Comparative studies of intracellular transport of secretory proteins

Synthesis of interphotoreceptor retinoid-binding protein (IRBP) by monkey retina in organ culture: effect of monensin

Whole monkey retinas were incubated in short-term organ culture with either radiolabeled amino acids or glucosamine. Soluble retinal proteins and proteins in the culture medium were analyzed by SDS-poly-acrylamide gel electrophoresis. Fluorography showed that the interphotoreceptor retinoid-binding protein (IRBP), a 146,000 Mr glycoprotein localized in the extracellular matrix, is synthesized by the neural retina and rapidly secreted into the medium. Secretion is blocked by 10-5M monensin. No significant IRBP synthesis was observed in the pigment-epithelium-choroid complex. IRBP is thus the major component synthesized and secreted by the neural retina into the interphotoreceptor space. This, and its affinity for retinoid makes it a prime candidate for an extracellular retinoid transport vehicle.

RNA and protein synthesis inhibitors: effects on sexual behavior in female rats

Biochemical evidence indicates that many steroid hormone effects on target tissues are mediated via actions on the genome. Studies on the hormonal control of reproductive behavior have demonstrated several effects of RNA or protein synthesis inhibitors on female sexual behavior in rats. In female rats treated with estrogen and progesterone, intracranial application of actinomycin-D (an RNA synthesis inhibitor) can disrupt lordosis responding if the drug is given in conjunction with estrogen, but not with progesterone. Protein synthesis inhibitors (cycloheximide or anisomycin) applied intracranially at the time of estrogen also disrupt lordosis, with anisomycin antagonizing the progesterone-facilitation of lordosis. Possible neural sites of action of these drugs are considered, as are alternative modes of action of these synthesis inhibitors. Whereas the effects of estrogen on lordosis include the synthesis of some, as yet unidentified, proteins, the exact role for protein synthesis in the mediation of progesterone's actions on lordosis remains less certain.

Effect of monensin on the Golgi apparatus of absorptive cells in the small intestine of the rat. Morphological and cytochemical studies

The effect of short-time treatment with the ionophore monensin, administered intraluminally at concentrations of 5 and 10 microM, was studied on the Golgi apparatus of absorptive cells in the small intestine of the rat. At 2-3 min after treatment most of the Golgi stacks exhibited dilated cisternae. At 4-5 min stacked cisternae were absent; they were replaced by groups of smooth-surfaced vacuoles. Dilatation and vacuolization occurred in the entire stacks without preferential effect on any particular Golgi subcompartment. Monensin did not influence the cytochemical Golgi reaction of thiamine pyrophosphatase and acid phosphatase. The characteristic staining pattern of these two enzymes in all Golgi cisternae of absorptive cells in the proximal small intestine, and the reactivity restricted to trans cisternae in distal segments of the small intestine, were unchanged after treatment with monensin. In the distal small intestine, the cytochemical pattern allowed the monensin-induced vacuoles to be attributed to the former cis- or trans-Golgi face. Further, the cytochemical results demonstrate that vacuolization is not restricted to the stacked cisternae, but includes the trans-most cisterna. The latter, usually located at some distance from the Golgi stacks, has been defined as belonging to the GERL system in several types of cells. The clear response to monensin, an agent that selectively affects the Golgi apparatus, indicates common properties between trans-most and stacked Golgi cisternae.

An overview of pancreatic exocrine secretion

Genes for all proteins have encoded in their DNA sequences, information that specifies where these proteins will localize within the cell. Nascent translation products of transcripts of these genes, that possess a specific NH2-terminal signal sequence, are able to translocate into a specialized membranous conducting system called the endoplasmic reticulum (ER), or can be incorporated directly into a target organelle (i.e. mitochondrion). Nascent polypeptides lacking this signal sequence remain in the cytosol. Once segregation into the ER has occurred each protein appears to migrate at a characteristic rate to a connecting organelle; the Golgi Complex. Here, enzymatic modifications of these proteins determines the organelle with which each will eventually become associated. In secretory tissues such as the exocrine pancreas, many different proteins are directed into specialized secretory structures called zymogen granules. These granules are maintained in a "ready-to-release" state by steroid hormones. In the absence of estrogens and glucocorticoids the zymogen granules disappear. Physiologically, secretion from the exocrine pancreas is brought about by parasympathetic nerve stimulation, or by the gut hormone cholecystokinin (CCK). Interaction of acetylcholine or CCK with specific receptors on pancreatic acinar cells initiates the process of exocytosis; that is, fusion of the zymogen granule membrane with the plasma membrane resulting in extracellular release of the contents of the secretory granule.

