Erythrocyte membrane protein band 3: its biosynthesis and incorporation into membranes

Band 3, a transmembrane protein that provides the anion channel of the erythrocyte plasma membrane, crosses the membrane more than once and has a large amino terminal segment exposes on the cytoplasmic side of the membrane. The biosynthesis of band 3 and the process of its incorporation into membranes were studied in vivo in erythroid spleen cells of anemic mice and in vitro in protein synthesizing cell-free systems programmed with polysomes and messenger RNA (mRNA). In intact cells newly synthesized band 3 is rapidly incorporated into intracellular membranes where it is glycosylated and it is subsequently transferred to the plasma membrane where it becomes sensitive to digestion by exogenous chymotrypsin. The appearance of band 3 in the cell surface is not contingent upon its glycosylation because it proceeds efficiently in cells treated with tunicamycin. The site of synthesis of band 3 in bound polysomes was established directly by in vitro translation experiments with purified polysomes or with mRNA extracted from them. The band-3 polypeptide synthesized in an mRNA- dependent system had the same electrophoretic mobility as that synthesized in cells treated with tunicamycin. When microsomal membranes were present during translation, the in vitro synthesized band-3 polypeptide was cotranslationally glycosylated and inserted into the membranes. This was inferred from the facts that when synthesis was carried out in the presence of membranes the product had a lower electrophoretic mobility and showed partial resistance to protease digestion. Our observations indicate that the primary translation product of band-3 mRNA is not proteolytically processed either co- or posttranslationally. It is, therefore, proposed that the incorporation of band 3 into the endoplasmic reticulum (ER) membrane is initiated by a permanent insertion signal. To account for the cytoplasmic exposure of the amino terminus of the polypeptide we suggest that this signal is located within the interior of the polypeptide. a mechanism that explains the final transmembrane disposition of band 3 in the plasma membrane as resulting from the mode of its incorporation into the ER is presented.     

Evidence against an essential role of COPII-mediated cargo transport to the endoplasmic reticulum-Golgi intermediate compartment in the formation of the primary membrane of vaccinia virus

Vaccinia virus assembles two distinct lipoprotein membranes. The primary membrane contains nonglycosylated proteins, appears as crescents in the cytoplasm, and delimits immature and mature intracellular virions. The secondary or wrapping membrane contains glycoproteins, is derived from virus-modified trans-Golgi or endosomal cisternae, forms a loose coat around some intracellular mature virions, and becomes the envelope of extracellular virions. Although the mode of formation of the wrapping membrane is partially understood, we know less about the primary membrane. Recent reports posit that the primary membrane originates from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). According to this model, viral primary membrane proteins are cotranslationally inserted into the ER and accumulate in the ERGIC. To test the ERGIC model, we employed Sar1(H79G), a dominant negative form of the Sar1 protein, which is an essential component of coatomer protein II (COPII)-mediated cargo transport from the ER to the ERGIC and other post-ER compartments. Overexpression of Sar1(H79G) by transfection or by a novel recombinant vaccinia virus with an inducible Sar1(H79G) gene resulted in retention of ERGIC 53 in the ER but did not interfere with localization of viral primary membrane proteins in factory regions or with formation of viral crescent membranes and infectious intracellular mature virions. Wrapping of intracellular mature virions and formation of extracellular virions did not occur, however, because some proteins that are essential for the secondary membrane were retained in the ER as a consequence of Sar1(H79G) overexpression. Our data argue against an essential role of COPII-mediated cargo transport and the ERGIC in the formation of the viral primary membrane. Instead, viral membranes may be derived directly from the ER or by a novel mechanism.     

