Preferential release of catecholamine from permeabilized PC12 cells by alpha- and beta-type protein kinase C subspecies

Impaired motor learning in the vestibulo-ocular reflex in mice with multiple climbing fiber input to cerebellar Purkinje cells

A unique feature of the cerebellar architecture is that Purkinje cells in the cerebellar cortex each receive input from a single climbing fiber. In mice deficient in the gamma isoform of protein kinase C (PKCgamma-/- mice), this normal architecture is disrupted so that individual Purkinje cells receive input from multiple climbing fibers. These mice have no other known abnormalities in the cerebellar circuit. Here, we show that PKCgamma-/- mice are profoundly impaired in vestibulo-ocular reflex (VOR) motor learning. The PKCgamma-/- mice exhibited no adaptive increases or decreases in VOR gain at training frequencies of 2 or 0.5 Hz. This impairment was present across a broad range of peak retinal slip speeds during training. We compare the results for VOR motor learning with previous studies of the performance of PKCgamma-/- mice on other cerebellum-dependent learning tasks. Together, the results suggest that single climbing fiber innervation of Purkinje cells is critical for some, but not all, forms of cerebellum-dependent learning, and this may depend on the region of the cerebellum involved, the organization of the relevant neural circuits downstream of the cerebellar cortex, as well as the timing requirements of the learning task.

Activation of protein kinase C enhances 1-methyl-4-phenylpyridinium ion (MPP+)-induced hydroxylradical generation in rat striatum

The present study examined the effect of chelerlythrine, a protein kinase C (PKC) inhibitor, on 1-methyl-4-phenylpyridine (MPP+)-induced hydroxyl radicals (*OH) in rat striatum. Rats were anesthetized, and sodium salicylate (0.5 mM or 0.5 nmol/microl/min) was infused through a microdialysis probe to detect the *OH generation as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (2,3-DHBA) in the striatum. Dopamine (DA)-selective neurotoxin, MPP+, infusion into the striatum of rats induces *OH formation, trapped as 2,3-DHBA. The application of chelerythrine, a potent and selective protein kinase C (PKC) inhibitor, suppressed MPP+ -induced *OH formation. The results in the present study suggests the protective effect of chelerythrine on *OH generation induced by MPP+.

Repeated amphetamine treatment induces neurite outgrowth and enhanced amphetamine-stimulated dopamine release in rat pheochromocytoma cells (PC12 cells) via a protein kinase C- and mitogen activated protein kinase-dependent mechanism

Repeated intermittent treatment with amphetamine (AMPH) induces both neurite outgrowth and enhanced AMPH-stimulated dopamine (DA) release in PC12 cells. We investigated the role of protein kinases in the induction of these AMPH-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (100 nm and 300 nm), Ro31-8220 (300 nm) and the MAP kinase kinase inhibitor, PD98059 (30 micro m) inhibited the ability of AMPH to elicit both neurite outgrowth and the enhanced AMPH-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 250 nm) mimicked the ability of AMPH to elicit neurite outgrowth and enhanced DA release. On the contrary, a selective PKA inhibitor, 100 micro m Rp-8-Br-cAMPS, blocked only the development of AMPH-stimulated DA release but not the neurite outgrowth. Treatment of the cells with acute AMPH elicited an increase in the activity of PKC and MAP kinase but not PKA. These results demonstrated that AMPH-induced increases in MAP kinase and PKC are important for induction of both the enhancement in transporter-mediated DA release and neurite outgrowth but PKA was only required for the enhancement in AMPH-stimulated DA release. Therefore the mechanisms by which AMPH induces neurite outgrowth and the enhancement in AMPH-stimulated DA release can be differentiated.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Effects of ethanol on protein kinase C activity induced by filamentous actin

