Localization of glycosylation sites in the Golgi apparatus using immunolabeling and cytochemistry

ST3GAL3 mutations impair the development of higher cognitive functions

The genetic variants leading to impairment of intellectual performance are highly diverse and are still poorly understood. ST3GAL3 encodes the Golgi enzyme β-galactoside-α2,3-sialyltransferase-III that in humans predominantly forms the sialyl Lewis a epitope on proteins. ST3GAL3 resides on chromosome 1 within the MRT4 locus previously identified to associate with nonsyndromic autosomal recessive intellectual disability. We searched for the disease-causing mutations in the MRT4 family and a second independent consanguineous Iranian family by using a combination of chromosome sorting and next-generation sequencing. Two different missense changes in ST3GAL3 cosegregate with the disease but were absent in more than 1000 control chromosomes. In cellular and biochemical test systems, these mutations were shown to cause ER retention of the Golgi enzyme and drastically impair ST3Gal-III functionality. Our data provide conclusive evidence that glycotopes formed by ST3Gal-III are prerequisite for attaining and/or maintaining higher cognitive functions.

Plant N-glycan processing enzymes employ different targeting mechanisms for their spatial arrangement along the secretory pathway

The processing of N-linked oligosaccharides in the secretory pathway requires the sequential action of a number of glycosidases and glycosyltransferases. We studied the spatial distribution of several type II membrane-bound enzymes from Glycine max, Arabidopsis thaliana, and Nicotiana tabacum. Glucosidase I (GCSI) localized to the endoplasmic reticulum (ER), alpha-1,2 mannosidase I (ManI) and N-acetylglucosaminyltransferase I (GNTI) both targeted to the ER and Golgi, and beta-1,2 xylosyltransferase localized exclusively to Golgi stacks, corresponding to the order of expected function. ManI deletion constructs revealed that the ManI transmembrane domain (TMD) contains all necessary targeting information. Likewise, GNTI truncations showed that this could apply to other type II enzymes. A green fluorescent protein chimera with ManI TMD, lengthened by duplicating its last seven amino acids, localized exclusively to the Golgi and colocalized with a trans-Golgi marker (ST52-mRFP), suggesting roles for protein-lipid interactions in ManI targeting. However, the TMD lengths of other plant glycosylation enzymes indicate that this mechanism cannot apply to all enzymes in the pathway. In fact, removal of the first 11 amino acids of the GCSI cytoplasmic tail resulted in relocalization from the ER to the Golgi, suggesting a targeting mechanism relying on protein-protein interactions. We conclude that the localization of N-glycan processing enzymes corresponds to an assembly line in the early secretory pathway and depends on both TMD length and signals in the cytoplasmic tail.

Glycosylation and sorting pathways of lysosomal enzymes in mussel digestive cells

Our aim was to contribute to the understanding of the synthesis, maturation and activation of lysosomal enzymes in an invertebrate cellular model: the endo-lysosomal system (ELS) of mussel digestive cells. The activities of 5'-nucleotidase (AMPase), arylsulphatase (ASase) and acid phosphatase (AcPase), which are transported towards acidic compartments as membrane proteins, were localised by enzyme cytochemistry. AcPase activity was found within large heterolysosomes and residual bodies. ASase was located in endosomes, endolysosomes and heterolysosomes. AcPase and ASase activities were recorded within small vesicles and cisterns of the trans-Golgi network. Conversely, AMPase activity was primarily found in microvilli and apical vesicles and, less conspicuously, in lysosomes and the cis-side of the Golgi and the cis-Golgi network (CGN). In order to understand the processes of synthesis and maturation of these lysosomal enzymes, selected glycoconjugates were localised after lectin cytochemistry. N-acetylglucosamine, mannose and fucose residues were almost ubiquitous in the ELS, as were galactose residues, which were apparently less abundant. N-acetylglucosamine residues occurred in the inner membrane co-localised with mannose residues within the lysosomal and pre-lysosomal acidic compartments. Based on these results, glycosylation and sorting pathways are proposed for both soluble and membrane enzymes. Unlike in mammalian cells, O-glycosylation is fully completed in the CGN, mannose addition in N-glycosylation extends beyond the CGN and galactose addition is fully achieved at the intermediate side. Sorting of soluble lysosomal enzymes, as in crustaceans, is mediated by the indirect transport of membrane-linked proteins with GlcNAc1-P6Man residues that are removed in endolysosomes and heterolysosomes.

