Detection of low molecular mass GTP-binding proteins in chromaffin granules and other subcellular fractions of chromaffin cells

Myosins II and V in chromaffin cells: myosin V is a chromaffin vesicle molecular motor involved in secretion

The presence of myosin II and V in chromaffin cells and their subcellular distribution is described. Myosin II and V distribution in sucrose density gradients showed only a strong correlation between the distribution of myosin V and secretory vesicle markers. Confocal microscopy images demonstrated colocalization of myosin V with dopamine beta-hydroxylase, a chromaffin vesicle marker, whereas myosin II was present mainly in the cell cortex. Cell depolarization induced, in a Ca2+ and time-dependent manner, the dissociation of myosin V from chromaffin vesicles suggesting that this association was not permanent but determined by secretory cycle requirements. Myosin II was also found in the crude granule fraction, however, its distribution was not affected by cell depolarization. Myosin V head antibodies were able to inhibit secretion whereas myosin II antibodies had no inhibitory effect. The pattern of inhibition indicated that these treatments interfered with the transport of vesicles from the reserve to the release-ready compartment, suggesting the involvement of myosin V and not myosin II in this transport process. The results described here suggest that myosin V is a molecular motor involved in chromaffin vesicle secretion. However, these results do not discard an indirect role for myosin II in secretion through its interaction with F-actin networks.

Regulated secretion in chromaffin cells: an essential role for ARF6-regulated phospholipase D in the late stages of exocytosis

ARFs constitute a family of structurally related proteins that forms a subset of the ras GTPases. In chromaffin cells, secretagogue-evoked stimulation triggers the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. Both PLD activation and catecholamine secretion are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6. ARNO, a potential guanine nucleotide exchange factor for ARF6, is expressed and localized in the plasma membrane of chromaffin cells. Using permeabilized cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits both PLD activation and catecholamine secretion. Chromaffin cells express PLD1 at the plasma membrane. We found that microinjection of the catalytically inactive PLD1(K898R) dramatically reduces catecholamine secretion monitored by amperometry, most likely by interfering with a late postdocking step of calcium-regulated exocytosis. We propose that ARNO-ARF6 participate in the exocytotic reaction by controlling the plasma membrane-bound PLD1. By generating fusogenic lipids at the exocytotic sites, PLD1 may represent an essential component of the fusion machinery in neuroendocrine cells.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Differential expression pattern of Rab-GDI isoforms during the parotid gland secretion cycle

Rab GDP dissociation inhibitor (GDI) plays an important role in regulating the GDP/GTP cycle of small GTP binding proteins of the Rab family. It also regulates their association to membranes. The small family of Rab-GDI consists of several closely related isoforms, the functional differences between which are still unknown. Here we show that multiple GDI isoforms are expressed in rat parotid gland and that the individual GDI isoforms have a characteristic expression both at the RNA and at the protein level, during the parotid secretory cycle. GDIalpha, the major isoform in brain, is expressed throughout the secretory process and is equally distributed between cytoplasmic and membranous fractions. In contrast, an isoform related to, but different from GDIbeta is found predominantly in the cytoplasmic fraction and its expression is detected only after beta-adrenergic stimulation of the gland, at the end of the secretion phase, when exocytosis is already completed. The induction of such a GDI isoform at the beginning of the recovery stage correlates with the expression pattern of Rab1 and Rab5, but not Rab2 and Rab4. Our results suggest different functional roles for multiple GDI isoforms along the secretion and recovery phases in rat parotid gland.

Several small GTP-binding proteins are strongly down-regulated in simian virus 40 (SV40) transformed human keratinocytes and may be required for the maintenance of the normal phenotype