Protein kinase C (PKC) can be activated by interaction with filamentous actin (F-actin) in the absence of membrane lipids (S.J. Slater, S.K. Milano, B.A. Stagliano, K.J. Gergich, J.P. Curry, F.J. Taddeo and C.D. Stubbs, Biochemistry 39 (2000) 271-280). Here, the effects of ethanol on the F-actin-induced activities of a panel of PKC isoforms consisting of 'conventional' (cPKC) alpha, betaI, gamma, 'novel' (nPKC) delta, epsilon and 'atypical' (aPKC) zeta were investigated using purified PKC and F-actin. Ethanol was found to inhibit the Ca2+- and phorbol ester-dependent activities of cPKCalpha and betaI, and the Ca2+- and phorbol ester-independent activity of cPKCgamma, whereas the activities of nPKCdelta, epsilon and aPKCzeta were unaffected. Although the activities of cPKCalpha and betaI induced by saturating levels of phorbol ester were inhibited by ethanol, the binding of these isozymes to F-actin was unaffected within the same phorbol ester concentration range. Conversely, within submaximal levels of phorbol ester, cPKCalpha and betaI activities were unaffected by ethanol whereas binding to F-actin was inhibited. The potency of the inhibition of F-actin-induced cPKCbetaI activity increased with n-alkanol chain length up to n-hexanol, after which it declined. The results indicate that PKC activities associated with F-actin, and therefore cellular processes involving the actin cytoskeleton, are potential targets for ethanol action. The effects of ethanol on these processes may differ according to the particular regulating PKC isoform, its intracellular localization and the presence of activators and cofactors.

Ziskind-Somerfeld Research Award. Protein kinase C signaling in the brain: molecular transduction of mood stabilization in the treatment of manic-depressive illness

Understanding the biology of the pharmacological stabilization of mood will undoubtedly serve to provide significant insight into the pathophysiology of manic-depressive illness (MDI). Accumulating evidence from our laboratories and those of other researchers has identified the family of protein kinase C isozymes as a shared target in the brain for the long-term action of both lithium and valproate. In rats chronically treated with lithium, there is a reduction in the hippocampus of the expression of two protein kinase isozymes, alpha and epsilon, as well as a reduction in the expression of a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. In addition, we have been investigating the down-stream impact of these mood stabilizers on another kinase system, GSK-3 beta and on the AP-1 family of transcription factors. Further studies have generated promising preliminary data in support of the antimanic action of tamoxifen, and antiestrogen that is also a PKC inhibitor. Future studies must address the therapeutic relevance of these protein targets in the brain using innovative strategies in both animal and clinical investigations to ultimately create opportunities for the discovery of the next generations of mood stabilizers for the treatment of MDI.

Calcium sensors in regulated exocytosis

Neurotransmitter release, hormone secretion and a variety of other secretory process are tightly regulated with exocytotic fusion of secretory vesicles being triggered by a rise in cytosolic Ca2+ concentration. A series of proteins that act as part of a conserved core machinery for vesicle docking and fusion throughout the cell have been identified. In regulated exocytosis this core machinery must be controlled by Ca(2+)-sensor proteins that allow rapid activation of the fusion process following elevation of cytosolic Ca2+ concentration. The properties of such Ca2+ sensors are known from physiological studies but their molecular identity remains to be unequivocally established. The multiple Ca(2+)-dependent steps in the exocytotic pathway suggest the likely involvement of several Ca(2+)-binding proteins with distinct properties. Functional evidence for the role of various Ca(2+)-binding proteins and their possible sites of action is accumulating but a definitive identification of the major Ca(2+)-sensor in the final step of Ca(2+)-triggered membrane fusion in different cell types awaits further analysis.

Inhibition of expression of PKC-alpha by antisense mRNA is associated with diminished cell growth and inhibition of amylase secretion by AR4-2J cells

AR4-2J pancreatoma cells were stably transfected with an expression vector containing the cDNA for PKC-alpha in the antisense orientation. Transfectants designated antisense-alpha AA1, AA2, and AA3 exhibited marked reductions in PKC-alpha expression and decrements in cell growth. The magnitude of the decrement in cell growth paralleled the reduction in PKC-alpha expression, i.e., AA3 > AA1 > AA2. The ability of dexamethasone to induce cell differentiation as assessed by a rise in cellular amylase levels was not markedly affected by the reduction in PKC-alpha expression. Unstimulated amylase release was attenuated in AA1 cells and almost completely blocked in AA2 transfectants. The AA2 transfectant cell line failed to elicit a secretory response to caerulein, and the AA1 transfectant exhibited a lack of the secondary phase of stimulated amylase secretion. These findings demonstrate that PKC-alpha is involved in the mechanisms regulating growth and secretion in AR4-2J cells, but is not necessary for the induction of amylase stores following differentiation.

