Site of synthesis of membrane and nonmembrane proteins of vesicular stomatitis virus

Biogenesis of epithelial cell plasma membranes

Polarized monolayers of cultured epithelial cells, such as the kidney-derived MDCK cell line, when infected with enveloped viruses, provide a convenient model system for study of the intracellular routes followed by newly synthesized glycoproteins to reach specific domains of the plasma membrane. The polarized nature of the monolayers is reflected in the asymmetric assembly of enveloped viruses, some of which, such as influenza and simian virus 5 (SV5), bud from the apical surfaces of the cells, while others, such as vesicular stomatitis virus (VSV), emerge from the basolateral surfaces. MDCK cells can sustain double infection with viruses of different budding polarity, and within such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at different sites. Immunoelectron microscopic observations of doubly infected cells show that glycoproteins of influenza and VSV traverse the same Golgi apparatus. This indicates that critical sorting steps must take place during or after passage of the glycoproteins through the organelle. Following passage through the Golgi, the HA glycoprotein accumulates almost exclusively at the apical surface, where the influenza virions assemble. Significant amounts of the G protein, however, are detected on both plasma membranes in singly and doubly infected cells, although VSV virion assembly is limited to basolateral domains. These observations indicate that the site of VSV budding is not exclusively determined by the presence of G polypeptides on a given cell-surface domain. It is possible that other cellular or viral components are responsible for the selection of the appropriate budding domain or that the G protein found on the apical surface must be transferred to the basolateral domain before it becomes competent for assembly.

Kinectin-dependent assembly of translation elongation factor-1 complex on endoplasmic reticulum regulates protein synthesis

Kinectin is an integral membrane protein with many isoforms primarily found on the endoplasmic reticulum. It has been found to bind kinesin, Rho GTPase, and translation elongation factor-1delta. None of the existing models for the quaternary organization of the elongation factor-1 complex in higher eukaryotes involves kinectin. We have investigated here the assembly of the elongation factor-1 complex onto endoplasmic reticulum via kinectin using in vitro and in vivo assays. We established that the entire elongation factor-1 complex can be anchored to endoplasmic reticulum via kinectin, and the interacting partners are as follows. Kinectin binds EF-1delta, which in turn binds EF-1gamma but not EF-1beta; EF-1gamma binds EF-1delta and EF-1beta but not kinectin. In vivo splice blocking of the kinectin exons 36 and 37 produced kinectin lacking the EF-1delta binding domain, which disrupted the membrane localization of EF-1delta, EF-1gamma, and EF-1beta on endoplasmic reticulum, similar to the disruptions seen with the overexpression of kinectin fragments containing the EF-1delta binding domain. The disruptions of the EF-1delta/kinectin interaction inhibited expression of membrane proteins but enhanced synthesis of cytosolic proteins in vivo. These findings suggest that anchoring the elongation factor-1 complex onto endoplasmic reticulum via EF-1delta/kinectin interaction is important for regulating protein synthesis in eukaryotic cells.

Cell-free transfer of the vesicular stomatitis virus G protein from an endoplasmic reticulum compartment of baby hamster kidney cells to a rat liver Golgi apparatus compartment for Man8-9 to Man5 processing