Morphodynamics of the secretory pathway

A careful scrutiny of the dynamics of secretory compartments in the entire eukaryotic world reveals many common themes. The most fundamental theme is that the Golgi apparatus and related structures appear as compartments formed by the act of transporting cargo. The second common theme is the pivotal importance for endomembrane dynamics of shifting back and forth the equilibrium between full and perforated cisternae along the pathway. The third theme is the role of a continuous membrane flow in anterograde transfer of molecules from the endoplasmic reticulum through the Golgi apparatus. The last common theme is the self-regulatory balance between anatomical continuities and discontinuities of the endomembrane system. As this balance depends on secretory activity, it provides a source of morphological variability among cell types or, for a given cell type, according to environmental conditions. Beyond this first source of variability, it appears that divergent strategies pave the evolutionary routes in different eukaryotic kingdoms. These divergent strategies primarily affect the levels of stacking, of stabilization, and of clustering of the Golgi apparatus. They presumably underscore a trade-off between versatility and stability to adapt the secretory function to the degree of environmental variability. Nonequilibrium secretory structures would provide yeasts, and plants to a lesser extent, with the required versatility to cope with ever changing environments, by contrast to the stabler milieu intérieur of homeothermic animals.

Structural requirements for Arabidopsis beta1,2-xylosyltransferase activity and targeting to the Golgi

Characterization of a beta1,2-xylosyltransferase from Arabidopsis thaliana (AtXylT) was carried out by expression in Sf9 insect cells using a baculovirus vector system. Serial deletions at both the N- and C-terminal ends proved that integrity of a large domain located between amino acid 31 and the C-terminal lumenal region is required for AtXylT activity expression. The influence of N-glycosylation on AtXylT activity has been evaluated using either tunicamycin or mutagenesis of potential N-glycosylation sites. AtXylT is glycosylated on two of its three potential N-glycosylation sites (Asn51, Asn301, Asn478) and the occupancy of at least one of these two sites (Asn51 and Asn301) is necessary for AtXylT stability and activity. Contribution of the N-terminal part of AtXylT in targeting and intracellular distribution of this protein was studied by expression of variably truncated, GFP-tagged AtXylT forms in tobacco cells using confocal and electron microscopy. These studies have shown that the transmembrane domain of AtXylT and its short flanking amino acid sequences are sufficient to specifically localize a reporter protein to the medial Golgi cisternae in tobacco cells. This study is the first detailed characterization of a plant glycosyltransferase at the molecular level.

The cytoplasmic tail of infectious bronchitis virus E protein directs Golgi targeting

We have previously shown that the E protein of the coronavirus infectious bronchitis virus (IBV) is localized to the Golgi complex when expressed exogenously from cDNA. Here, we report that neither the transmembrane domain nor the short lumenal domain of IBV E is required for Golgi targeting. However, an N-terminal truncation containing only the cytoplasmic domain (CTE) was efficiently localized to the Golgi complex, and this domain could retain a reporter protein in the Golgi. Thus, the cytoplasmic tail of the E protein is necessary and sufficient for Golgi targeting. The IBV E protein is palmitoylated on one or two cysteine residues adjacent to its transmembrane domain, but palmitoylation was not required for proper Golgi targeting. Using C-terminal truncations, we determined that the IBV E Golgi targeting information is present between tail amino acids 13 and 63. Upon treatment with brefeldin A, both the E and CTE proteins redistributed to punctate structures that colocalized with the Golgi matrix proteins GM130 and p115 instead of being localized to the endoplasmic reticulum like Golgi glycosylation enzymes. This suggests that IBV E is associated with the Golgi matrix through interactions of its cytoplasmic tail and may have interesting implications for coronavirus assembly in early Golgi compartments.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Cytochemical images of secretion in Saccharomyces cerevisiae and animal cells are different

