Interferons are the main effectors of the antiviral immune response and are released from virus-infected cells to stimulate the transcription of antiviral genes by triggering phosphorylation, dimerization, and nuclear import of signal transducer and activator of transcription 1 (STAT1) and STAT2. Because Ebola virus blocks the interferon-mediated antiviral response, the virus replicates extremely rapidly and interferon treatment is ineffective. Most proteins imported into the nucleus contain a classical nuclear localization signal (cNLS) sequence that is recognized by members of the karyopherin-α (also known as KPNA or importin-α) family and mediates their nuclear import. However, STATs do not contain a cNLS. Instead, they rely on a largely uncharacterized nonclassical NLS (ncNLS) that is formed through the dimerization of phosphorylated STATs and recognized by KPNA1, KPNA5, and KPNA6, a subset of the KPNA family of nuclear cargo receptors. The VP24 protein of Ebola virus (eVP24) binds to KPNA1, KPNA5, and KPNA6 to block the nuclear import of phosphorylated STAT1 (pSTAT1). Xu et al. solved the crystal structure of eVP24 bound to the C-terminal portion of KPNA5, shedding light not only on how the virus blocks the host antiviral response but also on how KPNAs recognize ncNLS-containing proteins. The structure showed that eVP24 bound to domains of KPNA5 distinct from the domains through which KPNA5 binds to cNLS-containing proteins. The residues of KPNA5 that mediated binding to eVP24 were conserved in KPNA1 and KPNA6 but not conserved in KPNAs that bind only cNLS-containing proteins. These eVP24-binding residues also mediated pSTAT1 binding to KPNA5 in vitro. Mutating residues of eVP24 that contacted KPNA5 reduced nuclear accumulation of pSTAT1 and compromised the ability of eVP24 to inhibit expression of an interferon-inducible reporter gene. Coimmunoprecipitation experiments indicated that eVP24 competed with pSTAT1 for binding to KPNA5 in cells, but eVP24 did not compete with a cNLS peptide for binding to KPNA5, implying that eVP24 does not interfere with the nuclear trafficking of viral or host proteins that rely on a cNLS for nuclear import. Marburg virus, another filovirus closely related to Ebola virus, does not antagonize interferon signaling. Structural and sequence comparisons showed that Marburg virus VP24 (mVP24) differed from eVP24 in regions that mediated binding of eVP24 to KPNA5. Commentary by Daugherty and Malik notes that ncNLS-mediated nuclear import of STATs may have evolved to ensure that during viral infection STAT1 nuclear import and cNLS-dependent nuclear import mechanisms do not compete.

W. Xu, M. R. Edwards, D. M. Borek, A. R. Feagins, A. Mittal, J. B. Alinger, K. N. Berry, B. Yen, J. Hamilton, T. J. Brett, R. V. Pappu, D. W. Leung, C. F. Basler, G. K. Amarasinghe, Ebola virus VP24 targets a unique NLS binding site on karyopherin alpha 5 to selectively compete with nuclear import of phosphorylated STAT1. Cell Host Microbe 16, 187–200 (2014). [PubMed]

M. D. Daugherty, H. S. Malik, How a virus blocks a cellular emergency access lane to the nucleus, STAT! Cell Host Microbe 16, 150–152 (2014). [PubMed]