We report the reconstitution of the transfer of a membrane glycoprotein (vesicular stomatitis virus glycoprotein, VSV-G protein) from endoplasmic reticulum to Golgi apparatus and its subsequent Man8-9GlcNAc2 to Man5GlcNAc2 processing in a completely cell-free system. The acceptor was Golgi apparatus from rat liver immobilized on nitrocellulose. The endoplasmic reticulum donor was from homogenates of VSV-G-infected BHK cells. Nucleoside triphosphate plus cytosol-dependent transfer and processing of radiolabeled VSV-G protein was observed with donor from BHK cells infected at 37 degrees C with wild-type VSV or at the permissive temperature of 34 degrees C with the ts045 mutant. With Golgi apparatus as acceptor, specific transfer at 37 degrees C in the presence of nucleoside triphosphate was eightfold that at 4 degrees C or in the absence of ATP. About 40% of the VSV-G protein transferred was processed to the Man5GlcNAc2 form. Processing was specific for cis Golgi apparatus fractions purified by preparative free-flow electrophoresis. Fractions derived from the trans Golgi apparatus were inactive in processing. With the ts045 temperature-sensitive mutant, transfer and processing were much reduced even in the complete system when microsomes were from cells infected with mutant virus and incubated at the restrictive temperature of 39.5 degrees C but were able to proceed at the permissive temperature of 34 degrees C. Thus, Man8-9GlcNAc2 to Man5GlcNAc2 processing of VSV-G protein occurs following transfer in a completely cell-free system using immobilized intact Golgi apparatus or cis Golgi apparatus cisternae as the acceptor and shows temperature sensitivity, donor specificity, requirement for ATP, and response to inhibitors similar to those exhibited by transfer and processing of VSV-G protein in vivo.

Chinese hamster ovary cells with reduced hexokinase activity maintain normal GDP-mannose levels

Parental Chinese hamster ovary (CHO) cells were mutagenized and subjected first to a mannose suicide selection technique and second to a screen of individual colonies grown on polyester discs for reduced mannose incorporation into protein. The incorporation of radioactivity for the selection and the screen was conducted at 41.5 degrees C instead of the normal growth temperature of 34 degrees C in order to allow for the isolation of temperature-sensitive lesions. This selection/screening procedure resulted in the isolation of M15-4 cells, which had three- to five-fold lower incorporation of [2-3H]mannose into mannose 6-phosphate, mannose 1-phosphate, GDP-mannose, oligosaccharide-lipid, and glycoprotein at 41.5 degrees C. We detected no difference in the qualitative pattern of mannose-labeled lipid-linked oligosaccharides compared to parental cells. M15-4 cells synthesized dolichol. The defect of M15-4 cells was determined to be in hexokinase activity; crude cytosolic extracts were eight- to nine-fold lower in hexokinase activity in M15-4 cells compared to parental cells. As a result of this defect, incorporation of labeled mannose from the medium was significantly decreased. However, the level of GDP-mannose in M15-4 cells was 70% of normal. The phenotype of M15-4 was a lower specific activity of labeled GDP-mannose, not a substantial reduction in the level of GDP-mannose. Consistent with these results, no alterations in the glycosylation of a model glycoprotein, G protein of vesicular stomatitis virus, were observed. These cells grew slower than parental cells, especially in low-glucose medium.

Reproduction of lyssaviruses: ultrastructural composition of lyssavirus and functional aspects of pathogenesis

Lyssaviruses are considerably adapted to neural tissue, although they can also be replicated in muscle and glandular cells. In neural tissue their reproduction takes place almost exclusively in neurons, and in the course of their dissemination they make use of the structural peculiarities of this highly differentiated cell type. The replication takes place completely in the cytoplasm, although rhabdovirus leader RNA enters the nucleus and by blocking host DNA and RNA synthesis promotes viral synthetic processes. In the cytoplasm the two phases of viral reproduction, the synthesis of nucleocapsids and the formation of the envelope together with the assembly of the virion, are separate in time and space. By this separation the transmission of infection by the incomplete form of the virus, i.e., by the synaptic transfer of ribonucleoprotein-transcriptase complexes is also possible. The formation of viral envelope and assembly of full viruses on the cisternal system of the host neurons is a highly complex process, as presented here in a three-dimensional analysis. Due to the high complexity of virus assembly, defects in construction are frequent, accounting for the high yield of defective interfering particles in the course of the reproduction of lyssaviruses.