Heat shock-induced changes in lipid and protein metabolism in the endoplasmic reticulum of barley aleurone layers

Heat shock in barley aleurone layers induces heat shock protein synthesis and suppresses secretory protein synthesis by selectively destabilizing their mRNAs. In addition, the endoplasmic reticulum (ER) membranes upon which secretory protein mRNAs are translated become vesiculated during heat shock, leading to the hypothesis that ER dissociation and targeted mRNA destabilization are linked mechanistically. Supporting this, ER can be heat adapted, and heat-adapted ER has higher levels of fatty acid saturation in membrane phospholipids which do not vesiculate upon heat shock. Secretory protein mRNAs are also more stable in heat-adapted cells. To understand better heat shock-induced changes in ER membranes, we examined ER membrane proteins and enzymes involved in phosphatidylcholine biosynthesis and phospholipid turnover in heat-shocked aleurone cells. Heat shock significantly increased the activity of phospholipases A2 and D, and shortly thereafter significant but gradual increases in choline kinase and phosphocholine glyceride transferase activities and a sharp increase in phosphorylcholine citidyl transferase activity were observed. Only minor changes were observed in SDS-PAGE analyses of proteins from sonicated ER membranes fractionated on continuous sucrose gradients. Overall, heat shock reduced total lipid in ER membranes relative to protein, and in intact, ultracentrifuged aleurone cells examined by light and electron microscopy the ER band appeared to increase in density. The changes in phospholipid metabolism coupled with the suppression of secretory protein synthesis indicate that in addition to inducing a classic heat shock response, high temperature also induces a classic unfolded protein response in the ER of this secretory cell.     

The endoplasmic reticulum membrane is permeable to small molecules

The lumen of the endoplasmic reticulum (ER) differs from the cytosol in its content of ions and other small molecules, but it is unclear whether the ER membrane is as impermeable as other membranes in the cell. Here, we have tested the permeability of the ER membrane to small, nonphysiological molecules. We report that isolated ER vesicles allow different chemical modification reagents to pass from the outside into the lumen with little hindrance. In permeabilized cells, the ER membrane allows the passage of a small, charged modification reagent that is unable to cross the plasma membrane or the lysosomal and trans-Golgi membranes. A larger polar reagent of approximately 5 kDa is unable to pass through the ER membrane. Permeation of the small molecules is passive because it occurs at low temperature in the absence of energy. These data indicate that the ER membrane is significantly more leaky than other cellular membranes, a property that may be required for protein folding and other functions of the ER.     

Role of the nuclear envelope in synthesis, processing, and transport of membrane glycoproteins

The outer nuclear membrane is morphologically similar to rough endoplasmic reticulum. The presence of ribosomes bound to its cytoplasmic surface suggests that it could be a site of synthesis of membrane glycoproteins. We have examined the biogenesis of the vesicular stomatitis virus G protein in the nuclear envelope as a model for the biogenesis of membrane glycoproteins. G protein was present in nuclear membranes of infected Friend erythroleukemia cells immediately following synthesis and was transported out of nuclear membranes to cytoplasmic membranes with a time course similar to transport from rough endoplasmic reticulum (t 1/2 = 5-7 min). Temperature-sensitive mutations in viral membrane proteins which block transport of G protein from endoplasmic reticulum also blocked transport of G protein from the nuclear envelope. Friend erythroleukemia cells and NIH 3T3 cells differed in the fraction of newly synthesized G protein found in nuclear membranes, apparently reflecting the relative amount of nuclear membrane compared to endoplasmic reticulum available for glycoprotein synthesis. Nuclear membranes from erythroleukemia cells appeared to have the enzymatic activities necessary for cleavage of the signal sequence and core glycosylation of newly synthesized G protein. Signal peptidase activity was detected by the ability of detergent-solubilized membranes of isolated nuclei to correctly remove the signal sequence of human preplacental lactogen. RNA isolated from the nuclear envelope was highly enriched for G protein mRNA, suggesting that G protein was synthesized on the outer nuclear membrane rather than redistributing to nuclear membranes from endoplasmic reticulum before or during cell fractionation. These results suggest a mechanism for incorporation of membrane glycoproteins into the nuclear envelope and suggest that in some cell types the nuclear envelope is a major source of newly synthesized membrane glycoproteins.     

Retention of mRNA on the endoplasmic reticulum membranes after in vivo disassembly of polysomes by an inhibitor of initiation

Membrane-bound ribosomes and messenger RNA remained associated with the microsomal membranes of human fibroblasts after cultures were treated with Verrucarin A, an inhibitor of initiation which led to extensive run-off of ribosomes from polysomal structures. When a membrane fraction from Verrucarin-treated cells containing such inactive ribosomes and mRNA was suspended in a medium of high salt concentration, extensive release of ribosomal subunits occurred without the need for puromycin. The mRNA nevertheless remained associated with the membranes. These results add support to the conclusion that, in human fibroblasts, mRNA is bound directly to ER membranes, independently of the ribosomes and nascent polypeptide chains.     