Replication and morphogenesis of avian coronavirus in Vero cells and their inhibition by monensin

Avian infectious bronchitis virus (IBV) was adapted to Vero cells by serial passage. No significant inhibition of IBV replication was observed when infected Vero cells were treated with alpha-amanitin or actinomycin D. In thin sections of infected cells, assembly of IBV was observed at the rough endoplasmic reticulum (RER), and mature IBV particles were located in dilated cisternae of the RER as well as in smooth cytoplasmic vesicles. In addition to typical IBV particles, enveloped particles containing numerous ribosomes were identified at later times postinfection. Monensin, a sodium ionophore which blocks glycoprotein transport to plasma membranes at the level of the Golgi complex, was found to inhibit the formation of infectious IBV. In thin sections of infected Vero cells treated with the ionophore, IBV particles were located in dilated cytoplasmic vesicles, but fewer particles were found when compared to controls. A similar pattern of virus-specific proteins was detected in control or monensin-treated IBV-infected cells, which included two glycoproteins (170 000 and 24 000 daltons) and a polypeptide of 52 000 daltons. These results suggest that the ionophore inhibits assembly of a virus which matures at intracellular membranes.

Biosynthesis of renin: multiplicity of active and intermediate forms

Processing of renin involves sequential proteolytic cleavages of a preproform to the active mature forms. Preprorenin is rapidly internalized cotranslationally into the rough endoplasmic reticulum and hydrolyzed by signal peptidase to produce prorenin. In the Golgi, prorenin is converted (within 15 min) to a form of renin that is enzymatically active. Over the next 12 hr, a slow intracellular process removes a dipeptide near the carboxyl terminus, converting the one-chain renin into two chains joined by a single disulfide bond. This conversion occurs during formation, condensation, and packaging of renin granules. The resultant two-chain renin is approximately one-sixth as active as the one-chain form. The intact renin molecule is obligatory for enzymatic activity because heavy chain alone has little or no activity. Both one- and two-chain renins are secreted, but prorenin is not. Multiple isoelectric forms of prorenin, one-chain renin, and two-chain renin are also observed. This microheterogeneity probably results from minor differences in amino acid composition as a consequence of variations in cleavage positions during processing. Thus, these data suggest that renin synthesis and secretion is complex and may be subject to regulation at multiple steps. Furthermore, based on the results of this study, we also propose that renin can be secreted by two different pathways.

Reappearance of beta-adrenergic receptors after isoproterenol treatment in intact C6-cells

The reappearance of beta-adrenergic receptors in C6-glioma cells after desensitization with isoproterenol was studied using the antagonist [3H]CGP-12177. Reappearance had the following properties: (a) it occurred in intact cells only, (b) it was temperature dependent, (c) it required an Na+/H+ gradient, low intracellular Ca2+ activity, and (d) it required ATP, and (e) intact lysosomes. The results suggest endocytosis and recycling of the beta-adrenergic receptor after agonist treatment.

Effect of nigericin on distribution of sodium, potassium and calcium ion in rabbit lens

Nigericin, a carboxylic monovalent cation ionophore, induced opacification in the rabbit lens within 2 hr of incubation at 37 degrees C at a concentration of 10(-7)M. The opacification was in the subcapsular region where a marked degree of Ca2+ accumulation and change in the monovalent cation balance occurred. Nigericin may accumulate and act locally at the surface region of the lens, presumably due to the high lipid solubility of nigericin. Nigericin induced an opacification of the lens when applied either to the anterior chamber or to the vitreous of rabbit eye.

Effect of carboxylic ionophores on lysosomal protein degradation in rat hepatocytes

Three different carboxylic ionophores (monensin, nigericin and lasalocid) were each found capable of causing a relatively complete block of the lysosomal (i.e., methylamine-sensitive) protein degradation in isolated rat hepatocytes. Monensin was found to be the most specific in action, as it had no effect on non-lysosomal degradation and did not bring about any substantial inhibition of protein synthesis. Morphometric examination of electron micrographs revealed that monensin causes an accumulation of early forms of autophagic vacuoles and blocks the swelling of lysosomes seen in the presence of methylamine. The results indicate that monensin inhibits lysosomal protein degradation by affecting lysosomal pH.