Like in animal cells, the major secretory pathway of the ascomycetous budding yeast Saccharomyces (s.) cerevisiae consists of membrane-bound compartments which transport soluble and membrane (glyco)peptides to lysosomal vacuoles, cell wall, or out of the cell. The established model of the cellular machinery of the yeast secretory pathway was deduced largerly from molecular ex situ analyses and for budding yeast cells it was assumed to be identical with that of secretory animal cells. Interphase yeast cells were never considered. Glycosylation of peptides was detected in the endoplasmic reticulum (ER) and the putative Golgi cisternae. Coated membrane vesicles were assumed to transport intermediates into and within the Golgi cascade. Proteolytic trimming would occur in the last Golgi compartment. Golgi-derived membrane vesicles would serve for exocytosis or fuse with lysosomal vacuoles. In contrast to this notion, yeast cytologists showed specific features of secretion in S. cerevisiae and other Ascomycetes. Cytochemical observations in situ of both dividing and interphase yeast showed direct communication between nuclear envelope, ER and segregated Golgi cisternae. A new class of constitutive conveyors, coated protein globules smaller than membrane vesicles, was shown to exist throughout the cell cycle. The function of Golgi-derived membrane vesicles was constrained to promotion of exocytosis in budding yeast. Some of the Golgi apparatus functions were detected in both these classes of exocytotic conveyors. Uptake (phagocytosis) of transport conveyors and lipoprotein condensates has been shown to deliver enzymes and secretory compounds into vacuoles. This simplified machinery of secretion, postulated for S. cerevisiae, does not include the Golgi cascade.

Auxin deprivation induces synchronous Golgi differentiation in suspension-cultured tobacco BY-2 cells

To date, the lack of a method for inducing plant cells and their Golgi stacks to differentiate in a synchronous manner has made it difficult to characterize the nature and extent of Golgi retailoring in biochemical terms. Here we report that auxin deprivation can be used to induce a uniform population of suspension-cultured tobacco (Nicotiana tabacum cv BY-2) cells to differentiate synchronously during a 4-d period. Upon removal of auxin, the cells stop dividing, undergo elongation, and differentiate in a manner that mimics the formation of slime-secreting epidermal and peripheral root-cap cells. The morphological changes to the Golgi apparatus include a proportional increase in the number of trans-Golgi cisternae, a switch to larger-sized secretory vesicles that bud from the trans-Golgi cisternae, and an increase in osmium staining of the secretory products. Biochemical alterations include an increase in large, fucosylated, mucin-type glycoproteins, changes in the types of secreted arabinogalactan proteins, and an increase in the amounts and types of molecules containing the peripheral root-cap-cell-specific epitope JIM 13. Taken together, these findings support the hypothesis that auxin deprivation can be used to induce tobacco BY-2 cells to differentiate synchronously into mucilage-secreting cells.

The mammalian Golgi apparatus during M-phase

The Golgi apparatus in mammalian cells disassembles into several thousand vesicles as cells enter M-phase. Disassembly is dependent on the action of cdc2-kinase and at least two pathways contribute to the fragmentation: One involves the budding of COP-coated vesicles from Golgi cisternae with concomitant inhibition of fusion with their target membranes, the other is a less well characterised COP-independent pathway. During telophase, the Golgi fragments reassemble and fuse into a fully functional Golgi stack, using at least two distinct fusion pathways. The morphological changes of the Golgi apparatus during M-phase offer an ideal system to study how cellular organelles are generated and how their structure is maintained during interphase.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

A cisternal maturation mechanism can explain the asymmetry of the Golgi stack

Morphological data suggest that Golgi cisternae form at the cis-face of the stack and then progressively mature into trans-cisternae. However, other studies indicate that COPI vesicles transport material between Golgi cisternae. These two observations can be reconciled by assuming that cisternae carry secretory cargo through the stack in the anterograde direction, while COPI vesicles transport Golgi enzymes in the retrograde direction. This model provides a mechanism for cisternal maturation. If Golgi enzymes compete with one another for packaging into COPI vesicles, we can account for the asymmetric distribution of enzymes across the stack.

