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J Virol. 2017 Jan 25. pii: JVI.00082-17. doi: 10.1128/JVI.00082-17. [Epub ahead of print]

The FACT complex promotes avian leukosis virus DNA integration.

Author information

  • 1Department of Biology, Johns Hopkins University, Baltimore, MD 21218.
  • 2Center for Retrovirus Research and Comprehensive Cancer Center, College of Pharmacy, The Ohio State University, Columbus, OH 43210.
  • 3Department of Biological Sciences, The University of Texas at Dallas, Richardson, TX 75080.
  • 4Department of Biology, Johns Hopkins University, Baltimore, MD 21218


All retroviruses need to integrate a DNA copy of their genome into the host chromatin. Cellular proteins regulating and targeting lentiviral and gammaretroviral integration in infected cells have been discovered, but the factors that mediate alpharetroviral avian leukosis virus (ALV) integration are unknown. Here, we have identified the FACT protein complex, which consists of SSRP1 and Spt16, as a principal cellular binding partner of ALV integrase (IN). Biochemical experiments with purified recombinant proteins show that SSRP1 and Spt16 are able to individually bind ALV IN, but only the FACT complex effectively stimulates ALV integration activity in vitro Likewise, in infected cells, the FACT complex promotes ALV integration activity with proviral integration frequency varying directly with cellular expression levels of the FACT complex. An increase in 2-LTR circles in the depleted FACT complex cell line indicates that this complex regulates the ALV life cycle at the level of integration. This regulation is shown to be specific to ALV, as disruption of the FACT complex did not inhibit either lentiviral or gammaretroviral integration in infected cells.


The majority of human gene therapy approaches utilize HIV-1 or MLV based vectors, which preferentially integrate near genes and regulatory regions; thus, insertional mutagenesis is a substantial risk. In contrast, ALV integrates more randomly throughout the genome, which decreases the risks of deleterious integration. Understanding how ALV integration is regulated could facilitate the development of ALV-based vectors for use in human gene therapy. Here we show that the FACT complex directly binds and regulates ALV integration efficiency in vitro and in infected cells.

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