Transmembrane movement of oligosaccharide-lipids during glycoprotein synthesis

The transport of sugar residues into the endoplasmic reticulum (ER) during glycoprotein synthesis was studied by examining the transmembrane orientations of the oligosaccharide-lipid precursors of asparagine-linked oligosaccharides. Using the lectin concanavalin A, the lipid-linked oligosaccharides Man3-5GlcNAc2 were found on the cytoplasmic side of ER-derived vesicles in vitro while lipid-linked Man6-9GlcNAc2 and Glc1-3Man9GlcNAc2 were found facing the lumen. These results suggest that Man5GlcNAc2-lipid is synthesized on the cytoplasmic side of the ER membrane and then translocated to the luminal side. Glc3Man9GlcNAc2-lipid is then completed on the luminal side where it serves as the donor in peptide glycosylation. Translocation of Man5GlcNAc2-lipid offers a mechanism for the export of sugar residues from the cytoplasm during glycoprotein synthesis. This translocation may be the reason for the participation of lipid-linked mono- and oligosaccharides in glycoprotein synthesis.     

Evidence that biosynthesis of phosphatidylethanolamine, phosphatidylcholine, and triacylglycerol occurs on the cytoplasmic side of microsomal vesicles

Experiments were performed to localize the hepatic microsomal enzymes of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol biosynthesis to the cytoplasmic or lumenal surface of microsomal vesicles. Greater than 90 percent of the activities of fatty acid-CoA ligase (EC 6.2.1.3), sn-glycerol 3-phosphate acyltransferase (EC 2.3.1.15), lysophosphatidic acid acyltransferase, diacylglycerol acyltransferase (EC 2.3.1.20), diacylglycerol cholinephosphotransferase (EC 2.7.8.2), and diacylglycerol ethanolaminephosphotransferase (EC 2.7.8.1) was inactivated by proteolysis of intact microsomal vesicles. The phosphatidic acid phosphatase (EC 3.1.3.4) was not inactivated by any of the protease tested. Under conditions employed, <5 percent of the luminal mannose-6-phosphatase (EC 3.1.3.9) activity was lost. After microsomal integrity was disrupted with detergents, protease treatment resulted in a loss of >74 percent of the mannose-6-phosphatase activity. The latency of the mannose-6-phosphatase activity was not affected by protease treatment. Mannose-6-phosphatase latency was not decreased by the presence of the assay components of several of the lipid biosynthetic activities, indicating that those components did not disrupt the microsomal vesicles. None of the lipid biosynthetic activities appeared latent. The presence of a protease-sensitive component of these biosynthetic activities on the cytoplasmic surface of microsomal vesicles, and the absence of latency for any of these biosynthetic activities suggest that the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol occurs asymmetrically on the cytoplasmic surface of the endoplasmic reticulum. The location of biosynthetic activities within the transverse plane of the endoplasmic reticulum is of particular interest for enzymes whose products may be either secreted or retained within the cell. Phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol account for the vast majority of hepatic glycerolipid biosynthesis. The phospholipids are utilized for hepatic membrane biogenesis and for the formation of lipoproteins, and the triacylglycerols are incorporated into lipoproteins or accumulate within the hepatocyte in certain disease states (14). The enzymes responsible for the biosynthesis of these glycerolipids (Scheme I) from fatty acids and glycerol-3P have all been localized to the microsomal subcellular fraction (12, 16, 29, 30). Microsomes are derived from the endoplasmic reticulum and are sealed vesicles which maintain proper sidedness. (11, 22). The external surface of these vesicles corresponds to the cytoplasmic surface of the endoplasmic reticulum. Macromolecules destined for secretion must pass into the lumen of the endoplasmic reticulum (5, 23). Uncharged molecules of up to approximately 600 daltons are able to enter the lumen of rat liver microsomes, but macromolecules and charged molecules of low molecular weight do not cross the vesicle membrane (10, 11). Because proteases neither cross the microsomal membrane nor destroy the permeability barrier of the microsomal vesicles, only the enzymes and proteins located on the cytoplasmic surface of microsomal vesicles are susceptible to proteolysis unless membrane integrity is disrupted (10, 11). By use of this approach, several enzymes and proteins have been localized in the transverse plane of microsomal membranes (11). With the possible exception of cytochrome P 450, all of the enzymes and proteins investigated were localized asymmetrically by the proteolysis technique (11). By studies of this type, as well as by product localization, glucose-6-phosphate (EC 3.1.3.9) has been localized to the luminal surface of microsomal vesicles (11) and of the endoplasmic reticulum (18, 19). All microsomal vesicles contain glucose-6-phosphatase (18, 19) which can effectively utilize mannose-6-P as a substrate, provided the permeability barrier of the vesicles has been disrupted to allow the substrate access to the active site located on the lumenal surface (4). An exact correspondence between mannose- 6-phosphate activity and membrane permeability to EDTA has been established (4). The latency of mannose-6-phosphatase activity provides a quantitative index of microsomal integrity (4.) Few of the microsomal enzymes in the synthesis of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol have been solubilized and/or purified, and little is known about the topography of these enzymes in the transverse or lateral planes of the endoplasmic reticulum. An asymmetric location of these biosynthetic enzymes on the cytoplasmic or lumenal surface of microsomal vesicles may provide a mechanism for regulation of the glycerolipids to be retained or secreted by the cell, and for the biogenesis of asymmetric phospholipid bilayers. In this paper, we report investigations on the localization of all seven microsomal enzymes (Scheme I) in the biosynthesis of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine, using the protease technique with mannose-6-phosphatase serving as luminal control activity. The latency of these lipid biosynthetic enzymes was also investigated, using the latency of mannose-6-phosphatase as an index of microsomal integrity.     