Interaction between G protein-operated receptors eliciting secretion in rat adrenals. A possible role of protein kinase C

Catecholamine release induced by angiotensin II, histamine, bradykinin and methacholine from the rat adrenal gland perfused in vitro was studied under conditions in which the activity of protein kinase C (PKC) was modified. Perfusion of glands with 10 nM bradykinin abolished, in a reversible way, the secretion induced by short pulses of angiotensin II, histamine and methacholine but did not modify the release evoked by 23.6 mM KCl (high K+). Perfusion with histamine or methacholine (30 microM) inhibited the secretion induced by the other agents by 30-50%, whereas incubation with angiotensin II (100 nM) caused little or no reduction in the release evoked by the other agents. The treatment of glands with 1 nM of the PKC activator phorbol 12,13-dibutyrate (PDBu) suppressed the responses induced by angiotensin II, histamine and methacholine, did not affect those evoked by bradykinin, and potentiated the secretion evoked by high K+. The adenylate cyclase stimulator forskolin (1 microM) did not affect the basal secretion but strongly potentiated the release evoked by all secretagogues used, suggesting a role for protein kinase A (PKA) downstream of the receptor. The PKC inhibitor Ro-31-8220 partially reversed the inhibitory effect of bradykinin. Our results suggest that angiotensin II, histamine and muscarinic receptors share some common transduction mechanism that is regulated by PKC. PKC activity was enhanced by these agents PDBu >> bradykinin = histamine > methacholine = angiotensin II. Bradykinin receptor transduction does not appear to be regulated by PKC.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Signal transduction of the gonadotropin releasing hormone (GnRH) receptor: cross-talk of calcium, protein kinase C (PKC), and arachidonic acid

1. The decapeptide neurohormone gonadotropin releasing hormone (GnRH) is the first key hormone of the reproductive system. Produced in the hypothalamus, GnRH is released in a pulsatile manner into the hypophysial portal system to reach the anterior pituitary and stimulates the release and synthesis of the gonadotropin hormones LH and FSH. GnRH, a Ca2+ mobilizing ligand, binds to its respective binding protein, which is a member of the seven transmembrane domain receptor family and activates a G-protein (Gq). 2. The alpha subunit of Gq triggers enhanced phosphoinositide turnover and the elevation of multiple second messengers required for gonadotropin release and biosynthesis. 3. The messenger molecules IP3, diacylglycerol, Ca2+, protein kinase C, arachidonic acid and leukotriene C4 cross-talk in a complex networks of signaling, culminating in gonadotropin release and gene expression.

Protein kinase C isoenzymes in mouse harderian gland. Differential expression of the alpha- and epsilon-isoforms during pregnancy. Protein kinase C-OC

Protein kinase C (PKC) is known to be involved in the regulation of exocytosis in different cell lines and tissues. Experiments were designed to determine whether the Harderian gland of CD-1 mouse produces PKC isoenzymes and whether the expression of the isoforms changes during pregnancy. The presence of the isoenzymes was assessed by immunoblotting experiments using extract of total Harderian gland and polyclonal antisera specific for nine different PKC isoforms. Antisera giving a positive staining on Western blots were subsequently used for immunohistochemical investigation using a secondary antibody conjugated to alkaline phosphatase. Immunoblotting experiments revealed that the Harderian gland from female mouse expresses PKC isoforms-alpha, -epsilon, -zeta and -eta. These isoforms were also detected in the Harderian gland from 13-day pregnant mouse; however, striking quantitative changes were seen concerning the alpha- and epsilon-isoforms. The 80-kDa native from of PKC-alpha almost doubled in the pregnant mouse in comparison with normal female mouse whereas the amount of 50-kDa catalytic domain did not change. Protein kinase C-epsilon appeared as a 92- to 93-kDa form and a 67-kDa form. While the 92- to 93-kDa protein was expressed to a similar extent in both types of mouse, the 67-kDa form was more abundant in the Harderian gland from normal female mouse. These data were corroborated by immunohistochemical experiments and showing a diffuse and granular staining of the adenomeres. These observations demonstrate for the first time (to our knowledge) that the mouse Harderian gland produces several PKC isoenzymes that could be involved in the regulation of exocytosis and/or other functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of protein kinase C and cytosolic Ca2+ on exocytosis in the isolated perfused rat liver