Solubility of vesicular stomatitis virus M protein in the cytosol of infected cells or isolated from virions

The peripheral membrane M protein of vesicular stomatitis virus purified by detergent extraction of virions and ion-exchange chromatography was determined to be a monomer in the absence of detergent at high salt concentrations. Reduction of the ionic strength below 0.2 M resulted in a rapid aggregation of M protein. This self-association was reversible by the detergent Triton X-100 even in low salt. However, aggregation was not reversible by high salt concentration alone. M protein is initially synthesized as a soluble protein in the cytosol of infected cells, thus raising the question of how the solubility of M protein is maintained at physiological ionic strength. Addition of radiolabeled M protein purified from virions to unlabeled cytosol from either infected or uninfected cells inhibited the self-association reaction. Cytosolic fractions from infected or uninfected cells were equally effective at preventing the self-association of M protein. Self-association could also be prevented by an irrelevant protein such as bovine serum albumin. Sedimentation velocity analysis indicated that most of the newly synthesized M protein is monomeric, suggesting that the solubility of M protein in the cytosol is maintained by either low-affinity interaction with macromolecules in the cytosol or interaction of a small population of M-protein molecules with cytosolic components.

Distribution of M protein and nucleocapsid protein of vesicular stomatitis virus in infected cell plasma membranes

The association of M protein and the nucleocapsid (N) protein of vesicular stomatitis virus (VSV) with the cytoplasmic surface of plasma membranes prepared from infected cells was examined by double label immunofluorescence. M protein in association with the cytoplasmic surface of the plasma membrane was distributed in two distinct labeling patterns. Punctate labeling of M protein in the plasma membrane was observed in association with corresponding labeling for the nucleocapsid protein. Diffusely labeled M protein was distributed in areas of the plasma membrane that were devoid of any detectable labeling for the nucleocapsid protein. Similar results were obtained with two different cell types at 4 h and later times postinfection. The diffuse label for M protein was present in membranes prepared from cells infected with a temperature-sensitive M protein mutant at the nonpermissive temperature, but neither the punctate label for M protein nor labeling for the nucleocapsid protein was observed. Upon shift to permissive temperature, both the punctate label for M protein and labeling for the nucleocapsid protein began to reappear in membranes prepared from cells infected with the M protein mutant. These results indicate that M protein can associate with the plasma membrane without prior binding to nucleocapsids and that association of functional M protein with the plasma membrane is required for the stable association of nucleocapsids with the membrane during the process of viral budding.

Difference in cell binding patterns of two monoclonal antibodies recognizing distinct epitopes on a human melanoma-associated oncofetal antigen

Two monoclonal antibodies (MAbs), 140.240 and 96.5, generated independently in different laboratories, have been shown to detect the target structures of 87,000 (gp87) and 97,000 (p97) glycoproteins, respectively, both strongly expressed by melanoma cells and fetal small intestine. To determine whether MAb 140.240 and MAb 696.5 recognized a same target structure, they were tested in immunoprecipitation/SDS-PAGE using NP-40 lysates of melanoma cells labelled with [35S]methionine for 18 hr. Both antibodies precipitated a single band with Mr = 87,000. Reciprocal immunodepletion studies showed that neither of the two antibodies detected the 87,000 band in the lysate immuno depleted by either antibody, suggesting that these two antibodies recognize the same or extremely similar molecules. Two-dimensional tryptic peptide mapping analysis showed that the two identified molecules shared the same finger-printing pattern. A 40,000 fragment of the 87,000 molecule produced by protease digestion was precipitated by MAb 96.5 but not MAb 140.240, indicating that the epitopes recognized by the two antibodies are localized at discrete sites on the molecule. Serological studies on these two antibodies revealed slightly different binding patterns in the MAb 140.240 exhibited a more melanoma-restricted specificity, while MAb 96.5 had a specificity to melanoma and to some other cell types. The observed difference in epitope specificity may be important in the clinical applications of these antibodies.

