Assembly of membrane glycoproteins studied by phenotypic mixing between mutants of vesicular stomatitis virus and retroviruses

Pseudotyped Vesicular Stomatitis Virus-Severe Acute Respiratory Syndrome-Coronavirus-2 Spike for the Study of Variants, Vaccines, and Therapeutics Against Coronavirus Disease 2019

World Health Organization (WHO) has prioritized the infectious emerging diseases such as Coronavirus Disease (COVID-19) in terms of research and development of effective tests, vaccines, antivirals, and other treatments. Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2), the etiological causative agent of COVID-19, is a virus belonging to risk group 3 that requires Biosafety Level (BSL)-3 laboratories and the corresponding facilities for handling. An alternative to these BSL-3/-4 laboratories is to use a pseudotyped virus that can be handled in a BSL-2 laboratory for study purposes. Recombinant Vesicular Stomatitis Virus (VSV) can be generated with complementary DNA from complete negative-stranded genomic RNA, with deleted G glycoprotein and, instead, incorporation of other fusion protein, like SARS-CoV-2 Spike (S protein). Accordingly, it is called pseudotyped VSV-SARS-CoV-2 S. In this review, we have described the generation of pseudotyped VSV with a focus on the optimization and application of pseudotyped VSV-SARS-CoV-2 S. The application of this pseudovirus has been addressed by its use in neutralizing antibody assays in order to evaluate a new vaccine, emergent SARS-CoV-2 variants (delta and omicron), and approved vaccine efficacy against variants of concern as well as in viral fusion-focused treatment analysis that can be performed under BSL-2 conditions.

The M2 ectodomain is important for its incorporation into influenza A virions

M2 is an integral protein of influenza A virus that functions as an ion channel. The ratio of M2 to HA in influenza A virions differs from that found on the cell surface, suggesting selective incorporation of M2 and HA into influenza virions. To examine the sequences that are important for M2 incorporation into virions, we used an incorporation assay that involves expressing M2 from a plasmid, transfecting the plasmid into recipient cells, and then infecting those cells with influenza virus. To test the importance of the different regions of the protein (extracellular, transmembrane, and cytoplasmic) in determining M2 incorporation, we created chimeric mutants of M2 and Sendai virus F proteins, exchanging corresponding extracellular, transmembrane, and cytoplasmic domains. Of the six possible chimeric mutants, only three were expressed on the cell surface. Of these three chimeric proteins, only one mutant (with the extracellular domain from M2 and the rest from F) was incorporated into influenza virions. These results suggest that the extracellular domain of M2 is important for its incorporation into virions.

Effect of phospholipids on adsorption and penetration of human T-cell leukemia virus type I

We have studied the ability of some phospholipids (PLs) and phospholipases (PLases) to interfere with infection of human T-cell leukemia virus type I (HTLV-I). Plating of pseudotype of vesicular stomatitis virus (VSV) bearing envelope antigens of HTLV-I, VSV(HTLV-I), was markedly inhibited by treatment of the cells with cardiolipin (CL) after, but not before, infection. Treatment of the cells with CL after infection also inhibited the plating of VSV pseudotype of bovine leukemia virus (BLV), but scarcely affected VSV infection. Furthermore, the plating of VSV(HTLV-I) was markedly enhanced by treatment with PLCase after infection. Treatment with PLCase, however, did not affect the plating of VSV. These results were also confirmed by polymerase chain reaction (PCR): Formation of proviral DNA was inhibited when indicator cells were treated with CL after cell-free infection of HTLV-I, but not before, and enhanced when indicator cells were treated with PLCase after HTLV-I infection. These findings suggested that PLs might play a role at the early stage of HTLV-I infection.

A multistep process of leukemogenesis in Moloney murine leukemia virus-infected mice that is modulated by retroviral pseudotyping and interference

Mixed retroviral infections frequently exhibit pseudotyping, in which the genome of one virus is packaged in a virion containing SU proteins encoded by another virus. Infection of mice by Moloney murine leukemia virus (M-MuLV), which induces lymphocytic leukemia, results in a mixed viral infection composed of the inoculated ecotropic M-MuLV and polytropic MuLVs generated by recombination of M-MuLV with endogenous retroviral sequences. In this report, we describe pseudotyping which occurred among the polytropic and ecotropic MuLVs in M-MuLV-infected mice. Infectious center assays of polytropic MuLVs released from splenocytes or thymocytes of infected mice revealed that polytropic MuLVs were extensively pseudotyped within ecotropic virions. Late in the preleukemic stage, a dramatic change in the extent of pseudotyping occurred in thymuses. Starting at about 5 weeks, there was an abrupt increase in the number of thymocytes that released nonpseudotyped polytropic viruses. A parallel increase in thymocytes that released ecotropic M-MuLV packaged within polytropic virions was also observed. Analyses of the clonality of preleukemic thymuses and thymomas suggested that the change in pseudotyping characteristics was not the result of the emergence of tumor cells. Examination of mice infected with M-MuLV, Friend erythroleukemia virus, and a Friend erythroleukemia virus-M-MuLV chimeric virus suggested that the appearance of polytropic virions late in the preleukemic stage correlated with the induction of lymphocytic leukemia. We discuss different ways in which pseudotypic mixing may facilitate leukemogenesis, including a model in which the kinetics of thymic infection, modulated by pseudotyping and viral interference, facilitates a stepwise mechanism of leukemogenesis.

