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Fv-1 restriction and its effects on murine leukemia virus integration in vivo and in vitro
1Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0502.
Abstract:
We have investigated the mechanisms by which alleles at the mouse Fv-1 locus restrict replication of murine leukemia viruses. Inhibition of productive infection is closely paralleled by reduced accumulation of integrated proviral DNA as well as by reduced levels of linear viral DNA in a cytoplasmic fraction. Nevertheless, viral DNA is present at nearly normal levels in a nuclear fraction, and total amounts of viral DNA are only mildly affected in restrictive infections, suggesting a block in integration to account for reduced levels of proviral DNA. However, integrase (IN)-dependent trimming of 3' ends of viral DNA occurs normally in vivo during restrictive infections, demonstrating that not all IN-mediated events are prevented in vivo. Furthermore, viral integration complexes present in nuclear extracts of infected restrictive cells are fully competent to integrate their DNA into a heterologous target in vitro. Thus, the Fv-1-dependent activity that restricts integration in vivo may be lost in vitro; alternatively, Fv-1 restriction may prevent a step required for integration in vivo that is bypassed in vitro.
Insights
The Fv-1 locus restricts murine leukemia virus replication by blocking viral DNA integration. This restriction occurs despite normal integrase activity and integration complex competence in vitro, suggesting a specific in vivo block.
Area of Science:
- Virology
- Genetics
- Molecular Biology
Background:
- The Fv-1 locus in mice controls susceptibility to murine leukemia viruses (MLVs).
- Understanding Fv-1 restriction mechanisms is crucial for controlling retroviral infections.
Purpose of the Study:
- To elucidate the molecular mechanisms underlying Fv-1 mediated restriction of MLV replication.
- To determine the stage at which Fv-1 blocks productive viral infection.
Main Methods:
- Analysis of viral DNA levels (linear, nuclear, cytoplasmic, integrated proviral DNA) in permissive and restrictive cells.
- Assessment of integrase (IN) activity and viral integration complex competence in vitro.
Main Results:
- Fv-1 restriction correlates with reduced accumulation of integrated proviral DNA and cytoplasmic linear viral DNA.
- Viral DNA is present in the nucleus, and integration complex activity is normal in vitro.
- Integrase-dependent trimming of viral DNA 3' ends occurs normally in vivo during restriction.
Conclusions:
- Fv-1 restriction likely blocks a critical step in viral DNA integration in vivo.
- The restriction mechanism may involve a factor lost or bypassed during in vitro assays.
- Further research is needed to identify the precise Fv-1-dependent integration block.