Subcellular and submitochondrial localization of phospholipid-synthesizing enzymes in Saccharomyces cerevisiae.

Using highly enriched membrane preparations from lactate-grown Saccharomyces cerevisiae cells, the subcellular and submitochondrial location of eight enzymes involved in the biosynthesis of phospholipids was determined. Phosphatidylserine decarboxylase and phosphatidylglycerolphosphate synthase were localized exclusively in the inner mitochondrial membrane, while phosphatidylethanolamine methyltransferase activity was confined to microsomal fractions. The other five enzymes tested in this study were common both to the outer mitochondrial membrane and to microsomes. The transmembrane orientation of the mitochondrial enzymes was investigated by protease digestion of intact mitochondria and of outside-out sealed vesicles of the outer mitochondrial membrane. Glycerolphosphate acyltransferase, phosphatidylinositol synthase, and phosphatidylserine synthase were exposed at the cytosolic surface of the outer mitochondrial membrane. Cholinephosphotransferase was apparently located at the inner aspect or within the outer mitochondrial membrane. Phosphatidate cytidylyltransferase was localized in the endoplasmic reticulum, on the cytoplasmic side of the outer mitochondrial membrane, and in the inner mitochondrial membrane. Inner membrane activity of this enzyme constituted 80% of total mitochondrial activity; inactivation by trypsin digestion was observed only after preincubation of membranes with detergent (0.1% Triton X-100). Total activity of those enzymes that are common to mitochondria and the endoplasmic reticulum was about equally distributed between the two organelles. Data concerning susceptibility to various inhibitors, heat sensitivity, and the pH optima indicate that there is a close similarity of the mitochondrial and microsomal enzymes that catalyze the same reaction.     

Biogenesis of endoplasmic reticulum membrane in rat liver cells. II. Discharge of the nascent peptides of NADPH-cytochrome c reductase and cytochrome b5 on the cytoplasmic side of the endoplasmic reticulum membrane.

The direction of discharge of the nascent peptides of NADPH-cytochrome c reductase and cytochrome b5 from bound polyribosomes of rough microsomes was investigated in order to elucidate the mechanism of separation of these membrane proteins from secretory proteins, which are also synthesized by the same class of ribosomes of rough endoplasmic reticulum. The nascent peptides of NADPH-cytochrome c reductase and cytochrome b5 in intact rough microsomes were accessible to externally added 125I-Fab's against these proteins, and were susceptible to trypsin digestion, whereas the nascent peptides of serum albumin were not. The nascent peptides of these two microsomal proteins were released into the cytoplasm by puromycin treatment of intact rough microsomes, while the nascent peptides of serum albumin were retained in the microsomal lumen. These observations suggest that the nascent peptides of microsomal proteins, which are present on the cytoplasmic surface of the endoplasmic reticulum membrane, are exposed on the surface of microsomal vesicles, while those of secretory proteins are enclosed inside the vesicles. Therefore, the topographical separation of microsomal membrane proteins from secretory proteins is accomplished at the step of their synthesis by the bound polyribosomes of rough endoplasmic reticulum.     