Inhibition of adsorption of human T-cell-leukemia virus type 1 by a plant lectin, wheat-germ agglutinin

Thirty-six lectins that recognize various sugar chains were examined for inhibitory activities against infection with human T-cell leukemia virus type I (HTLV-I). Wheat-germ agglutinin (WGA) was the most inhibitory among them: plating of the pseudotype of vesicular-stomatitis virus (VSV) bearing envelope antigens of HTLV-I was markedly inhibited by treatment of indicator cells with WGA just before adsorption, but not by treatment after virus adsorption. Treatment with WGA before adsorption, however, could not inhibit the plating of VSV, VSV pseudotypes of bovine leukemia virus, Moloney murine leukemia virus and human immunodeficiency virus type I. Syncytium formation induced by HTLV-I was also inhibited by WGA upon co-cultivation of U-251 MG human glioma cells or MOLT-4 human T-cells with HTLV-I-producing C91/PL cells. Formation of proviral DNA detected one day after infection was also inhibited when indicator cells had been treated with WGA before adsorption of HTLV-I, but not after its adsorption. These findings indicated that WGA specifically inhibits plating of HTLV-I when added to culture just before adsorption and suggested that a substance(s) containing sugar chains recognized by WGA might be involved in an adsorption step of HTLV-I.

Cytoplasmic domain requirement for incorporation of a foreign envelope protein into vesicular stomatitis virus

Incorporation of human immunodeficiency virus type 1 (HIV-1) envelope proteins into vesicular stomatitis virus (VSV) particles was studied in a system that allows expressed envelope proteins to rescue phenotypically a temperature-sensitive mutant of VSV (tsO45). This mutant exhibits defective transport of its own envelope glycoprotein (G) and can be rescued by simultaneous expression of wild-type G protein from cDNA. We report here that a hybrid HIV-1-VSV protein containing the extracellular and transmembrane domains of the HIV-1 envelope protein fused to the cytoplasmic domain of VSV G protein was able to rescue the tsO45 mutant lacking the G protein, while the wild-type HIV-1 envelope protein was not. The VSV(HIV) pseudotypes obtained infected only CD4+ cells and were neutralized specifically by anti-HIV-1 sera. Our results indicate that the cytoplasmic tail of the VSV glycoprotein contains an independent signal capable of directing a foreign protein into VSV particles. The VSV(HIV) pseudotypes generated here were prepared in the absence of HIV-1 and should be useful for identifying molecules that block HIV-1 entry.

Membrane association of functional vesicular stomatitis virus matrix protein in vivo

The matrix (M) protein of vesicular stomatitis virus (VSV) is a major structural component of the virion which is generally believed to bridge between the membrane envelope and the ribonucleocapsid (RNP) core. To investigate the interaction of M protein with cellular membranes in the absence of other VSV proteins, we examined its distribution by subcellular fractionation after expression in HeLa cells. Approximately 90% of M protein, expressed without other viral proteins, was soluble, whereas the remaining 10% was tightly associated with membranes. A similar distribution in VSV-infected cells has been observed previously. Conditions known to release peripherally associated membrane proteins did not detach M protein from isolated membranes. Membrane-associated M protein was soluble in the detergent Triton X-114, whereas soluble M protein was not, suggesting a chemical or conformational difference between the two forms. Membranes containing associated M protein were able to bind RNP cores, whereas membranes lacking M protein were not. We suggest that this membrane-bound M fraction constitutes a functional subset of M protein molecules required for the attachment of RNP cores to membranes during normal virus budding.

