DDX6 (Rck/p54) Is Required for Efficient Hepatitis C Virus Replication but Not for Internal Ribosome Entry Site-Directed Translation

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Abstract
DDX6 (Rck/p54) is an evolutionarily conserved member of the SF2 DEAD-box RNA helicase family that contributes to the regulation of translation and storage and the degradation of cellular mRNAs. It interacts with multiple proteins and is a component of the micro-RNA (miRNA)-induced silencing complex (miRISC). Since miRNA-122 (miR-122) is essential for efficient hepatitis C virus (HCV) replication, we investigated the requirement for DDX6 in HCV replication in cultured hepatoma cells. Small interfering RNA (siRNA)-mediated knockdown of DDX6 and rescue with an siRNA-resistant mutant demonstrated that DDX6 expression is indeed required for optimal HCV replication. However, DDX6 knockdown did not impair miR-122 biogenesis or alter HCV responsiveness to miR-122 supplementation. Overexpression of DDX6 fused to EYFP (EYFP-DDX6) enhanced replication, whereas a helicase-deficient mutant with a substitution in the conserved DEAD-box motif II (DQAD) had a dominant-negative effect, reducing HCV yields. Coimmunoprecipitation experiments revealed an intracellular complex containing DDX6, HCV core protein, and both viral and cellular RNAs, the formation of which was dependent upon the C-terminal domain of DDX6 but not DDX6 helicase activity. However, since DDX6 abundance influenced the replication of subgenomic HCV RNAs lacking core sequence, the relevance of this complex is uncertain. Importantly, DDX6 knockdown caused minimal reductions in cellular proliferation, generally stimulated cellular translation ([(35)S]Met incorporation), and did not impair translation directed by the HCV internal ribosome entry site. Thus, DDX6 helicase activity is essential for efficient HCV replication, reflecting essential roles for DDX6 in HCV genome amplification and/or maintenance of cellular homeostasis.