Macromolecular requirements for abrogation of Fv-1 restriction by murine leukemia viruses

Antiretroviral restriction factors in mice

One of the most exciting areas in contemporary retrovirus research is the discovery of "restriction factors". These are cellular proteins that act after virus entry to inhibit infection by or replication of retroviruses (and other viruses and intracellular pathogens). We briefly discuss here three antiretroviral restriction factors in mice: Fv1, APOBEC3, and tetherin, touching on both biological and molecular aspects of these restriction systems.

Retroviral restriction factors Fv1 and TRIM5alpha act independently and can compete for incoming virus before reverse transcription

The restriction factors Fv1 and TRIM5alpha provide dominant blocks to retroviral infection, targeting incoming capsids at a postentry, preintegration step. They both restrict N-tropic murine leukemia virus with similar specificity yet act at different points in the viral life cycle. TRIM5alpha-restricted virus is usually unable to reverse transcribe, whereas Fv1-restricted virus reverse transcribes normally. Here we investigate the relationship between these two restriction factors by expressing Fv1 alleles in human cells. We demonstrate that Fv1 is able to compete with TRIM5alpha for virus before reverse transcription. In human cells expressing Fv1(b), N-tropic restricted virus becomes less infectious but reverse transcribes more efficiently, indicating competition between the two antiviral molecules and protection of the virus from TRIM5alpha by Fv1. Our findings suggest that, like TRIM5alpha, Fv1 interacts with virus before reverse transcription, but the consequences of this interaction are not realized until a later stage of the life cycle. We also demonstrate that Fv1 is functionally independent of TRIM5alpha when expressed in human cells.

Genetic control of retrovirus susceptibility in mammalian cells

Host cellular genes can have profound effects on retrovirus replication. Many of these genes encode restriction factors that block virus infection; others encode positive factors that are exploited by the viruses. Recently, a number of such genes have been cloned and characterized, bringing into sharper focus the mechanisms and pathways exploited by these viruses. The major host factors involved in the early phase of the viral life cycle are discussed.

Retrovirus restriction factors

A number of cellular genes have recently been identified that actively inhibit retrovirus replication and so protect cells from infection. The genes target many distinct steps in the viral life cycle: entry, viral DNA synthesis, intracellular movement of viral nucleic acids, and viral gene expression. These restriction systems constitute newly appreciated components of an innate immunity that may be important for survival of a host exposed to retrovirus infection. It may someday be possible to enhance or activate these systems to induce antiviral states.

Characterization of murine leukemia virus restriction in mammals

Restriction of murine leukemia virus (MLV) was examined in cells from a range of mammals. All nonmurine restrictions were saturable blocks to N-tropic MLV infection, and several were prior to reverse transcription. We demonstrate restriction in cells from bat and show that if we express the murine restriction factor Fv1 in human cells, then Fv1, not the human host, defines the stage at which infection is blocked.

Abrogation of Fv-1 restriction by genome-deficient virions produced by a retrovirus packaging cell line

The Fv-1b-mediated restriction of N-tropic retrovirus vector infection of BALB/3T3 cells was partially abrogated by prior infection with N-tropic murine leukemia virus. Likewise, abrogation of the Fv-1b restriction of N-tropic murine leukemia virus replication was accomplished by prior infection with genome-deficient virions produced by an N-tropic murine leukemia virus packaging cell line. The latter observation suggests that the Fv-1 target in genome-deficient virions abrogates Fv-1 restriction in the absence of any viral genome-directed processes.

Development and characterization of an Fv-1-sensitive retrovirus-packaging system: single-hit titration kinetics observed in restrictive cells

We have constructed an RNA-packaging-deficient mutant of N-tropic murine leukemia virus WN1802N by removal of 330 nucleotides located between the upstream long terminal repeat and the start of the gag gene region. Transfection into mink CCL64 cells produced a cell line capable of packaging retrovirus vectors into ecotropic, Fv-1 N-tropic virions. Using retrovirus vectors that confer resistance to the antibiotic G418, we demonstrated that the magnitude of restriction in BALB/3T3 and SIM.R cells (both Fv-1b/b) and in RFM/3T3 cells (Fv-1nr/nr) is approximately 100-fold compared with that in AKR or NIH 3T3 cells (both Fv-1n/n). Furthermore, titration kinetics were single hit in restrictive cells. Colonies of antibiotic-resistant cells recovered after infection of genotypically restrictive cultures were phenotypically restrictive when reinfected, ruling out selection of stably nonrestrictive subpopulations. These results suggest that the ability to infect some fraction of cells in a genotypically restrictive culture does not require specific abrogation and that multihit kinetics may not be an essential feature of Fv-1 restriction.

Murine leukemia virus mutant with a frameshift in the reverse transcriptase coding region: implications for pol gene structure

The molecular defect in the nonconditional B-tropic MuLV pol mutant, clone 23 (Gerwin et al., J. Virol. 31:741-751, 1979), has been characterized by recombinant DNA technology. The entire mutant genome was cloned from an EcoRI digest of integrated cellular DNA into bacteriophage lambda Charon 4A and then subcloned at the EcoRI site of pBR322. NIH-3T3 cells transfected with the plasmid clone, termed pRTM (RTM, reverse transcriptase mutant), reproduced the properties of clone 23 virus-infected cells. In vivo ligation experiments involving cotransfection of subclones of pRTM and wild-type murine leukemia virus localized the defect in the clone 23 genome to an approximately 400-base-pair region in the pol gene between the SalI and XhoI sites. Sequence analysis of this region in the wild-type and mutant genomes revealed that the mutant has one additional C residue located 231 bases downstream of the last base of the SalI recognition site. This 1-base insertion brings three TGA termination codons into phase. Thus, the mutation in clone 23 leads to premature termination of translation, explaining the presence in clone 23 virions of a truncated polymerase with low levels of enzymatic activity. It was previously shown that the gag precursor is cleaved normally in clone 23-infected cells; therefore, if a virus-coded protease is involved in this cleavage, it must be encoded by sequences upstream of the reverse transcriptase region of the pol gene. This consideration, coupled with the observed molecular weight of the mutant polymerase and our precise determination of its C terminus, have led to a proposal for the genetic organization of the murine leukemia virus pol gene.