Search

The host response to murine ecotropic leukemia viruses is mainly controlled by the mouse Fv-1 gene. This locus controls virus replication at an intracellular stage and prevents provirus integration. Biological studies suggest that the Fv-1 effector molecule recognizes at least one virion structural protein. We have produced host range variants of B-tropic Friend murine leukemia virus in order to study the primary structure of potential viral target proteins. Our results show that conversion of B-tropism to NB-tropism is associated with changes in the primary structure of three gag proteins--p15, p12, and p30. These results suggest that host range conversion is due to a recombinational event, presumably between the parental virus and an endogenous murine virus. They also open the possibility that p12 and p30 may be involved in host range restriction.

Host restriction of exogenous infection by murine leukemia viruses is controlled in vitro predominantly by the murine Fv-1 locus. The mechanism of this host restriction was investigated by comparing the early events in the replication of N-tropic versus B-tropic Friend leukemia virus in NIH 3T3 cells. These cells, which are Fv-1nn in type, are permissive for the N-tropic strain, but nonpermissive for the B-tropic strain, which replicates permissively in Balb/c cells. We have studied the synthesis, intracellular location, and molecular form of virus-specific DNA early in replication by means of molecular hybridization with a virus-specific DNA probe. Our results suggest that in the permissive infection viral DNA rapidly becomes integrated with cellular DNA. However, in the nonpermissive infection, although almost equal amounts of both positive and negative strand viral DNA are synthesized, integration of the provirus does not occur.

Concanavalin A (Con A) has been used to rapidly and selectively agglutinate murine and avian oncornavirions from culture medium or plasma. The agglutinated virus was concentrated rapidly and gently by low-speed centrifugation and solubilization with alpha-methyl mannoside. Infectious virus was purified 2.3 times with respect to nucleic-acid content, and more than 60% of its infectivity was recovered. Infectious particles of densities 1.18 and 1.16 g/cm(3) were found in mouse cells infected with Friend virus. Con A reacted only with particles of density 1.16 g/cm(3), indicating heterogeneity with respect to carbohydrate content or structure as well as buoyant density. Electron microscopy of virus agglutinated with Con A showed a zone of Con A-glycoprotein complexes averaging 12-15 nm in thickness.

Host restriction of oncogenesis of RNA tumor viruses in vivo is associated with several gene loci. One of these genes, the Fv-1 locus in mice, is expressed in vitro and may be studied in mouse-embryo cultures that are restrictive or permissive for replication of Friend leukemia virus. Two strains of Friend leukemia virus, N-or B-tropic, show reciprocal ability to replicate successfully in either NIH Swiss (N-type) or BALB/c (B-type) cells that differ at the Fv-1 locus. These two strains of virus and two cell lines form a system to measure host restriction in vitro. Measurement of adsorption of Friend leukemia virus to permissive or restrictive cells reveals no difference in rate or total amount of virus bound. Furthermore, studies with virions of vesicular stomatitis virus phenotypically mixed within an envelope containing Friend leukemia virus protein show no differences in penetration or replication of vesicular stomatitis virus. These results strongly suggest that host restriction of Friend leukemia virus is due to an intracellular event in the viral replication cycle.