparations. In NHP terminals, Ba2+ ions behave as weak substitutes for Ca2+. Exocytotic events occur principally during depolarizing pulses, i.e., events are "stimulus-coupled" to Ba2+ entry through voltage-gated Ca2+ channels. Stimulus-coupled exocytosis apparently requires elevated submembrane cation concentrations that dissipate rapidly on hyperpolarization-induced Ca(2+)-channel closure. Intracellular dialysis of NHP terminals with Ba2+ does not evoke exocytosis, nor does it interfere with depolarization-evoked Ca2+ influx and exocytosis. In chromaffin cells, Ba2+ ions evoke a small quantity of stimulus-coupled secretion, but the dominant response is an additional pronounced poststimulus capacitance increase that outlasts channel closures by 20-50 sec. "Stimulus-decoupled" exocytosis is slow (approximately 25-40 fF/sec) compared with Ca(2+)-evoked stimulus-coupled exocytosis (approximately 1000 fF/sec). Decoupled secretion is not attributable to Ba2+ displacement of intracellular Ca2+ ions, because it is insensitive to 10 mM EGTA or thapsigargin. Slow exocytosis is initiated by inclusion of Ba2+ ions in the recording pipette and continues steadily for 5-12 min, producing a total increase of several thousand fF, which ultimately doubles or triples the original cell-surface area. We propose that two pathways of regulated exocytosis with distinct kinetics and divalent cation sensitivity exist in chromaffin cells. Only a single kinetic pattern is detected in NHP terminals, suggesting that mechanisms for secretion are not universally distributed in excitable cells.

S29-2 - Mechanism of Exocytosis in Chromaffin Cells: Roles of Myosin Light Chain Kinase and Cytoskeletal Proteins.

S29-2 - Mechanism of Exocytosis in Chromaffin Cells: Roles of Myosin Light Chain Kinase and Cytoskeletal Proteins.

A peptide that mimics the carboxy-terminal domain of SNAP-25 blocks Ca 2+-dependent exocytosis in chromaffin cells

SNAP-25, a synaptosomal associated membrane protein of 25 kDa, participates in the presynaptic process of vesicle-plasma membrane fusion that results in neurotransmitter release at central nervous system synapses. SNAP-25 occurs in neuroendocrine cells and, in analogy to its role in neurons, has been implicated in catecholamine secretion, yet the nature of the underlying mechanism remains obscure. Here we use an anti-SNAP-25 monoclonal antibody to show that SNAP-25 is localized at the cytosolic surface of the plasma membrane of chromaffin cells. This antibody inhibited the Ca 2+-evoked catecholamine release from digitonin-permeabilized chromaffin cells in a time- and dose-dependent manner. Remarkably, a 20-mer synthetic peptide representing the sequence of the C-terminal domain of SNAP-25 blocked Ca 2+-dependent catecholamine release with an IC 50 = 20 μM. The inhibitory activity of the peptide was sequence-specific as evidenced by the inertness of a control peptide with the same amino acid composition but random order. The C-terminal segment of SNAP-25, therefore, plays a key role in regulating Ca 2+-dependent exocytosis, presumably mediated via interactions with other protein components of the fusion complex.

A role for soluble NSF attachment proteins (SNAPs) in regulated exocytosis in adrenal chromaffin cells.