Digestive enzyme secretion in Stomoxys calcitrans (Diptera: Muscidae)

Summary Enzyme assays and morphological and histological studies show that the opaque zone midgut cells of the haematophagous fly Stomoxys calcitrans are responsible for the production of proteolytic digestive enzymes and that these are secreted into the gut lumen via membrane bound vesicles (MBV). The secretory cycle can be summarized as follows; initially the rough endoplasmic reticulum is stacked and the apices of the cells are packed with MBV. This is followed by a period of release characterized first by cytoplasmic extrusions containing high densities of MBV, then by microvesiculation of the microvilli combined with a progressive distribution of rough endoplasmic reticulum and lightening of the cellular cytoplasm. Glycogen appears in the cells at this stage and is gradually lost as the rough endoplasmic reticulum becomes stacked once more and the numbers of MBV build up again. The cycle which occurs regularly and synchronously in the cells of the zone repeats itself many times up to the completion of digestion of the blood meal. The secretory cycle is discussed with reference to activity in other secretory tissues.The author is indebted to the Science Research Council for financial support     

Direct Formation of Vaccinia Virus Membranes from the Endoplasmic Reticulum in the Absence of the Newly Characterized L2-Interacting Protein A30.5

Crescents consisting of a single lipoprotein membrane with an external protein scaffold comprise the initial structural elements of poxvirus morphogenesis. Crescents enlarge to form spherical immature virions, which enclose viroplasm consisting of proteins destined to form the cores of mature virions. Previous studies suggest that the L2 protein participates in the recruitment of endoplasmic reticulum (ER)-derived membranes to form immature virions within assembly sites of cytoplasmic factories. Here we show that L2 interacts with the previously uncharacterized 42-amino-acid A30.5 protein. An open reading frame similar in size to the one encoding A30.5 is at the same genome location in representatives of all chordopoxvirus genera. A30.5 has a putative transmembrane domain and colocalized with markers of the endoplasmic reticulum and with L2. By constructing a complementing cell line expressing A30.5, we isolated a deletion mutant virus that exhibits a defect in morphogenesis in normal cells. Large electron-dense cytoplasmic inclusions and clusters of scaffold protein-coated membranes that resemble crescents and immature virions devoid of viroplasm were seen in place of normal structures. Crescent-shaped membranes were continuous with the endoplasmic reticulum membrane and oriented with the convex scaffold protein-coated side facing the lumen, while clusters of completed spherical immature-virion-like forms were trapped within the expanded lumen. Immature-virion-like structures were more abundant in infected RK-13 cells than in BS-C-1 or HeLa cells, in which cytoplasmic inclusions were decorated with scaffold protein-coated membrane arcs. We suggest that the outer surface of the poxvirus virion is derived from the luminal side of the ER membrane.     

Experimental Determination of the Membrane Topology of the Plasmodium Protease Plasmepsin V

The malaria parasite exports hundreds of proteins into its host cell. The majority of exported proteins contain a Host-Targeting motif (also known as a Plasmodium export element) that directs them for export. Prior to export, the Host-Targeting motif is cleaved by the endoplasmic reticulum-resident protease Plasmepsin V and the newly generated N-terminus is N-α-acetylated by an unidentified enzyme. The cleaved, N-α-acetylated protein is trafficked to the parasitophorous vacuole, where it is translocated across the vacuole membrane. It is clear that cleavage and N-α-acetylation of the Host-Targeting motif occur at the endoplasmic reticulum, and it has been proposed that Host-Targeting motif cleavage and N-α-acetylation occur either on the luminal or cytosolic side of the endoplasmic reticulum membrane. Here, we use self-associating ‘split’ fragments of GFP to determine the topology of Plasmepsin V in the endoplasmic reticulum membrane; we show that the catalytic protease domain of Plasmepsin V faces the endoplasmic reticulum lumen. These data support a model in which the Host-Targeting motif is cleaved and N-α-acetylated in the endoplasmic reticulum lumen. Furthermore, these findings suggest that cytosolic N-α-acetyltransferases are unlikely to be candidates for the N-α-acetyltransferase of Host-Targeting motif-containing exported proteins.     