Synthesis and secretion of proteoglycans by cultured chondrocytes. Effects of monensin, colchicine and beta-D-xyloside

Chondrocytes, isolated from elastic ear cartilage of young rabbits, were grown in monolayer cultures in Ham's F-12 medium. Synthesis and secretion of macromolecules were monitored by labelling with radioactive precursors and the effect of monensin and other experimental agents was investigated. Monensin caused an inhibition of the incorporation of precursors into macromolecular material and a moderate intracellular accumulation when used in higher concentrations. The effect was more pronounced for 35SO4 than for 3H-labelled glucose or proline. p-Nitrophenyl-beta-D-xyloside alleviated this inhibition to some extent, but there was a concomitant increase in the amount of intracellular labelled material. Colchicine and monensin together caused a severe inhibition of the incorporation of 35SO4 and a marked shift of the label to the intracellular compartment. Colchicine also increased the sensitivity of the cells to monensin, lowering the minimal effective concentration about one order of magnitude. The latter results are consistent with the idea that cytoplasmic microtubules have a stabilizing function on the secretory pathways and, that their removal by colchicine, causing a 'randomizing' of the Golgi complex, makes these pathways more vulnerable to monensin.

Monensin inhibits receptor-mediated endocytosis of asialoglycoproteins in rat hepatocytes

Isolated rat liver parenchymal cells incubated in the presence of monensin exhibited a reduced uptake of 125I-asialofetuin (125I-AF). Binding studies indicated that the effect was due to a rapid reduction in the number of active surface receptors for the asialoglycoprotein. Monensin had no effect on receptor internalization, but apparently interrupted the recycling of receptors back to the cell surface. Monensin also inhibited the degradation of 125I-AF previously bound to the cells; this inhibition was probably not due to a direct effect on intralysosomal proteolysis, as no lysosomal accumulation of undegraded ligand could be demonstrated in subcellular fractionation studies by means of sucrose gradients. It is more likely that monensin inhibits transfer of the labelled ligand from endocytic vesicles to lysosomes, as indicated by the accumulation of radioactivity in the former and by the ability of monensin to prevent the normally observed time-dependent increase in the buoyant density of endocytic vesicles. Whereas the effect of monensin on binding and uptake of asialofetuin was reversible, the effect on asialofetuin degradation could not be reversed.

Apoprotein E is synthesized and secreted by resident and thioglycollate-elicited macrophages but not by pyran copolymer- or bacillus Calmette-Guerin-activated macrophages

Macrophages are active secretory cells that display functionally distinct phenotypes that are regulated by inflammation. We have found that apoprotein E (ApoE), a component of plasma lipoproteins, was synthesized and secreted by resident and nonspecifically stimulated macrophages elicited with thioglycollate broth, but not by activated macrophages obtained from mice treated with bacillus Calmette-Guerin, pyran copolymer, whole Corynebacterium parvum, or bacterial endotoxin. ApoE represented approximately 1% of the newly synthesized protein and approximately 10% of secreted protein of resident and thioglycollate-elicited macrophages. ApoE from thioglycollate-elicited macrophages was indistinguishable from ApoE in mouse plasma lipoproteins, as determined by immunoreactivity, peptide mapping, and molecular weight. When specific antibodies were used to localize cell-associated ApoE, strong immunofluorescence was seen in the Golgi region of resident and thioglycollate-elicited macrophages immediately after removal from the peritoneal cavity, as well as after culture for up to 7 d. In contrast, activated macrophages did not synthesize or secrete ApoE to an appreciable extent and had no immunocytochemically detectable intracellular ApoE. When activated macrophages were cultured in medium containing serum, their activated state, as judged by production of H2O2, declined within 48-72 h in parallel with the induction of synthesis and secretion of ApoE and detection of intracellular ApoE by immunofluorescence. During prolonged culture the rate of synthesis and secretion of ApoE increased in both resident and activated macrophages. Therefore, the synthesis and secretion of ApoE may serve as markers for the functional state of macrophages.

Monensin inhibits initial spreading of cultured human fibroblasts

The monovalent ionophore, monensin, inhibits the secretion of both pro-collagen and fibronectin in cultured human fibroblasts and other cell types. The block to secretion is due to the ability of monensin to suppress the export of these secretory proteins from the Golgi apparatus. As such proteins are known to be implicated in the adhesion, spreading and movement of cultured fibroblasts, it might be expected that monensin treatment would interfere with these processes. However, it has recently been reported that monensin-treated human embryonal fibroblasts attached and spread onto glass substrata to the same extent as untreated cells, although at later stages they fail to develop focal adhesion sites. However, these experiments were performed using medium supplemented with fetal calf serum (FCS). We now demonstrate that in the absence of FCS, while monensin has little or no effect on the initial adhesion of fibroblasts to the substratum, subsequent spreading is much reduced. The inhibition of spreading is noticeable within 30 min of plating and is maintained for at least 100 min in monensin-free medium following prolonged pre-incubation of the cells with monensin.

Disparate effects of monensin and colchicine on intracellular processing of secretory proteins in cultured rat hepatocytes

We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, alpha 1-protease inhibitor and alpha 2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, alpha 1-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with Mr 51 000, and then processed to two endoglycosidase-H-resistant forms having Mr 51 000 and 56 000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for alpha 2u-globulin. In the cells treated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed alpha 1-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized alpha 1-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51 000-Mr forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant alpha 1-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH4Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins.