Density of newly synthesized plasma membrane proteins in intracellular membranes. I. Stereological studies

As the spike proteins of Semliki Forest virus (SFV) pass from their site of synthesis in the endoplasmic reticulum (ER) to the cell surface, they must be concentrated and freed from endogenous proteins. To determine the magnitude of this sorting process we have measured the density of spike proteins in membranes of the intracellular transport pathway. In this first paper, using stereological procedures, we have estimated the surface areas of the ER, Golgi complex, and plasma membrane of infected and mock-infected baby hamster kidney cells. First, we estimated the mean cell volume in absolute units. This was done using a novel in situ method which is described in detail. Infection by SFV was found to have no effect on any of the parameters measured. In the accompanying paper ( Quinn , P., G. Griffiths, and G. Warren, 1984, J. Cell Biol., 2142-2147) these stereological estimates were combined with biochemical estimates of the amount of spike proteins in ER, Golgi complex, and plasma membrane to determine the density in the membranes of these compartments.     

Rapid activation of endothelial NO synthase by estrogen: evidence for a steroid receptor fast-action complex (SRFC) in caveolae

Estrogen has important atheroprotective and vasoactive properties related to its capacity to stimulate nitric oxide (NO) production by endothelial NO synthase. Previous work has shown that these effects are mediated by estrogen receptor (ER) alpha functioning in a nongenomic manner via calcium-dependent, MAP kinase-dependent mechanisms. Recent studies have demonstrated that estradiol (E(2)) activates eNOS in isolated endothelial plasma membranes in the absence of added calcium, calmodulin or eNOS cofactors. Studies of blockade by ICI 182,780 and by ER alpha antibody, and also immunoidentification experiments indicate that the process is mediated by a subpopulation of plasma membrane-associated ER alpha. Fractionation of endothelial cell plasma membranes has further revealed that ER alpha protein is localized to caveolae, and that E(2) causes stimulation of eNOS in isolated caveolae which is ER-dependent and calcium-dependent, whereas noncaveolae membranes are insensitive. Furthermore, in intact endothelial cells the activation of eNOS by E(2) is prevented by pertussis toxin, and exogenous GDP beta S inhibits the response in isolated plasma membranes. Coimmunoprecipitation studies have shown that E(2) exposure causes interaction between ER alpha and G(alpha i) on the plasma membrane, and eNOS activation by E(2) is enhanced by overexpression of G(alpha i) and attenuated by expression of a protein regulator of G protein signaling (RGS), RGS4. Thus, a subpopulation of ER alpha is localized to caveolae in endothelial cells, where they are coupled via G(alpha i) to eNOS in a functional signaling module. Emphasizing the dependence on cell surface-associated receptors, these observations provide evidence for the existence of a steroid receptor fast-action complex, or SRFC, in caveolae.     

Integral Membrane Proteins of the Nuclear Envelope Are Dispersed throughout the Endoplasmic Reticulum during Mitosis

We have analyzed the fate of several integral membrane proteins of the nuclear envelope during mitosis in cultured mammalian cells to determine whether nuclear membrane proteins are present in a vesicle population distinct from bulk ER membranes after mitotic nuclear envelope disassembly or are dispersed throughout the ER. Using immunofluorescence staining and confocal microscopy, we compared the localization of two inner nuclear membrane proteins (laminaassociated polypeptides 1 and 2 [LAP1 and LAP2]) and a nuclear pore membrane protein (gp210) to the distribution of bulk ER membranes, which was determined with lipid dyes (DiOC₆ and R6) and polyclonal antibodies. We found that at the resolution of this technique, the three nuclear envelope markers become completely dispersed throughout ER membranes during mitosis. In agreement with these results, we detected LAP1 in most membranes containing ER markers by immunogold electron microscopy of metaphase cells. Together, these findings indicate that nuclear membranes lose their identity as a subcompartment of the ER during mitosis. We found that nuclear lamins begin to reassemble around chromosomes at the end of mitosis at the same time as LAP1 and LAP2 and propose that reassembly of the nuclear envelope at the end of mitosis involves sorting of integral membrane proteins to chromosome surfaces by binding interactions with lamins and chromatin.     