High resolution two-dimensional (2-D) gel electrophoresis in combination with the blot overlay nucleotide binding assay was used to reveal low molecular weight GTP-binding proteins expressed by primary cultured, normal human keratinocytes. Forty one small GTP-binding proteins (30 isoelectric focusing, IEF; and 11 nonequilibrium pH gradient electrophoresis, NEPHGE) ranging in molecular weights from 18,000 to 30,000 and isoelectric points from 4.4 to 8.0 were detected and mapped in the master human keratinocyte database. Four GTP-binding proteins were identified by 2-D gel immunoblotting and these correspond to rap1 and 2 and two forms of rab6. ras-Proteins are most likely present in the [alpha 32P]GTP 2-D gel blots but their levels may be too low to be detected by immunoblotting. Quantitative changes in the relative expression levels of [alpha 32P]GTP-binding proteins in normal proliferating and simian virus 40 (SV40) transformed human keratinocytes (K 14) were determined by scintillation counting of the radioactive spots excised from the nitrocellulose blots. The results showed that thirteen of these proteins were not expressed in transformed K14 keratinocytes, implying that they may play a role in the maintenance of the normal cell phenotype.

Expression and localization of two low molecular weight GTP-binding proteins, Rab8 and Rab10, by epitope tag

Small GTP-binding proteins of the YPT/SEC4/Rab family have been shown to play an essential role in intracellular membrane trafficking. In mammals, Rab8 and Rab10 are the two small GTP-binding proteins identified so far that are closest to SEC4, an essential gene product involved in post-Golgi constitutive secretion in the yeast Saccharomyces cerevisiae. To study the localization of Rab proteins, we have expressed the cDNAs with an influenza virus hemagglutinin (HA) epitope tag at the N terminus. The feasibility of this method was tested by using yeast SEC4. HA-tagged SEC4 functionally complemented a temperature-sensitive sec4 mutant similarly to wild-type SEC4, indicating that the modified protein retained functional integrity. Monoclonal antibody 12CA5, raised against the HA tag, was used to determine the expression and localization of HA-tagged proteins after transfection. In stably transfected CHO and Swiss 3T3 cells, HA-tagged Rab8 was localized to the cell periphery, with the highest concentration in the ruffling areas. In contrast, epitope-tagged Rab10 expressed in CHO and BHK cells was concentrated on membranes in the perinuclear region. By light microscopy, the staining partially overlapped with that of a Golgi marker, beta-COP. Thus, despite the high degree homology of Rab8 and Rab10 (66% identity), the two proteins are localized to distinct cellular compartments. This approach should provide a general tool for the analyses of other members of the YPT/SEC4/rab gene family.

Exocytosis from permeabilized bovine adrenal chromaffin cells is differently modulated by guanosine 5'-[gamma-thio]triphosphate and guanosine 5'-[beta gamma-imido]triphosphate. Evidence for the involvement of various guanine nucleotide-binding proteins

1. In bovine adrenal chromaffin cells made permeable either to molecules less than or equal to 3 kDa with alphatoxin or to proteins less than or equal to 150 kDa with streptolysin O, the GTP analogues guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) differently modulated Ca(2+)-stimulated exocytosis. 2. In alphatoxin-permeabilized cells, p[NH]ppG up to 20 microM activated Ca(2+)-stimulated exocytosis. Higher concentrations had little or no effect. At a free Ca2+ concentration of 5 microM, 7 microM-p[NH]ppG stimulated exocytosis 6-fold. Increasing the free Ca2+ concentration reduced the effect of p[NH]ppG. Pretreatment of the cells with pertussis toxin prevented the activation of the Ca(2+)-stimulated exocytosis by p[NH]ppG. 3. In streptolysin O-permeabilized cells, p[NH]ppG did not activate, but rather inhibited Ca(2+)-dependent catecholamine release under all conditions studied. In the soluble cytoplasmic material that escaped during permeabilization with streptolysin O, different G-protein alpha-subunits were detected using an appropriate antibody. Around 15% of the cellular alpha-subunits were detected in the supernatant of permeabilized control cells. p[NH]ppG or GTP[S] stimulated the release of alpha-subunits 2-fold, causing a loss of about 30% of the cellular G-protein alpha-subunits under these conditions. Two of the alpha-subunits in the supernatant belonged to the G(o) type, as revealed by an antibody specific for G(o) alpha. 4. GTP[S], when present alone during stimulation with Ca2+, activated exocytosis in a similar manner to p[NH]ppG. Upon prolonged incubation, GTP[S], in contrast to p[NH]ppG, inhibited Ca(2+)-induced exocytosis from cells permeabilized by either of the pore-forming toxins. This effect was resistant to pertussin toxin. 5. The p[NH]ppG-induced activation of Ca(2+)-stimulated release from alphatoxin-permeabilized chromaffin cells may be attributed to one of the heterotrimeric G-proteins lost during permeabilization with streptolysin O. The inhibitory effect of GTP[S] on exocytosis is apparently not mediated by G-protein alpha-subunits, but by another GTP-dependent process still occurring after permeabilization with streptolysin O.