Ultrastructural changes of hepatocyte organelles induced by chemicals and their relation to fat accumulation in the liver

Fatty liver was induced in the rats shortly after administration of cycloheximide, ethionine, orotic acid, monensin or colchicine. It was strongly suggested that derangements in one or more of the hepatic lipoprotein metabolic steps, which occur at the levels of endoplasmic reticulum, Golgi apparatus and secretory vacuoles lead to an accumulation of triglyceride within hepatocytes.

Biosynthesis and secretion of procollagenase by rabbit synovial fibroblasts. Inhibition of procollagenase secretion by monensin and evidence for glycosylation of procollagenase

Monolayer cultures of rabbit synovial fibroblasts stimulated with phorbol myristate acetate to produce large amounts of collagenase (EC 3.4.24.7) were used to study the biosynthesis and secretion of this enzyme. [3H]Leucine was added to cell cultures for pulse-chase and continuous-labelling experiments. The labelled procollagenase synthesized was identified by immunoprecipitation followed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and fluorography. The amounts of intracellular and extracellular proenzyme were quantified by measuring radioactivity incorporated into the proteins. procollagenase was synthesized as doublet proteins of Mr 57 000 and Mr 61 000. Immunoprecipitable proenzyme proteins were first detected in culture medium 35 min after [3H]leucine was added to the cells. Monensin treatment of the cells inhibited procollagenase secretion and led to intracellular accumulation of the proenzyme. Cells treated with tunicamycin produced only the 57 000-Mr form, indicating that in rabbit synovial cells the 61 000-Mr form was post-translationally modified by addition of oligosaccharides to asparagine residues. The ratios of glycosylated to unglycosylated forms in cell lysates and in culture medium were 0.22:1 and 0.07:1 respectively.

The mechanism of receptor-mediated degradation of insulin in isolated rat adipocytes: indirect evidence for a non-lysosomal pathway

Receptor-bound insulin is substrate for a degradation leading to the release of about half the cell-associated [125I]monoiodoinsulin as [125I]monoiodotyrosine. Classical lysosomal inhibitors of the amine type (cloroquine, methylamine and NH+4) only partly inhibited this receptor-mediated degradation. Leupeptin, which is very effective in other systems, was without any effect in the present system. The degradation could not be reduced by lowering the ATP content of the cells. Sulphydryl reagents strongly inhibited the degradation as has also been shown for the cytosolic insulin-specific protease. Microtubules and microfilaments are probably not involved since inhibitors of the cytoskeleton were without marked effects. It is suggested that in the rat adipocyte only a minor part of the receptor-mediated degradation of insulin takes place via the classical endocytotic lysosomal pathway.

Reduced temperature prevents transfer of a membrane glycoprotein to the cell surface but does not prevent terminal glycosylation

The transport kinetics of the influenza virus hemagglutinin from its site of synthesis to the apical plasma membrane of Madin-Darby canine kidney cells, a polarized epithelial cell line, were studied by a sensitive tryptic assay. Hemagglutinin acquired terminal sugars, as judged by sensitivity to endo-beta-N-acetylglucosaminidase H, 10-15 min after synthesis, and first appeared on the apical domain 15 min later. None of the pulse-labeled hemagglutinin accumulated on the basolateral domain. At 20 degrees C, terminal glycosylation continued, but no hemagglutinin was detected on the cell surface within 2 hr. If the incubation temperature was raised from 20 degrees C to 37 degrees C, hemagglutinin was quickly externalized, demonstrating that the inhibition at low temperature was reversible.

The bioenergetics of Golgi apparatus function: evidence for an ATP-dependent proton pump

The energy requirement for the processing of newly-synthesized proteins by the Golgi was examined. Rat liver Golgi preparations enriched more than 100-fold have high ATPase activity that co-purified with the Golgi marker enzyme galactosyl transferase. The ATPase activity was 80% inhibited by dicyclohexylcarbodiimide and may represent a proton pump. Evidence is presented for a functional role of the ATPase in Golgi. First, measurement of [14C]methylamine uptake demonstrated ATP-dependent acidification. Second, inhibition of the ATPase with dicyclohexylcarbodiimide resulted in a 3-fold accumulation of newly-synthesized protein in the Golgi.