Analysis of HIV-1 envelope mutants and pseudotyping of replication-defective HIV-1 vectors by genetic complementation

Infectious HIV-1 particles containing replication-defective vectors that express the hygromycin B phosphotransferase gene were generated by transient complementation in COS-1 cells. A defective vector dependent only on trans-complementation with an env gene and a small vector containing a deletion of almost all of the trans region were used to examine pseudotyping of HIV-1 by an amphotropic murine retrovirus. Although pseudotyping by the heterologous envelope glycoprotein occurred with efficiency, no pseudotyping at the RNA level was observed. Genetic complementation was used to rapidly analyze the effect of env mutations in the V3, proteolytic processing site, fusion domain, and cytoplasmic tail on viral infectivity. Mutations decreasing syncytium formation usually also lowered infectivity. However, a mutation in the cytoplasmic tail and a separate mutation adjacent to the fusion domain dramatically decreased viral particle infectivity but did not appreciably decrease envelope glycoprotein-mediated cell-to-cell fusion. These results may indicate that these regions of the transmembrane peptide are necessary for acquisition of envelope glycoprotein by budding virus particles or for virus entry.

Pseudotype formation of murine leukemia virus with the G protein of vesicular stomatitis virus

Mixed infection of a cell by vesicular stomatitis virus (VSV) and retroviruses results in the production of progeny virions bearing the genome of one virus encapsidated by the envelope proteins of the other. The mechanism for the phenomenon of pseudotype formation is not clear, although specific recognition of a viral envelope protein by the nucleocapsid of an unrelated virus is presumably involved. In this study, we used Moloney murine leukemia virus (MoMLV)-based retroviral vectors encoding the gene for neomycin phosphotransferase to investigate the interaction between the VSV G protein and the retroviral nucleocapsid during the formation of MoMLV(VSV) pseudotypes. Our results show that VSV G protein can be incorporated into the virions of retrovirus in the absence of other VSV-encoded proteins or of retroviral envelope protein. Infection of hamster cells by MoMLV(VSV) pseudotypes gave rise to neomycin phosphotransferase-resistant colonies, and addition of anti-VSV serum to the virus preparations completely abolished the infectivity of MoMLV(VSV) pseudotypes. It should be possible to use existing mutants of VSV G protein in the system described here to identify the signals that are important for the formation of MoMLV(VSV) pseudotypes.

Pseudotyping with human T-cell leukemia virus type I broadens the human immunodeficiency virus host range

Several epidemiologic and clinical studies suggest that patients coinfected with human immunodeficiency virus (HIV), the primary etiologic agent in AIDS, and other viruses, such as cytomegalovirus or human T-cell leukemia virus (HTLV), have a more severe clinical course than those infected with HIV alone. Cells infected with two viruses can, in some cases, give rise to phenotypically mixed virions with altered or broadened cell tropism and could therefore account for some of these findings. Such pseudotypes could alter the course of disease by infecting more tissues than are normally infected by HIV. We show here that HIV type 1 (HIV-1) efficiently incorporates the HTLV type I (HTLV-I) envelope glycoprotein and that both HIV-1 and HTLV-II accept other widely divergent envelope glycoproteins to form infectious pseudotype viruses whose cellular tropisms and relative abilities to be transmitted by cell-free virions or by cell contact are determined by the heterologous envelope. We also show that the mechanism by which virions incorporate heterologous envelope glycoproteins is independent of the presence of the homologous glycoprotein or heterologous gag proteins. These results may have important implications for the mechanism of HIV pathogenesis.

Role of heterologous and homologous glycoproteins in phenotypic mixing between Sendai virus and vesicular stomatitis virus

Phenotypic mixing between Sendai virus and vesicular stomatitis virus (VSV) or the mutant VSV ts045 was studied. Conditions were optimized for double infection, as shown by immunofluorescence microscopy. Virions from double-infected cells were separated by sequential velocity and isopycnic gradient centrifugations. Two types of particles with mixed protein compositions were found. One type was VSV particles with Sendai virus spikes, i.e., phenotypically mixed particles. A second type was Sendai virus-VSV associations, which in plaque assays also behaved as phenotypically mixed particles. The ratio of VSV G protein to Sendai virus glycoproteins on the cell surface was varied, using the VSV mutant ts045 in double infections. Thus, different amounts of the VSV G protein were allowed to reach the cell surface at 32, 38, and 39 degrees C in Sendai virus-infected cells. However, a fixed number of Sendai virus spikes was always found in the ts045 virions. This represented 12 to 16% of the number of G proteins present in normal VSV. Furthermore, the yield of ts045 virions was radically reduced during double infection when the temperature was raised to block G-protein transport to the cell surface, suggesting that the Sendai virus glycoproteins were not able to compensate for G protein in budding. These results emphasize the role of the G protein in VSV assembly.

Prospects for the control of AIDS patients by introducing defective-HIV harbouring leukocytes

Introduction of leukocytes harbouring an artificially constructed defective HIV provirus into AIDS patients may result in inducing superinfection resistance against HIV and interfering with HIV receptors or replication of HIV. All these may slow down progression of the disease.

