Temperature-Sensitive Mutants of Vesicular Stomatitis Virus: Synthesis of Virus-Specific Proteins

Viral proteins synthesized in L cells infected with temperature-sensitive (ts) mutants of vesicular stomatitis (VS) virus at permissive (31 C) and nonpermissive (39 C) temperatures were compared by polyacrylamide gel electrophoresis. Mutant ts 5, deficient in synthesis of viral ribonucleic acid (RNA⁻), failed to synthesize any of the five identifiable viral proteins at 39 C. Each of three RNA⁺ mutants, representing three separate complementation groups, showed distinctive patterns of viral protein synthesis at nonpermissive temperature. Equivalent amounts of ³H-amino acids were incorporated into the five viral proteins made in cells infected with RNA⁺ mutant ts 45 at 31 and 39 C. Complete virions of ts 45 could be identified by electron microscopy of infected cells incubated at the nonpermissive temperature; the defect in ts 45 appeared to be due in part to greater thermolability of virions as compared with the wild-type. RNA⁺ mutant ts 23 was deficient in synthesis of viral envelope protein S and failed to make detectable virions at the nonpermissive temperature. Infection of cells at 39 C with the third RNA⁺ mutant, ts 52, resulted in synthesis of all five viral proteins, but the peak of radioactivity representing the viral membrane glycoprotein migrated more rapidly on gels than coelectrophoresed authentic virion ¹⁴C-glycoprotein or viral ³H-glycoprotein extracted from cells infected at 31 C. These data and results of experiments on incorporation of radioactive glucosamine suggest that the primary defect in mutant ts 52 at nonpermissive temperature is failure of glycosylation of the viral glycoprotein. The viral structural proteins made in cells infected with ts 52 at the nonpermissive temperature did not assemble into sedimentable components as they did at permissive temperature; this observation indicates failure of insertion of the nonglycosylated protein (G′) into cell membrane. In support of this hypothesis was the finding that antiviral-antiferritin hybrid antibody did not detect VS viral antigen on the plasma membrane of L cells infected at 39 C with ts 52. In contrast, VS viral antigen localized in plasma membrane of L cells infected at 39 C with mutants ts 23 and ts 45 was readily detected by electron microscopy and fluorescence microscopy.     

Preliminary Physiological Characterization of Temperature-Sensitive Mutants of Vesicular Stomatitis Virus

Different temperature-sensitive mutants of vesicular stomatitis virus have been characterized in terms of their ability to induce synthesis of viral ribonucleic acid (RNA) in BHK-21 cells at 39 C (the restrictive temperature for these mutants). Mutants belonging to complementation groups I and IV (and probably II) did not induce actinomycin-resistant RNA synthesis in infected cells incubated at 39 C. All three mutants comprising complementation group III induced viral RNA synthesis at 39 C. The temperature sensitivity of the defective viral functions has also been studied by temperature-shift experiments. The functions associated with the mutants of groups I, II, and IV were required early, whereas the function associated with the group III mutants was not required until a late stage of the viral cycle. The heat sensitivity of extracellular virion was not correlated with complementation group.     

Defective Particles in BHK Cells Infected with Temperature-Sensitive Mutants of Vesicular Stomatitis Virus

Defective particles were the major product after undiluted passage of certain temperature-sensitive (ts) mutants of the Indiana C strain of vesicular stomatitis virus in BHK-21 cells at the permissive temperature (31 C). Essentially homogeneous preparations of defective particles were obtained with the wild-type and individual ts mutants. The defective particles associated with some of the ts mutants, however, were morphologically and physically distinguishable from wild type and from each other. All varieties of defective particle interfered with the multiplication of mutant and wild-type virus at the permissive temperature at early times of infection but failed to complement virions of different complementation groups at the restrictive temperature (39 C) at any time during infection.     

Temperature-Sensitive Mutants of Vesicular Stomatitis Virus: In Vitro Studies of Virion-Associated Polymerase

The temperature dependence of the virion-associated polymerase activity of six temperature-sensitive (ts) mutants of vesicular stomatitis virus (tsW10, 11, 14, 16B, 28, and 29) has been examined in vitro and compared to the heat-resistant parent (HR). The polymerase of five of the mutants (tsW10, 11, 14, 16B, and 28) appears to be significantly more ts than that of HR. Because certain pairs of these five mutants can complement each other's in vitro polymerase activity, it appears that in vitro some components involved in the polymerase of one virion can be utilized by another virion. Examination of 19 revertants of tsW11 and tsW16B which had regained their ability to replicate at 38 C showed that their in vitro polymerase activity had also become less ts. Furthermore, it was found that the pairs of mutants which showed in vitro complementation of polymerase activity at 38 C were those which had shown complementation in yielding infectious progeny in mixedly infected cells. These two observations suggest that the ts behavior of the in vitro polymerase activity of the five mutants is related to their inability to replicate at the nonpermissive temperature.     

