Picornavirus internal ribosome entry site elements target RNA cleavage events induced by the herpes simplex virus virion host shutoff protein

The herpes simplex virus (HSV) virion host shutoff (vhs) protein (UL41 gene product) is a component of the HSV virion tegument that triggers shutoff o f host protein synthesis and accelerated mRNA degradation during the early stages of HSV infection. vhs displays weak amino acid sequence similarity t o the fen-l family of nucleases and suffices to induce accelerated RNA turn over through endoribonucleolytic cleavage events when it is expressed as th e only HSV protein in a rabbit reticulocyte in vitro translation system. Al though vhs selectively targets mRNAs in vivo, the basis for this selectivit y remains obscure, since in vitro activity is not influenced by the presenc e of a 5' cap or 3' poly(A) tail. Here,ve show that vhs activity is greatly altered by placing an internal ribosome entry site (IRES) from encephalomy ocarditis virus or poliovirus in the RNA substrate. Transcripts bearing the IRES mere preferentially cleaved by the vhs-dependent endoribonuclease at multiple sites clustered in a narrow zone located immediately downstream of the element in a reaction that did not require ribosomes. Targeting was ob served when the IRES was located at the 5' end or placed at internal sites in the substrate, indicating that it is independent of position or sequence context, These data indicate that the vhs-dependent nuclease can be select ively targeted by specific cis-acting elements in the RNA substrate, possib ly through secondary structure or a component of the translational machiner y.