Studies on the export of viral mRNA by the herpes simplex virus type 1 regulatory protein ICP27

In this study, the ability of the Herpes Simplex Virus Type 1 (HSV-1) infected cell protein 27 (ICP27) to interact with components of cellular mRNA export pathway in order to facilitate the nuclear export of viral intronless mRNAs was examined. It has been previously reported that ICP27 binds RNA and can shuttle between the nucleus and the cytoplasm by interacting with the cellular export factors Aly/REF and TAP/NXF1; thus it was predicted that ICP27 is involved in viral mRNA export. A global analysis of viral mRNA localization was performed to determine if ICP27 exploits these interactions to stimulate the export of viral transcripts in infected cells. Using poly(A)+ in situ hybridization and microarray analysis it was found that ICP27 must be able to interact with both RNA and TAP/NXF1 for viral mRNAs to be efficiently exported during viral infection. Thus the export of the vast majority of viral transcripts is ICP27-dependent. The importance of the TAP/NXF1 export receptor in ICP27's shuttling and mRNA export activities was determined utilizing RNA interference and a dominant negative mutant of TAP/NXF1. Knock down of the export adapter protein Aly/REF by RNA interference slightly impaired the localization and export efficiency of ICP27, but did not affect viral replication or protein expression. Thus TAP/NXF1 is essential for the ICP27-mediated export of viral mRNAs while Aly/REF is dispensable, indicating ICP27 is the major export adaptor protein responsible for the export of viral mRNAs. UAP56 is a member of the human TREX complex, along with Aly/REF and the Tho complex. The TREX complex couples mRNA transcription and export. It was found that ICP27 could bind UAP56 both in vivo and in vitro and Aly/REF stabilized this interaction in vitro. UAP56 also cross-linked to viral mRNAs, suggesting that UAP56 binds to viral messages and may be involved in viral mRNA export. We predict that ICP27's interaction with Aly/REF facilitates the recruitment of the TREX complex to sites of viral transcription to aid in viral gene expression and interfere with host mRNA synthesis and processing.