Digitonin-permeabilized chromaffin cells secrete catecholamines by exocytosis in response to micromolar Ca2+ concentrations, but lose the ability to secrete in response to Ca2+ as the cells lose soluble proteins through the plasma membrane pores. Such secretory run-down can be retarded by cytosolic fractions, thus providing an assay for proteins potentially involved in the exocytotic process. We have used this assay to investigate the role of N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) in regulated exocytosis. Recombinant alpha- and gamma-SNAP stimulated Ca(2+)-dependent exocytosis, although recombinant NSF was ineffective, despite the fact that NSF and alpha-SNAP leak from the permeabilized cells with similar time courses. However, around one third of cellular NSF was found to be present in a non-cytosolic form and so it is possible that this is sufficient for exocytosis and that exogenous SNAPs stimulate the exocytotic mechanism by acting on the leakage-insensitive NSF. The stimulatory effect of alpha-SNAP displayed a biphasic dose-response curve and was maximal at 20 micrograms/ml. The effect of alpha-SNAP was Ca(2+)- and MgATP-dependent and was inhibited by N-ethylmaleimide and botulinum A neurotoxin, indicating a bona fide action on the exocytotic mechanism. Furthermore, Ca2+ concentrations which trigger catecholamine secretion acted to prevent the leakage of NSF and alpha-SNAP from permeabilized cells. These findings provide functional evidence for a role of SNAPs in regulated exocytosis in chromaffin cells.

I 1-imidazoline receptor modulating substances influence ouabain-induced cardiac arrhythmia

The blockade of exocytosis induced by the putative endogenous ligand for imidazoline receptors, agmatine, was studied by using on-line measurement of catecholamine release in bovine adrenal medullary chromaffin cells. Agmatine inhibited the acetylcholine-evoked release of catecholamines in a concentration-dependent manner (IC50=366 μM); the K+-evoked release of catecholamines was unaffected. Clonidine (100 μM) and moxonidine (100 μM) also inhibited by 75% and 50%, respectively, the acetylcholine-evoked response. In cells voltage-clamped at −80 mV, the intermittent application of acetylcholine pulses elicited whole-cell inward currents (IACh) that were blocked 63% by 1 mM agmatine. The onset of blockade was very fast (τon=31 ms); the recovery of the current after washout of agmatine also occurred very rapidly (τoff=39 ms). Efaroxan (10 μM) did not affect the inhibition of IACh elicited by 1 mM agmatine. IACh was blocked 90% by 100 μM clonidine and 50% by 100 μM moxonidine. The concentration–response curve for acetylcholine to elicit inward currents was shifted to the right in a non-parallel manner by 300 μM agmatine. The blockade of IACh caused by agmatine (100 μM) was similar at various holding potentials, around 50%. When intracellularly applied, agmatine did not block IACh. At 1 mM, agmatine blocked INa by 23%, IBa by 14%, IK(Ca) by 16%, and IK(VD) by 18%. In conclusion, agmatine blocks exocytosis in chromaffin cells by blocking nicotinic acetylcholine receptor currents. In contrast to previous views, these effects seem to be unrelated to imidazoline receptors.

Essential role of myosin light chain kinase in the mechanism for MgATP- dependent priming of exocytosis in adrenal chromaffin cells

Ca(2+)-induced exocytosis in chromaffin cells now seems to consist of at least two distinct steps:MgATP-dependent Ca(2+)-dependent priming of the secretory apparatus, and Ca(2+)-dependent MgATP-independent step that triggers exocytosis (Bittner and Holz, 1992). Recently we found that a specific inhibitor of myosin light chain kinase (MLCK), wortmannin, inhibits Ca(2+)-induced catecholamine release from digitonin-permeabilized chromaffin cells, suggesting an implication of MLCK in the mechanisms of Ca(2+)-induced exocytosis (Imaizumi et al., 1992b). To elucidate further the implication of MLCK in the mechanism of exocytosis, we studied the effects of wortmannin and a peptide inhibitor (SM-1) corresponding to the pseudosubstrate domain of MLCK on MgATP-dependent and MgATP-independent release in digitonin-permeabilized chromaffin cells. Ca(2+)-induced exocytosis from the permeabilized cells in the presence of MgATP was inhibited by both SM-1 and wortmannin. Inhibitory effect of wortmannin on the rate of release induced by 10 microM Ca2+ in the presence of MgATP was much prominent in the later phase (1-10 min), although the initial rate was also decreased. SM-1 strongly inhibited ATP-dependent release without affecting Ca(2+)-dependent ATP-independent release at all. In addition, priming effect of MgATP that underlies Ca(2+)-dependent ATP-independent release was remarkably reduced by both wortmannin and SM-1. These results suggest that MLCK plays an essential role in ATP-dependent priming of Ca(2+)-induced exocytosis in chromaffin cells.