Uptake and incorporation of an epitope-tagged sialic acid donor into intact rat liver Golgi compartments. Functional localization of sialyltransferase overlaps with beta-galactosyltransferase but not with sialic acid O-acetyltransferase

The transfer of sialic acids (Sia) from CMP-sialic acid (CMP-Sia) to N-linked sugar chains is thought to occur as a final step in their biosynthesis in the trans portion of the Golgi apparatus. In some cell types such Sia residues can have O-acetyl groups added to them. We demonstrate here that rat hepatocytes express 9-O-acetylated Sias mainly at the plasma membranes of both apical (bile canalicular) and basolateral (sinusoidal) domains. Golgi fractions also contain 9-O-acetylated Sias on similar N-linked glycoproteins, indicating that O-acetylation may take place in the Golgi. We show here that CMP-Sia-FITC (with a fluorescein group attached to the Sia) is taken up by isolated intact Golgi compartments. In these preparations, Sia-FITC is transferred to endogenous glycoprotein acceptors and can be immunochemically detected in situ. Addition of unlabeled UDP-Gal enhances Sia-FITC incorporation, indicating a substantial overlap of beta-galactosyltransferase and sialyltransferase machineries. Moreover, the same glycoproteins that incorporate Sia-FITC also accept [3H]galactose from the donor UDP-[3H]Gal. In contrast, we demonstrate with three different approaches (double-labeling, immunoelectron microscopy, and addition of a diffusible exogenous acceptor) that sialyltransferase and O-acetyltransferase machineries are much more separated from one another. Thus, 9-O-acetylation occurs after the last point of Sia addition in the trans-Golgi network. Indeed, we show that 9-O-acetylated sialoglycoproteins are preferentially segregated into a subset of vesicular carriers that concentrate membrane-bound, but not secretory, proteins.

Development of electrophoretic conditions for the characterization of protein glycoforms by capillary electrophoresis-electrospray mass spectrometry

A capillary electrophoresis (CE) method using acidic buffers and capillaries coated with Polybrene, a cationic polymer has been developed for the separation of glycoproteins and glycopeptides. Electrophoretic conditions have been optimized to provide resolution of individual glycoforms observed for different glycoprotein preparations. These conditions were found to be entirely compatible with the operation of electrospray mass spectrometry (ESMS), which facilitated the assignments of possible carbohydrate compositions of glycopeptides arising from digests of glycoproteins. By using operating conditions enhanced the formation of oxonium fragment ions prior to mass spectral analysis, selective identification of glycopeptides was achieved for complex samples such as those from proteolytic digests or chemical cleavages. Examples of applications are presented for ribonuclease B, ovalbumin, horseradish peroxidase, and a lectin from Erithrina corallodendron using both CE-ESMS and CE with ultraviolet detection (CE-UV).

Trans-Golgi network (TGN) of different cell types: three-dimensional structural characteristics and variability

BACKGROUND

The trans-Golgi network (TGN) is generally considered as a distinct and permanent structural compartment of the Golgi apparatus of various cell types. To verify this postulate we examined and compared the three-dimensional characteristics of the TGNs of 14 different mammalian cell types as presented in our various publications since 1979 when we initially described the trans-tubular network of Sertoli cells.

METHODS

In all these studies we used low and high voltage electron microscopes on thin or thick sections of tissues fixed with glutaraldehyde and postfixed with reduced osmium. The sections were stained with uranyl acetate and lead citrate. Stereopairs, prepared from photographs of tilted specimens, permitted a direct observation of the three-dimensional structure of the various elements of the Golgi apparatus.

RESULTS

The TGNs are multilayered and extensive in cells which do not form large typical secretory granules (Sertoli cells, nonciliated cells of ductuli efferentes, spinal ganglion cells) but have an extensive lysosomal system. The TGN is absent in cells forming very large secretory granules (secretory cells of seminal vesicles and lactating mammary glands). The TGNs are small in cells producing small to medium-size secretory granules and/or appear as residual fragments on the trans aspect of the Golgi stacks (e.g., mucous cells of Brunner's gland, pancreatic acinar cells, etc.). In cells with multiple and extensive TGNs, a continuity of these tubular networks with the two or three transmost saccules of the stack is observed but there are seemingly no connections between the TGNs. Whenever the TGNs are present, they do not form a continuous structure along the Golgi ribbon. However, they do present, in all cases, configurations suggestive of desquamation and renewal.