Both protein kinase C and cytosolic Ca2+ are involved in the regulation of exocytosis in a number of cell types. However, the relative importance of each of these for apical exocytosis in the hepatocyte is unknown. To investigate this, we studied the effects of protein kinase C and Ca2+ agonists on horseradish peroxidase excretion in the isolated perfused rat liver. Vasopressin increased both horseradish peroxidase concentration and net horseradish peroxidase excretion in bile, and these effects were abolished by the protein kinase C inhibitor H-7. The protein kinase C activator phorbol dibutyrate also increased both net excretion and the concentration of biliary horseradish peroxidase. In contrast, the Ca2+ ionophore A23187 and the Ca2+ mobilizing agent 2,5'-di(tertbutyl)-1,4-benzohydroquinone both had minimal effects on horseradish peroxidase concentration and inhibited the rate of horseradish peroxidase excretion. These results suggest that protein kinase C stimulates apical exocytosis in the hepatocyte, whereas increased Cai2+ per se does not influence exocytosis and inhibits excretion only transiently by reducing bile flow.

Biosynthesis of proteins of large dense-core vesicles in rat PC12 cells: regulation by forskolin and phorbol ester

We have investigated the influence of various second messengers on the biosynthesis of large dense-core vesicle constituents in rat PC12 cells. After treatment with forskolin, phorbol ester or a combination of both substances for up to six days, the messenger RNA levels of several vesicle components were determined by northern blotting. Forskolin increased the expression of messenger RNA encoding the soluble proteins chromogranin B, neuropeptide Y and VGF. Addition of phorbol ester markedly enhanced the effects of forskolin. On the other hand, the expression of two further soluble proteins, chromogranin A and secretogranin II, remained fairly unchanged with all treatments tested. Amongst partly membrane-bound vesicle components, the biosynthesis of glycoprotein III and peptidylglycine alpha-amidating mono-oxygenase was significantly up-regulated by combined treatment with forskolin plus phorbol ester. The carboxypeptidase H messenger RNA increased due to phorbol ester and after long-term application of both drugs. In contrast, phorbol ester alone or plus forskolin down-regulated the expression of dopamine beta-hydroxylase. Essentially the same applies to the intrinsic membrane protein cytochrome b-561, whose messenger RNA level declined in all treatment groups. In conclusion, our results show that forskolin and phorbol ester can regulate the composition of large dense-core vesicles in quite distinct patterns.

Analysis of protein kinase C requirement for exocytosis in permeabilized rat basophilic leukaemia RBL-2H3 cells: a GTP-binding protein(s) as a potential target for protein kinase C

The role of protein kinase C in calcium-dependent exocytosis was investigated in permeabilized rat basophilic leukaemia cells. When protein kinase C was down-regulated by phorbol myristate acetate (1 microM for 3-6 h) or inhibited by pharmacological agents such as calphostin C (1 microM) or a protein kinase C-specific pseudo-substrate peptide inhibitor (100-200 microM), cells lost the ability to secrete in response to 10 microM free Ca2+. In contrast, a short treatment (15 min) with phorbol myristate acetate, which maximally activates protein kinase C, potentiated the effects of calcium. Biochemical analysis of protein kinase C-deprived cells indicated that loss of the Ca(2+)-induced secretory response correlated with disappearance of protein kinase C-alpha. In addition, at the concentrations effective for exocytosis, calcium caused translocation of protein kinase C-alpha to the membrane fraction and stimulated phospholipase C, suggesting that, in permeabilized cells, protein kinase C can be activated by calcium through generation of the phospholipase C metabolite diacylglycerol. The delta, epsilon and zeta Ca(2+)-independent protein kinase C isoenzymes were insensitive to phorbol myristate acetate-induced down-regulation and did not, as expected, translocate to the particulate fraction in response to calcium. Interestingly, secretory competence was restored in cells depleted of protein kinase C or in which protein kinase C itself was inhibited by non-hydrolysable GTP analogues, but not by GTP, suggesting that protein kinase C might regulate the ability of a G protein(s) directly controlling the exocytotic machinery to be activated by endogenous GTP.