Mapping regions of the matrix protein of vesicular stomatitis virus which bind to ribonucleocapsids, liposomes, and monoclonal antibodies

The matrix (M) protein of vesicular stomatitis virus (VSV) appears to function as a bridge between the ribonucleocapsid (RNP) core and the envelope in assembly of the virion. Two such properties would necessitate at least one site for interaction with the nucleocapsid and one with the envelope. In this study M protein was found to mediate the in vitro binding to RNP cores of phospholipid vesicles, representing membrane structures. The M protein could bind initially to either the vesicles or the RNP cores to promote RNP-vesicle association. A trypsin-resistant fragment (MT) of M protein, missing the initial 43 amino acids from its amino terminus, reconstituted with acidic phospholipid vesicles with the same binding efficiency as did whole M protein, suggesting that the carboxy-terminal 81% retained those regions of the M protein which interact with a lipid bilayer. The MT protein, however, was considerably less efficient than intact M protein as an inhibitor of in vitro virus transcription; almost 2.5-fold more MT protein than intact M protein was required for 50% inhibition of VSV transcription, indicating that a site for interaction with the RNP core may have been lost. A monoclonal antibody which is able to reverse the in vitro inhibition of transcription by M protein did not react by immunoblotting with MT protein. Partial tryptic digests of the M protein probed with this monoclonal antibody indicated that epitope 1 lies between amino acid residues 18 and 43. This region appears to be a site that promotes interaction of the M protein with the RNP core of VSV. Monoclonal antibodies to epitopes 2 and 3, which exhibit some overlap in binding to M protein but do not reverse transcription inhibition, were mapped by cleavage with N-chlorosuccinimide at regions in a carboxy direction from epitope 1.

Structural characterization and biosynthesis of gp87, a melanoma-associated oncofetal antigen defined by monoclonal antibody 140.240

The monoclonal antibody 140.240, which reacts with a melanoma-associated oncofetal antigen, identifies an epitope present on an 87 kd molecule present on the surface of melanoma cells. This molecule is a single-chain monomer, which by tunicamycin treatment and two-dimensional tryptic mapping has been shown to arise from a 77 kd polypeptide precursor (p77). This precursor is rapidly glycosylated to an 83 kd intermediate (gp83) which in turn is rapidly further glycosylated into the mature 87 kd glycopolypeptide (gp87). Neuraminidase treatment of the glycopeptides indicates that only gp87 contains accessible sialic acid moieties.

Fine structure of a membrane anchor domain

We describe a detailed deletion analysis of the anchoring domain of a model membrane protein. Removal of the 23 contiguous uncharged amino acids from the carboxy terminus of the bacteriophage fl gene III protein (pIII) converts it from an integral membrane protein to a secreted periplasmic form. Deletions that remove six or fewer residues of the hydrophobic core result in no diminution of the protein's capacity to anchor in the membrane. Longer deletions into this hydrophobic domain gradually destablize the protein-membrane association. pIII derivatives with over half of the hydrophobic core deleted retain substantial residual anchor function. The basic residues, arginine and lysine, which provide a carboxy-terminal boundary for this domain, can be deleted without loss of anchoring capacity.

Neutralization of Newcastle disease virus by monoclonal antibodies to the hemagglutinin-neuraminidase glycoprotein: requirement for antibodies to four sites for complete neutralization

The neutralizing characteristics of monoclonal antibodies directed to four antigenic sites on the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus were determined. Neutralization by each antibody resulted in a persistent fraction of nonneutralized virus which varied from 1 to 17% depending on the hemagglutinin-neuraminidase site recognized, but not on the antibody. The addition of antibodies to all four sites on the hemagglutinin-neuraminidase glycoprotein was required to give a level of neutralization comparable with that obtained with polyclonal mouse antiserum. The high persistent fractions were not due to viral aggregates, a high level of variants in the virus stock, the use of insufficient antibody, low antibody avidity, or an effect peculiar to the use of the chicken cells as host. The addition of rabbit anti-mouse immunoglobulin to the persistent fraction left by any of the antibodies resulted in a further reduction in infectivity, often by as much as two logs. Thus, some viral particles are capable of binding antibody while retaining their infectivity. The implications of these findings to the mechanism of neutralization are discussed.

