Identification of host translation inhibitory factor of Campoletis sonorensis ichnovirus on the tobacco budworm, Heliothis virescens

Parasitization of a wasp, Campoletis sonorensis, against the larvae of Heliothis virescens depresses synthesis of specific host proteins related to growth and immunity. It has been suggested that the inhibition of host gene expression is targeted at a posttranscriptional level. This study aimed to verify the identity of host translation inhibitory factor (HTIF) derived from wasp parasitization. To identify HTIF, the proteins in the parasitized host were fractionated using different protein purification methods, and each fraction's HTIF activity was assessed. In the course of the protein purification steps, HTIF activity was highly correlated with the fractions containing VHv1.4 protein, which has a conserved cysteine-motif and is encoded in C. sonorensis ichnovirus (CsIV). Purified VHv1.4 protein using an immunoaffinity column exhibited a significant HTIF effect, while the heat-inactivated VHv1.4 did not. Both recombinant VHv1.4 and VHv1.1 (another cys-motif protein encoded in CsIV) proteins were synthesized in Sf9 cells through a baculovirus expression system. The purified recombinant VHv1.4 and VHv1.1 exhibited significant HTIF activities in a nanomolar range. However, VHv1.4 protein showed about four times higher HTIF activity than did VHv1.1 protein. Both HTIFs acted directly on translation machinery because they inhibited a cell-free in vitro translation system using rabbit reticulocyte lysate. Both HTIFs are likely to discriminate specific target mRNAs because they inhibited translation of RNA extracts from the Tn368 cell line, but not from Sf9 cells. In addition, they inhibited translation of RNAs from fat body, hemocytes, and testis, but not from epidermis, gut, labial gland, and nerve tissues of H. virescens. These results indicate that both cys-motif proteins of VHv1.4 and VHv1.1 play a role as HTIF in C. sonorensis parasitization.