Isolation and Characterization of Temperature-Sensitive Mutants of Vesicular Stomatitis Virus, New Jersey Serotype

Forty-eight temperature-sensitive (ts) mutants have been isolated from a wild-type strain of the New Jersey serotype of vesicular stomatitis virus (VSV) after exposure to the base analogue mutagen 5-fluorouracil. Of these mutants, 47 have been classified into 6 nonoverlapping complementation groups containing 21, 17, 4, 3, 2, and 1 mutant, respectively (1 mutant remaining unallocated). The ribonucleic acid (RNA) phenotype of 23 of these mutants has been established. Four of the six groups contain one or more mutants unable to synthesize detectable amounts of viral RNA under restrictive conditions (39 C). No complementation was observed in mixed infection with ts mutants from the five established complementation groups of the Indiana serotype of VSV.     

Temperature-Sensitive Mutants of Complementation Group E of Vesicular Stomatitis Virus New Jersey Serotype Possess Altered NS Polypeptides

In vesicular stomatitis virus New Jersey serotype polyacrylamide gel electrophoresis was unable to distinguish the polypeptides of the temperature-sensitive (ts) mutants of complementation groups A, B, C, and F from those of the wild-type virus. However, the NS polypeptide of the representative mutant of group E, ts E1, had a significantly greater electrophoretic mobility than that of the wild-type virus NS polypeptide. The electrophoretic mobilities of the NS polypeptides of the three mutants of complementation group E varied, being greatest in the case of ts E1, slightly less for ts E2, and only a little greater than that of wild-type virus NS polypeptide in the case of ts E3. Since the NS polypeptides of the revertant clones ts E1/R1 and ts E3/R1 have mobilities identical to that of wild-type NS polypeptide, the observed altered mobilities of the group E mutants are almost certainly the direct result of the ts mutations in the E locus. The electrophoretic mobilities of the intracellular NS polypeptides of the group E mutants were indistinguishable from those of their virion NS polypeptides. The electrophoretic mobilities of the NS polypeptides of the group E mutants synthesized in vitro using mRNA synthesized in vitro by TNP were identical to those of the NS polypeptides of their purified virions. The NS polypeptides of all three mutants were labeled with (32)P(i) to approximately the same extent as wild-type virus NS polypeptide, indicating that gross differences in phosphorylation of this polypeptide are unlikely to account for the altered mobilities. We propose a model in which the NS polypeptide consists of at least three loops held in this configuration by hydrophobic or ionic forces or both and stabilized by phosphodiester bridges. If a mutation affects one of the amino acids to which the phosphate is covalently linked, the phosphodiester bridge cannot be formed, and, as a result, in the presence of sodium dodecyl sulfate the affected loop opens and thus the NS polypeptide migrates further into the gel. Such a configuration may also explain the multifunctional nature of the NS polypeptide.     

Enhanced Mutability Associated with a Temperature-Sensitive Mutant of Vesicular Stomatitis Virus

