title: Host and viral determinants for MxB restriction of HIV-1 infection.
abstract: BACKGROUND
  Interferon-induced cellular proteins play important roles in the host response against viral infection. The Mx family of dynamin-like GTPases, which include MxA and MxB, target a wide variety of viruses. Despite considerable evidence demonstrating the breadth of antiviral activity of MxA, human MxB was only recently discovered to specifically inhibit lentiviruses. Here we assess both host and viral determinants that underlie MxB restriction of HIV-1 infection.
  
  
  RESULTS
  Heterologous expression of MxB in human osteosarcoma cells potently inhibited HIV-1 infection (~12-fold), yet had little to no effect on divergent retroviruses. The anti-HIV effect manifested as a partial block in the formation of 2-long terminal repeat circle DNA and hence nuclear import, and we accordingly found evidence for an additional post-nuclear entry block. A large number of previously characterized capsid mutations, as well as mutations that abrogated integrase activity, counteracted MxB restriction. MxB expression suppressed integration into gene-enriched regions of chromosomes, similar to affects observed previously when cells were depleted for nuclear transport factors such as transportin 3. MxB activity did not require predicted GTPase active site residues or a series of unstructured loops within the stalk domain that confer functional oligomerization to related dynamin family proteins. In contrast, we observed an N-terminal stretch of residues in MxB to harbor key determinants. Protein localization conferred by a nuclear localization signal (NLS) within the N-terminal 25 residues, which was critical, was fully rescuable by a heterologous NLS. Consistent with this observation, a heterologous nuclear export sequence (NES) abolished full-length MxB activity. We additionally mapped sub-regions within amino acids 26-90 that contribute to MxB activity, finding sequences present within residues 27-50 particularly important.
  
  
  CONCLUSIONS
  MxB inhibits HIV-1 by interfering with minimally two steps of infection, nuclear entry and post-nuclear trafficking and/or integration, without destabilizing the inherent catalytic activity of viral preintegration complexes. Putative MxB GTPase active site residues and stalk domain Loop 4 -- both previously shown to be necessary for MxA function -- were dispensable for MxB antiviral activity. Instead, we highlight subcellular localization and a yet-determined function(s) present in the unique MxB N-terminal region to be required for HIV-1 restriction.
authors.affiliation: Department of Cancer Immunology and AIDS, Dana Farber Cancer Institute and Department of Medicine, Harvard Medical School, Boston, MA, 02215, USA. kmatreyek@gmail.com.|Present address: Department of Genome Sciences, University of Washington, Seattle, WA, 98195, USA. kmatreyek@gmail.com.
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Matreyek
authors.affiliation: Department of Cancer Immunology and AIDS, Dana Farber Cancer Institute and Department of Medicine, Harvard Medical School, Boston, MA, 02215, USA. weifeng_wang@dfci.harvard.edu.
authors.forename: Weifeng
authors.initials: W
authors.lastname: Wang
authors.affiliation: Department of Cancer Immunology and AIDS, Dana Farber Cancer Institute and Department of Medicine, Harvard Medical School, Boston, MA, 02215, USA. erik_serrao@dfci.harvard.edu.
authors.forename: Erik
authors.initials: E
authors.lastname: Serrao
authors.affiliation: Section on Eukaryotic Transposable Elements, Program in Cellular Regulation and Metabolism, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA. parmit.singh@nih.gov.
authors.forename: Parmit Kumar
authors.initials: PK
authors.lastname: Singh
authors.affiliation: Section on Eukaryotic Transposable Elements, Program in Cellular Regulation and Metabolism, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA. levinh@mail.nih.gov.
authors.forename: Henry L
authors.initials: HL
authors.lastname: Levin
authors.affiliation: Department of Cancer Immunology and AIDS, Dana Farber Cancer Institute and Department of Medicine, Harvard Medical School, Boston, MA, 02215, USA. alan_engelman@dfci.harvard.edu.
authors.forename: Alan
authors.initials: A
authors.lastname: Engelman
mesh_terms: D021581:Active Transport, Cell Nucleus; D000998:Antiviral Agents / Q000378:metabolism*; D002460:Cell Line; D015497:HIV-1 / Q000276:immunology* / Q000502:physiology*; D006801:Humans; D064547:Myxovirus Resistance Proteins / Q000378:metabolism*; D016662:Virus Integration
chemical_list: D000998:Antiviral Agents; C579068:MX2 protein, human; D064547:Myxovirus Resistance Proteins