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

Intracellular behavior of rabies virus matrix protein (M) is determined by the viral glycoprotein (G)

To investigate the nature and intracellular behavior of the matrix (M) protein of an avirulent strain (HEP-Flury) of rabies virus, we cloned and sequenced the cDNA of the protein. Using expression vectors pZIP-NeoSV(X)1 and pCDM8, the cDNA was transfected to animal cells (BHK-21 and COS-7) with or without coexpression of viral glycoprotein (G). When M protein alone was expressed in the cells, it displayed homogeneous distribution in the whole cell including the nucleus. In contrast, coexpression with G protein resulted in the abolishment of nuclear distribution of M antigen, and both of the antigens displayed a colocalized distribution in the cell, especially at the cellular membrane as seen in the virus-infected cells, while the distribution of G antigen was not affected by coexpressed M antigen. Immunoprecipitation studies revealed that M protein was coprecipitated with G protein by anti-G antibody, and vice versa, although cross-linking with dithiobis(succinimidyl propionate) was necessary for coprecipitation because of their easier dissociation in the presence of sodium deoxycholate. These results suggest that M protein intimately associates with G protein, which may affect or regulate the behavior (e.g., intracellular localization) of M protein. Studies with deletion mutants of M protein indicate that an internal region around the amino acids from 115 to 151 is essential for the M protein to preserve its binding ability to G protein.

Viral liposomes released from insect cells infected with recombinant baculovirus expressing the matrix protein of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) has been found to promote assembly and budding of virions as well as down-regulating of VSV transcription. Large quantities of M protein can be produced in insect cells infected with recombinant baculovirus expressing the VSV M gene under control of the polyhedrin promoter. Analysis by pulse-chase experiments and density gradient centrifugation revealed that the [35S]methionine-labeled M protein synthesized in insect cells is released into the extracellular medium in association with lipid vesicles (liposomes). Electron microscopy and immunogold labeling showed that M protein expressed in insect cells induced the formation on plasma membrane of vesicles containing M protein, which are released from the cell surface in the form of liposomes. The baculovirus vector itself or recombinants expressing VSV glycoprotein (G) or nucleocapsid (N) protein did not produce the formation of vesicles in infected cells. The baculovirus-expressed M protein retains biological activity as demonstrated by its capacity to inhibit transcription when reconstituted with VSV nucleocapsids in vitro. These data suggest that M protein has the capacity to associate with the plasma membrane of infected cells and, in so doing, causes evagination of the membrane to form a vesicle which is released from the cell. This observation leads to the postulate, which requires further proof, that the VSV M protein can induce the formation and budding of liposomes from the cell membrane surface.

Alterations to influenza virus hemagglutinin cytoplasmic tail modulate virus infectivity

The influenza virus hemagglutinin (HA) contains a cytoplasmic domain that consists of 10 to 11 amino acids, of which five residues have sequence identity for 10 of 13 HA subtypes. To investigate properties of these conserved residues, oligonucleotide-directed mutagenesis was performed, using an HA cDNA of influenza virus A/Udorn/72 (H3N2) to substitute the conserved cysteine residues with other residues, to delete the three C-terminal conserved residues, or to remove the entire cytoplasmic domain. The altered HAs were expressed in eukaryotic cells, and the rates of intracellular transport were examined. It was found that substitution of either conserved cysteine residue within the cytoplasmic domain did not affect the rate of intracellular transport, whereas deletion of residues within the C-terminal domain resulted in delayed cell surface expression. All the altered HAs were biologically active in hemadsorption and fusion assays. To investigate whether the wild-type HA and HAs with altered cytoplasmic tails could complement the influenza virus temperature-sensitive transport-defective HA mutant A/WSN/33 ts61S, the HA cDNAs were expressed by using a transient expression system and released virus was assayed by plaque analysis. The wild-type HA expression resulted in a release of approximately 10(3) PFU of virus per ml. Antibody neutralization of complemented virus indicated that the infectivity was due to incorporation of wild-type H3 HA into ts61S virions. Sucrose density gradient analysis of released virions showed that each of the HA cytoplasmic domain mutants was incorporated into virus particles. Virions containing HAs with substitution of the cysteine residues in the cytoplasmic domain were found to be infectious. However, no infectivity could be detected from virions containing HAs that had deletions in their cytoplasmic domains. Possible roles of the HA cytoplasmic domain in forming protein-protein interactions in virions and their involvement in the initiation of the infection process in cells are discussed.

