The Double-Stranded RNA-Binding Protein PACT Functions as a Cellular Activator of RIG-I to Facilitate Innate Antiviral Response

Kin-Hang Kok, Pak-Yin Lui, Ming-Him James Ng, Kam-Leung Siu, Shannon Wing Ngor Au, and Dong-Yan Jin

Cell Host & Microbe 9, 299–309, 2011

Speaker: Chun-Yang Lin (林峻暘)                                Time: 13:00~14:00, Aug. 14, 2011

Commentator: Dr. Yee-Shin Lin (林以行老師)            Place: Room 602

Abstract:

Detection of viral nucleic acids by the cytoplasmic sensor RIG-I triggers signals to the production of type I interferons (IFNs) and inflammatory cytokines. RIG-I is a DExD/H box RNA helicase bearing structural similarity with Dicer, an RNase III-type nuclease that mediates RNAinterference. Human Dicer requires dsRNA-binding protein partners TRBP and PACT for optimal activity in RNAi (1). Furthermore, a C. elegans protein named p110, which is an homolog of RIG-I, was found to interact with PACT homolog RDE4 (2). It prompts authors to examine the interaction between human RIG-I and PACT. First, Co-immunoprecipitation experiments confirm the formation of a RIG-I-PACT protein complex inside cells. Further analysis with truncated GST-RIG-I mutants reveals that the C-terminal repression domain (CTD) of RIG-I is sufficient for binding with PACT. Second, reporter assays show that coexpression of RIG-I and PACT enhance RIG-induced activation of IFN-b promoter. PACT does not potentiate RIG-I-induced activation of NF-kB. A549, IMR90, and MEF cells harboring PACT siRNA result in increased VSV titer. Finally, to determine the involvement of Dicer in RIG-I activation, depletion of Dicer does not have an impact on SeV-induced activation of IFN-b production. Knockdown of PKR has no effect on PACT-mediated activation of RIG-I. Together, this study demonstrates that PACT is a key regulator to amplify RIG-I signaling to IFN-b. In the future, it remains to be elucidated how PACT finetunes RIG-I signaling to differential activation of IRF3 and NF-kB.

References:

1.         Kok, K.H., Ng, M.H., Ching, Y.P., and Jin, D.Y. 2007. Human TRBP and PACT directly interact with each other and associate with dicer to facilitate the production of small interfering RNA. J Biol Chem 282:17649-17657.

2.         Takahasi, K., Yoneyama, M., Nishihori, T., Hirai, R., Kumeta, H., Narita, R., Gale, M., Jr., Inagaki, F., and Fujita, T. 2008. Nonself RNA-sensing mechanism of RIG-I helicase and activation of antiviral immune responses. Mol Cell 29:428-440.