CONCLUSIONS

The structure of the TGN varies considerably from one cell type to another, being extensive in cells not showing typical secretory granules but having an extensive lysosomal system, while in secretory cells showing small or large secretory granules the TGN is either small or even entirely absent.

A normal rabbit serum containing Golgi-specific autoantibodies identifies a novel 74-kDa trans-Golgi resident protein

A normal rabbit serum has been identified which contains Golgi-specific autoantibodies. In indirect immunofluorescence experiments the serum was found to stain the juxtanuclear Golgi complex in a variety of cell lines, including human skin fibroblasts, rat osteoblasts, rat myoblasts (L6), baby hamster kidney epithelial cells, and human embryonic kidney cells (293). Thus, the antigen(s) recognized by this serum seems to be well conserved and universally expressed in various mammalian cell types. Immunoelectron microscopy revealed that the epitope resides in the luminal side of the Golgi membranes, and that the antigen is concentrated in the trans-face of the Golgi stacks. In agreement with these results, brefeldin A treatment did not release the antigen from the membranes, but caused its redistribution partly into the endoplasmic reticulum but also into the juxtanuclear area, similarly as with other proteins known to be present in the trans-Golgi cisternae or trans-Golgi network. Our immunoprecipitation studies in human skin fibroblasts demonstrated that the serum recognizes specifically only a single protein with a molecular size of 74 kDa. This protein also cosedimented with a known trans-Golgi-specific marker protein, galactosyltransferase, after fractionation of subcellular organelles by Nycodenz gradient centrifugation. The widespread and polarized expression of this 74-kDa trans-Golgi resident protein suggests that it is required for the late Golgi functions in different mammalian cell types.

Identification of a novel glycosaminoglycan core-like molecule. II. Alpha-GalNAc-capped xylosides can be made by many cell types

The accompanying article (Manzi, A., Salimath, P. V., Spiro, R. C., Keifer, P. A., and Freeze, H. H. (1995) J. Biol. Chem. 270, 9154-9163) reported the complete structure of a novel molecule made by human melanoma cells incubated with 1 mM 4-methylumbelliferyl-beta Xyl (Xyl beta MU). The product resembles a common pentasaccharide core region found in chondroitin/dermatan sulfate glycosaminoglycans, except that a terminal alpha-Gal-NAc residue is found in a location normally occupied by beta-GalNAc in these chains or alpha-GlcNAc in heparan sulfate chains. In this paper we show that several other human cancer cell lines and Chinese hamster ovary cells also make alpha-GalNAc-capped xylosides. The [6-3H]galactose-labeled Xyl beta MU product binds to immobilized alpha-GalNAc-specific lectin from Helix pomatia and the binding is competed by GalNAc, but not by Glc. Binding to the lectin is destroyed by digestion with alpha-N-acetylgalactosaminidase, but not beta-hexosaminidase. The nature of the aglycone influences the amount and relative proportion of this material made, with p-nitrophenyl-beta-xyloside being a better promoter of alpha-GalNAc-terminated product than Xyl beta MU. This novel oligosaccharide accounts for 45-65% of xyloside-based products made by both human melanoma and Chinese hamster ovary cells when they are incubated with 30 microM Xyl beta MU, but at 1 mM both the total amount and the proportion decreases to only 5-10%. In both cell lines this product is replaced by a corresponding amount of Sia alpha 2,3Gal beta 4Xyl beta MU. Preferential synthesis of the alpha-GalNAc-capped material at very low xyloside concentration argues that it is a normal biosynthetic product and not an experimental artifact. This pentasaccharide may be a previously unrecognized intermediate in glycosaminoglycan chain biosynthesis. Since this alpha-GalNAc residue occurs at a position that determines whether chondroitin or heparan chains are added to the acceptor, it may influence the timing, type, and extent of further chain elongation.