Biosynthesis, intracellular processing and secretion of haptoglobin in cultured rat hepatocytes

Biosynthesis, intracellular processing and secretion of the hetero-tetrameric (alpha 2 beta 2) glycoprotein, haptoglobin, were studied in primary cultured rat hepatocytes. The results obtained from pulse-chase experiments demonstrated that haptoglobin was initially synthesized as a larger precursor (pro-form), a single polypeptide chain comprising both the alpha- and beta-subunits, and immediately cleaved into subunits during intracellular transport, although about 8% of the newly synthesized haptoglobin was secreted as a pro-form. Monensin which impedes the secretory process at the Golgi complex blocked the complete glycosylation of beta-subunit but rather accelerated the conversion of the pro-form to subunits. These results indicate that the proteolytic processing of the haptoglobin precursor takes place at an early stage before the Golgi complex of the intracellular transport.

Suppressor cells of the human NK activity: characterization of the cells and mechanism of action

The surface marker characteristics and mechanism of action of small- to medium-sized NK suppressor lymphocytes, which can be found in both umbilical cord blood and adult peripheral blood, have been studied. Evidence suggestive of T-cell origin of the lymphocytes consisted of E-rosette formation, reactivity with OKT3 monoclonal antibody, and dot-like acid alpha-naphthyl acetate esterase (ANAE) staining pattern typical of T cells. Furthermore, no reactivity was seen with OKT6 and OKM1 monoclonal antibodies and the presence of intracytoplasmic immunoglobulin was excluded by indirect immunofluorescence microscopy, making the involvement of monocytes, B cells, and thymocytes less likely. As regards the mechanism of action, the role of prostaglandins was unlikely since indomethacin had no effect on the level of suppression. The role of soluble mediators was further examined by blocking cell secretion with monensin. In these experiments monensin treatment of the suppressor cells did not unwind suppression, suggesting that mechanisms other than secretion of suppressive factors were operative. The importance of cell-to-cell contact was demonstrated by the following observations: (i) A short contact of effector lymphocytes with suppressor lymphocytes, followed by their physical separation, resulted in decreased cytotoxic activity of the effector cells. (ii) Suppression could be mediated through Nuclepore filters, which allowed cell processes to pass through the filter, but not through filters which did not allow cell-to-cell contact. The suppressor cells were resistant to irradiation (2500 rad) and treatment with dexamethasone and puromycin. Viable cells were not needed, since paraformaldehyde-fixed suppressor cells could also mediate inhibition of K562 killing.

The Golgi apparatus mediates the transport of phytohemagglutinin to the protein bodies in bean cotyledons

When developing cotyledons of Phaseolus vulgaris L. were labeled with [(3)H]fucose, fucose-labeled phytohemagglutinin (PHA) was found in organelles with average densities of 1.13 g cm(-3) and 1.22 g cm(-3). The position of these organelles on isopycnic sucrose gradients was independent of the presence of MgCl2 and ethylenediaminetetraacetate in the media, indicating that the fucose-labeled PHA was not associated with the rough endoplasmic reticulum (ER). The organelles with a density of 1.13 g cm(-3) were identified as membranes of the Golgi apparatus on the basis of the similarity of their sedimentation properties and those of the Golgi marker enzyme, inosine diphosphatase, in both isopycnic and rate-zonal sucrose gradients. The organelles with a density of 1.22 g cm(-3) were identified as small (0.1-0.4 μm), electron-dense vesicles with a protein content similar to that of the protein bodies. Pulsechase experiments with [(3)H]fucose indicated that fucose-labeled PHA first appeared in the Golgi-apparatus-derived membranes and later in the dense vesicles. Fucose-labeled PHA chased out of the Golgi apparatus first, then out of the dense vesicles, and accumulated in the soluble portion of the homogenate which contained the contents of the broken protein bodies. Fucose-labeled PHA chased out of the two types of organelles with a t 1/2 of 20-30 min, a rate three to four times faster than newly synthesized PHA chases out of the bulk of the ER (Chrispeels, M.J., Bollini, R., 1982, Plant Physiol. 70, 1425-1428). This result indicates that the Golgi apparatus is a much smaller compartment than the ER in the storage parenchyma cells. The sodium ionophore, monensin, which interferes with the function of the Golgi apparatus of animal cells, blocks the biosynthesis and-or transport of fucose- and galactose-labeled macromolecules to the cotyledon cell walls. Monensin also blocks the transport of labeled PHA out of the Golgi apparatus and into the protein bodies. These results provide the first biochemical evidence that a specific storage protein which accumulates in seeds is modified in, and passes through, the Golgi apparatus on its way to the protein bodies.