Intracellular processing of the vesicular stomatitis virus glycoprotein and the Newcastle disease virus hemagglutinin-neuraminidase glycoprotein

The kinetics of intracellular transport of the vesicular stomatitis virus (VSV) glycoprotein (G) and the Newcastle disease virus (NDV) hemagglutinin-neuraminidase (HN) glycoprotein in chicken embryo cells were compared. To assay for the appearance of pulse-labelled glycoprotein at the cell surface, an antibody-binding assay was developed which allowed the precipitation of only those molecules on the outside surfaces of infected cells. Using this assay, it was found that pulse-labelled VSV G protein appeared at the cell surface with a half-time of approximately 27 min, while pulse-labelled NDV HN glycoprotein reached the cell surface with a half-time of approximately 78 min. To determine the transit time of these glycoproteins to trans-Golgi membranes, the kinetics of the acquisition of endoglycosidase H resistance was analyzed. The half-time of the transit of the G protein to the trans-Golgi membranes was found to be approximately 13 min while that of the HN glycoprotein was found to be approximately 60 min. Since the G protein migrates to the trans-Golgi membranes with a half-time of 13 min, and the cell surface with a half-time of 27 min, the half-time for the transit between the trans-Golgi membrane and the plasma membrane must be approximately 14 min. In a similar analysis, the half-time for the transit of the HN glycoprotein from the trans-Golgi membrane to the plasma membrane must be approximately 18 min, a time not significantly different from that of the G protein. Thus the difference in the kinetics of the intracellular transport of these two glycoproteins resides primarily in the transit from the rough endoplasmic reticulum to the trans-Golgi membranes. These results argue against a non-selective mechanism for the transport of plasma membrane glycoproteins to the cell surface.

A stop transfer sequence confers predictable transmembrane orientation to a previously secreted protein in cell-free systems

We have combined molecular genetic and cell-free reconstitution approaches to study the mechanism of membrane assembly. The coding region for the carboxy-terminal transmembrane sequence of membrane IgM heavy chain has been inserted between the coding regions for lactamase and globin domains of a fusion protein previously shown to be completely translocated across microsomal membranes in a cell-free transcription-linked translation system. The resulting fusion protein behaves as an integral transmembrane protein of predicted asymmetry: all of the membrane integrated copies display lactamase within the lumen and globin on the cytoplasmic face of the vesicles. In another construction, this transmembrane coding region replaces that of the signal sequence. The resulting fusion protein is not translocated across membranes. These data provide strong evidence that there are stop transfer sequences whose ability to arrest chain translocation and achieve asymmetric transmembrane orientation is independent of the size of the subsequent carboxy-terminal domain to be localized in the cytosol; and that signal and stop transfer sequences are functionally distinct.

Analysis of the hemagglutinin glycoprotein from mutants of vaccinia virus that accumulates on the nuclear envelope

We isolated mutants whose vaccinia hemagglutinin (HA) accumulates on nuclear envelopes and the rough endoplasmic reticulum. Mutant HA must be blocked at a pre-Golgi step because it has high-mannose-type carbohydrates but no fucose. Neither N- nor O-glycosidically linked carbohydrates are involved in the transport defect of the mutant HA, because tunicamycin, an inhibitor of N-type glycosylation, has no effect, and O-type glycosylation takes place in the Golgi organelle. The unglycosylated form of the mutant HA synthesized in the presence of tunicamycin is 3000 daltons larger than the wild type. This higher molecular weight is related to the transport defect. HAs translated in vitro also show this difference, evidence that it reflects mutation in the HA structural gene. Portions of HAs that project into the cytoplasm seem to account for this weight difference. Thus the cytoplasmic tail of glycoprotein has an important function in transport out of the rough endoplasmic reticulum.