Mapping the distribution of Golgi enzymes involved in the construction of complex oligosaccharides

The distribution of beta 1,2 N-acetylglucosaminyltransferase I (NAGT I), alpha 1,3-1,6 mannosidase II (Mann II), beta 1,4 galactosyltransferase (GalT), alpha 2,6 sialyltransferase (SialylT) was determined by immuno-labelling of cryo-sections from HeLa cell lines. Antibody labelling in the HeLa cell line was made possible by stable expression of epitope-tagged forms of these proteins or forms from species to which specific antibodies were available. NAGT I and Mann II had the same distribution occupying the medial and trans cisternae of the stack. GalT and SialylT also had the same distribution but they occupied the trans cisterna and the trans-Golgi network (TGN). These results generalise our earlier observations on the overlapping distribution of Golgi enzymes and show that each of the trans compartments of the Golgi apparatus in HeLa cells contains unique mixtures of those Golgi enzymes involved in the construction of complex, N-linked oligosaccharides.

Subcellular distribution of terminal alpha-D- and beta-D-galactosyl residues in Ehrlich tumour cells studied by lectin-gold techniques

We have studied by high resolution in situ light and electron microscopic lectin-gold techniques the subcellular distribution of alpha-D-Gal residues using the Griffonia simplicifolia I-B4 isolectin and compared it with that of beta-D-Gal residues as detected with the Datura stramonium lectin in Ehrlich tumour cells grown as ascites or monolayer. The microvillar but not the smooth plasma membrane regions were labelled with the Griffonia simplicifolia I-B4 isolectin whereas both plasma membrane regions were equally well labelled with the Datura stramonium lectin. Elements of the endocytotic/lysosomal system such as coated membrane invaginations and vesicles, early and late endosomes and secondary lysosomes were positive for both alpha-D-Gal and beta-D-Gal residues. A particular feature of Ehrlich tumour cells is an elaborate tubular membrane system located in the pericentriolar region which is labelled throughout by both lectins and represents part of the endosomal system. In the Golgi apparatus labelling with both lectins was observed to commence in trans cisternae which is indirect evidence for a joint distribution of the sequentially acting beta 1,4 and alpha 1,3-galactosyl-transferases.

A novel method to co-localize glycosaminoglycan-core oligosaccharide glycosyltransferases in rat liver Golgi. Co-localization of galactosyltransferase I with a sialyltransferase

4-Methylumbelliferyl-beta-xyloside (Xyl beta MU) primes glycosaminoglycan synthesis by first serving as an acceptor for the addition of 2 galactoses and 1 glucuronic acid residue to make the typical core structure, GlcUA beta 1, 3Gal beta 1,3Gal beta 1,4Xyl beta MU. To investigate the relative localization of these biosynthetic enzymes, intact and properly oriented rat liver Golgi preparations were incubated with Xyl beta MU and 1 microM UDP-[3H]Gal and then chased with 5 microM of unlabeled UDP-Gal, UDP-GlcUA, UDP-GlcNAc, UDP-GalNAc, and CMP-Neu5Ac. Under these conditions, no intervesicular transport occurs and acceptor labeling depends entirely upon transporter-mediated delivery of the labeled sugar nucleotides into the lumen of a vesicle and co-localization of the appropriate glycosyltransferases. The labeled products were isolated from the incubation medium and from within the Golgi and their structures analyzed by C18, anion-exchange, and amine adsorption high performance liquid chromatography in combination with glycosidase digestions. Surprisingly, the major products within the Golgi were two sialylated xylosides (Sia alpha 2,3Gal beta 1,4Xyl-beta MU and Sia alpha 2,8Sia alpha 2,3Gal beta 1,4Xyl beta MU) rather than the expected group of partially completed GAG core structures. Less than 10% of the products within the Golgi are the expected core structures containing a second Gal residue or, in addition, GlcUA. The amount of the sialylated products is only partially decreased if the chase is omitted or if the chase is done in the absence of added CMP-Sia, suggesting a pool of previously transported CMP-Sia drives synthesis of the major products. Conversely, when detergent permeabilized vesicles are provided with high concentration of the same sugar nucleotides, the ratio of sialylated products is reduced and replaced by an increase in GAG-like products. These results argue that GAG core-specific Ga1 transferase I and II are not extensively co-localized within the same Golgi compartment. By contrast, glycosaminoglycan core Gal transferase I is substantially co-localized with an alpha-2,3-sialyltransferase and an alpha-2,8-sialyltransferase. Incubating intact Golgi vesicles with exogenous diffusible acceptors offers a novel method to assess the functional co-localization of glycosyltransferases of multiple pathways within the Golgi compartments.