Amylase release from streptolysin O-permeabilized pancreatic acinar cells. Effects of Ca2+, guanosine 5'-[gamma-thio]triphosphate, cyclic AMP, tetanus toxin and botulinum A toxin

The molecular requirements for amylase release and the intracellular effects of botulinum A toxin and tetanus toxin on amylase release were investigated using rat pancreatic acinar cells permeabilized with streptolysin O. Micromolar concentrations of free Ca2+ evoked amylase release from these cells. Maximal release was observed in the presence of 30 microM free Ca2+. Ca(2+)-stimulated, but not basal, amylase release was enhanced by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) (3-4 fold) or cyclic AMP (1.5-2 fold). Neither the two-chain forms of botulinum A toxin and tetanus toxin, under reducing conditions, nor the light chains of tetanus toxin, inhibited amylase release triggered by Ca2+, or combinations of Ca2+ + GTP[S] or Ca2+ + cAMP. The lack of inhibition was not due to inactivation of botulinum A toxin or tetanus toxin by pancreatic acinar cell proteolytic enzymes, as toxins previously incubated with permeabilized pancreatic acinar cells inhibited Ca(2+)-stimulated [3H]noradrenaline release from streptolysin O-permeabilized adrenal chromaffin cells. These data imply that clostridial neurotoxins inhibit a Ca(2+)-dependent mechanism which promotes exocytosis in neural and endocrine cells, but not in exocrine cells.

Cytoskeleton and molecular mechanisms in neurotransmitter release by neurosecretory cells

The process of exocytosis is a fascinating interplay between secretory vesicles and cellular components. Secretory vesicles are true organelles which not only store and protect neurotransmitters from inactivation but also provide the cell with efficient carriers of material for export. Different types of secretory vesicles are described and their membrane components compared. Associations of several cytoplasmic proteins and cytoskeletal components with secretory vesicles and the importance of such associations in the mechanism of secretion are discussed. A description of possible sites of action for Ca2+ as well as possible roles for calmodulin, G-proteins and protein kinase C in secretion are also presented. Important aspects of the cytoskeleton of neurosecretory cells are discussed. The cytoskeleton undergoes dynamic changes as a result of cell stimulation. These changes (i.e. actin filament disassembly) which are a prelude to exocytosis, play a central role in secretion. Moreover, advanced electrophysiological techniques which allow the study of secretory vesicle-plasma membrane fusion in real-time resolution and at the level of the single secretory vesicle, have also provided a better understanding of the secretory process.

Identification of ras and ras-related low-molecular-mass GTP-binding proteins associated with rat lung lamellar bodies

Recent evidence from genetic experiments in yeast and from studies using guanosine triphosphate (GTP) analogues in mammalian cells suggests a key role for low-molecular-mass GTP-binding proteins (LMM-GBPs) (Mr 19 to 28 kD) in processes of intracellular vesicular sorting and secretion. Assembly and exocytosis of the lamellar body (LB), the secretory organelle of the pulmonary alveolar type 2 pneumocyte, may be regulated by LMM-GBPs. We used [alpha-32P]GTP binding to Western blotted proteins, ultraviolet crosslinking of [alpha-32P]GTP to membrane proteins, immunoblotting with specific antisera, and botulinum exoenzyme C3-catalyzed ADP ribosylation to detect LMM-GBPs in LB. With the first two techniques, we have identified six LMM-GBPs of approximately 27, 25.5, 24.5, 23, 22, and 21 kD that are enriched in a highly purified LB fraction compared with type 2 pneumocyte homogenate, crude membranes, and cytosol. Further characterization of the LB LMM-GBPs by immunoblotting revealed that ras p21 is greatly enriched in the LB fraction compared with other type 2 pneumocyte fractions. In addition, botulinum exoenzyme C3 catalyzed the ADP ribosylation of 20- to 21-kD proteins that were similarly enriched in the LB fraction. In contrast, a monospecific antibody to ADP-ribosylation factor reacted with a 19-kD protein only in the type 2 pneumocyte homogenate and cytosol fractions. Monospecific antibodies to yeast Sec4 protein and to rab 3A did not react with any type 2 pneumocyte proteins. The LMM-GBPs specifically associated with LB may participate in intracellular events required for surfactant packaging and secretion.