The effect of ammonium chloride and tunicamycin on the glycoprotein content and infectivity of herpes simplex virus type 1

Infectious virions of MP, a syncytial strain of herpes simplex virus type 1, are formed in the presence of 50 mM NH4Cl. Underglycosylated virion glycoproteins are synthesized in infected cells and are incorporated into virions in the presence of the same concentration of NH4Cl. We conclude that fully glycosylated glycoproteins are not required for viral infectivity. Virus particles, deficient in glycosylated glycoproteins, are assembled in the presence of tunicamycin but they are not infectious. The decrease in infectivity could be due to the decreased amount of the gB or possibly other peptides and/or to the lack of the high-mannose saccharides of precursor glycoproteins.

Protein synthesis directed by polyadenylated and non-polyadenylated RNA isolated from membrane-bound and free polysomes of Tetrahymena pyriformis

Free and membrane-bound polysomes were prepared from the protozoa Tetrahymena pyriformis using a procedure which gives good recovery and practically no cross-contamination. Polysomes are intact as analysed by sedimentation analysis. Poly(A)-rich RNA and poly(A)-free RNA, isolated from both populations of polysomes, show similar electrophoretic patterns. These RNAs were translated in the rabbit reticulocyte lysate cell-free system and the translation products were analysed by one-dimensional and two-dimensional gel electrophoresis. The most striking differences were found in the two-dimensional electrophoretic analysis namely: (a) a group of polypeptides (10) is synthesized mainly on membrane-bound polysomes, (b) a second abundant group is synthesized mainly in free polysomes (c) and a third class of polypeptides is synthesized on both kinds of polysomes. Poly(A)-free RNAs, isolated from free polysomes, are also able to promote synthesis of some polypeptides. The results are discussed taking into account the fact that T. pyriformis is a non-secretory cell.

Expression from cloned cDNA of cell-surface secreted forms of the glycoprotein of vesicular stomatitis virus in eucaryotic cells

A cDNA clone of the mRNA encoding the glycoprotein (G) of vesicular stomatitis virus was inserted into plasmid vectors under the control of either the SV40 early promoter (pSV2G) or the SV40 late promoter (pSVGL). Synthesis of G protein was observed in mouse L cells injected with pSV2G DNA or in COS1 cells transfected with pSVGL DNA. Immunofluorescent staining of G protein produced in both cell types showed a pattern of internal and cell-surface staining indistinguishable from that seen in cells infected with vesicular stomatitis virus. The G protein produced in transfected COS1 cells was the size of normal G protein and was glycosylated. Expression of a G protein lacking 79 amino acids from the COOH terminus was also examined. This G protein lacks the transmembrane domain and the hydrophilic COOH terminus, which, we postulated, anchor G protein in the lipid bilayer. This "anchorless" protein is glycosylated and is secreted, albeit slowly.

RNA synthesis by Newcastle disease virus temperature-sensitive mutants in two RNA-negative complementation groups

The temperature-sensitive RNA-negative mutants of Newcastle disease virus comprise two complementation groups, group A (seven members) and group E (one member). The RNA-synthesizing activities of four representative members of group A and the single member of group E were compared with the activity of the wild type. These mutants were defective to varying extents in primary transcription at the nonpermissive temperature, ranging from mutant A1, which had no activity, to mutant E1, which lost only 50% of its activity. All of the mutants were also defective in a postprimary transcriptive process since after preincubation at the permissive temperature in the presence of cycloheximide, there was no subsequent RNA synthesis at the nonpermissive temperature upon removal of the cycloheximide. Similarly, in experiments in which cycloheximide was not used, shifts from the permissive temperature to the nonpermissive temperature before 3 h postinfection did not result in RNA synthesis. However, later shifts to the nonpermissive temperature did allow RNA synthesis. With the exception of mutant A1, all of the mutants maintained this RNA-synthetic ability for at least 3 h, suggesting that RNA synthesis from progeny genomes was not the major postprimary transcriptive defect in these mutants. In contrast, the RNA-synthetic ability of mutant A1 rapidly decayed at the nonpermissive temperature, suggesting that the A gene product is involved in RNA synthesis from progeny genomes. The postprimary transcriptive defect(s) of the other mutants may be in the processing or stability of a protein, in the processing of mRNA, or in replication. Plaque-forming revertants (ts+) of all of the mutants coreverted for RNA synthesis. This finding strengthens the relationship between temperature sensitivity for plaquing and both the primary and postprimary RNA-negative phenotypes.