Glycoprotein cytoplasmic domain sequences required for rescue of a vesicular stomatitis virus glycoprotein mutant

We have used transient expression of the wild-type vesicular stomatitis virus (VSV) glycoprotein (G protein) from cloned cDNA to rescue a temperature-sensitive G protein mutant of VSV in cells at the nonpermissive temperature. Using cDNAs encoding G proteins with deletions in the normal 29-amino-acid cytoplasmic domain, we determined that the presence of either the membrane-proximal 9 amino acids or the membrane-distal 12 amino acids was sufficient for rescue of the temperature-sensitive mutant. G proteins with cytoplasmic domains derived from other cellular or viral G proteins did not rescue the mutant, nor did G proteins with one or three amino acids of the normal cytoplasmic domain. Rescue correlated directly with the ability of the G proteins to be incorporated into virus particles. This was shown by analysis of radiolabeled particles separated on sucrose gradients as well as by electron microscopy of rescued virus after immunogold labeling. Quantitation of surface expression showed that all of the mutated G proteins were expressed less efficiently on the cell surface than was wild-type G protein. However, we were able to correct for differences in rescue efficiency resulting from differences in the level of surface expression by reducing wild-type G protein expression to levels equivalent to those observed for the mutated G proteins. Our results provide evidence that at least a portion of the cytoplasmic domain is required for efficient assembly of the VSV G protein into virions during virus budding.

Binding and entry of animal viruses

Viruses are infectious agents capable of packaging and delivering nucleic acids and proteins to specific populations of cells. To initiate infection, viruses bind to sites, or receptors, on the cell surface and transfer their genome across the limiting membrane of the cell. The mechanisms underlying these events, and viral tropism for particular host cells, are becoming increasingly well understood. Several cell surface proteins have now been identified as viral receptors, and analyses of intact virus particles and sub-viral components are revealing the structures of the binding determinants on the viruses themselves. For many viruses, the events leading to penetration and delivery involve constitutive endocytic properties of the host cell, and the low pH environment in endocytic compartments is a crucial trigger in the penetration process. The knowledge of viral tropism, binding and entry suggests strategies which may be applied to the design of targeted therapeutic agents with appropriate specificities and effective delivery mechanisms.

Transcription inhibition site on the M protein of vesicular stomatitis virus located by marker rescue of mutant tsO23(III) with M-gene expression vectors

The matrix (M) protein of vesicular stomatitis virus serves as an endogenous inhibitor of viral transcription, a function missing or deficient in M proteins of temperature-sensitive (ts) mutants assigned to complementation group III. Previous studies with mutant tsO23(III) and vaccinia virus M-gene expression vectors revealed that the temperature-sensitive phenotype is due to a mutation leading to substitution of phenylalanine for leucine at amino acid III, whereas loss of the major antigenic determinant (epitope 1) of the mutant M protein results from the substitution of glutamic acid for the wild-type amino acid glycine at position 21 (Y. Li, L. Luo, R. M. Snyder, and R. R. Wagner, J. Virol. 62:3729-3737, 1988). We demonstrate here that transcription inhibition activity is restored to rescued tsO23 virus only when the rescuing vaccinia virus recombinant expresses M protein with glycine and not glutamic acid at amino acid 21. These experiments indicate the importance of the conformational integrity of the amino-terminal domain in determining the capacity of the vesicular stomatitis virus M protein to down regulate endogenous transcription.

Site-specific mutations in vectors that express antigenic and temperature-sensitive phenotypes of the M gene of vesicular stomatitis virus

