A molecular mechanism underlies grass carp (Ctenopharyngodon idella) TARBP2 regulating PKR-mediated cell apoptosis

Interferon-inducible double-stranded RNA-dependent protein kinase (PKR) is one of the key antiviral arms in the innate immune system. The activated PKR performs its antiviral function by inhibiting protein translation and inducing apoptosis. In our previous study, we identified grass carp TARBP2 as an inhibitor of PKR activity, thereby suppressing cell apoptosis. This study aimed to explore the effects of grass carp TARBP2 on PKR activity and cell apoptosis. Grass carp TARBP2 comprises two N-terminal dsRBDs and a C-terminal C4 domain. Subcellular localization analysis conducted in CIK cells revealed that TARBP2-FL (full-length TARBP2), TARBP2-Δ1 (lack of the first dsRBD), and TARBP2-Δ2 (lack of the second dsRBD) are predominantly located in the cytoplasm, while TARBP2-Δ3 (lack of the two dsRBDs) is distributed both in the nucleus and cytoplasm. Colocalization and immunoprecipitation assays confirmed the interaction of TARBP2-FL, TARBP2-Δ1, and TARBP2-Δ2 with PKR, while TARBP2-Δ3 showed no binding. Furthermore, our findings suggested that the inhibitory effect of TARBP2-Δ1 or TARBP2-Δ2 on the PKR-eIF2α pathway is depressed compared to TARBP2-FL. In cell apoptosis assays, it was observed that TARBP2-FL inhibits PKR-mediated cell apoptosis. TARBP2-Δ1 or TARBP2-Δ2 exhibits decreased inhibition to PKR-mediated cell apoptosis, whereas TARBP2-Δ3 nearly completely loses this inhibitory effect. These findings highlight the critical importance of two dsRBDs of TARBP2 in interaction with PKR, as well as in the inhibition of PKR activity, resulting in the suppression of cell apoptosis triggered by prolonged PKR activation.