Temperature-sensitive mutants of vesicular stomatitis virus: comparison of the in vitro RNA polymerase defects of group I and group IV mutants

Characterization of vesicular stomatitis virus mutants by partial proteolysis

Structural proteins of temperature-sensitive (ts) mutants of vesicular stomatitis virus, Indiana serotype, were compared with those of wild-type and revertant virions by electrophoresis on polyacrylamide gels of partial digests with Staphylococcus aureus V8 protease. Mutants of complementation groups III (tsG31 and tsG33), II (tsG22), and IV (tsG41) differed from the wild-type virion in peptide profiles of their M, NS, and N proteins, respectively. The differences were only detectable over a narrow range of enzyme-substrate ratios and were due to peptides transiently generated during incomplete digestion. Proteins of revertants to tsG31, tsG22, and tsG41 exhibited the wild-type virion peptide pattern, indicating that reversion had restored their original conformation. However, in the case of tsG22, the NS peptide profile reverted to the wild-type phenotype only partially, suggesting that a silent mutation might have taken place during either the original chemical mutagenesis or the following repeated laboratory passages. The apparent alteration in protein conformation and its restoration upon reversion of the mutants indicated that the lesions of groups III and IV were located in the M and N proteins, respectively. Moreover, for the first time, the site of mutation of group II could be positively identified as the NS protein cistron.

Neurovirulence mutant of vesicular stomatitis virus with an altered target cell tropism in vivo

Intracerebral infection of weanling Swiss mice with a temperature-sensitive (ts) mutant of vesicular stomatitis virus (VSV), ts pi364, resulted in a unique neuropathological syndrome not previously described with other VSV mutants. Mice infected with wild-type VSV died from an acute encephalitis characterized by neuronal necrosis and efficient virus replication in both brain and spinal cord. In contrast, with VSV ts pi364, the most prominent histopathological feature was destruction of the ependyma of the lateral ventricles. Virus antigen was also limited to the leptomeninges and the lateral ventricles. Infected mice survived and developed hydrocephalus. Replication of ts pi364 in the brain was 10- to 100- fold less than that of wild-type VSV, and appearance of virus in the spinal cord was delayed. VSV ts pi364 was isolated from mouse cells persistently infected with VSV. Another VSV ts pi mutant, isolated from the same persistent infection, behaved in vivo like wild-type VSV, even though both mutants were very similar in plaque size, reversion frequency, cut-off temperature, and synthesis of virus-specific proteins at semipermissive temperature. These results strongly suggest that VSV ts pi364 has a second, non-ts mutation which results in a restricted target cell range in vivo; wild-type VSV can infect both neurons and ependymal cells, whereas ts pi364 does not replicate in neurons.

Temperature-sensitive defect of vesicular stomatitis virus in complementation group II

The prototype member of the complementation group II temperature-sensitive (ts) mutants of vesicular stomatitis virus, ts II 052, has been investigated. In ts II 052-infected HeLa cells at the restrictive temperature (39.5 degrees C), reduced viral RNA synthesis was observed by comparison with infections conducted at the permissive temperature (30 degrees C). It was found that for an infection conducted at 39.5 degrees C, no 38S RNA or intracytoplasmic nucleocapsids were present. For nucleocapsids isolated from ts II 052 purified virions or from ts II 052-infected cells at 30 degrees C, the RNA was sensitive to pancreatic RNase after an exposure at 39.5 degrees C in contrast to the resistance observed for wild-type virus. The nucleocapsid stability of wild-type virus when heated to 63 degrees C or submitted to varying pH was not found in nucleocapsids extracted from ts II 052 purified virions. The data suggest that for ts II 052 there is an altered relationship between the viral 38S RNA and the nucleocapsid protein(s) by comparison with wild-type virus. Such results argue for the complementation group II gene product being N protein, so that the ts defect in ts II 052 represents an altered N protein.

Maturation of viral proteins in cells infected with temperature-sensitive mutants of vesicular stomatitis virus

Maturation of viral proteins in cells infected with mutants of vesicular stomatitis virus was studied by surface iodination and cell fractionation. The movement of G, M, and N proteins to the virion bud appeared to be interdependent. Mutations thought to be in G protein prevented its migration to the cell surface, allowed neither M nor N protein to become membrane bound, and blocked formation of viral particles. Mutant G protein appeared not to leave the endoplasmic reticulum at the nonpermissive temperature, but this defect was partially reversible. In cells infected with mutants that caused N protein to be degraded rapidly or prevented its assembly into nucleocapsids, M protein did not bind to membranes and G protein matured to the cell surface, but never entered structures with the density of virions. Mutations causing M protein to be degraded prevented virion formation, and G protein behaved as in cells infected by mutants in N protein. These results are consistent with a model of virion formation involving coalescence of soluble nucleocapsid and soluble M protein with G protein already in the plasma membrane.

Analysis of the defects of temperature-sensitive mutants of vesicular stomatitis virus: intracellular degradation of specific viral proteins

The metabolism of viral RNA and proteins has been studied in cells infected with temperature-sensitive mutant strains of vesicular stomatitis virus. Certain viral proteins encoded by the mutant strains, usually the putative mutant protein for the assigned complementation group, were shown to be degraded more rapidly at the nonpermissive temperature than were the wild-type proteins. Group III mutants (tsG33, tsM301) encode M proteins which are degraded three- to fourfold faster than the wild-type protein. This defect cannot be fully rescued by coinfection with wild-type virus, and thus the defect appears to be in the M protein itself. Mutants tsM601 (VI) and tsG41(IV) encode N proteins which are degraded much faster than the wild-type protein and also share the property of being defective in replication of viral RNA, suggesting a correlation between these phenotypic properties. Furthermore, the L proteins of tsG11(I) and tsG13(I) are more labile than the wild-type protein at the nonpermissive temperature. The G protein of tsM501(V) did not undergo the change in electrophoretic mobility previously shown to be the result of sialylation, suggesting that it is defective in maturation or glycosylation at the nonpermissive temperature. Three of the mutants previously isolated in this laboratory, tsM502(V), tsM601(VI), and tsM602(VI), were shown to be defective in viral RNA synthesis at the nonpermissive temperature. Mutant tsM601(VI) was defective mainly in viral RNA replication, whereas tsM502(V) appeared to be totally defective for viral RNA transcription and replication at the nonpermissive temperature.

RNA- temperature-sensitive mutants of vesicular stomatitis virus: L-protein thermosensitivity accounts for transcriptase restriction of group I mutants

In vitro transcriptase activity of three group I temperature-sensitive (ts) mutants of vesicular stomatitis virus restricted at 39 C was restored by L-protein fractions derived from wild-type (wt) vesicular stomatitis virion nucleo-capsids. Soluble NS protein from wt nucleocapsids did not reconstitute restricted transcriptions of the group I RNA-ts mutants. NS protein activity, but not L protein activity, was purified from the group I ts mutants; this NS fraction always displayed the wt phenotype in reconstitution assays. Neither the L nor the NS protein was capable of restoring the defective transcriptive activity of the group IV vesicular stomatitis virus mutant ts W16B.