GAP-43 controls the availability of secretory chromaffin granules for regulated exocytosis by stimulating a granule-associated G0.

Besides having a role in signal transduction, heterotrimeric G proteins may also be involved in membrane trafficking events as suggested by their presence in specific intracellular compartments. In chromaffin cells, G alpha 0 is associated with secretory organelles, and its activation inhibits exocytosis. Although plasma membrane-bound G proteins are activated by cell-surface receptors, the intracellular proteins controlling organelle-associated G proteins are currently unknown. GAP-43, a neuronal protein enriched in axonal growth cones and presynaptic terminals, is one possible candidate since it can directly stimulate purified G0. We have investigated the interaction of adrenal medullary GAP-43 with chromaffin granule-associated G0 and its effect on catecholamine secretion. Cytosolic and depalmitoylated membrane-extracted GAP-43 were found to stimulate guanine nucleotide binding and exchange activity in chromaffin granule membranes. In permeabilized chromaffin cells, both forms of GAP-43 blocked calcium-dependent exocytosis, and this effect was inhibited by specific antibodies against G alpha 0. A synthetic peptide corresponding to the GAP-43 domain that interacts with G0 inhibited catecholamine secretion. This effect could be selectively reversed by the COOH-terminal peptide of G alpha 0. These results indicate that GAP-43 may be an endogenous pseudoreceptor for the secretory granule-bound form of G0 and can thereby control calcium-regulated exocytosis in chromaffin cells.

Synaptobrevin cleavage by the tetanus toxin light chain is linked to the inhibition of exocytosis in chromaffin cells

Exocytosis of secretory granules by adrenal chromaffin cells is blocked by the tetanus toxin light chain in a zinc specific manner. Here we show that cellular synaptobrevin is almost completely degraded by the tetanus toxin light chain within 15 min. We used highly purified adrenal secretory granules to show that synaptobrevin, which can be cleaved by the tetanus toxin light chain, is localized in the vesicular membrane. Proteolysis of synaptobrevin in cells and in secretory granules is reversibly inhibited by the zinc chelating agent dipicolinic acid. Moreover, cleavage of synaptobrevin present in secretory granules by the tetanus toxin light chain is blocked by the zinc peptidase inhibitor captopril and by synaptobrevin derived peptides. Our data indicate that the tetanus toxin light chain acts as a zinc dependent protease that cleaves synaptobrevin of secretory granules, an essential component of the exocytosis machinery in adrenal chromaffin cells.

Role of Phosphatidylserine and Diacylglycerol in the Fusion of Chromaffin Granules with Target Membranes

The role of phosphatidylserine and diacylglycerol in the fusion of chromaffin granules with target membranes was investigated in vitro, monitoring the mixing of membrane lipids with the octadecyl rhodamine B fluorescence dequenching assay. Polylysine was able to induce fusion of chromaffin granule ghosts with plasma membrane vesicles in the absence of Ca 2+. This fusion was maximal at 1.6 μM polylysine and the kinetics were dependent on the amount of plasma membrane added. Polylysine lowered the Ca 2+ threshold concentration for inducing fusion, increased the extent of fusion at a given Ca 2+ concentration and acted synergistically with Ca 2+ when added prior to the cation. Removal of membrane proteins by trypsinization of chromaffin granule ghost and/or plasma membranes increased the extent of polylysine induced fusion. Incorporation of diacylglycerol into the plasma membrane promoted Ca 2+-induced fusion. Chromaffin granule ghosts were induced to fuse with model membranes of different complexities from one resembling the inner monolayer of the erythrocyte membrane to one composed solely of pure phosphatidylserine. The characteristics of the fusion with model membranes were qualitatively similar to that observed with target plasma membrane, although differences in kinetics and stoichiometries were found. Interestingly, considerably low (μM) Ca 2+ concentrations were able to induce fusion of chromaffin granule ghosts with diacylglycerol containing phosphatidylserine vesicles in the presence of polylysine. Our results indicate that acidic phospholipid-like phosphatidylserine may have an important role in the process of fusion and suggest that diacylglycerol, as a destabilizing lipid, may have a role by itself in promoting exocytosis in chromaffin cells.