Ultracytochemical evidence of Golgi functions in microvesicles at all phases of cell cycle in Saccharomyces cerevisiae

The topical question of Golgi compartment identity in the ascomycetous yeast Saccharomyces cerevisiae is illustrated by a multiple ultracytochemical approach. For this eucaryotic single-cell organism the established scheme of secretory transport via a cascade of cisternae housing different functions of Golgi apparatus has been deduced principally of genetic and molecular analyses ex situ and confirms the mammalian secretion scheme. Nevertheless, ultracytochemical in situ localizations of enzyme activities engaged in secretion represented evidence for localization of important steps of secretory glycoprotein maturation in two morphologically distinct populations of transport microvesicles formed from endoplasmic reticulum and Golgi cisternae. Both types of microvesicles function in exocytosis or transport into lysosomal vacuoles and have identical charge. However, their presence differs in interphase and in budding cells of S. cerevisiae. Smooth, larger membrane bound microvesicles are conspicuous at the onset of budding and at construction of scars, while the coated, smaller microvesicles of globular ultrastructure are present constitutively, throughout the cell cycle. Because the established model of the yeast secretory path considers only the part of the budding phase preceding the onset of mitosis, an alternative scheme for the cellular mechanism of glycoprotein secretion in S. cerevisiae that distinguishes interphase and budding yeast, has been established. The lumen of microvesicles contains proteases catalysing maturation of the mating pheromone alpha-factor (yscIV, yscF), vacuolar protease yscY, alkaline phosphohydrolase, polyphosphorylated components of the bud scar and glycoproteins. The in situ approach also reveals a minimum level of alpha-factor precursor processing proteolytic activity at the budding phase of cells, a transient presence of polyphosphorylated compounds in the bud scars and their transport by microvesicles. Ultracytochemical reactions suggest that the nuclear envelope lumen houses certain functions attributed to endoplasmic reticulum and that some steps of outer-chain glycosylation may occur in microvesicles. Microvesicles which contain proteases and polyphosphorylated intermediates also appear in juvenile vacuoles (lysosomes). Ultracytochemical findings show the Golgi compartment of S. cerevisiae to consist not only of discrete endoplasmic cisternae, immunodetected by others as sites of outer chain alpha-1,6-mannosylation and of the Golgi membrane marker proteins Sec7p and Ypt1p, but also of microvesicles moving either to the cell plasma membrane or to vacuoles. The previously hypothesized hierarchy of segregated yeast Golgi cisternae was not revealed by ultracytochemical findings.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.

Proteins of the Golgi apparatus. Purification to homogeneity, N-terminal sequence, and unusually large Stokes radius of the membrane-bound form of UDP-galactose:N-acetylglucosamine beta 1-4galactosyltransferase from rat liver

The Golgi marker enzyme, UDP-galactose:N-acetylglucosamine beta 1-4galactosyltransferase (beta 1-4GalT) was purified 44300-fold in its intact, membrane-bound form from rat liver membranes. The protein was isolated from detergent extracts as a high-M(r) form, having a Stokes radius approximating a globular protein of M(r) 440,000. It is comprised of a single protein component as observed on SDS/polyacrylamide gels, having an M(r) near 51,000, and does not have intermolecular disulfide cross-links. N-terminal sequencing of the enzyme demonstrated that it contains an N-terminal hydrophobic stretch deduced previously from cDNA encoding for the enzyme. Previous studies have indicated that the protein may be translated at either of two AUG sites near the 5' end of the mRNA [Russo, R. N., Shaper, N. L. & Shaper, J. H. (1990) J. Biol. Chem. 265, 3324-3331], giving rise to two polypeptides, one appended with 13 amino acids. In the work described here, evidence was only found for the sequence of the short form, missing a single methionine at the N-terminus. Mild proteolytic treatment cleaved the enzyme, giving rise to low-M(r) forms which were fully catalytically active and which, upon sequencing, were missing a 66-amino-acid stretch from the N-terminus (as compared to the mouse cDNA). Proteolytic treatment was accompanied by conversion of the form having a large Stokes radius to one approximating a globular protein with M(r) near 50,000. The N-terminal stretch appears to contribute to maintenance of the form having a large Stokes radius. This may be the result of interaction with a detergent micelle, dimerization or oligomerization, or interaction with some other large, non-protein molecule, although a detergent exchange still resulted in a form having a large Stokes radius.