Full-length cDNA copies of mRNAs coding for the matrix (M) proteins of vesicular stomatitis virus and its mutant tsO23(III) were cloned in pBSM13- (BlueScribe). The authenticity of these clones was demonstrated by restriction enzyme mapping, DNA sequencing, and in vitro transcription and translation to identify the two M proteins by Western immunoblotting with epitope-specific monoclonal antibodies. Site-directed mutants were constructed by primer extension of synthetic oligodeoxynucleotides with one or two nucleotide changes to alter the glycine at amino acid 21 of the wild-type (wt) M gene to glutamic acid, alanine, or proline. Similarly, a revertant was created in the M gene of mutant tsO23 by a Glu-21----Gly substitution. A series of wt- and mutant-M-gene chimeras was also constructed to create mutant and revertant clones with Leu----Phe and His----Tyr alterations at amino acids 111 and 227, respectively. We then moved the wt and tsO23 M genes and their site-specific mutants and chimeras cloned in pBSM13- into the eucaryotic expression vector pTF7 directed by the T7 bacteriophage RNA polymerase of the vaccinia virus recombinant vTF1-6,2. Western blot analysis of the M proteins transiently expressed in CV-1 cells by plasmids carrying M genes altered at amino acid 21 revealed that the critical antigenic determinant (epitope 1) is expressed only by the Gly-21 M protein and not by Glu-21, Ala-21, or Pro-21 M proteins. Of particular interest is an apparent conformational change, evidenced by slightly but significantly retarded electrophoretic migration, in plasmid-expressed M proteins with amino acids substituted for glycine at position 21. The glutamic acid at position 21 of tsO23 is not responsible for its temperature-sensitive phenotype, because a tsO23 revertant plasmid with glycine substituted at position 21 fails to rescue tsO23 virus in cells infected at the restrictive temperature; conversely, plasmids expressing wt M protein with substitutions of glutamic acid, alanine, or proline at position 21 are just as effective in marker rescue of tsO23 as is the Gly-21 wt M protein. Marker rescue experiments with wt- and mutant-M-gene chimeras support the hypothesis of K. Morita, R. Vanderoef, and J. Lenard (J. Virol. 61:256-263, 1987) that the temperature-sensitive phenotype of tsO23 is due to a phenylalanine substituted for leucine at amino acid 111, rather than the His-227----Tyr substitution or the Gly-21----Glu substitution, which independently accounts for the loss of epitope 1 in the mutant M protein of tsO23.(ABSTRACT TRUNCATED AT 400 WORDS)

Phenotypic revertants of temperature-sensitive M protein mutants of vesicular stomatitis virus: sequence analysis and functional characterization

Twenty-five spontaneous temperature-stable revertants of four different temperature-sensitive (ts) M protein mutants (complementation group III: tsG31, tsG33, tsO23, and tsO89) were sequenced and tested for their ability to inhibit vesicular stomatitis virus RNA polymerase activity in vitro. Consensus sequences of the coding region of each M protein gene were determined, using total viral RNA as template. Fifteen different sequences were found among the 25 revertants; 14 differed from their ts parent by a single amino acid (one nucleotide), and 1 differed by two amino acids (two nucleotides). Amino acids were altered in various positions between residues 64 and 215, representing over 60% of the polypeptide chain. Resequencing of the Glasgow and Orsay wild types and the four ts mutants confirmed previously published differences (Y. Gopalakrishana and J. Lenard, J. Virol., 56:655-659, 1985), and one or two additional differences were found in each. The relative charges of the revertant M proteins, as determined by nonequilibrium pH gradient electrophoresis, were consistent with the deduced sequences in every case. The ability of each revertant M protein to inhibit the RNA polymerase activity of nucleocapsids prepared from its parent ts mutant was also tested. Only 13 of the 25 revertants had M protein with high (wild type-like) polymerase-inhibiting activity, while 5 had low (ts-like) activity, and 7 had intermediate activity, demonstrating that this property is not an essential concomitant of the temperature-stable phenotype. It is concluded that the high reversion frequency observed for these mutants arises from a very high incidence of pseudoreversion, i.e., many different molecular changes can repair the ts phenotype.

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.

Sequence alterations in temperature-sensitive M-protein mutants (complementation group III) of vesicular stomatitis virus

Sequences were determined of the coding regions of the M-protein genes of the Glasgow and Orsay strains of vesicular stomatitis virus (Indiana serotype) and of two group III (M-protein) mutants derived from each wild type. Synthetic primers were annealed with viral genomic RNA and extended with reverse transcriptase. The resulting high-molecular-weight cDNA was sequenced directly. Both Glasgow and Orsay wild types differed in 13 bases from a clone of the San Juan strain sequenced by J. K. Rose and C. J. Gallione (J. Virol. 39:519-528, 1981). Six of these base changes caused amino acid changes in each wild type, whereas seven were degenerate. The Orsay and Glasgow sequences resembled each other more closely than either resembled that of Rose and Gallione, differing in eight nucleotides and four amino acids. Each of the four mutants, however, differed from its parent wild type in only one or two point mutations. Every mutation caused a change either from or to a charged amino acid; the change for tsG31 was Lys (position 215) to Glu, the change for tsO23 was Gly (position 21) to Glu, the change for tsO89 was Ala (position 133) to Asp, the changes for tsG33 were Lys (position 204) to Thr and Glu (position 214) to Lys. The charge differences predicted from these amino acid changes was confirmed by nonequilibrium pH gradient electrophoresis for tsG31, tsG33, tsO23, and the two wild types. These mutations affect residues spanning nearly 85% of the linear sequence, although the mutants possess nearly identical phenotypic properties.