Heterogeneous distribution of glycosyltransferases in the endoplasmic reticulum of castor bean endosperm

Endoplasmic reticulum membranes stripped of attached ribosomes were isolated from homogenates of germinating castor bean (Ricinus communis L.) endosperm by sucrose density gradient centrifugation. The isolated endoplasmic reticulum fraction was further separated into two major membrane subfractions by centrifugation on a flotation gradient. Both subfractions appeared to be derived from the endoplasmic reticulum inasmuch as they share several enzymic markers including cholinephosphotransferase, NADH-cytochrome c reductase, and glycoprotein fucosyl-transferase and phase separation of membrane polypeptides using Triton X-114 revealed a striking similarity in both their hydrophilic and hydrophobic protein components. The endoplasmic reticulum membrane subfractions contain glycoproteins which were readily labeled by incubating intact endosperm tissue with radioactive sugars prior to fractionation.

Castor bean endosperm endoplasmic reticulum apparently exhibits a degree of enzymic heterogeneity, however, since the enzymes responsible for the synthesis of dolicholpyrophosphate N-acetylglucosamine and dolicholmonophosphate mannose together with their incorporation into the oligosaccharide-lipid precursor of protein N-glycosylation were largely recovered in a single endoplasmic reticulum subfraction.

     

Topography of glycerolipid synthetic enzymes. Synthesis of phosphatidylserine, phosphatidylinositol and glycerolipid intermediates occurs on the cytoplasmic surface of rat liver microsomal vesicles

The topography of glycerolipid biosynthetic enzymes within the transverse plane of rat liver microsomal vesicles was investigated: (1) by use of the impermeant inhibitor, mercury-dextran; (2) by use of proteases; and (3) by determining whether the enzyme activities are latent. The seven enzyme activities investigated (dihydroxyacetone-phosphate acyltransferase, acyldihydroxyacetone-phosphate oxidoreductase, phosphatidic acid : CTPcytidyltransferase, CDPdiacylglycerol : inositol phosphatidyltransferase, 2-monoacylglycerol acyltransferase, diacylglycerol kinase, and the serine base exchange enzyme) function in phosphatidylinositol and phosphatidylserine synthesis and at intermediate levels in glycerolipid synthesis including steps of ether lipid synthesis. Mercury-dextran inhibited four of these enzymes greater than 60% in intact microsomal vesicles. One or more of the proteases employed (chymotrypsin, trypsin and pronase) inactivated each of the seven enzyme activities in intact microsomal vesicles. These two approaches indicate that each of these enzymes has important domains located on the cytoplasmic surface of microsomal vesicles. These enzyme activities could be assayed in intact microsomal vesicles. None appeared to be highly latent, indicating that substrates have free access to active sites. One substrate for each of these enzymes had been shown previously to be unable to cross the microsomal membrane. These data indicate that the active sites of these enzymes are located on the cytoplasmic surface of microsomal vesicles. It is concluded that the synthesis of phosphatidylserine and phosphatidylinositol, intermediates of ether lipid formation and other intermediates of glycerolipid synthesis occur asymmetrically on the cytoplasmic surface of the endoplasmic reticulum. These findings and our previous investigations on the topography of seven enzymes of triacylglycerol, phosphatidylcholine and phosphatidylethanolamine biosynthesis (Ballas, L.M. and Bell, R.M., Biochim. Biophys. Acta 602, (1980) 578-590) indicate that the synthesis of the major cellular glycerolipids occurs asymmetrically on the cytoplasmic surface of the endoplasmic reticulum.     

Effect of anion-specific inhibitors on the utilization of sugar nucleotides for N-linked carbohydrate unit assembly by thyroid endoplasmic reticulum vesicles

The effect of anion-specific inhibitors on the utilization of the sugar nucleotides (UDP-glucose, GDP-mannose, and UDP-N-acetylglucosamine) required for the formation of the oligosaccharide-lipid involved in N-glycosylation has been studied in intact endoplasmic reticulum (ER) vesicles from thyroid. Of the reagents tested, the nonpenetrating probe DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid) and its dihydro derivative (H2DIDS) were the most effective, causing a pronounced impairment in the synthesis from UDP-Glc of dolichyl phosphate (Dol-P) glucose (50% reduction at 60 microM DIDS) and in the incorporation of glucose into oligosaccharide-lipid and N-glycosylated protein; in contrast, no inhibition was observed in the formation from UDP-Glc of a glycogen-like proteoglucan. The specificity of the DIDS effect was indicated by the finding that methyl isothiocyanate, a nonanionic amino-reactive agent, demonstrated negligible inhibition. While DIDS also effected a block in the formation of Dol-P-P-GlcNAc from UDP-GlcNAc, no impairment in the utilization of GDP-Man for Dol-P-Man synthesis was observed. Since the DIDS inhibition of UDP-Glc and UDP-GlcNAc utilization was maintained after disruption of the ER vesicles with Triton, even when the incubations were supplemented with Dol-P, it appears that this reagent does not interact with sugar nucleotide translocator proteins but rather with the cytoplasmically oriented anion binding sites of glycosyltransferases (UDP-Glc- and UDP-GlcNAc:Dol-P glucosyl- and GlcNAc-1-P transferases). This is consistent with the protease sensitivity of these enzymes in the intact ER vesicles. Incubation of the vesicles with tritiated H2DIDS (8 microM) introduced radioactivity into membrane polypeptides with molecular weights of about 52,000 and 31,000 as observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that this inhibitor may prove useful as an affinity label in further studies of some of the glycosyltransferases involved in the synthesis of lipid-monosaccharide intermediates.     