Monensin inhibits intracellular dissociation of asialoglycoproteins from their receptor

Treatment of short-term monolayer cultures of rat hepatocytes with the proton ionophore, monensin, abolishes asialoglycoprotein degradation, despite little effect of the drug on either surface binding of ligand or internalization of prebound ligand. Centrifuging cell homogenates on Percoll density gradients indicates that, as a result of monensin treatment, ligand does not enter lysosomes but sediments instead in a lower density subcellular fraction that is likely an endocytic vesicle. Analyzing the degree of receptor association of intracellular ligand revealed that monensin prevents the dissociation of the receptor-ligand complex that normally occurs subsequent to endocytosis. The weak base, chloroquine, also blocks this intracellular dissociation. Evidence from sequential substitution experiments is presented, indicating that monensin and chloroquine act at the same point in the sequence of events leading to ligand dissociation. These data are discussed in terms of a pH-mediated dissociation of the receptor-ligand complex within a prelysosomal endocytic vesicle.

The rough endoplasmic reticulum and the Golgi apparatus visualized using specific antibodies in normal and tumoral prolactin cells in culture

Antibodies directed against membrane components of dog pancreas rough endoplasmic reticulum (A-RER) and rat liver Golgi apparatus (A-Golgi) (Louvard, D., H. Reggio, and G. Warren, 1982, J. Cell Biol. 92:92-107) have been applied to cultured rat prolactin (PRL) cells, either normal cells in primary cultures, or clonal GH3 cells. In normal PRL cells, the A-RER stained the membranes of the perinuclear cisternae as well as those of many parallel RER cisternae. The A-Golgi stained part of the Golgi membranes. In the stacks it stained the medial saccules and, with a decreasing intensity, the saccules of the trans side, as well as, in some cells, a linear cisterna in the center of the Golgi zone. It also stained the membrane of many small vesicles as well as that of lysosomelike structures in all cells. In contrast, it never stained the secretory granule membrane, except at the level of very few segregating granules on the trans face of the Golgi zone. In GH3 cells the A-RER stained the membrane of the perinuclear cisternae, as well as that of short discontinuous flat cisternae. The A-Golgi stained the same components of the Golgi zone as in normal PRL cells. In some cells of both types the A-Golgi also stained discontinuous patches on the plasma membrane and small vesicles fusing with the plasma membrane. Immunostaining of Golgi membranes revealed modifications of membrane flow in relation to either acute stimulation of PRL release by thyroliberin or inhibition of basal secretion by monensin.

Effects of monensin on posttranslational processing of myelin proteins

Rat brain slices were incubated with [3H]palmitic acid and [14C]glycine to label the lipid and protein moieties, respectively, of myelin proteolipid protein (PLP). The effects of monensin on posttranslational processing of proteins were examined by measuring the appearance of [14C]glycine- and [3H]palmitate-labeled proteins in myelin and myelin-like fractions. At 0.01 and 0.10 microM, monensin did not appreciably affect total lipid or protein synthesis; higher concentrations caused increased inhibition. Monensin at 0.10 microM markedly decreased the appearance of [14C]glycine-labeled PLP in myelin, but had little effect on the 14C basic proteins or the incorporation of [3H]palmitic acid into total or myelin PLP. The same relative effect was apparent at higher monensin concentrations. In the myelin-like fraction, monensin at 0.10 microM also depressed entry of [14C]glycine into protein comigrating with PLP, and again had no effect on incorporation of [3H]palmitic acid. In addition, monensin increased the [3H]palmitate label associated with two high-molecular-weight proteins in the myelin-like fraction with no concomitant increase in [14C]glycine label.

Inhibition of glycosylation of herpes simplex virus glycoproteins: identification of antigenic and immunogenic partially glycosylated glycopeptides on the cell surface membrane

