Characterising the relationship between the herpes simplex virus tegument proteins VP22 and vhs

Abstract

Structural protein VP22 is a major component of the virus tegument of herpes simplex virus-1 (HSV-1). VP22 is not essential for the structure of the virus but has been associated with several functions, including the regulation of the virion host shut off (vhs) endoribonuclease activity and the promotion of optimal late protein synthesis. Many of its activities are believed to function through its interaction with a second tegument protein, VP16, which is known to help downregulate vhs activity during infection. It has been proposed that when the VP22-encoding gene is deleted, HSV-1 acquires spontaneous secondary mutations within vhs rendering it inactive. Here we show that a wild-type vhs is not inherently lethal for virus replication in the absence of VP22, as we generated a replication-competent Δ22 virus by homologous recombination which maintains a wild-type vhs gene and has no other gross mutations. By contrast, replication-competent Δ22 viruses recovered from a bacterial artificial chromosome contain multiple amino acid changes within a conserved region of vhs. Hence, we conclude that the mode of virus rescue influences the acquisition of secondary mutations. Nonetheless, we demonstrate that wild-type vhs is poorly expressed in the absence of VP22 in infection, a defect that is attributed to poor translation rather transcription. While VP22 has been shown to bind to vhs only in the presence of VP16, it is shown here that this VP22-VP16 complex is neither sufficient nor required for the efficient translation of vhs. Moreover, using primary human fibroblasts as a physiologically relevant model system, VP22 is shown to enhance the translation of additional virus proteins, revealing a general role in infected cell translation. Finally, and unlike the situation in permissive Vero cells, the Δ22 virus fails to form plaque in primary human fibroblasts, indicating that VP22 is essential for virus replication in this highly relevant cell type.