Exocytosis in chromaffin cells: evidence for a MgATP-independent step that requires a pertussis toxin-sensitive GTP-binding protein.

We have previously described that mastoparan, an amphiphilic tetradecapeptide that activates heterotrimeric G-proteins, inhibits Ca(2+)-induced MgATP-dependent secretion from streptolysin-O-permeabilized chromaffin cells [Vitale, Mukai, Rouot, Thiersé, Aunis and Bader (1993) J. Biol. Chem. 268, 14715-14723]. Our observations suggest the involvement of an inhibitory G(o)-protein, possibly located on the membrane of secretory granules, in the final stages of the exocytotic pathway in chromaffin cells. Here, we demonstrate that mastoparan is also able to stimulate the Ca(2+)-dependent secretion of catecholamines in the absence of MgATP in the medium. This MgATP-independent secretion is totally blocked by tetanus toxin, a potent inhibitor of exocytosis in all neurosecretory cells so far investigated, suggesting that the mastoparan target is a component of the exocytotic machinery. Mas17, a mastoparan analogue inactive on G-proteins, had no effect on catecholamine secretion whereas both Mas7, a highly active analogue of mastoparan, and AlF4-, which selectively activates trimeric G-proteins, triggered MgATP-independent secretion. Non-hydrolysable GTP analogues (GTP[S] and p[NH]ppG) mimicked the dual effects of mastoparan on secretion: they inhibited exocytosis in the presence of MgATP and stimulated MgATP-independent secretion. The different potencies displayed by these two analogues suggest the involvement of two distinct G-proteins. Accordingly, the mastoparan-induced MgATP-independent secretion is highly sensitive to pertussis toxin (PTX) whereas the inhibition by mastoparan of secretion in the presence of MgATP is resistant to PTX treatment. When permeabilized cells were incubated with mastoparan, the release of arachidonic acid increased in a PTX-sensitive manner. 7,7-Dimethyl-5,8-eicosadienoic acid, a potent inhibitor of intracellular phospholipase A2, inhibited both the arachidonate release and the MgATP-independent catecholamine secretion evoked by mastoparan. In contrast, neomycin, an inhibitor of phospholipase C, had no significant effect on either the release of arachidonic acid or the secretion of catecholamines provoked by mastoparan. We conclude that two distinct heterotrimeric G-proteins act in series in the exocytotic pathway in chromaffin cells: one controls an ATP-dependent priming step through an effector pathway that remains to be determined, and the second is involved in a late Ca(2+)-dependent step which does not require MgATP but possibly involves the generation of arachidonic acid.

Antibodies against zinc-binding epitopes restore exocytosis in chromaffin cells treated with tetanus and botulinum A neurotoxins: H. Bergel, F. Bartels and H. Bigalke (Medical School of Hannover, Institute of Toxicology, 30625 Hannover, Germany)

Antibodies against zinc-binding epitopes restore exocytosis in chromaffin cells treated with tetanus and botulinum A neurotoxins: H. Bergel, F. Bartels and H. Bigalke (Medical School of Hannover, Institute of Toxicology, 30625 Hannover, Germany)

Specific antibodies against the Zn(2+)-binding domain of clostridial neurotoxins restore exocytosis in chromaffin cells treated with tetanus or botulinum A neurotoxin.