The surface membranes of cells infected with herpes simplex virus type 1 (HSV-1), strain KOS, contain three principal glycoproteins, gC (apparent Mr 129k), gB (apparent Mr 120k), and gD (apparent Mr 58k). Infections carried out in the presence of the glycosylation inhibitor 2-deoxy-D-glucose result in the loss of the mature species with the concurrent appearance of lower-molecular-weight polypeptides which are presumably partially glycosylated forms of the fully processed glycoproteins. Specific immunoprecipitation of radiolabeled cytoplasmic extracts of 2-deoxy-D-glucose-inhibited infections identified partially glycosylated proteins designated DG92, DG88, and DG53, which are antigenically related to the corresponding mature forms gB, gC, and gD. Cell surface radioiodination, in combination with specific immunoprecipitation, revealed that DG88 and DG53 were the principal species transported to the cell surface in 2-deoxy-D-glucose-inhibited infections. DG92 was readily detected in the cytoplasm but not on the plasma membrane. Cells infected with the KOS mutant, syn LD70, did not synthesize glycoprotein gC. In glycosylation-inhibited syn LD70 infections, DG88 was not detected in either the cytoplasm or plasma membrane, demonstrating a genetic relationship between DG88 and gC. Polyclonal and monoclonal antibodies directed against the glycoproteins gC, gB, and gD sensitized infected cells to complement-mediated immune cytolysis. Cells infected in the presence of the inhibitor were sensitized to lysis only by antibody specific for gC and gD. The glycosylation-inhibited cells were insensitive to immunolysis by anti-gB monoclonal antibody. These findings confirm that the glycosylation-deficient forms of gC and gD, but not gB reach the cell surface in the presence of inhibitor and that the inhibitor-induced alterations in glycosylation do not cause a complete loss of antigenicity. Inoculation of mice with syngeneic 3T3 cells infected in the presence or absence of inhibitor-induced cytolytic and neutralizing antibody. A major portion of the cytolytic antibody was directed against gC, but anti-gC antibody appeared to play a minor role in virus neutralization. While the serum induced by the control infected cells contained precipitating antibodies for gC, gB, and gD, the serum derived from mice inoculated with inhibitor-treated infected cells had only weak immunoprecipitating activity against gB. Together, these findings have identified partially glycosylated forms of the major HSV glycoproteins and show that complete glycosylation is not required for transport of some of these partially glycosylated polypeptides to the cell surface. Moreover, complete glycosylation of the glycopeptides is not essential for maintenance of antigenicity or immunogenicity, indicating that at least some determinants recognized by antibodies directed against the mature glycoproteins are not affected by 2-deoxy-D-glucose-induced carbohydrate alterations.

Glycosylation as a criterion for defining subpopulations of fast-transported proteins

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.

Expression of membrane IGM by a human B lymphoblastoid cell line in the presence of monensin

The effect of the carboxylic ionophore, monensin, on the synthesis and expression of membrane IgM in the human lymphoblastoid cell line, Daudi, was investigated. The normal processing events in the maturation of mu chains and k chains were altered in monensin treated Daudi cells; the immunoglobulin chains did not appear to undergo complete terminal glycosylation modifications. Transport of the glycoprotein to the plasma membrane could be demonstrated indicating that the interference of intracellular processing of the IgM by monensin did not significantly influence the membrane expression of the IgM.

O-linked oligosaccharides are acquired by herpes simplex virus glycoproteins in the Golgi apparatus

The O-linked oligosaccharides on mature forms of herpes simplex virus type 1 (HSV1) glycoproteins were characterized, and were found to account largely for the lower electrophoretic mobilities of these forms relative to the mobilities of immature forms. Other posttranslational modifications of HSV1 glycoproteins (designated gB, gC, gD and gE) were related temporally to the discrete shifts in electrophoretic mobilities that signal acquisition of the O-linked oligosaccharides. Fatty acid acylation (principally of gE) could be detected just prior to the shifts, whereas conversion of high-mannose-type N-linked oligosaccharides to the complex type occurred coincident with the shifts. The addition of O-linked oligosaccharides did not occur in cells treated with the ionophore monensin or in a ricin-resistant cell line defective in the processing of N-linked oligosaccharides. We conclude that extension of O-linked oligosaccharide chains on HSV1 glycoproteins, and probably also attachment of the first O-linked sugars, occurs as a late posttranslational modification in the Golgi apparatus.

Dissection of the Golgi complex. I. Monensin inhibits the transport of viral membrane proteins from medial to trans Golgi cisternae in baby hamster kidney cells infected with Semliki Forest virus

Baby hamster kidney (BHK) cells were infected with Semliki Forest virus (SFV) and, 2 h later, were treated for 4 h with 10 microM monensin. Each of the four to six flattened cisternae in the Golgi stack became swollen and separated from the others. Intracellular transport of the viral membrane proteins was almost completely inhibited, but their synthesis continued and they accumulated in the swollen Golgi cisternae before the monensin block. In consequence, these cisternae bound large numbers of viral nucleocapsids and were easily distinguished from other swollen cisternae such as those after the block. These intracellular capsid-binding membranes (ICBMs) were not stained by cytochemical markers for endoplasmic reticulum (ER) (glucose-6-phosphatase) or trans Golgi cisternae (thiamine pyrophosphatase, acid phosphatase) but were labeled by Ricinus communis agglutinin I (RCA) in thin, frozen sections. Since this lectin labels only Golgi cisternae in the middle and on the trans side of the stack (Griffiths, G., R. Brands, B. Burke, D. Louvard, and G. Warren, 1982, J. Cell Biol., 95:781-792), we conclude that ICBMs are derived from Golgi cisternae in the middle of the stack, which we term medial cisternae. The overall movement of viral membrane proteins appears to be from cis to trans Golgi cisternae (see reference above), so monensin would block movement from medial to the trans cisternae. It also blocked the trimming of the high-mannose oligosaccharides bound to the viral membrane proteins and their conversion to complex oligosaccharides. These functions presumably reside in trans Golgi cisternae. This is supported by data in the accompanying paper, in which we also show that fatty acids are covalently attached to the viral membrane proteins in the cis or medial cisternae. We suggest that the Golgi stack can be divided into three functionally distinct compartments, each comprising one or two cisternae. The viral membrane proteins, after leaving the ER, would all pass in sequence from the cis to the medial to the trans compartment.