Monoclonal antibodies to the M protein of vesicular stomatitis virus (Indiana serotype) and to a cDNA M gene expression product

Twenty-nine independent hybridomas producing monoclonal antibodies to the matrix (M) protein of vesicular stomatitis virus (Indiana serotype) were prepared by fusion of SP2/0 myeloma cells with spleen lymphocytes obtained from BALB/c mice which had been immunized with the purified M protein. The specific reactivity of each monoclonal antibody was determined by an enzyme-linked immunosorbent assay and a competitive binding assay. Most of the antibodies were of the immunoglobulin G2a and G2b isotypes, although some were immunoglobulin M. By measuring the competitive binding of 125I-antibody, we identified four antigenic determinants in the M protein of the virus; two of these determinants, however, exhibited a large degree of overlap. Western blot analysis revealed little or no cross-reactivity of the antibodies with other viral proteins or with the M protein of the New Jersey serotype. Prolonged trypsin proteolysis removed the first 43 amino acids from the amino-terminal region of the M protein, but it retained its reactivity with monoclonal antibodies to each epitope, except for diminished reactivity with one. To aid in future mapping of these epitopes, we inserted a cDNA clone of the mRNA encoding the M protein of vesicular stomatitis virus into an inducible lac expression vector; the M protein produced in the JM103 strain of Escherichia coli under induced conditions was found to be approximately the same size as native M protein and was recognized by the monoclonal antibodies. These monoclonal antibodies and the cDNA clone should be useful for studying the role of M protein in virus maturation and the regulation of viral transcription.

Further characterization of proteins assembled by vesicular stomatitis virus from human tumor cells

Vesicular stomatitis virus (VSV), when reproduced in human tumor cell lines, assembled a specific subset of cell-derived proteins. These were detected by 35S]methionine labeling of cells prior to infection and subsequent immunoprecipitation of VSV grown in these cells, as well as by direct immunoprecipitation of labeled cell extracts with antiserum directed against the VSV-assembled proteins. Their molecular weight (Mr) ranged between 15K and 180K; the larger proteins were glycosylated. Two of the major protein species (gp88 and gp130) were common to all four cell lines used (HeLa-cervical carcinoma, T47D-breast carcinoma, and HMB2 and SK1477-two melanoma cell lines). Proteins of other molecular weights were detected only in one or two of the cell lines. The melanoma cell lines (even in the absence of VSV) shed large particulate material which had contained the same spectrum of proteins that were assembled by VSV. The major protein component had an Mr of 30K. Some of the VSV-assembled proteins might possibly serve as specific tumor markers. It is also conceivable that the proteins assembled by VSV as well as the large particulate material might be products of defective endogenous human retroviruses.

Sequence of the envelope glycoprotein gene of type II human T lymphotropic virus

The sequence of the envelope glycoprotein gene of type II human T lymphotropic virus (HTLV) is presented. The predicted amino acid sequence is similar to that of the corresponding protein of HTLV type I, in that the proteins share the same amino acids at 336 of 488 residues, and 68 of the 152 differences are of a conservative nature. The overall structural similarity of these proteins provides an explanation for the antigenic cross-reactivity observed among diverse members of the HTLV retrovirus family by procedures that assay for the viral envelope glycoprotein, for example, membrane immunofluorescence.

Monoclonal antibody against an antigen selectively assembled into vesicular stomatitis virus virions from HeLa cells

A mouse hybridoma cell line IIB9, secreting IgG2b antibody specific for a HeLa cell antigen, was obtained by fusion of a mouse myeloma cell line with spleen cells from mice immunized with purified VSV tsO45 mutant (defective in assembly of G protein) which had been reproduced at a non-permissive temperature in HeLa cells. The monoclonal antibody IIB9 was strictly specific for HeLa cells in two tests: (1) reaction with VSV or Chandipura virus phenotypically mixed with host cell antigen, (2) complement-dependent cytotoxicity test (51Cr-release).

Pseudotypes of human T-cell leukemia virus types 1 and 2: neutralization by patients' sera

Pseudotypes of vesicular stomatitis virus (VSV) bearing envelope antigens of human T-cell leukemia virus (HTLV) types 1 and 2 were prepared by propagating VSV in cells lines productively infected with HTLV. Plaque assays of VSV (HTLV) pseudotypes were employed to determine the presence of (i) HTLV receptors on cells and (ii) neutralizing antibodies in the serum of patients with adult T-cell leukemia-lymphoma (ATLL). Cell surface receptors for HTLV-1 and HTLV-2 were found on nonlymphoid cells of human and mammalian origin. Neutralizing antibodies specific to VSV(HTLV-1) were found in sera of ATLL patients in titers varying from 1:50 to 1:30,000 and did not correlate closely with antibody titers for internal viral antigens. Sera from ATLL patients in the United Kingdom (Caribbean immigrants), United States, and Japan completely neutralized VSV (HTLV-1), indicating that the HTLV isolates from these distinct geographic regions represent a single envelope serotype. Neutralization of VSV (HTLV-1) was more specific and more sensitive than assays of syncytium inhibition. No cross-neutralization was observed between bovine leukosis virus and HTLV, and only limited cross-reaction was found for envelope antigens of HTLV-1 and HTLV-2. These studies show that VSV (HTLV) pseudotypes can be readily used to screen for neutralizing antibodies in patients' sera and to distinguish HTLV envelope serotypes.