Cell-Free Transfer of Phosphatidylinositol between Membrane Fractions Isolated from Soybean

Transfer of phosphatidylinositol (PI) between membranes was reconstituted in a cell-free system using membrane fractions isolated from dark-grown soybean (Glycine max [L.] Merr.). Donor membrane vesicles contained [3H]myo-inositol-labeled PI. A fraction enriched in endoplasmic reticulum was a more efficient donor than its parent microsomal membrane fraction. As acceptor, cytoplasmic side-out plasma membrane vesicles were more efficient than cytoplasmic side-in plasma membrane vesicles. Endoplasmic reticulum was also an efficient acceptor, suggesting that transfer occurred to cytoplasmic membrane leaflets. PI transfer was time and temperature dependent but did not require cytosolic proteins, ATP, GTP, cytosol, and acyl-coenzyme A. These results suggest that neither lipid transfer proteins nor transition vesicles, similar to those involved in vesicle trafficking from endoplasmic reticulum to the Golgi apparatus, were involved. In the presence of Mg2+ and ATP, endoplasmic reticulum PI was not metabolized, whereas PI transferred to the plasma membrane was metabolized into phosphatidylinositol monophosphate and phosphatidylinositol bisphosphate. To summarize, the cell-free transfer of endoplasmic reticulum-derived PI was distinct from, for example, vesicle transport from endoplasmic reticulum to Golgi apparatus, not only in its regulation but also in its acceptor unspecificity.     

A protein-conducting channel in the endoplasmic reticulum

The existence of a protein-conducting channel in the endoplasmic reticulum membrane was demonstrated by electrophysiological techniques. Pancreatic rough microsome (RM) vesicles were fused to one side (cis) of a planar lipid bilayer separating two aqueous compartments of 50 mM salt. This exposed the cytoplasmic surface of the RMs, with its attached ribosomes, to the cis chamber. Addition of 100 microM puromycin to the cis side caused a large increase in membrane conductance, presumably the result of puromycin-induced clearance of nascent protein chains from the lumen of protein-conducting channels. When puromycin was added at low concentrations (0.33 microM), single channels of 220 pS were observed. These closed when the salt concentration was raised to levels at which ribosomes detach from the membrane (150-400 mM), indicating that the attached ribosome keeps the channel in an open conformation. A mechanism for a complete cycle of opening and closing of the protein-conducting channel is suggested.     

Genetic and biochemical evaluation of eucaryotic membrane protein topology: multiple transmembrane domains of Saccharomyces cerevisiae 3-hydroxy-3-methylglutaryl coenzyme A reductase.

Both 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase isozymes of the yeast Saccharomyces cerevisiae are predicted to contain seven membrane-spanning domains. Previous work had established the utility of the histidinol dehydrogenase protein domain, encoded by HIS4C, as a topologically sensitive monitor that can be used to distinguish between the lumen of the endoplasmic reticulum and the cytoplasm. This study directly tested the structural predictions for HMG-CoA reductase by fusing the HIS4C domain to specific sites in the HMG-CoA reductase isozymes. Yeast cells containing the HMG-CoA reductase-histidinol dehydrogenase fusion proteins grew on histidinol-containing medium if the HIS4C domain was present on the cytoplasmic side of the endoplasmic reticulum membrane but not if the HIS4C domain was targeted to the endoplasmic reticulum lumen. Systematic exchanges of transmembrane domains between the isozymes confirmed that both isozymes had equivalent membrane topologies. In general, deletion of an even number of putative transmembrane domains did not interfere with the topology of the protein, but deletion or duplication of an odd number of transmembrane domains inverted the orientation of the protein. The data confirmed the earlier proposed topology for yeast HMG-CoA reductase, demonstrated that the yeast enzymes are core glycosylated, and provided in vivo evidence that the properties of transmembrane domains were, in part, dependent upon their context within the protein.     

