Ebola Virus VP24 Binds Karyopherin α1 and Blocks STAT1 Nuclear Accumulation

Abstract

Ebola virus (EBOV) infection blocks cellular production of alpha/beta interferon (IFN-␣Further, VP24 is found to specifically interact with karyopherin ␣1, the nuclear localization signal receptor for PY-STAT1, but not with karyopherin ␣2, ␣3, or ␣4. Overexpression of VP24 results in a loss of karyopherin ␣1-PY-STAT1 interaction, indicating that the VP24-karyopherin ␣1 interaction contributes to the block to IFN signaling. These data suggest that VP24 is likely to be an important virulence determinant that allows EBOV to evade the antiviral effects of IFNs.The filoviruses, Ebola virus (EBOV) and Marburg virus, cause periodic outbreaks of severe hemorrhagic fever in humans. In EBOV outbreaks consisting of more than 10 reported cases, mortality rates have ranged from 40 to 90% (41), and Marburg virus outbreaks have had reported case fatality rates ranging from 25 to 80% (13). This extreme virulence has made Ebola and Marburg viruses of concern both as naturally emerging pathogens and as potential bioweapons (41).The molecular mechanisms contributing to the severe pathogenesis of filovirus infection are poorly understood. Several potential mechanisms contributing to EBOV virulence have been reviewed (41). These include cytotoxicity of the viral glycoprotein, the production of proinflammatory cytokines, and the dysregulation of the coagulation cascade due to the production of tissue factor (14,20,21,62,64). Infection also appears to induce a general immune suppression (11, 53). Possible mechanisms contributing to this suppression include inhibition of dendritic cell activation and an induction of lymphocyte apoptosis (2,8,18,22,43). Each of these pathogenic processes likely occurs as a result of the active replication of the virus. Thus, the ability of the virus to counteract early antiviral responses, including those of the host's interferon system, likely plays an important role in EBOV virulence (41).EBOV encodes mechanisms to counteract the host interferon (IFN) response by blocking both production of IFN-␣/␤ and cellular responses to IFN-␣/␤ or -␥ treatment (6,24,26,27). We previously demonstrated that the EBOV VP35 protein suppresses IFN-␣/␤ production by inhibiting the activation of interferon regulatory factor 3 (IRF-3) (5, 7, 51), and subsequent studies confirm that VP35 exerts this function (8, 28). However, the manner in which EBOV blocks signaling from the IFN-␣/␤ or -␥ receptor has remained incompletely defined.IFN-␣/␤, a family of structurally related proteins, and IFN-␥ bind to two distinct receptors but activate similar signaling pathways (reviewed in reference 38). For both pathways, ligand binding activates receptor-associated Jak family tyrosine kinases. These undergo auto-and transphosphorylation and phosphorylate the cytoplasmic domains of the receptor subunits. The receptor-associated phosphotyrosine residues then serve as docking sites for the SH2 domains of STAT proteins. The receptor-associated STATs then undergo tyrosine-phosphorylation and form homo-or heterodimers via reciprocal SH2 domai...

Post-exposure treatments for Ebola and Marburg virus infections

“…EBOV VP24 disrupts signal transducer and activator of transcription 1 (STAT1) translocation to the nucleus to prevent transcription of interferon response genes 110,111 . MARV VP40 directly blocks phosphorylation of Janus kinase 1 (JAK1), STAT1 and STAT2 (REF.…”

How Ebola and Marburg viruses battle the immune system

“…Recently, filoviruses were found to be capable of disabling at least some of the host IFN pathway: the filoviral protein VP35 prevents the production of type I IFNs (that is, IFNα and IFNβ) 19,20 and VP24 interferes with the ability of IFNα, IFNβ and IFNγ to induce an antiviral state in cells 21 (FIG. 2).…”

“…Additional forms of soluble glycoprotein arise post-translationally from proteolysis and/or failure to establish disulphide bonds between naturally occurring glycoprotein fragments GP1 and GP2; the role of soluble forms of glycoprotein in confounding the immune response remains hypothetical. The unchecked replication in primate cells can be explained in large part by the recently described capacity of VP35 to inhibit interferon (IFN)-regulatory factor 3 (IRF3) and thereby prevent infected cells from synthesizing type I IFNs 19 , and the ability of VP24 to interrupt the nuclear accumulation of tyrosine-phosphorylated STAT1 (signal transducer and activator of transcription 1), rendering cells insensitive to the antiviral effects of interferons 21 . EBOV and MARV are cytopathic in most cell types, but direct viral damage to cells is slow enough that host-cell pathways, including protein synthesis, continue to function for many hours, albeit abnormally.…”

Filovirus Replication and Transcription

“…The viral protein VP24 is presumed to be a minor matrix protein involved in nucleocapsid formation and assembly [13][14][15][16][17]. Since VP24 plays a crucial role in host tropism determination and is able to counteract the type I interferon response, it is considered to be an important virulence factor [18][19][20]. The four remaining proteins are tightly associated with the viral RNA genome, forming the nucleocapsid [21].…”