Identification of the Gross cell surface antigen associated with murine leukemia virus-infected cells.

The Gross cell surface antigen (GCSA) is produced by cells that are either exogenously infected with murine leukemia virus (MuLV) or are expressing endogenous MuLV genomes. In immune precipitation assays, GCSA was resolved into two serologically distinct 85,000- and 95,000-dalton viral proteins. These antigenic components are glycosylated forms of the polyprotein precursors of the MuLV internal structural proteins.     

Identification of an FMR cell surface antigen associated with murine leukemia virus-infected cells.

FMR antigens are found on the surface of cells infected with Friend, Moloney, and Rauscher murine leukemia viruses (MuLV). These antigens are serologically distinct from the G cell surface antigens that are found on cells infected with endogenous MuLV (AKR and Gross virus). Cell surface antigens of both virus groups are immunogenic in mice, and immunization with appropriate virus-infected cells leads to the production of cytotoxic antisera. The cytotoxic activity of FMR antisera can be absorbed by disrupted preparations of Rauscher MuLV, but not by AKR MuLV. FMR antisera precipitate the viral envelope proteins gp70, pl5(E), and p12(E) from detergent-disrupted preparations of [3H]leucine-labeled MuLV. The reaction of these antisera with p15(E) and p12(E) proteins is directed against group-specific antigens and can be absorbed with AKR MuLV; in contrast, the reaction of these antisera with gp70 is directed against type-specific antigens and is absorbed only by viruses of the FMR group. In immune precipitation assays with detergent-disrupted 125I surface-labeled cells, FMR antisera react only with type-specific antigens of the viral envelpe protein. On the basis of these findings we conclude that the FMR cell surface antigen is a determinant on the MuLV env gene product.     

Induction of GIX antigen and gross cell surface antigen after infection by ecotropic and xenotropic murine leukemia viruses in vitro.

A number of ecotropic and xenotropic murine leukemia viruses were examined for their ability to induce the GIX antigen and Gross cell surface antigen (GCSA) in tissue culture fibroblasts. GIX appears to be a constituent of murine leukemia virus gp70; a molecular characterization of GCSA has not yet been reported. Antigen induction was measured by the ability of productively infected cells to absorb cytotoxic activity from the standard GIX- and GCSA-typing antisera. Cells infected by ecotropic viruses displayed four distinct phenotypes GIX:+/GCSA++, GIX-/GCSA++, GIX++/GCSA+, and GIX-/GSCA+; cells infected by xenotropic viruses were either GIX-/GCSA+ or GIX-/GCSA-. GIX induction appeared to be a type-specific property of some but not all Gross-AKR type ecotropic viruses. Differences in the degree of absorption of the GCSA antiserum by ecotropic virus- and xenotropic virus-infected cells indicated that GCSA may comprise multiple antigenic determinants.     

Structural studies of the RNA pseudoknot required for readthrough of the gag-termination codon of murine leukemia virus.

Retroviruses, such as murine leukemia virus (MuLV), whose gag and pol genes are in the same reading frame but separated by a UAG stop codon, require that 5-10 % of ribosomes decode the UAG as an amino acid and continue translation to synthesize the Gag-Pol fusion polyprotein. A specific pseudoknot located eight nucleotides 3' of the UAG is required for this redefinition of the UAG stop codon. The structural probing and mutagenic analyses presented here provide evidence that loop I of the pseudoknot is one nucleotide, stem II has seven base-pairs, and the nucleotides 3' of stem II are important for function. Stem II is more resistant to single-strand-specific probes than stem I. Sequences upstream of the UAG codon allow formation of two competing structures, a stem-loop and the pseudoknot.     

Possible cell surface receptor for Friend murine leukemia virus isolated with viral envelope glycoprotein complexes.

Water-soluble multimeric complexes of Friend leukemia virus envelope glycoprotein gp85 bind specifically to C57BL/6 mouse spleen leukocytes. Such complexes were used to isolate cell surface receptors for the virus, using an immunoprecipitation technique. The putative rceptor has a molecular weight of 14,000. Mouse H-2 histocompatibility antigens, which are receptors for Semliki Forest virus, are not receptors for Friend leukemia virus.     

The 17 nucleotides downstream from the env gene stop codon are important for murine leukemia virus packaging

We have identified a previously unknown nucleotide sequence important for the packaging of murine leukemia virus. This nucleotide sequence is located downstream from the stop codon of the env gene but does not overlap the polypurine tract. Deletion of 17 bp from this region resulted in a more than 10-fold decrease in viral titer. Consistent with this result, the deletion mutant showed a 20- to 30-fold drop in the amount of virion RNA in the culture supernatant. The total amount of virion protein in the culture supernatant was comparable for the deletion mutant and the parental virus, suggesting that the mutant construct could release the empty viral particles. These results suggested that the packaging signal sequence might be present at the two extreme sites of the viral genome, one in the region around the splice donor sequence downstream from the 5' long terminal repeat (LTR) and the other immediately upstream from the 3' LTR. Implications for gene therapy, especially in regard to construction of retroviral vectors and packaging constructs, are discussed.