Routing of the G Protein During Maturation of Festuca Leaf Streak Rhabdovirus in its Plant Host

By immunogold labelling the location of Festuca leaf streak virus glycoprotein (FLSV-G) was investigated in developing phloem and mature leaf parenchyma of Festuca gigantea infected with Festuca leaf streak virus (FLSV: Rhabdotiridae). In developing phloem cells, FLSV-G was detected in endoplasmic reticulum (ER). at perinuclear membranes, and in assembled virions, but neither in Golgi stacks and Golgi vesicles nor at the plasma membrane of infected cells. These results indicate that FLSV-G stays in the ER after transmembrane synthesis, and is not routed through the secretory pathway in F. gigantea. The membranous inclusions, present in infected mature leaf parenchyma cells were found to contain FLSV-G. It is suggested that the, virus-induced membranous inclusions have developed from FLSV-G-containing ER. The residence of FLSV-G in ER (present study) is in contrast to results with vesicular stomatitis virus (VSV; vertebrate rhabdovirus). Here the G protein is known to be routed to the plasma membrane through the secretory pathway.     

Identification of a Region within the Placental Alkaline Phosphatase mRNA That Mediates p180-dependent Targeting to the Endoplasmic Reticulum

In both eukaryotic and prokaryotic cells, it has been recently established that mRNAs encoding secreted and membrane proteins can be localized to the surface of membranes via both translation-dependent and RNA element-mediated mechanisms. Previously, we showed that the placental alkaline phosphatase (ALPP) mRNA can be localized to the ER membrane independently of translation, and this localization is mediated by p180, an mRNA receptor present in the ER. In this article, we aimed to identify the cis-acting RNA element in ALPP. Using chimera constructs containing fragments of the ALPP mRNA, we demonstrate that the ER-localizing RNA element is present within the 3′ end of the open reading frame and codes for a transmembrane domain. In addition, we show that this region requires p180 for efficient ER anchoring. Taken together, we provide the first insight into the nature of cis-acting ER-localizing RNA elements responsible for localizing mRNAs on the ER in mammalian cells.     

Cholesterol is required for efficient endoplasmic reticulum-to-Golgi transport of secretory membrane proteins

Although cholesterol is synthesized in the endoplasmic reticulum (ER), compared with other cellular membranes, ER membrane has low cholesterol (3-6%). Most of the molecular machinery that regulates cellular cholesterol homeostasis also resides in the ER. Little is known about how cholesterol itself affects the ER membrane. Here, we demonstrate that acute cholesterol depletion in ER membranes impairs ER-to-Golgi transport of secretory membrane proteins. Cholesterol depletion is achieved by a brief inhibition of cholesterol synthesis with statins in cells grown in cholesterol-depleted medium. We provide evidence that secretory membrane proteins vesicular stomatitis virus glycoprotein and scavenger receptor A failed to be efficiently transported from the ER upon cholesterol depletion. Fluorescence photobleaching recovery experiments indicated that cholesterol depletion by statins leads to a severe loss of lateral mobility on the ER membrane of these transmembrane proteins, but not loss of mobility of proteins in the ER lumen. This impaired lateral mobility is correlated with impaired ER-to-Golgi transport. These results provide evidence for the first time that cholesterol is required in the ER membrane to maintain mobility of membrane proteins and thus protein secretion.     