Identification of some of the brain Gn27 as the ral gene product. Comparison between the brain and platelet Gn-proteins

Two major Gn-proteins, Gn27 and Gn26, were detected in the 100,000 x gav particulate fraction of rabbit and bovine brain. The Gn26 protein was also present in significant amounts (approximately 50% of total) in the brain supernatant fraction. An antiserum raised against recombinant simian ralA recognized a 27-kDa brain protein with the same apparent molecular mass as the Gn27 protein. In further analysis by two-dimensional polyacrylamide gel electrophoresis, the brain particulate Gn-proteins were resolved into 6 major forms, four of 27 kDa (Gn27a-d) and two of 26 kDa (Gn26a and Gn26b). Minor GTP-binding components were also observed at 25 kDa and 24 kDa. The ralA antibody reacted strongly with the brain Gn27b form and weakly with the Gn27a and Gn27c but not with Gn27d or any of the other Gn-proteins. In addition, comparison of human platelet and bovine brain particulate Gn-proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated a tissue/cell-type specific expression of the various forms of Gn-proteins.

Redistribution of 23 kDa tubulovesicle-associated GTP-binding proteins during parietal cell stimulation

Small GTP-binding proteins are important regulators of intracellular traffic. The presence of several small GTP-binding proteins was documented in subfractions of rabbit parietal cells. Upon maximal stimulation of the cells with a combination of histamine and forskolin, one 23 kDa GTP-binding band was observed to decrease in a 50,000 g membrane fraction while increasing in 4000 g membranes. The 23 kDa band resolved into one major and two minor species on two-dimensional gels. GTP-binding species of 23 kDa, 24 kDa and 25 kDa were present in purified preparations of tubulovesicles. The three isoelectric species of the 23 kDa proteins observed in parietal cell 50,000 g microsomes were enriched in tubulovesicle preparations. None of the tubulovesicle-associated GTP-binding proteins were substrates for ADP-ribosylation by a preparation of botulinum D toxin. These results indicate that tubulovesicles contain discrete small GTP-binding proteins which redistribute during parietal cell stimulation.

Biogenesis of secretory granules. Implications arising from the immature secretory granule in the regulated pathway of secretion

In endocrine cells the regulated secretion of hormones, peptides, enzymes and neurotransmitters into the external medium occurs when mature secretory granules fuse with the plasma membrane. Secretory granules form at the trans-Golgi network (TGN) by envelopment of the dense-core aggregate of regulated secretory proteins by a specific membrane. The secretory granules initially formed at the TGN, referred to here as immature secretory granules, are morphologically and biochemically distinct from mature secretory granules. The functional similarities and differences between the immature secretory granule and the mature secretory granule, and the events involved in the maturation of the secretory granules are briefly discussed.

Biochemical and biological comparison of HIV-1 NEF and ras gene products

Human immunodeficiency virus type 1 (HIV-1) NEF protein has been reported to share certain biochemical and structural properties with known oncoproteins like src or rats. To determine whether this is a general property of NEF from various HIV isolates, three different NEF proteins were expressed in Escherichia coli using a thermoinducible expression system previously exploited to overproduce functionally active p21 ras proteins. ras and NEF proteins expressed in this manner were evaluated in parallel to compare their biochemical and biological properties. In contrast to ras, our NEF protein preparations had no detectable GTP binding but showed autophosphorylation activity when incubated in the presence of either GTP or ATP. This putative autokinase activity was higher in NEF proteins containing threonine at position 15 than in those carrying alanine at that position. Two different NEF genes also failed to induce oncogenic transformation of permanently transfected NIH 3T3 cells under conditions that led to oncogenic transformation using activated ras genes. Also, unlike ras, the NEF gene products failed to induce meiotic maturation when injected into fully grown Xenopus oocytes.

