The virion host shutoff protein (vhs) of herpes simplex virus triggers accelerated degradation of cellular and viral mRNAs while sparing other cytoplasmic RNA species. Previous work has shown that vhs forms a complex with translation initiation factor eIF4H, which displays detectable RNase activity in the absence of other viral or host proteins. However, the contributions of eIF4H and other host factors to the activity and mRNA targeting properties of vhs have not yet been directly examined. An earlier report from our laboratory demonstrated that rabbit reticulocyte lysate (RRL) contains one or more factors that strongly stimulate the RNase activity of vhs produced in Saccharomyces cerevisiae. We report here that such yeast extracts display significant vhs-dependent RNase activity in the absence of mammalian factors. This activity differs from that displayed by vhs generated in RRL in that it is not targeted to the encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES). Activity was strongly enhanced by the addition of RRL, eIF4H, or the related translation factor eIF4B. RRL also reconstituted strong targeting to the EMCV IRES, resulting in a major change in the RNA cleavage pattern. In contrast, eIF4H and eIF4B did not reconstitute IRES-directed targeting. These data indicate that eIF4B and 4H stimulate the nuclease activity of vhs, and they provide evidence that additional mammalian factors are required for targeting to the EMCV IRES.
Amino Acid Sequence Animals