Crucian carp HERC7 is an RNA binding protein that selectively targets IRF7 mRNA to downregulate IFN response

Crucian carp (Carassius auratus) HERC7 is a recently identified, fish-specific member of small HERC family that plays a pivotal role in negatively regulating the interferon (IFN) antiviral response by targeting three RLR signaling factors, STING, MAVS and IRF7. Mechanistically, crucian carp HERC7 facilitates proteasome-dependent protein degradation to downregulate STING- and MAVS-mediated IFN response. Although HERC7's role in degrading IRF7 protein is known, its potential regulation of IRF7 mRNA has not been clarified. In this study, we further characterized crucian carp HERC7 as an unorthodox RNA binding protein (RBP) that was unique to IRF7 coding RNA, exactly to its N-terminal coding region. These features endowed crucian carp HERC7 to facilitate IRF7 mRNA decay, thereby downregulating IRF7-mediated IFN response. Despite lacking canonical RNA-binding domains (RBDs), the N-terminal RLD domain of crucian carp HERC7 protein exhibited a binding potential to IRF7 mRNA in vitro. However, either the full length or the E3 ligase activity was required for crucian carp HERC7 to bind IRF7 mRNA in living cells. Functional comparisons between wild-type crucian carp HERC7 and its E3 ligase mutant (C958A) further revealed that, crucian carp HERC7 inhibited IRF7-mediated IFN response primarily by targeting IRF7 mRNA decay, which required its E3 ligase activity and RNA binding activity, while C958A did so by targeting IRF7 protein degradation independently of both activities. Taken together, our results suggest that crucian carp HERC7 has acquired RNA-binding activity and E3 ligase activity to fine-tune fish IFN response through two distinct mechanisms.