Cell type-dependent variations in the subcellular distribution of alpha-mannosidase I and II

alpha-mannosidases I and II (Man I and II) are resident enzymes of the Golgi complex involved in oligosaccharide processing during N-linked glycoprotein biosynthesis that are widely considered to be markers of the cis- and medial-Golgi compartments, respectively. We have investigated the distribution of these enzymes in several cell types by immunofluorescence and immunoelectron microscopy. Man II was most commonly found in medial- and/or trans- cisternae but showed cell type-dependent variations in intra-Golgi distribution. It was variously localized to either medial (NRK and CHO cells), both medial and trans (pancreatic acinar cells, enterocytes), or trans- (goblet cells) cisternae, or distributed across the entire Golgi stack (hepatocytes and some enterocytes). The distribution of Man I largely coincided with that of Man II in that it was detected primarily in medial- and trans-cisternae. It also showed cell type dependent variations in its intra-Golgi distribution. Man I and Man II were also detected within secretory granules and at the cell surface of some cell types (enterocytes, pancreatic acinar cells, goblet cells). In the case of Man II, cell surface staining was shown not to be due to antibody cross-reactivity with oligosaccharide epitopes. These results indicate that the distribution of Man I and Man II within the Golgi stack of a given cell type overlaps considerably, and their distribution from one cell type to another is more variable and less compartmentalized than previously assumed.

Combined lectin binding and PAS/alcian blue staining in glycol methacrylate sections

Evaluation of cryofixation and paraffin and glycol methacrylate embedding showed that lectin binding was essentially independent of the embedding medium. Fluorescence intensity increased in the following order: glycol methacrylate, paraffin and cryostat sections. The optical resolution increased in the reverse order. Semi-thin glycol methacrylate sections provided satisfactory fluorescence intensities and the best resolution of all embedding techniques applied. Furthermore the lectin treated sections can be stained further using routine histological or specific histochemical methods. The potassium hydroxide/alcian blue/periodic acid-phenylhydrazine-Schiff method was used successfully to demonstrate sulfated and nonsulfated sialomucins. Lectins combined with mucin histochemistry allowed visualization of specific sugar residues in the same glycol methacrylate plastic section.

The endoplasmic reticulum-Golgi intermediate compartment

The recent identification of an endoplasmic reticulum-Golgi intermediate compartment has added to the complexity of the structural and functional organization of the early secretory pathway. Protein sorting along the endoplasmic reticulum-Golgi pathway depends on different signals and mechanisms, some of which guarantee recycling from various levels of the Golgi apparatus to biosynthetically earlier compartments.

Comparisons of Golgi structure and dynamics in plant and animal cells

The Golgi apparatus of both higher plant and animal cells sorts and packages macromolecules which are in transit to and from the cell surface and to the lysosome (vacuole). It is also the site of oligosaccharide and polysaccharide synthesis and modification. The underlying similarity of function of plant and animal Golgi is reflected in similar morphological features, such as cisternal stacking. There are, however, several fundamental differences between the Golgi of plant and animal cells, reflecting, in large part, the fact that the extracellular matrices and lysosomal systems differ between these kingdoms. These include 1) the form and replication of the Golgi during cell division; 2) the disposition of the Golgi in the interphase cell; 3) the nature of "anchoring" the Golgi in the cytoplasm; 4) the genesis, extent, and nature of membranes at the trans side of the stack; 5) targeting signals to the lysosome (vacuole); and 6) physiological regulation of secretion events (constitutive vs. regulated secretion). The degree of participation of the Golgi in endocytosis and membrane recycling is becoming clear for animal cells, but has yet to be explored in detail for plant cells.