Ebolavirusprotein VP35 binds dsRNA , blocks IRF-3 activation

http://jvi.asm.org/content/80/11/5168.abstract
http://vir.sgmjournals.org/content/91/2/352.full.pdf

Ebola Virus VP35 Protein Binds Double-Stranded RNA and Inhibits Alpha/Beta Interferon Production Induced by RIG-I Signaling

1. Washington B. Cárdenas1,et al.

+ Author Affiliations

1. ¹Department of Microbiology, Mount Sinai School of Medicine, New York, New York 10029
2. ²Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas
3. ³Special Pathogens Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road, MS G-14, Atlanta, Georgia 30329
4. ⁴Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Rd., La Jolla, California 92037

ABSTRACT

The Ebola virus (EBOV) VP35 protein blocks the virus-induced phosphorylation and activation of interferon regulatory factor 3 (IRF-3), a transcription factor critical for the induction of alpha/beta interferon (IFN-α/β) expression. However, the mechanism(s) by which this blockage occurs remains incompletely defined. We now provide evidence that VP35 possesses double-stranded RNA (dsRNA)-binding activity. Specifically, VP35 bound to poly(rI) · poly(rC)-coated Sepharose beads but not control beads. In contrast, two VP35 point mutants, R312A and K309A, were found to be greatly impaired in their dsRNA-binding activity. Competition assays showed that VP35 interacted specifically with poly(rI) · poly(rC), poly(rA) · poly(rU), or in vitro-transcribed dsRNAs derived from EBOV sequences, and not with single-stranded RNAs (ssRNAs) or double-stranded DNA. 

We then screened wild-type and mutant VP35s for their ability to target different components of the signaling pathways that activate IRF-3. These experiments indicate that VP35 blocks activation of IRF-3 induced by overexpression of RIG-I, a cellular helicase recently implicated in the activation of IRF-3 by either virus or dsRNA. Interestingly, the VP35 mutants impaired for dsRNA binding have a decreased but measurable IFN antagonist activity in these assays. Additionally, wild-type and dsRNA-binding-mutant VP35s were found to have equivalent abilities to inhibit activation of the IFN-β promoter induced by overexpression of IPS-1, a recently identified signaling molecule downstream of RIG-I, or by overexpression of the IRF-3 kinases IKKε and TBK-1. These data support the hypothesis that dsRNA binding may contribute to VP35 IFN antagonist function. However, additional mechanisms of inhibition, at a point proximal to the IRF-3 kinases, most likely also exist.

• Received 19 October 2005.