Although tetanus and botulinum A neurotoxins are ineffective in cultured chromaffin cells, they will inhibit carbachol-induced release of noradrenaline provided they gain access to the cytosol either through artificial pores generated in the plasma membrane or by binding to incorporated exogenous gangliosides. The block of exocytosis persists for weeks followed by a slow recovery of cell function. When specific anti-botulinum A toxin antibodies are introduced into cells through pores after manifestation of the block by botulinum A neurotoxin, restoration of exocytotic function is accelerated and fully reestablished within 4 days. The same time course of restoration is seen with anti-tetanus toxin antibodies in cells poisoned by tetanus toxin. Since the light chains of the toxins are enzymatically active, we have introduced polyclonal and monoclonal anti-light chain antibodies into the cytosol. Of all light chain antibodies tested, only those directed against the peptide homologous to the zinc-binding sequence, which is present in both neurotoxins, restored exocytosis regardless of which toxin caused the block. These results indicate that the zinc-binding domain is directly involved in the interaction of the light chains with their substrates and that the toxins have to be present continuously to maintain the block.

Identification of Exo2 as the catalytic subunit of protein kinase A reveals a role for cyclic AMP in Ca(2+)-dependent exocytosis in chromaffin cells.

Digitonin-permeabilized chromaffin cells secrete catecholamines by exocytosis in response to micromolar Ca2+ concentrations, but lose the ability to secrete in response to Ca2+ as the cells lose soluble proteins through the plasma membrane pores. We have previously shown [Morgan and Burgoyne (1992) Nature, 355, 833-836] that cytosol can retard this loss of secretory competence and that two distinct stimulatory activities (Exo1 and Exo2) are present in cytosol. Here we report that Exo2 behaved as a single peak of activity through purification on hydroxyapatite, ammonium sulfate precipitation and gel filtration and the activity correlated with a single polypeptide of approximately 44 kDa on SDS gels. Protein sequencing of this band revealed it to be the catalytic subunit of cyclic AMP-dependent protein kinase (PKA). Both cyclic AMP and the commercially available catalytic subunit of PKA stimulated exocytosis in a dose-dependent manner which was absolutely dependent on the presence of micromolar Ca2+. These data show that PKA (Exo2) regulates Ca(2+)-dependent exocytosis in bovine adrenal chromaffin cells.

Exocytosis in chromaffin cells. Possible involvement of the heterotrimeric GTP-binding protein G(o)

The use of non-hydrolyzable analogues of GTP in permeabilized secretory cells suggests that guanine nucleotide-binding regulatory proteins (G proteins) may be involved in regulated exocytosis. Because GTP analogues are known to modulate both monomeric low molecular mass G proteins and heterotrimeric G proteins, we have examined the effect of mastoparan, an activator of heterotrimeric G proteins, on secretion from intact and permeabilized chromaffin cells. In intact cells, mastoparan inhibited catecholamine secretion evoked by nicotine but had no effect on release induced by other secretagogues. In permeabilized cells, mastoparan inhibited calcium-dependent secretion providing that the pores created in the plasma membrane allow the penetration of the peptide into the cytoplasm. These results indicate that mastoparan blocks the exocytotic machinery through an intracellular target protein that may not be located just beneath the plasma membrane. Accordingly, mastoparan was able to stimulate G proteins associated with purified chromaffin granule membranes, in a range of concentration and Mg2+ requirement that was similar to its inhibitory effect on secretion. Mas 17, a mastoparan analogue inactive on purified G proteins, neither modified catecholamine secretion nor stimulated chromaffin granule G proteins. The substance P-related peptide, GPAnt-2, known to antagonize the effects of mastoparan on G(o), blocked both the inhibitory effect of mastoparan on secretion and the mastoparan-stimulated GTPase activity in chromaffin granule membranes. Moreover, specific antibodies raised against the carboxyl terminus of G(o) alpha reversed in a dose-dependent manner the inhibition by mastoparan on catecholamine release and the stimulation by mastoparan of chromaffin granule-associated G proteins. These results suggest that the secretory machinery in chromaffin cells can be blocked by activating a G(o) protein. Consistent with this finding, two other known activators of heterotrimeric G proteins, aluminum fluoride and benzalkonium chloride, inhibited calcium-evoked catecholamine secretion in streptolysin O-permeabilized chromaffin cells. We conclude that an inhibitory G(o) protein, possibly located on the membrane of secretory granules, is involved in the final stages of exocytosis in chromaffin cells.