Segregation of specific classes of messenger RNA into free and membrane-bound polysomes

Analysis of mRNA populations from rat liver rough microsomes and free polysomes by homologous and heterologous cDNA . mRNA hybridization shows that the two mRNA populations are distinct, demonstrating that specific mRNA classes are efficiently segregated for translation in association with endoplasmic reticulum membranes. We estimate that approximately 90% of the mRNA in membrane-bound polysomes contains a diverse set of messengers with a minimum of 500--2000 different species although approximately 5--8 messengers may constitute 25--30% of the mRNA mass. The complexity of the mRNA population of free polysomes appears to be comparable to that estimated for total liver poly(A) + mRNA by other investigators, and is likely to be substantially greater than that of the bulk of bound mRNA. In addition, mRNA in free polysomes lacks the high abundance class characteristic of mRNA-bound polysomes. The substantial complexity of the bound mRNA population suggests that the segregation of polysomes in rough microsomes is not limited to a small class specialized in manufacturing secretory proteins, but extends to polysomes engaged in the synthesis of proteins for intracellular distribution. The segregation of specific messengers into the free and membrane-bound classes was abolished when polysome disassembly was induced by administration of ethionine. Thus, messenger RNA molecules themselves lacked the capacity for segregation, although they contain information for segregation which is expressed during translation. These findings are consistent with the presence of signal sequences in nascent polypeptides which determine the attachment of ribosomes to endoplasmic reticulum membranes.

Messenger RNA is translated when associated with the cytoskeletal framework in normal and VSV-infected HeLa cells

When the cytoskeletal framework is prepared from suspension-grown HeLa by extraction with nonionic detergent, all the polyribosomes are associated with the framework while 80% of tRNA and the major portion of monoribosomes as well as 75% of the cell proteins are found in the soluble fraction. The mRNA of polyribosomes is bound to the cytoskeleton and these molecules remain attached even after polyribosomes are disassembled in vivo prior to extraction. Although all actively translating message molecules are attached to the framework, about one quarter of the poly(A)+ mRNA is free of the framework. The binding of message to the skeleton may be obligatory for translation. Upon infection with VSV, all the viral polyribosomes but not all the viral messages of the infected cell are associated with the cytoskeletal framework. Pulse-chase labeling shows that VSV messages initially associate with the framework and then later detach and cease translation. The mRNA for the viral glycoprotein (G), known to translate only on ribosomes bound to endoplasmic reticulum, is also retained by the detergent-extracted structure. It appears that the protein substructure of the endoplasmic reticulum which binds polyribosomes is a component of the cytoskeletal framework.

Synthesis, stability, and cleavage of Newcastle disease virus glycoproteins in the absence of glycosylation

Polypeptides synthesized in Newcastle disease virus (NDV)-infected CHO cells in the absence of glycosylation were characterized. Incorporation of either [3H]mannose of [3H]glucosamine into NDV polypeptides was inhibited to greater than 99% by the antibiotic tunicamycin. Under these conditions, infected cells synthesized proteins which comigrated on polyacrylamide gels with the viral L protein, nucleocapsid protein, membrane protein, and a polypeptide with a molecular weight of 55,000 (P55). These cells did not synthesize polypeptides with the size of the hemagglutinin-neuraminidase (HN) protein or the fusion (F0) protein. They did, however, synthesize new polypeptides with molecular weights of 75,000 (P75), 67,000 (P67), and 52,000 (P52). Peptide analysis revealed that P75 was a host cell protein whose synthesis is enhanced by tunicamycin. P67 corresponded to the unglycosylated forms of the glycoproteins were found to be relatively stable in infected cells. P55, previously thought to correspond to the cleaved form of F0, was found to be a unique viral protein which is associated with intracellular nucleocapsid structures.

