Identification of three host translation inhibitory factors encoded in Cotesia glomerata bracovirus

An endoparasitoid wasp, Cotesia glomerata (Braconidae: Hymenoptera), parasitizes larvae of the diamondback moth, Plutella xylostella (Yponomeutidae: Lepidoptera). Parasitized P. xylostella exhibits immunosuppression and developmental alteration. In this study, the parasitized larvae exhibited a marked suppression in storage proteins (SPs) especially at SP1 out of three SPs, although there was significant mRNA level of SP1. In addition, the parasitization significantly inhibited spreading behavior of hemocytes, in which a host translation inhibitory factor (HTIF)-like protein was detected in cytoplasm. A bracovirus is a symbiotic polydnavirus of C. glomerata (CgBV) and has been suspected as a main parasitic factor manipulating host physiological processes. Here, we cloned three putative HTIFs derived from CgBV: Cg-HTIF(I), Cg-HTIF(II) and Cg-HTIF(III). All Cg-HTIFs share sequence homologies with eukaryotic initiation factors for translation as well as with two other known HTIFs of Cotesia plutellae bracovirus. Quantitative RT-PCR and immunoblotting assays indicated that these genes were mostly expressed at late developmental stage of P. xylostella parasitized by C. glomerata. Transfection of a recombinant Cg-HTIF(I) in P. xylostella by microinjection expressed the gene as early as 12 h and maintained until 72 h, at which hemocyte behavior was impaired and the SP levels were significantly reduced. However, the expression of Cg-HTIF(I) did not inhibit transcription of SP1. Co-injection of the recombinant vector along with double-stranded RNA specific to Cg-HTIF(I) notably recovered the hemocyte-spreading behavior and SP1 translation.