Intracellular neutralization of botulinum A neurotoxin restores exocytosis in chromaffin cells: F. Bartels, 1H. Bigalke1 and J. L. Middlebrook2 ( 1Medical School of Hannover, Institute of Toxicology, 3000 Hannover 61, F.R.G.; and 2Department of Toxinology, Pathology Division, U.S. Army Medical Research Institute of Infectious Diseases, Frederick, M.D. 21701-5011, U.S.A.)

Intracellular neutralization of botulinum A neurotoxin restores exocytosis in chromaffin cells: F. Bartels, 1H. Bigalke1 and J. L. Middlebrook2 ( 1Medical School of Hannover, Institute of Toxicology, 3000 Hannover 61, F.R.G.; and 2Department of Toxinology, Pathology Division, U.S. Army Medical Research Institute of Infectious Diseases, Frederick, M.D. 21701-5011, U.S.A.)

Two GTP-binding Proteins Control Calcium-dependent Exocytosis in Chromaffin Cells.

The effect of guanosine triphosphate analogues on catecholamine secretion from permeabilized bovine chromaffin cells was examined. Guanosine 5'-[gamma-thio]triphosphate was demonstrated to produce a dual effect on calcium-evoked secretion, enhancing the release through a mechanism involving protein kinase C and inhibiting secretion by a protein kinase C-independent pathway. We propose that two functionally distinct G-proteins control the stimulus - secretion coupling in chromaffin cells.

Characterization of cytoplasmic proteins essential for exocytosis in chromaffin cells

Characterization of cytoplasmic proteins essential for exocytosis in chromaffin cells

A pertussis-toxin-sensitive protein controls exocytosis in chromaffin cells at a step distal to the generation of second messengers

The role of GTP-binding proteins (G-proteins) in the secretory process in chromaffin cells was investigated by studying the effects of pertussis toxin (PTX) on catecholamine release and generation of various second messengers. PTX was found to stimulate the catecholamine secretion induced by nicotine, 59 mM-K+ or veratridine. PTX also potentiated Ca2(+)-evoked catecholamine release from permeabilized chromaffin cells, suggesting that PTX substrate(s) regulate the exocytotic machinery at a step distal to the rise in intracellular Ca2+. We have investigated the possible intracellular pathways involved in the stimulation of secretion by PTX. PTX did not modify the translocation of protein kinase C (PKC) to membranes in intact or permeabilized cells; in addition, neither inhibitors nor activators of PKC had any effect on catecholamine release induced by PTX. Thus it seems unlikely that the effect of PTX on secretion is mediated by activation of PKC. The effect of PTX is also cyclic AMP-independent, as PTX did not change cytoplasmic cyclic AMP levels. The relationship between PTX treatment and arachidonic acid release was also examined. We found that an increase in cytoplasmic arachidonic acid concentration enhanced Ca2(+)-evoked catecholamine release in permeabilized cells, but arachidonic acid did not mimic the effect of PTX on the Ca2(+)-dose-response curve for secretion. Furthermore, PTX did not significantly modify the release of arachidonic acid measured in resting or stimulated chromaffin cells, suggesting that the stimulatory effect of PTX on secretion is not mediated by an activation of phospholipase A2. Taken together, these results suggest that PTX may modulate the intracellular machinery of secretion at a step distal to the generation of second messengers. In alpha-toxin-permeabilized cells, full retention of the PTX-induced activation of secretion was observed even 30 min after permeabilization. In contrast, when chromaffin cells were permeabilized with streptolysin-O (SLO), there was a marked progressive loss of the PTX effect. We found that SLO caused the rapid leakage of three G-protein alpha-subunits which are specifically ADP-ribosylated by PTX. We propose that a PTX-sensitive G-protein may play an inhibitory role in the final stages of the Ca2(+)-evoked secretory process in chromaffin cells.

Evolutionary conservation and structural determinants of the calelectrins (annexins)

Evolutionary conservation and structural determinants of the calelectrins (annexins)