Mediation of the attachment or fusion step in vesicular transport by the GTP-binding Ypt1 protein

The function of the guanosine triphosphate (GTP)-binding protein Ypt1 in regulating vesicular traffic was studied in a cell-free system that reconstitutes transport from the endoplasmic reticulum to the Golgi. Blocking the Ypt1 protein activity resulted in accumulation of vesicles that act as an intermediate passing between the two compartments. The Ypt1 protein was found on the outer side of these vesicles. The transport process is completed by fusion of these vesicles with the acceptor compartment, and Ypt1 protein activity was needed for this step. Thus, a specific GTP-binding protein is required for either attachment or fusion (or both) of secretory vesicles with the acceptor compartment during protein secretion.

Association of the GTP-binding protein Rab3A with bovine adrenal chromaffin granules

The Rab3A protein belongs to a large family of small GTP-binding proteins that are present in eukaryotic cells and that share amino acid identities with the Ras proteins (products of the ras protooncogenes). Rab3A, which is specifically located in nervous and endocrine tissues, is suspected to play a key role in secretion. Its localization was investigated in bovine adrenal gland by using a polyclonal antibody. Rab3A was detected in adrenal medulla but not in adrenal cortex. In cultured adrenal medulla cells. Rab3A was specifically expressed in the catecholamine-secreting chromaffin cells. Subcellular fractionation suggested that Rab3A is about 30% cytosolic and that particulate Rab3A is associated with the membrane of chromaffin granules (the catecholamine storage organelles) and with a second compartment likely to be the plasma membrane. The Rab3A localization on chromaffin granule membranes was confirmed by immunoadsorption with an antibody against dopamine beta-hydroxylase. Rab3A was not extracted from this membrane by NaCl or KBr but was partially extracted by urea and totally solubilized by Triton X-100, suggesting either an interaction with an intrinsic protein or a membrane association through fatty acid acylation. This study suggests that Rab3A, which may also be located on other secretory vesicles containing noncharacterized small GTP-binding proteins, is involved in their biogenesis or in the regulated secretion process.

Identification and localization of low-molecular-mass GTP-binding proteins associated with synaptic vesicles and other membranes

GTP-binding proteins were studied in synaptic vesicles prepared from bovine brain by differential centrifugation and separated further from plasma membranes using gel permeation chromatography. Following separation by SDS-PAGE of proteins from the different fractions, and transfer to nitrocellulose sheets, the presence and localization of low-molecular-mass GTP-binding proteins were assessed by [alpha-32 P]GTP binding. The vesicle-membrane fraction (SV) was enriched in synaptophysin (p38, a synaptic vesicle marker) and contained low-molecular-mass GTP-binding proteins; these consisted of a major 27 kDa protein and minor components (Mr 26 and 24 kDa) which were trypsin-sensitive and immunologically distinguishable from ras p21 protein. GTP-binding proteins of low molecular mass, but displaying less sensitivity to trypsin, were also found in the plasma membrane fraction (PM; enriched in Na+/K(+)-ATPase). In addition, the PM fraction contained GTP-binding proteins with higher Mr (Gi alpha and G0 alpha), together with another GTP-binding protein, ras p21. Putative function(s) of these GTP-binding proteins with low mass are discussed.

Multiple GTP-binding proteins from cholinergic synaptic vesicles

Cholinergic synaptic vesicles purified from the electric organ of the marine ray, Discopyge ommata, contain 2 different size classes of GTP-binding proteins: one or more with an apparent molecular weight (MW) between 37 and 41 kDa, and 3 major and at least 2 minor proteins with MW between 20 and 29 kDa. These GTP-binding proteins were detectable using the alpha 32P-GTP overlay technique and covalent modification with bacterial toxins. The higher MW GTP-binding proteins are ADP-ribosylated by pertussis toxin and 2 of the lower MW GTP-binding proteins are sensitive to botulinum toxin.