Nipah Virus V Protein Evades Alpha and Gamma Interferons by Preventing STAT1 and STAT2 Activation and Nuclear Accumulation

Abstract

Interferons (IFNs) are the primary innate antiviral cytokines in mammalian cells and also regulate aspects of the adaptive immune response (3,5,27). Both IFN-␣/␤ and IFN-␥ can induce an antiviral state in cells as an end point of signal transduction through the JAK-STAT pathway. The general paradigm for this signaling pathway involves IFN-dependent receptor-mediated tyrosine phosphorylation of latent cytoplasmic STAT proteins to produce DNA-binding oligomers that are competent for nuclear translocation. IFN-␣/␤ responses are mediated primarily by an activated transcription complex, ISGF3, that consists of heterodimeric STAT1 and STAT2 in association with a DNA binding subunit, IRF9. ISGF3 binds to a DNA element, ISRE, in IFN-␣/␤ target gene promoters, inducing their transcription. IFN-␥ responses are also mediated by a STAT-containing transcription complex consisting of STAT1 homodimers. The STAT1 homodimer binds to a DNA element termed GAS to induce the transcription of a distinct but overlapping set of IFN-␥ target genes. In both cases, these IFN-stimulated gene products produce a general antiviral state in the cell that inhibits the replication of diverse virus species.The Paramyxoviridae family represents a wide variety of enveloped minus-strand RNA viruses including a number of established and emerging human and animal pathogens (12). Several paramyxoviruses, particularly those in the Rubulavirus genus, have been recently demonstrated to interfere with IFNinduced antiviral responses by targeting the STAT1 and/or STAT2 proteins for proteasomal degradation. For example, mumps virus and simian virus 5 (SV5) target STAT1 for degradation, while human parainfluenza virus type 2 (HPIV2) targets STAT2 (7,11,20).The exact mechanisms underlying STAT protein targeting by the rubulaviruses are not completely understood, but it is clear that the anti-STAT effects are mediated by a single viral protein, termed V (7). The V protein is not related to any cellular proteins and does not appear to be a protease itself; instead, it acts in concert with cellular factors to mediate STAT degradation. For SV5-induced STAT1 degradation and HPIV2-induced STAT2 degradation, the targeting complex requires at least V, STAT1, and STAT2 to be present in the host cell (21). The role for a nontarget STAT protein in a STAT-targeting complex is not apparent, but available evidence indicates that the requirement for the confederate STAT protein is completely independent of IFN-␣/␤ signal transduction or ISGF3 functions and can be supplied by a STAT protein N-terminal fragment (21). Moreover, the ability of SV5 to induce the degradation of STAT1 and to antagonize IFN signaling is species specific. IFN evasion by SV5 is very efficient in primate cells but restricted in murine cells (6). The cellular basis for the differential ability of murine and human cells to create an innate antiviral response to SV5 was recently uncovered (19). The failure of SV5 to antagonize IFN responses in the mouse system is not due to intrinsic defects in the SV5 pro...

Identification of the Nuclear Export Signal and STAT-Binding Domains of the Nipah Virus V Protein Reveals Mechanisms Underlying Interferon Evasion

“…Expression vectors for FLAG epitope-tagged Nipah virus and Hendra virus V proteins were obtained as described previously (24,25). To create truncated Nipah virus V proteins, PCR products were generated with BglII and NotI restriction sites and were ligated into a pEFN expression vector containing an in-frame N-terminal FLAG epitope tag.…”

“…A new genus of recently emerged paramyxoviruses, Henipavirus, was demonstrated to share V-dependent IFN signaling evasion properties with other paramyxoviruses (21,24,25). Encoded by a polycistronic gene, the 405-amino-acid V protein N terminus is shared with the co-amino-terminal P and W proteins (3).…”

“…As an IFN response modifier, the V protein is a virulence and pathogenesis-determining factor. In the case of SV5, species-specific restriction of STAT1 degradation has been shown to be a determinant of host species tropism, restricting SV5 replication in IFN-competent murine cells (20,31).A new genus of recently emerged paramyxoviruses, Henipavirus, was demonstrated to share V-dependent IFN signaling evasion properties with other paramyxoviruses (21,24,25). Encoded by a polycistronic gene, the 405-amino-acid V protein N terminus is shared with the co-amino-terminal P and W proteins (3).…”

“…This sequence conservation accounts for the functional similarity in the IFN evasion activities of the Nipah virus and Hendra virus V proteins. Both V proteins have been demonstrated to subvert IFN responses by sequestering STAT1 and STAT2 in high-molecular-weight cytoplasmic complexes (24,25). In addition to the ability to bind to both STAT1 and STAT2, the Henipavirus V proteins exhibit nuclear-cytoplasmic shuttling behavior that depends on CRM1-dependent nuclear export signals (NES).…”

“…In addition to the ability to bind to both STAT1 and STAT2, the Henipavirus V proteins exhibit nuclear-cytoplasmic shuttling behavior that depends on CRM1-dependent nuclear export signals (NES). Not only does this shuttling affect the steady-state subcellular distribution of the V protein, but it also alters the distribution of the latent STAT1, relocalizing the protein to the cytoplasm (24,25).…”