A role for protein kinase C subtypes alpha and epsilon in phorbol-ester-enhanced K(+)- and carbachol-evoked noradrenaline release from the human neuroblastoma SH-SY5Y

Protein kinase C (PKC) consists of a family of closely related subtypes which differ in their localization and activation properties. Our previous studies have suggested a role for PKC in the regulation of noradrenaline (NA) release from the human neuroblastoma SH-SY5Y. Here we have used two approaches to characterize the PKC subtypes present in SH-SY5Y cells. Firstly, the PCR was used to show that SH-SY5Y cells contain mRNA encoding PKC subtypes alpha, beta, gamma, delta, epsilon and zeta. Secondly, immunoblotting showed that SH-SY5Y cells express PKC subtypes alpha, epsilon and zeta at the protein level. Prolonged (48 h) exposure of cells to the phorbol ester phorbol 12-myristate 13-acetate (PMA; 100 nM) resulted in a marked decrease in the amounts of PKC-alpha and PKC-epsilon, with no change in levels of PKC-zeta. Prolonged PMA treatment had no significant effect on K(+)-evoked NA release from SH-SY5Y cells, whereas carbachol-evoked release was increased 2.2-fold. However, prolonged exposure to PMA completely inhibited the ability of acute (12 min) PMA treatment to enhance both K(+)- and carbachol-evoked NA release. The specific PKC inhibitor RO 31-7459 (10 microM) was found to inhibit K(+)- and carbachol-evoked release by 27% and 68% respectively. RO 31-7549 also completely inhibited the ability of acute PMA treatment to enhance release. These data suggest that PKC-alpha and/or PKC-epsilon play an essential role in the regulation of PMA-enhanced K(+)- and carbachol-evoked NA release in SH-SY5Y cells.

Long-term action of lithium: a role for transcriptional and posttranscriptional factors regulated by protein kinase C

Lithium, a simple monovalent cation, represents one of psychiatry's most important treatments and is the most effective treatment for reducing both the frequency and severity of recurrent affective episodes. Despite extensive research, the underlying biologic basis for the therapeutic efficacy this drug remains unknown, and in recent years, research has focused on signal transduction pathways to explain lithium's efficacy in treating both poles of manic-depressive illness. Critical to attributions of therapeutic relevance to any observed biochemical effect, however, is the observation that the characteristic prophylactic action of lithium in stabilizing the profound mood cycling of bipolar disorder requires a lag period for onset and is not immediately reversed upon discontinuation of treatment. Biochemical changes requiring such prolonged administration of a drug suggest alterations at the genomic level but, until recently, little has been known about the transcriptional and posttranscriptional factors regulated by chronic drug treatment, although long-term changes in neuronal synaptic function are known to be dependent upon the selective regulation of gene expression. In this paper, we will present evidence to show that chronic lithium exerts significant transcriptional and posttranscriptional effects, and that these actions of lithium may be mediated via protein kinase C (PKC)-induced alterations in nuclear transcription regulatory factors responsible for modulating the expression of proteins involved in long-term neural plasticity and cellular response. Such target sites for chronic lithium may help unravel the processes by which a simple monovalent cation can produce a long-term stabilization of mood in individuals vulnerable to bipolar illness.