Identity of HeLa cell determinants acquired by vesicular stomatitis virus with a tumor antigen

Growth of vesicular stomatitis virus (VSV) in HeLa cells results in progeny containing non-VSV antigens with a molecular weight around 75,000. The non-VSV antigens were detected by antiserums to HeLa cell determinants. These antiserums precipitate whole virions but do not neutralize them. Because one of the antiserums is directed to a tumor-specific surface antigen of HeLa cells, it appears that VSV specifically acquires such antigens during its passage through human tumor cells.

Human cell surface proteins selectively assembled into vesicular stomatitis virus virions

Vesicular stomatitis virus (VSV) selectively assembled proteins from human cells into progeny virions. These proteins can be surface labeled before infection with 125I, and when purified virus was examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, only two or three bands of proteins (Mr around 100K) were seen. Antisera to these proteins were produced, using as immunizing antigen VSV tsO45 mutant, defective in assembly of G protein, which had been made at the nonpermissive temperature in the three human tumor cell lines, HeLa (cervical carcinoma), T47D (breast carcinoma), and HMB2 (melanoma). After absorption with wild-type VSV, each of the antisera displayed a different pattern of reactivity; at least three antigenic specificities were detected. Two of them, corresponding to antigens selected by VSV from HeLa and T47D, were to some extent related and they showed an association mainly with epithelial cell-derived gynecological tumors, but they were absent in carcinomas of lung or of digestive tract. These (or related) antigens were expressed in a lower level in some normal tissues, mainly in ovaries. Antigen(s) assembled by VSV from the melanoma cell line was entirely different and appeared to be associated with cell growth. The grounds for selective assembly of these specific proteins by VSV are not clear; they either share with viral surface glycoproteins some physical or structural properties, which are critical for incorporation into the viral envelope, or conceivably they even may represent uncleaved precursor proteins coded by env genes of incomplete genomes of endogenous human retroviruses.

Rous-associated virus type 7 induces a syndrome in chickens characterized by stunting and obesity

Infection of 10-day-old chicken embryos with an avian retrovirus. Rous-associated virus type 7, resulted in a disease characterized by stunting and hyperlipidemia. By 20 days after hatch, infected chickens were smaller than hatchmates and developed ataxia and obesity over the next 30 days. Histological examinations of livers from infected chickens revealed a diffuse panlobular fatty infiltrate involving an accumulation of fat in microdroplets. Electron microscopic examinations of livers from infected chickens revealed hepatocytes with swollen mitochondria that lacked cristae. The thyroid and pancreas were infiltrated with lymphoblastoid cells by 1 week after hatch. An examination of the blood revealed a mild anemia, a frank lipemia, and high levels of uric acid. This syndrome induced by Rous-associated virus type 7 in chickens may be useful for elucidating the nature of several diseases, including that found in the fatty liver and kidney syndrome of chickens and that observed in a strain of obese chickens.

Purified matrix protein of vesicular stomatitis virus blocks viral transcription in vitro

One of the major structural proteins of vesicular stomatitis virus is a small, nonglycosylated, matrix protein which associates with the nucleocapsid core during final stages of morphogenesis and budding. Biochemical and genetic studies suggested that the matrix protein regulates RNA synthesis both in vitro and in vivo. We have purified biologically active matrix protein from the virus and have directly shown that it significantly inhibits RNA synthesis in vitro mediated by the virion-associated RNA polymerase at low ionic strength (0.02 M). The inhibition was greater than 80% when the ratio of matrix protein to the major nucleocapsid protein in the transcribing complex was 2:1 (wt/wt). The inhibition was found to be at the level of RNA chain elongation and not at the initiation step. Electron microscopic studies revealed that inhibition of transcription by matrix protein was accompanied by a profound structural change of the transcribing nucleocapsid from an extended structure to a highly compact form. At higher ionic strength (0.12 M), the matrix protein failed to interact with the nucleocapsid. The matrix protein appears to be involved in condensing the nucleocapsid and blocking transcription during maturation of the virus particle.

