Barrier-to-Autointegration Factor BAF Binds p55 Gag and Matrix and Is a Host Component of Human Immunodeficiency Virus Type 1 Virions

Department of Cell Biology, Johns Hopkins University, Baltimore, Maryland, United States
Journal of Virology (Impact Factor: 4.44). 01/2004; 77(24):13084-92. DOI: 10.1128/JVI.77.24.13084-13092.2003
Source: PubMed


Barrier-to-autointegration factor (BAF) is a conserved human chromatin protein exploited by retroviruses. Previous investigators showed that BAF binds double-stranded DNA nonspecifically and is a host component of preintegration complexes (PICs) isolated from cells infected with human immunodeficiency virus type 1 (HIV-1) or Moloney murine leukemia virus. BAF protects PIC structure and stimulates the integration of salt-stripped PICs into target DNA in vitro. PICs are thought to acquire BAF from the cytoplasm during infection. However, we identified two human tissues (of 16 tested) in which BAF mRNA was not detected: thymus and peripheral blood leukocytes, which are enriched in CD4(+) T lymphocytes and macrophage precursors, respectively. BAF protein was detected in activated but not resting CD4(+) T lymphocytes; thus, if BAF were essential for PIC function, we hypothesized that virions might "bring their own BAF." Supporting this model, BAF copurified with HIV-1 virions that were digested with subtilisin to remove microvesicle contaminants, and BAF was present in approximately zero to three copies per virion. In three independent assays, BAF bound directly to both p55 Gag (the structural precursor of HIV-1 virions) and its cleaved product, matrix. Using lysates from cells overexpressing Gag, endogenous BAF and Gag were coimmunoprecipitated by antibodies against Gag. Purified recombinant BAF had low micromolar affinities (1.1 to 1.4 micro M) for recombinant Gag and matrix. We conclude that BAF is present at low levels in incoming virions, in addition to being acquired from the cytoplasm of newly infected cells. We further conclude that BAF might contribute to the assembly or activity of HIV-1 PICs through direct binding to matrix, as well as DNA.

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    • "We conclude that BAF and MA do not directly interact. Then how might the previously reported interactions [15] be explained? BAF and MA both bind DNA and we propose the protein preparations contained sufficient DNA to allow an apparent interaction between BAF and MA through DNA binding. "
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    ABSTRACT: Barrier-to-autointegration factor is a cellular protein that protects retroviral DNA from autointegration. Its cellular role is not well understood, but genetic studies show that it is essential and depletion or knockout results in lethal nuclear defects. In addition to binding DNA, BAF interacts with the LEM domain, a domain shared among a family of lamin-associated polypeptides. BAF has also been reported to interact with several other viral and cellular proteins suggesting that these interactions may be functionally relevant. We find that, contrary to previous reports, BAF does not interact with HIV-1 MA, cone-rod homeobox (Crx) or MAN1-C. The reported interactions can be explained by indirect association through DNA binding and are unlikely to be biologically relevant. A mutation that causes a premature aging syndrome lies on the previously reported MAN1-C binding surface of BAF. The absence of direct binding of BAF to MAN1-C eliminates disruption of this interaction as the cause of the premature aging phenotype.
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    • "We also found that the expression of several genes such as BANF1, BTRC, CD209, APOBEC3F, and TAT-SF1 increased in HD-HIV cells. BANF1 is known for its ability to protect retroviruses from intra-molecular integration and there by promoting intermolecular integration into the host cell genome [42]. BTRC interacts with HIV-1 viral protein U (Vpu) and connects CD4 to the proteolytic machinery [43]. "
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    • "LEDGF/p75 (single-peptide “hit”; Table 1) is a transcription factor that can bind human immunodeficiency virus type 1 (HIV-1) integrase directly, and contributes significantly to the efficiency and location of HIV-1 integration sites in human chromosomes [79]. BAF binds the HIV-1 matrix and Gag proteins directly, and is a component of HIV-1 virions [80]. BAF and emerin were both reported to contribute significantly to the efficiency of HIV-1 integration into primary human macrophages [33]. "
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