A single nucleotide substitution in the 5'-untranslated region of the vaccinia N2L gene is responsible for both alpha-amanitin-resistant and temperature-sensitive phenotypes

The locus responsible for encoding resistance to alpha-amanitin was previously mapped to the vaccinia virus (VV) HindIII N fragment by using cloned wild-type VV DNA fragments to rescue the ability of an alpha-amanitin-resistant/temperature-sensitive VV mutant (alpha rts7) to replicate under nonpermissive conditions. DNA sequencing and transcriptional analyses of this region identified two leftward-reading open reading frames (ORFs), N2L and M1L, as candidates to encode the protein responsible for eliciting both phenotypes. In the present study, high-resolution marker rescue mapping and genomic sequencing techniques have been applied to identify the nature of the mutation within the HindIII N region of the alpha rts7 genome. Interestingly, a single G to T transversion mutation was noted at position -10 relative to the initiator ATG of the N2L ORF. Since transcription of the N2L gene starts at position -12/-13, this places the alpha rts7 mutation within the 5'-untranslated leader of the N2L transcript expressed early in infection and suggests that the transcriptional efficiency, mRNA stability, or translational efficiency must be altered in the mutant RNA. These results identify the N2L ORF as the gene responsible for conferring resistance to alpha-amanitin in the alpha rts7 mutant and suggest that the N2L gene product is the viral function that interacts with the host cell nucleus during VV infection.

Cerulenin, an inhibitor of lipid synthesis, blocks vesicular stomatitis virus RNA replication

The replication of genomes of animal viruses in the cytoplasm of susceptible cells is usually coupled to specialized membrane structures. The inhibitor of lipid synthesis cerulenin blocks the formation of vesicular stomatitis virus polypeptides when added to cells soon after virus entry, but has much less effect on viral translation, or the acylation of the glycoprotein G, when cerulenin is added later during infection. By contrast, cerulenin powerfully blocks viral RNA synthesis or the incorporation of glycerol into lipids when present at any time after VSV-infection. These findings suggest that the synthesis of VSV RNA is dependent on continuous synthesis of lipids.

Sequences of the vesicular stomatitis virus matrix protein involved in binding to nucleocapsids

The purpose of these experiments was to study the physical structure of the nucleocapsid-M protein complex of vesicular stomatitis virus by analysis of nucleocapsid binding by wild-type and mutant M proteins and by limited proteolysis. We used the temperature-sensitive M protein mutant tsO23 and six temperature-stable revertants of tsO23 to test the effect of sequence changes on M protein binding to the nucleocapsid as a function of NaCl concentration. The results showed that M proteins from wild-type, mutant, and three of the revertant viruses had similar NaCl titration curves, while the curve for M proteins from the other three revertants differed significantly. The altered NaCl dependence of M protein was correlated with a single amino acid substitution from Phe to Leu at position 111 compared with the original temperature-sensitive mutant and was not correlated with a substitution of Gly to Glu at position 21 in tsO23 and the revertants. To determine whether protease cleavage sites in the M protein were protected by interaction with the nucleocapsid, nucleocapsid-M protein complexes were subjected to limited proteolysis with trypsin, chymotrypsin, or Staphylococcus aureus V8 protease. The initial trypsin and chymotrypsin cleavage sites, located after amino acids 19 and 20, respectively, were as accessible to proteases when M protein was bound to the nucleocapsid as when it was purified, indicating that this region of the protein does not interact directly with the nucleocapsid. Furthermore, trypsin or chymotrypsin treatment released the M protein fragments from the nucleocapsid, presumably due to conformational changes following proteolysis. V8 protease cleaved the M protein at position 34 or 50, producing two distinct fragments. The M protein fragment produced by V8 protease cleavage at position 34 remained associated with the nucleocapsid, while the fragment produced by cleavage at position 50 was released from the nucleocapsid. These results suggest that the amino-terminal region of the M protein around amino acid 20 does not interact directly with the nucleocapsid and that conformational changes resulting from single-amino-acid substitutions at other sites in the M protein are important for this interaction.

Cells that express all five proteins of vesicular stomatitis virus from cloned cDNAs support replication, assembly, and budding of defective interfering particles

An alternative approach to structure-function analysis of vesicular stomatitis virus (VSV) gene products and their interactions with one another during each phase of the viral life cycle is described. We showed previously by using the vaccinia virus-T7 RNA polymerase expression system that when cells expressing the nucleocapsid protein (N), the phosphoprotein (NS), and the large polymerase protein (L) of VSV were superinfected with defective interfering (DI) particles, rapid and efficient replication and amplification of (DI) particle RNA occurred. Here, we demonstrate that all five VSV proteins can be expressed simultaneously when cells are contransfected with plasmids containing the matrix protein (M) gene and the glycoprotein (G) gene of VSV in addition to plasmids containing the genes for the N, NS, and L proteins. When cells coexpressing all five VSV proteins were superinfected with DI particles, which because of their defectiveness are unable to express any viral proteins or to replicate, DI particle replication, assembly, and budding were observed and infectious DI particles were released into the culture fluids. Omission of either the M or G protein expression resulted in no DI particle budding. The vector-supported DI particles were similar in size and morphology to the authentic DI particles generated from cells coinfected with DI particles and helper VSV and their infectivity could be blocked by anti-VSV or anti-G antiserum. The successful replication, assembly, and budding of DI particles from cells expressing all five VSV proteins from cloned cDNAs provide a powerful approach for detailed structure-function analysis of the VSV gene products in each step of the replicative cycle of the virus.

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.