百合花粉母细胞间染色质穿壁运动前次生胞间连丝形成的研究

百合花粉母细胞间染色质穿壁运动前(细线期到偶线期)的花药,用一般电镜制片法和铅沉淀法对酸性磷酸酶活性的细胞化学反应产物的定位实验,其结果总结如下:(1)形成次生胞间连丝通道水解作用所需的酶可能是由“类溶酶体”小泡或由内质网腔直接分泌的;(2)次生胞间连丝通道的水解作用,可在细胞壁的两边细胞同时开始,先形成半胞间连丝,然后贯穿??在一起;或从一侧开始,一直穿孔到另一边,最后两者都能形成胞间连丝;(3)用铅沉淀法进行的酸性磷酸酶细胞化学的定位实验表明:在质膜、内质网、类溶酶体小泡中的酶活性反应产物沉积的部位与一般电镜法制备的切片上看到的电子致密度物质的分布情况完全一致,(4)用X-射线微区能谱分析的结果表明:沉淀物中含有铅元素,确实是磷酸铅。因此我们推测所谓“类溶酶体”以及内质网所分泌的水解酶,可能具有果胶酶、纤维素酶和半纤维素酶的性质,它们都能降解、穿孔各自的细胞壁形成胞间连丝。     

Distribution of newly synthesized DT-diaphorase in rat liver

The distribution, synthesis transport, and glycosylation of rat-liver DT-diaphorase has been investigated. The enzyme could be isolated using specific antibodies, mainly from the soluble supernatant but also from microsomal vesicles, Golgi membrane, and mitochondria. 40% of the microsomal enzyme was located in the lumen or on the interior side of the membrane, the rest remaining as an integral non-extractable part of the membrane. Synthesis of DT-diaphorase takes place on both free and bound ribosomes, although it was found to be transported in a sequential manner from the rough to the smooth endoplasmic reticulum and also subsequently to the mitochondria. The rough and smooth microsomal DT-diaphorase contains covalently bound carbohydrate, but no sugar moiety could be detected bound to the cytoplasmic form of the enzyme.     

Electron microscopic studies of the epithelial reticular cells of the mouse thymus

Summary Electron microscopic studies have been made of the epithelial reticular cells of the thymus in mice of both sexes ranging in age from 5 to 8 weeks. The epithelial cells generally have long cytoplasmic processes by which they are interconnected and form a network throughout the organ. The processes adhere tightly to one another by desmosomes. At the surface of the organ the processes constitute a thin sheet, and a basement membrane is discernible close and parallel to the free surface of the epithelial sheet. In the cortex the meshes of the epithelial reticulum are filled with numerous lymphoid cells and relatively few mesenchymal reticular cells. The epithelial cells in the cortex are characterized by their slender cytoplasmic processes and by the presence of large round vesicles which contain coarsely granulated, dense material. By the presence of the vesicles as well as desmosomes at junctions of the cytoplasmic processes the epithelial cells can be distinguished from other cells. For comparison the cytological characteristics of the mesenchymal reticular cells are also described. In the medulla two types — reticular and hypertrophic — of epithelial cells are recognized. The cells of reticular type are irregularly stellated in shape with extended cytoplasmic processes. Their cytoplasm often contains considerable amounts of fine filaments in bundles. Due to the relative abundance of free ribonucleoprotein particles and other cytoplasmic components, the cytoplasm appears relatively electronopaque as compared with that of the cells of the other type. The plasma membrane of the cells of reticular type sometimes invaginates into the cytoplasm to enclose a lumen which contains substance of low density and sometimes fine filaments. A basement membrane-like layer is discernible close to the infolded plasma membrane in the lumen. The cells of hypertrophic type are relatively large and round with a few shorter cytoplasmic processes. They are characterized by the abundance of the smooth endoplasmic reticulum which appears as vesicle or sac of small size. These cells often possess peculiar vesicles the wall of which is provided with microvilli projecting into the lumen. Some of these vesicles carry cilia on their wall in addition to the microvilli. The cells of hypertrophic type often undergo degeneration. The degenerating cells are concentrically surrounded by a few neighboring cells of both hypertrophic and reticular types, and Hassall's corpuscles are formed.     

