HSV-1 ICP22 is a selective viral repressor of cellular RNA polymerase II-mediated transcription elongation

Abstract

The Herpes Simplex Virus (HSV-1) immediate early protein ICP22 interacts with cellular proteins to inhibit host cell gene expression and promote viral gene expression. ICP22 inhibits phosphorylation of Ser2 of the RNA polymerase II (pol II) carboxyl-terminal domain (CTD) and productive elongation of pol II. Here we show that ICP22 affects elongation of pol II through both the early-elongation checkpoint and the poly(A)-associated elongation checkpoint on a protein-coding gene model. Coimmunoprecipitation assays using tagged ICP22 expressed in human cells and pulldown assays with recombinant ICP22 in vitro coupled with mass spectrometry identify transcription elongation factors, including P-TEFb, additional CTD kinases and the FACT complex as interacting cellular factors. Using a photoreactive amino acid incorporated into ICP22, we found that L191, Y230 and C225 crosslink to both subunits of the FACT complex in cells. Our findings indicate that ICP22 physically interacts with critical elongation regulators to inhibit transcription elongation of cellular genes, which may be vital for HSV-1 pathogenesis. We also show that the HSV viral activator, VP16 has a region of structural similarity to the ICP22 region that interacts with elongation factors, suggesting a model where VP16 competes with ICP22 to deliver elongation factors to viral genes.

A Review of the Multipronged Attack of Herpes Simplex Virus 1 on the Host Transcriptional Machinery

“…This study also demonstrated that the viral protein ICP27 coprecipitated with Spt5 in a DRB-responsive manner, while another group further identified ICP22 as a major determinant for Spt5 localization to viral DNA [42]. Spt5 is also copurified with ICP22 in HeLa nuclear extracts [43]. Affected by at least three viral proteins and remaining associated with transcribing polymerases after the pause release, Spt5 would thus be an interesting focus of future work.…”

“…Interestingly, transiently expressed ICP22 was found on a cellular gene by ChIP, suggesting that the loss of CTD phosphorylation is not a result of failure to recruit CDK9 to sites of transcription [51]. Instead, ICP22 and CDK9 are recruited to sites of transcription, and at least for cellular genes, this leads to a local reduction in pS2 hyperphosphorylation and transcriptional elongation as measured by ChIP qPCR [43,51].…”

“…It is important to note that ICP22 has recently been demonstrated to enhance transcriptional elongation on viral genes by PRO-Seq [57], indicating that polymerases can exhibit different activities on cellular and viral chromatin, which may be facilitated through the recruitment of transcriptional Pol II co-factors by ICP22. Furthermore, ICP22 coprecipitates with another Ser2 kinase, CDK12, and numerous other transcription elongation factors, while the functional consequences during infection remain unclear [43]. An additional consideration is that VP16 can coprecipitate with CDK9, which might relieve the inhibitory activity of ICP22 [43,58].…”

“…Furthermore, ICP22 coprecipitates with another Ser2 kinase, CDK12, and numerous other transcription elongation factors, while the functional consequences during infection remain unclear [43]. An additional consideration is that VP16 can coprecipitate with CDK9, which might relieve the inhibitory activity of ICP22 [43,58]. ICP22 has been found to also bind the murine CD80 promoter by ChIP and inhibit its transcriptional activity in vitro and in vivo [59].…”

“…FACT is a heterodimeric histone chaperone composed of two subunits, Spt16 (suppressor of Ty 16) and SSRP1 (structure-specific recognition protein 1), which promote transcription elongation through nucleosomes (reviewed in [117]). It was identified that ICP22 interacts with both FACT subunits by Co-IP and mass spectrometry [42,43], while ICP8 is also purified with Spt16 [118]. FACT can act with P-TEFb to alleviate promoter-proximal pausing [119], and promote Pol II elongation through nucleosomes [120].…”