Laminin is produced by early rat astrocytes in primary culture

The production of laminin by early rat astrocytes in primary culture was investigated by double immunofluorescence staining for laminin and the glial fibrillary acidic protein (GFAP), a defined astrocyte marker. In early cultures (3 d in vitro; 3 DIV) cytoplasmic laminin was detected in all the GFAP-positive cells which formed the major population (80%) of the nonneuronal cells present in cultures from 20-21-d embryonic, newborn, or 5-d-old rat brains. Monensin treatment (10 microM, 4 h) resulted in accumulation of laminin in the Golgi region, located using labeled wheat germ agglutinin. Laminin started gradually to disappear from the cells with the time in culture, was absent in star-shaped, apparently mature astrocytes, but remained as pericellular matrix deposits. The disappearance of cellular laminin was dependent on the age of the animal and the time in culture so that it started earlier in cultures from 5-d-old rat brains (5 DIV) and approximately following the in vivo age difference in cultures from newborn (12 DIV) and embryonic (14 DIV) rat brains. Our results indicate that laminin is a protein of early astrocytes and also deposited by them in primary culture, thus suggesting a role for this glycoprotein in the development of the central nervous system.

Effects of monensin on photoreceptors of isolated frog retinas

Monensin induces the vacuolization of the Golgi apparatus in photoreceptors of isolated frog retinas and also, more slowly, produces a vacuolization of the pre-synaptic terminals. Accompanying these effects is an inhibition of transport of protein to the outer segment so that the radioactive bands normally detectable by autoradiography do not form. Monensin thus promises to be a useful tool in the study of intracellular transport in photoreceptors. The findings reported here indicate that impairment of the functioning of the Golgi apparatus considerably diminishes transport of membrane protein to the rod outer segment suggesting that passage through the Golgi apparatus is an obligatory step for completion of outer segment membrane or its transport to the outer segment.

Biosynthesis and secretion of fibronectin in human melanoma cells

The biosynthesis and secretion of cellular fibronectin from human melanoma cells have been investigated by pulse-chase/immunoprecipitation analysis. Melanoma cells synthesize endoglycosidase H (Endo H)-sensitive glycoprotein precursors of fibronectin glycoproteins which chase to an Endo H-resistant monomer with an apparent Mr of 240,000 (240 K). This molecule, which has a significantly higher molecular weight than normal plasma or cellular fibronectin, is rapidly secreted by melanoma cells, resulting in the secretion of 80% of newly synthesized fibronectin in 120 min, following a 10-min biosynthetic pulse. This active secretory process can be inhibited by brief exposure of melanoma cells to sodium monensin (10(-7) M), which also results in a modified fibronectin of lower apparent Mr. Monosaccharide-incorporation studies of melanoma fibronectin reveal that monensin significantly inhibits galactose and fucose incorporation into this glycoprotein, correlating with reported effects of monensin on Golgi apparatus functions. These studies indicate that this tumor-associated and biosynthetically altered cellular fibronectin is a rapidly secreted major N-linked glycoprotein of metastatic human melanoma cells.

Effect of monensin on Mason-Pfizer monkey virus glycoprotein synthesis

The effect of the monovalent carboxylic ionophore monensin on the biosynthesis, intracellular transport, and surface expression of the glycoproteins of Mason-Pfizer monkey virus was examined. Cells treated with monensin at concentrations of 10(-7) or 10(-6) M continued to synthesize virus particles, which from electron microscopic studies appeared to bud normally from the plasma membrane of the cells. However, the particles released had an altered buoyant density in sucrose gradients and were noninfectious. These noninfectious virions had a normal complement of non-glycosylated polypeptides but showed a significantly reduced amount of glycosylated proteins. The gp70 and gp20 polypeptides appeared to be completely absent, and a heterogeneous, higher-molecular-weight protein was observed on the virions instead. Studies on intracellular protein synthesis indicated that the precursor (Pr86env) to gp70 and gp20 is synthesized normally but is not cleaved to the mature proteins. Immunofluorescence studies showed, however, that the uncleaved molecule is expressed on the cell surface. In this system, therefore, Mason-Pfizer monkey virus glycoprotein migration appears to occur in the presence of monensin, whereas the cleavage and insertion of the glycoproteins into virions are inhibited.