3D tomography reveals connections between the phagophore and endoplasmic reticulum

Autophagosomes have been reported to form in the vicinity of the endoplasmic reticulum (ER). In many cases, the phagophore membrane is observed between two cisternae of rough ER, but it is not known whether these two membranes are directly connected. To investigate the relationship of the phagophore membrane and the ER, we used electron microscopic tomography of serum and amino acid starved normal rat kidney cells. The cells were fixed in glutaraldehyde and reduced osmium tetroxide and embedded in Epon. Dual axis tilt image series were acquired from two successive 250-nm sections. To analyze the three-dimensional (3D) morphology of phagophores and the associated rough ER, 3D tomograms were used to model the ER and phagophore membranes. The tomographic reconstructions revealed connections between the phagophore/autophagosome membrane and the closely located ER cisternae, especially with the ER located inside the autophagosome. The connections were typically formed by narrow extensions from the phagophore/autophagosome to the ER. This finding has potential implications on the origin of autophagosome membranes, and on the mechanism of phagophore membrane extension. In addition, we observed lipid droplets in very close contact with the phagophores/autophagosomes.     

Membrane-bound ribosomes of myeloma cells. VI. Initiation of immunoglobulin mRNA translation occurs on free ribosomes

Immunoglobulin heavy (Ig H) and light (Ig L) chain mRNA molecules have been released from the endoplasmic reticulum (ER) membranes as free (F) mRNP particles when MOPC 21 (P3K) mouse myeloma cells are exposed to a hypertonic initiation block (HIB). The subsequent fate of these mRNA sequences has been examined when the cells are returned to normal growth medium. Upon return to isotonicity, all previously translated mRNA molecules reassociate with ribosomes and form functional polysomes. Ig H mRNA is found incorporated first into F polysomes and then into membrane-bound (MB) polysomes. Kinetic studies indicate that the time of passage of Ig H mRNA in F polysomes is approximately 30 s, during which a nascent polypeptide chain of approximately 80 amino acids would have been completed. When the rate of polypeptide elongation is depressed with emetine during the recovery from HIB, both Ig H and L mRNA molecules accumulate in small F polysomes. These results indicate that the formation of Ig-synthesizing polysomes proceeds in the sequence: mRNA leads to F polysomes leads to MB polysomes. With the additional observation that during HIB recovery puromycin completely prevents the reassociation of Ig mRNA with the ER, these findings support a model of MB polysome formation in which the specificity of membrane attachment is determined by the nature of the N- terminal amino acid sequence of the nascent polypeptide chain.     

Involvement of the endoplasmic reticulum in the assembly and envelopment of African swine fever virus.

African swine fever (ASF) virus is a large enveloped DNA virus assembled in the cytoplasm of cells. In this study, the membrane compartments involved in the envelopment of ASF virus were investigated. A monoclonal antibody recognizing p73, the major structural protein of ASF virus, was generated to analyze the binding of p73 to membranes during the assembly of the virus. Approximately 50% of the intracellular pool of p73 associated with membranes as a peripheral membrane protein. Binding was rapid and complete within 15 min of synthesis. Subcellular membrane fractionation showed that newly synthesized p73 molecules cosedimented with endoplasmic reticulum (ER) membranes and remained associated with the ER during a 2-h chase. A similar distribution on gradients was recorded for p17, a structural membrane protein of ASF virus. The results suggested that the ER was involved in the assembly of ASF virus. A protease protection assay demonstrated a time-dependent envelopment of the membrane bound, but not cytosolic, pool of p73. Envelopment of p73 took place 1 h after binding to membranes and was completed 1 h before the first detection of p73 in virions secreted from cells. Envelopment was unaffected by brefeldin A and monensin, drugs that block membrane transport between the ER and Golgi. Taken together the results provide evidence for the binding of ASF virus structural proteins to a specific membrane compartment and implicate a role for the ER in the assembly and envelopment of ASF virus.     

Peroxisomal membrane ascorbate peroxidase is sorted to a membranous network that resembles a subdomain of the endoplasmic reticulum