Lithium decreases membrane-associated protein kinase C in hippocampus: selectivity for the alpha isozyme

We investigated the effects of lithium on alterations in the amount and distribution of protein kinase C (PKC) in discrete areas of rat brain by using [3H]phorbol 12,13-dibutyrate quantitative autoradiography as well as western blotting. Chronic administration of lithium resulted in a significant decrease in membrane-associated PKC in several hippocampal structures, most notably the subiculum and the CA1 region. In contrast, only modest changes in [3H]phorbol 12,13-dibutyrate binding were observed in the various other cortical and subcortical structures examined. Immunoblotting using monoclonal anti-PKC antibodies revealed an isozyme-specific 30% decrease in hippocampal membrane-associated PKC alpha, in the absence of any changes in the labeling of either the beta (I/II) or gamma isozymes. These changes were observed only after chronic (4 week) treatment with lithium, and not after acute (5 days) treatment, suggesting potential clinical relevance. Given the critical role of PKC in regulating neuronal signal transduction, lithium's effects on PKC in the limbic system represent an attractive molecular mechanism for its efficacy in treating both poles of manic-depressive illness. In addition, the decreased hippocampal membrane-associated PKC observed in the present study offers a possible explanation for lithium-induced memory impairment.

Regulated exocytosis

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Resolution of regulated secretion into sequential MgATP-dependent and calcium-dependent stages mediated by distinct cytosolic proteins

The biochemical events and components responsible for ATP-dependent Ca(2+)-activated secretion remain to be identified. To simplify the molecular dissection of regulated secretion, we have resolved norepinephrine (NE) secretion from semi-intact PC12 cells into two kinetically distinct stages, each of which was studied separately to discern its molecular requirements. The first stage consisted of MgATP-dependent priming of the secretory apparatus in the absence of Ca2+. MgATP-dependent priming was readily reversible and inhibited by a broad range of protein kinase inhibitors. The second stage consisted of Ca(2+)-triggered exocytosis which, in contrast to priming, occurred in the absence of MgATP. Both priming and triggering were found to be dependent upon or stimulated by cytosolic proteins. The priming and triggering activities of cytosol were functionally distinct as indicated by differing thermolability. Furthermore, active components in cytosol resolved by gel filtration were found to support either priming or triggering, but not both. For both priming and triggering reactions, several peaks of activity were detected; one of each type of factor was partially purified from rat brain cytosol, and found to be enriched for stage-specific activity. Two partially purified factors exhibiting stage-specific activity, a approximately 20-kD priming factor and approximately 300-kD triggering factor, were able to support regulated secretion as effectively as crude cytosol when used sequentially in the partial reactions. Further characterization of stage-specific cytosolic factors should clarify the nature of MgATP- and Ca(2+)-dependent events in the regulated secretory pathway.

Interaction between protein kinase C and Exo1 (14-3-3 protein) and its relevance to exocytosis in permeabilized adrenal chromaffin cells

The roles of protein kinase C (PKC) and Exo1 in exocytosis from digitonin-permeabilized adrenal chromaffin cells were explored by using exogenous purified proteins in a run-down/reconstitution system. The stimulatory action of Exo1 on exocytosis from run-down cells was found to be completely dependent on the continuous presence of exogenous MgATP, suggesting that it acts on the slow phase of exocytosis [Holz, Bittner, Peppers, Senter & Eberhard (1989), J. Biol. Chem. 264, 5412-5419]. Partially purified rat brain PKC was found to be able to stimulate Ca(2+)-dependent exocytosis from run-down cells in a dose-dependent manner. This effect was indeed due to PKC and not a contaminant in the PKC fraction, since the PKC activator phorbol 12-myristate 13-acetate (PMA), under conditions in which control secretion was not affected, potentiated the effect of the exogenous PKC in stimulating secretion. Furthermore, although either PKC or Exo1 alone could stimulate exocytosis from run-down cells, the effect of combining the fractions was synergistic, as had previously been observed using PMA treatment combined with Exo1 incubation [Morgan & Burgoyne (1992) Nature (London) 355, 833-836]. The observed synergy between PKC and Exo1 was not due to PKC-mediated phosphorylation of Exo1, and Exo1 was found not to affect PKC activity in enzyme assays. We conclude that PKC and Exo1 act synergistically in the slow phase of Ca(2+)-dependent exocytosis from adrenal chromaffin cells. Furthermore, PKC does not directly affect Exo1, but rather enhances the activity of Exo1 by a putative phosphorylation of another, unidentified, component of the exocytotic machinery which facilitates the action of Exo1 in exocytosis.