Signal peptides open protein-conducting channels in E. coli.

Plasma membrane vesicles and protoplasts of Escherichia coli were fused to planar lipid bilayers and studied with electrophysiological techniques. Large transmembrane aqueous channels were opened when 0.2 nM LamB signal peptide was added to the cytoplasmic side of the membrane. These aqueous pores are similar in conductance to those previously observed in mammalian endoplasmic reticulum when puromycin is used to release and thus unplug nascent translocating chains. Signal sequences have been previously shown to be necessary and sufficient for targeting proteins to cellular membranes. These results demonstrate that signal peptides are sufficient for opening the protein-conducting channels. We suggest that they are the physiological ligands that open protein-conducting channels at the initiation of protein translocation across prokaryotic plasma membrane and mammalian endoplasmic reticulum.     

Thematic review series: lipid posttranslational modifications. GPI anchoring of protein in yeast and mammalian cells, or: how we learned to stop worrying and love glycophospholipids

Glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins is the most complex and metabolically expensive of the lipid posttranslational modifications described to date. The GPI anchor is synthesized via a membrane-bound multistep pathway in the endoplasmic reticulum (ER) requiring >20 gene products. The pathway is initiated on the cytoplasmic side of the ER and completed in the ER lumen, necessitating flipping of a glycolipid intermediate across the membrane. The completed GPI anchor is attached to proteins that have been translocated across the ER membrane and that display a GPI signal anchor sequence at the C terminus. GPI proteins transit the secretory pathway to the cell surface; in yeast, many become covalently attached to the cell wall. Genes encoding proteins involved in all but one of the predicted steps in the assembly of the GPI precursor glycolipid and its transfer to protein in mammals and yeast have now been identified. Most of these genes encode polytopic membrane proteins, some of which are organized in complexes. The steps in GPI assembly, and the enzymes that carry them out, are highly conserved. GPI biosynthesis is essential for viability in yeast and for embryonic development in mammals. In this review, we describe the biosynthesis of mammalian and yeast GPIs, their transfer to protein, and their subsequent processing.     

Membrane-bound apolipoprotein B is exposed at the cytosolic surface of liver microsomes.

We have used a competitive enzyme-linked immunoassay with a panel of monoclonal antibodies to probe the topography of the membrane-bound form of apolipoprotein B (apo B) in rabbit microsomes. All epitopes investigated were found to be expressed at the cytosolic side of the microsomal membrane under conditions in which the vesicles remained sealed. These results indicate that the membrane-associated form of apolipoprotein B is either at the cytosolic side of the endoplasmic reticulum membrane or integrated into the membrane. From this site apo B may be translocated to the lumen for assembly into VLDL or may be degraded.     

Stereospecificity of NADH-ferricyanide reductase is a convenient marker for the endoplasmic reticulum of plant cells

The stereospecificity of NADH-ferricyanide reductase and NADH-cytochrome c reductase in the endoplasmic reticulum (ER) for the α-hydrogen on the nicotinamide ring is presented as a very sensitive and convenient assay to detect ER contamination in preparations of membranes lacking α-specific NADH-acceptor reductase, such as the plasma membrane and the tonoplast. The experimental details of the assay are given and the limitations explored (time-course, amount of protein, possible side reactions, speed, reproducibility, etc.). The NADH-ferricyanide reductase activity of plasma membranes from spinach and sugarbeet leaf was completely β-specific and always showed a latency (increase upon addition of Triton X-100), whereas the α-specificity in the ER was non-latent. This is consistent with the presence of mainly right-side-out vesicles in preparations of plasma membranes with the binding site for NADH and ferricyanide on the inner, cytoplasmic surface. In contrast, right-side-out ER vesicles have the binding site on the outer, cytoplasmic surface. The addition of as little as 1% of the α-specific ER (on an NADH-ferricyanide activity basis) to the spinach leaf plasma membrane could be detected with the stereospecificity assay. Wheat root plasma membrane showed some α-specificity (in addition to β-specificity) which was probably due to ER contamination since the activity was non-latent. The stereospecificity assay is also shown to be useful in monitoring the separation of tonoplast vesicles from ER vesicles by countercurrent distribution of a light microsomal fraction. It follows that the NADH-acceptor reductase activities in preparations of plasma membrane and tonoplast are due to distinct enzymes characteristic for those membranes.