Delayed appearance of pseudotypes between vesicular stomatitis virus influenza virus during mixed infection of MDCK cells

In intact Madin-Darby canine kidney (MDCK) cell monolayers, vesicular stomatitis virus (VSV) matures only at basolateral membranes beneath tight junctions, whereas influenza virus buds from apical cell surfaces. Early in the growth cycle, the viral glycoproteins are restricted to the membrane domain from which each virus buds. We report here that phenotypic mixing and formation of VSV pseudotypes occurred when influenza virus-infected MDCK cells were superinfected with VSV. Up to 75% of the infectious VSV particles from such experiments were neutralized by antiserum specific for influenza virus, and a smaller proportion (up to 3%) were resistant to neutralization with antiserum specific for VSV. The latter particles, which were neutralized by antiserum to influenza A/WSN virus, are designated as VSV(WSN) pseudotypes. During mixed infections, both wild-type viruses were detected 1 to 2 h before either phenotypically mixed VSV or VSV(WSN) pseudotypes. Coincident with the appearance of cytopathic effects in the monolayer, the yield of pseudotypes rose dramatically. In contrast, in doubly infected BHK-21 cells, which do not show polarity in virus maturation sites and are not connected by tight junctions, VSV(WSN) pseudotypes were detected as soon as VSV titers rose to the minimum levels which allowed detection of pseudotypes, and the proportion observed remained relatively constant at later times. Examination of thin sections of doubly infected MDCK monolayers revealed that polarity in maturation sites was preserved for both viruses until approximately 12 h after inoculation with influenza virus, when disruption of junctional complexes was evident. Even at later periods, the majority of each virus type was associated with its normal membrane domain, suggesting that the sorting mechanisms responsible for directing the glycoproteins of VSV and influenza virus to separate surface domains continue to operate in doubly infected MDCK cells. The time course of VSV(WSN) pseudotype formation and changes in virus maturation sites are compatible with progressive mixing of viral glycoproteins at either intracellular or plasma membranes of doubly infected cells.

Interactions of wild-type and mutant M protein of vesicular stomatitis virus with viral nucleocapsid and envelope in intact virions. Evidence from [125I]iodonaphthyl azide labeling and specific cross-linking

Four different temperature-sensitive M protein mutants (tsM) of vesicular stomatitis virus (VSV) were characterized with regard to the association of the mutated M protein either with nucleocapsids or with membranes in the intact virions. Virions were labeled with the photoreactive hydrophobic probe [125I]iodonaphthyl azide (INA) to assess interactions between viral proteins and the lipid envelope. In wild type (wt) virions, the three major structural proteins--G, M, and N--were labeled in the ratio ca. 1.0:0.4:0.2. INA labeled only the membrane-associated peptide of G protein, both in the intact virion and in reconstituted G protein--viral lipid vesicles, demonstrating the specificity of INA for lipid bilayer regions. Labeling of tsM virions with INA resulted in a 2--3-fold greater incorporation into M protein than was found for wt virions, suggesting increased M--membrane associations in the mutant virions. Temperature-stable revertants from tsM possessed wt labeling characteristics. Interaction of the M protein with nucleocapsids was assessed from the abundance of disulfide-linked M--N complexes found after disruption of the virions by sodium dodecyl sulfate solution under nonreducing conditions. The abundance of such complexes was 30--80% less from tsM virions than from wt virions, suggesting decreased M--nucleocapsid interactions in tsM virions. Temperature-stable revertants from tsM resembled wt in the abundance of M--N complex formed. We conclude that the mutations alter M protein in such a way as simultaneously to increase its association with membrane and to decrease its affinity for nucleocapsids in the intact virion.

Fluorescence photobleaching recovery measurements reveal differences in envelopment of Sindbis and vesicular stomatitis viruses

Fluorescence photobleaching recovery (FPR) measurements of virus glycoproteins on the surfaces of cells infected with vesicular stomatitis virus (VSV) and Sindbis virus showed that the VSV glycoprotein (G) remained mobile throughout the infectious cycle, whereas Sindbis virus glycoproteins (E1, E2) were partially mobile early after infection and immobile at later times when greater amounts of these proteins were on the cell surface. A highly mobile fraction of Sindbis virus glycoproteins was detected throughout the replication cycle of a temperature-sensitive mutant unable to form virus particles. This immobilization of E1 and E2 was the result of increasing surface glycoprotein concentrations and virus budding. Together with other data, which included the detection of E1 and E2 in particles as soon as these proteins were transported to the cell surface, the FPR results suggest that Sindbis virus assembly initiates on intracellular vesicles, where glycoproteins aggregate and bind nucleocapsids. In contrast, our FPR data on VSV support a model previously suggested by others, in which a small fraction of cell-surface G is immobilized into budding sites formed by interactions with virus matrix and nucleoproteins. FPR measurements also provide direct evidence for strong interactions between E1 and E2, as well as between E1 and PE2, the precursor form of E2.

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.