Sites of synthesis of viral proteins in avian sarcoma virus-infected chicken cells

We determined the sites of synthesis of avian sarcoma virus-specific proteins in infected chicken cells by immunoprecipitation of the products synthesized in vitro by free and membrane-bound polyribosomes; 85% of Pr76, the precursor of the viral internal structural proteins (group-specific antigens), was synthesized on free polyribosomes, and 15% was synthesized on membrane-bound polyribosomes. Pr92, the lycosylated precursor of the viral glycoproteins (gp85 and gp35), was synthesized exclusively on membrane-bound polyribomes, which is consistent with its role as a membrane protein. When we investigated the site of synthesis of pp60src, the product of the avian sarcoma virus src gene, we found that 90% was synthesized on free polyribosomes, whereas 10% was detected on membrane-bound polyribosomes. The implications of these results with respect to the subcellular location of pp60src are discussed.

Mutants of vesicular stomatitis virus blocked at different stages in maturation of the viral glycoprotein

Maturation of the vesicular stomatitis virus (VSV) glycoprotein (G) to the cell surface is blocked at the nonpermissive temperature in cells infected with temperature-sensitive mutants in the structural gene encoding for G. We show here that these mutants fall into two discrete classes with respect to the stage of post-translational processing at which the block occurs. In all cases the mutant glycoproteins are inserted normally into the endoplasmic reticulum membrane, receive the two-high-mannose oligosaccharides, and apparently lose the NH2-terminal signal sequence of 16 amino acids. In cells infected with one class of mutants, no further processing of the glycoprotein occurs, and we conclude that the mutant protein is blocked at a pre-Golgi stage. In cells infected with ts L511(V), however, addition of the terminal sugars galactose and sialic acid occurs normally. Thus the maturation of G proceeds through several Golgi functions but is blocked before its appearance on the cell surface. The oligosaccharide chain of ts L511(V) G, accumulated at either the permissive (where surface maturation occurs) or the nonpermissive temperature, lacks one saccharide residue, probably fucose. In addition, no fatty acid residues are added to the ts L511(V) G protein at the nonpermissive temperature, although addition does occur under permissive conditions.

Membrane-associated ribosomes in producing and nonproducing mouse myeloma cells

Mouse myeloma cell lines synthesize large amounts of immunoglobulin on their membrane-associated polyribosomes. Variants which no longer synthesize immunoglobulins have been studied and shown to have the same number of tightly bound membrane-associated polyribosomes as the parental cell lines. These polyribosomes are still active in the synthesis of nonimmunoglobulin proteins.

Polarized distribution of viral envelope proteins in the plasma membrane of infected epithelial cells

The surface distribution of the envelope glycoproteins of influenza, Sendai and Vesicular Stomatitis viruses was studied by immunofluorescence and immunoelectromicroscopy in infected epithelial cell monolayers, from which these viruses bud in a polarized fashion. It was found that before the onset of viral budding, the envelope proteins are exclusively localized into the same plasma membrane domains of the epithelial cells from which the virions ultimately bud: the glycoproteins of influenza and Sendai were detected at the apical surface, while the G protein of Vesicular Stomatitis virus was concentrated at the basolateral region. On the other hand, Sendai virus nucleocapsids, which can be easily identified in the cytoplasm before viral assembly, could be observed throughout the cell, not showing any preferential localization near the surface that the virions utilize for budding. These results are consistent with a model in which the asymmetric distribution of viral envelope proteins, rather than a polarized delivery of nucleocapsids, directs the polarity of viral budding. Furthermore, the asymmetric surface localization of viral glycoproteins suggests that these proteins share with intrinsic surface proteins of epithelial cells common biogenetic mechanisms and informational features or "sorting out" signals that determine their compartmentalization in the plasma membrane.