The HSV-1 regulatory protein ICP27 interacts with key host cell proteins to mediate the inhibition of pre-mRNA splicing and export of intronless viral mRNA

Infection of metazoan cells with some viruses alters the balance of cellular gene expression to favor viral gene expression. Here, evidence is presented to show that the herpes simplex virus type 1 (HSV-1) immediate-early protein, ICP27, which is essential for viral lytic infection, interacts with and alters key cellular proteins involved in the processing and export of mRNA. ICP27 contributes to the shut-off of host protein synthesis by inhibiting pre-mRNA splicing early in infection. To address the mechanism by which ICP27 performs this function, we demonstrate that ICP27 affects the reversible phosphorylation state of the essential, non-snRNP splicing factors termed SR proteins. In vitro splicing assays showed that ICP27 blocked spliceosome assembly at the pre-spliceosomal complex A stage, resulting in inhibition of the first splicing transesterification reaction. Furthermore, purified SR proteins were able to restore splicing in ICP27-inhibited extracts. In addition, we show that ICP27 interacted with and modulated the specificity of a kinase specific for the SR proteins, termed SRPK1, such that the phosphorylation of SR proteins was altered in vitro. Taken together, these results suggest that ICP27 alters the phosphorylation state of critical splicing factors such that they are unable to support spliceosome assembly during early HSV-1 infection.; ICP27 is required for viral late gene expression and abundant expression of some early genes. Our laboratory and others have shown that ICP27 shuttles between the nucleus and the cytoplasm and it mediates the export of some intronless viral messages, thus contributing to the expression of early and late HSV-1 genes. We demonstrate that ICP27 interacts with a cellular RNA export factor, termed Aly/REF that may provide ICP27 access to the cellular TAP mRNA export pathway. Furthermore, because Aly/REF is associated with the spliceosome, ICP27 may utilize its interaction with splicing factors and its ability to stall spliceosome assembly to access components of cellular splicing-dependent export pathways.