Temperature-sensitive (ts) mutant tsD1 of vesicular stomatitis virus, New Jersey serotype, is the sole representative of complementation group D. Clones derived from this mutant exhibited three different phenotypes with respect to electrophoretic mobility of the G and N polypeptides of the virion in sodium dodecyl sulfate-polyacrylamide gel. Analysis of non-ts pseudorevertants showed that none of the three phenotypes was associated with the temperature sensitivity of mutant tsD1. Additional phenotypes, some also involving the NS polypeptide, appeared during sequential cloning, indicating that mutations were generated at high frequency during replication of tsD1. Furthermore, mutations altering the electrophoretic mobility of the G, N, NS, and M polypeptides were induced in heterologous viruses multiplying in the same cells as tsD1. These heterologous viruses included another complementing ts mutant of vesicular stomatitis virus New Jersey and ts mutants of vesicular stomatitis virus Indiana and Chandipura virus. Complete or incomplete virions of tsD1 appeared to be equally efficient inducers of mutations in heterologous viruses. Analysis of the progeny of a mixed infection of two complementing ts mutants of vesicular stomatitis virus New Jersey with electrophoretically distinguishable G, N, NS, and M proteins yielded no recombinants and excluded recombination as a factor in the generation of the electrophoretic mobility variants. In vitro translation of total cytoplasmic RNA from BHK cells indicated that post-translational processing was not responsible for the aberrant electrophoretic mobility of the N, NS, and M protein mutants. Aberrant glycosylation could account for three of four G protein mutants, however. Some clones of tsD1 had an N polypeptide which migrated faster in sodium dodecyl sulfate-polyacrylamide gel than did the wild type, suggesting that the polypeptide might be shorter by about 10 amino acids. Determination of the nucleotide sequence to about 200 residues from each terminus of the N gene of one of these clones, a revertant, and the wild-type parent revealed no changes compatible with synthesis of a shorter polypeptide by premature termination or late initiation of translation. The sequence data indicated, however, that the N-protein mutant and its revertant differed from the parental wild type in two of the 399 nucleotides determined. These sequencing results and the phenomenon of enhanced mutability associated with mutant tsD1 reveal that rapid and extensive evolution of the viral genome can occur during the course of normal cytolytic infection of cultured cells.     

Neuroblastoma Cell Fusion by a Temperature-Sensitive Mutant of Vesicular Stomatitis Virus

A temperature sensitive mutant of vesicular stomatitis virus which does not mature properly when grown at 39 degrees C promoted extensive fusion of murine neuroblastoma cells at this nonpermissive temperature. Polykaryocytes apparently formed as a result of fusion from within the cells that requires low doses of infectious virions for its promotion and is dependent on viral protein synthesis. Although 90% of infected N-18 neuroblastoma cells were fused by 15 h after infection, larger polykaryocytes continued to form, leading to an average of 28 nuclei per polykaryocyte as a result of polykaryocytes fusing to each other. Two neuroblastoma cell lines have been observed to undergo fusion, whereas three other cell lines (BHK-21, CHO, and 3T3) were incapable of forming polykaryocytes, suggesting that nervous system-derived cells are particularly susceptible to vesicular stomatitis virus-induced fusion. Although the normal assembly of the protein components of this virus is deficient at 39 degrees C, the G glycoprotein was inserted into the infected cell membranes at this temperature. Two lines of evidence suggest that the expression of G at the cell surface promotes this polykaryocyte formation: (i) inhibition of glycosylation, which may be involved in the migration of the G protein to the cellular plasma membranes, will inhibit the cell fusion reaction; (ii) addition of antiserum, directed toward the purified G glycoprotein, will also inhibit cell fusion.     

Model for Vesicular Stomatitis Virus

Vesicular stomatitis virus contains single-stranded ribonucleic acid of molecular weight 3.6 x 10(6) and three major proteins with molecular weights of 75 x 10(3), 57 x 10(3), and 32.5 x 10(3). The proteins have been shown to be subunits of the surface projections, ribonucleoprotein, and matrix protein, respectively. From these values and from estimates of the proportions of the individual proteins, it has been calculated that the virus has approximately 500 surface projections, 1,100 protein units on the ribonucleoprotein strand, and 1,600 matrix protein units. Possible models of the virus are proposed in which the proteins are interrelated.     

Proteins of Vesicular Stomatitis Virus and of Phenotypically Mixed Vesicular Stomatitis Virus-Simian Virus 5 Virions

The identity of the glycoprotein of vesicular stomatitis virus (VSV) as the spike protein has been confirmed by the removal of the spikes with a protease from Streptomyces griseus, leaving bullet-shaped particles bounded by a smooth membrane. This treatment removes the glycoprotein but does not affect the other virion proteins, apparently because they are protected from the enzyme by the lipids in the viral membrane. The proteins of phenotypically mixed, bullet-shaped virions produced by cells mixedly infected with VSV and the parainfluenza virus simian virus 5 (SV5) have been analyzed by polyacrylamide gel electrophoresis. These virions contain all the VSV proteins plus the two SV5 spike proteins, both of which are glycoproteins. The finding of the SV5 spike glycoproteins on virions with the typical morphology of VSV indicates that there is not a stringent requirement that only the VSV glycoprotein can be used to form the bullet-shaped virion. On the other hand, the SV5 nucleocapsid protein and the major non-spike protein of the SV5 envelope were not detected in the phenotypically mixed virions, and this suggests that a specific interaction between the VSV nucleocapsid and regions of the cell membrane which contain the nonglycosylated VSV envelope protein is necessary for assembly of the bullet-shaped virion.