The peroxisomal isoform of ascorbate peroxidase (APX) is a novel membrane isoform that functions in the regeneration of NAD(+) and protection against toxic reactive oxygen species. The intracellular localization and sorting of peroxisomal APX were examined both in vivo and in vitro. Epitope-tagged peroxisomal APX, which was expressed transiently in tobacco BY-2 cells, localized to a reticular/circular network that resembled endoplasmic reticulum (ER; 3,3'-dihexyloxacarbocyanine iodide-stained membranes) and to peroxisomes. The reticular network did not colocalize with other organelle marker proteins, including three ER reticuloplasmins. However, in vitro, peroxisomal APX inserted post-translationally into the ER but not into other purified organelle membranes (including peroxisomal membranes). Insertion into the ER depended on the presence of molecular chaperones and ATP. These results suggest that regions of the ER serve as a possible intermediate in the sorting pathway of peroxisomal APX. Insight into this hypothesis was obtained from in vivo experiments with brefeldin A (BFA), a toxin that blocks vesicle-mediated protein export from ER. A transiently expressed chloramphenicol acetyltransferase-peroxisomal APX (CAT-pAPX) fusion protein accumulated only in the reticular/circular network in BFA-treated cells; after subsequent removal of BFA from these cells, the CAT-pAPX was distributed to preexisting peroxisomes. Thus, plant peroxisomal APX, a representative enzymatic peroxisomal membrane protein, is sorted to peroxisomes through an indirect pathway involving a preperoxisomal compartment with characteristics of a distinct subdomain of the ER, possibly a peroxisomal ER subdomain.     

Light rough endoplasmic reticulum membranes re-appear before heavy rough membranes after feeding starved MPC-11 cells

In 48 hr starved MPC-11 cells smooth (S) endoplasmic reticulum (ER) membranes accounted for more than 80% of the total ER membrane fraction. After 2 1/2 hr feeding the amount of S membranes was reduced to below 55% while light rough (LR) membranes had increased from about 16% of the total to 42%. The percentage of heavy rough (HR) membranes only showed a minor increase during the incubation period. The results show that following an accumulation of S membranes in starved cells the LR membranes 're-appear' more rapidly than the HR membranes. This difference in behaviour provides further support to the concept that LR and HR membranes represent distinct domains of the ER system.     

Subcellular localization of a high affinity binding site for D-myo-inositol 1,4,5-trisphosphate from Chenopodium rubrum

It is now generally accepted that a phosphoinositide cycle is involved in the transduction of a variety of signals in plant cells. In animal cells, the binding of D-myo-inositol 1,4,5-trisphosphate (InsP(3)) to a receptor located on the endoplasmic reticulum (ER) triggers an efflux of calcium release from the ER. Sites that bind InsP(3) with high affinity and specificity have also been described in plant cells, but their precise intracellular locations have not been conclusively identified. In contrast to animal cells, it has been suggested that in plants the vacuole is the major intracellular store of calcium involved in signal induced calcium release. The aim of this work was to determine the intracellular localization of InsP(3)-binding sites obtained from 3-week-old Chenopodium rubrum leaves. Microsomal membranes were fractionated by sucrose density gradient centrifugation in the presence and absence of Mg(2+) and alternatively by free-flow electrophoresis. An ER-enriched fraction was also prepared. The following enzymes were employed as specific membrane markers: antimycin A-insensitive NADH-cytochrome c reductase for ER, cytochrome c oxidase for mitochondrial membrane, pyrophosphatase for tonoplast, and 1,3-beta-D-glucansynthase for plasma membrane. In all membrane separations, InsP(3)-binding sites were concentrated in the fractions that were enriched with ER membranes. These data clearly demonstrate that the previously characterized InsP(3)-binding site from C. rubrum is localized on the ER. This finding supports previous suggestions of an alternative non-vacuolar InsP(3)-sensitive calcium store in plant cells.     

Maintenance of Golgi structure and function depends on the integrity of ER export.

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitative fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We found that all classes of Golgi components are dynamically associated with this organelle, contrary to the prediction of the stable organelle model. Enzymes and recycling components are continuously exiting and reentering the Golgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without disrupting COPII-dependent ER export machinery (by brefeldin A treatment or expression of Arf1[T31N]), the Golgi structure disassembles, leaving no residual Golgi membranes. Rather, all Golgi components redistribute into the ER, the cytoplasm, or to ER exit sites still active for recruitment of selective membrane-bound and peripherally associated cargos. A similar phenomenon is induced by the constitutively active Sar1[H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a juxtanuclear region. In cells expressing Sar1[T39N], a constitutively inactive form of Sar1 that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These results argue against the hypothesis that the Golgi apparatus contains stable components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose membranes can be nucleated and are maintained by the activities of the Sar1-COPII and Arf1-coatomer systems.